Initial working fully go release

.gitignore

@@ -5,3 +5,10 @@ interfaces/
 synthetic.json
 uopi.toml
 *.local.toml
+resources
+.claude
+.github
+.goreleaser.yaml
+.iocsh_history
+go.work.sum
+*.log

Makefile

@@ -1,4 +1,4 @@
-.PHONY: all frontend backend backend-epics backend-debug release release-epics test bench race lint clean run
+.PHONY: all frontend backend backend-debug release catools test bench race lint clean run
 
 # --------------------------------------------------------------------------- #
 # Sources — adding any file here triggers a frontend or backend rebuild        #
@@ -8,7 +8,9 @@ FRONTEND_SRCS := $(shell find web/src -name '*.tsx' -o -name '*.ts' -o -name '*.
                  $(wildcard web/vendor/*.mjs web/vendor/*.js web/vendor/*.css) \
                  tools/buildfrontend/main.go
 
-GO_SRCS := $(shell find cmd internal -name '*.go') web/embed.go go.mod go.sum
+GO_SRCS := $(shell find cmd internal -name '*.go') \
+           $(shell find pkg/ca -name '*.go') \
+           web/embed.go go.mod go.sum go.work
 
 # --------------------------------------------------------------------------- #
 # Outputs                                                                      #
@@ -16,16 +18,17 @@ GO_SRCS := $(shell find cmd internal -name '*.go') web/embed.go go.mod go.sum
 
 FRONTEND_OUT := web/dist/main.js
 BINARY       := dist/uopi
+CATOOLS      := dist/catools
 
 # --------------------------------------------------------------------------- #
 # Top-level targets                                                            #
 # --------------------------------------------------------------------------- #
 
-all: $(BINARY)
+all: $(BINARY) $(CATOOLS)
 
 frontend: $(FRONTEND_OUT)
 
-backend: $(BINARY)
+backend: $(BINARY) $(CATOOLS)
 
 # --------------------------------------------------------------------------- #
 # Build rules                                                                  #
@@ -36,127 +39,40 @@ $(FRONTEND_OUT): $(FRONTEND_SRCS)
 	@mkdir -p web/dist
 	go run ./tools/buildfrontend/main.go
 
-# Compile the self-contained binary (embeds web/dist at build time)
+# Compile the self-contained binary (embeds web/dist at build time).
+# Pure-Go CA client (pkg/ca) is always used; CGO_ENABLED=0 produces a fully
+# static binary with no external library dependencies.
 $(BINARY): $(GO_SRCS) $(FRONTEND_OUT)
 	@mkdir -p dist
-	go build -ldflags="-s -w" -o $(BINARY) ./cmd/uopi
+	CGO_ENABLED=0 go build -ldflags="-s -w" -o $(BINARY) ./cmd/uopi
 
-# Build with EPICS Channel Access support.
+# CA diagnostic tools (caget/caput/cainfo/camonitor equivalents)
-#
+$(CATOOLS): $(GO_SRCS)
-# Auto-detection: if EPICS_BASE is set in the environment, CGO flags are
-# derived automatically.  Override any variable on the command line:
-#
-#   make backend-epics EPICS_BASE=/path/to/epics/base EPICS_ARCH=linux-x86_64
-#
-# If EPICS_BASE is not set, set it to the directory that contains
-# include/cadef.h and lib/<arch>/libca.so.
-
-EPICS_BASE ?= $(EPICS_BASE)
-
-# Detect host arch from EPICS startup script if not provided.
-EPICS_ARCH ?= $(shell \
-  if [ -x "$(EPICS_BASE)/startup/EpicsHostArch" ]; then \
-    "$(EPICS_BASE)/startup/EpicsHostArch"; \
-  elif [ -x "$(EPICS_BASE)/lib" ]; then \
-    ls "$(EPICS_BASE)/lib" | grep linux | head -1; \
-  else \
-    echo "linux-x86_64"; \
-  fi)
-
-EPICS_CGO_CFLAGS  ?= -I$(EPICS_BASE)/include -I$(EPICS_BASE)/include/os/Linux -I$(EPICS_BASE)/include/compiler/gcc
-EPICS_CGO_LDFLAGS ?= -L$(EPICS_BASE)/lib/$(EPICS_ARCH) -lca -lCom -Wl,-rpath,$(EPICS_BASE)/lib/$(EPICS_ARCH)
-
-backend-epics: $(FRONTEND_OUT)
-	@if [ -z "$(EPICS_BASE)" ]; then \
-	  echo "ERROR: EPICS_BASE is not set."; \
-	  echo "  export EPICS_BASE=/path/to/epics/base"; \
-	  echo "  then re-run: make backend-epics"; \
-	  exit 1; \
-	fi
-	@echo "Building with EPICS support"
-	@echo "  EPICS_BASE = $(EPICS_BASE)"
-	@echo "  EPICS_ARCH = $(EPICS_ARCH)"
 	@mkdir -p dist
-	CGO_ENABLED=1 \
+	CGO_ENABLED=0 go build -ldflags="-s -w" -o $(CATOOLS) ./cmd/catools
-	  CGO_CFLAGS="$(EPICS_CGO_CFLAGS)" \
-	  CGO_LDFLAGS="$(EPICS_CGO_LDFLAGS)" \
-	  go build -tags epics -ldflags="-s -w" -o $(BINARY) ./cmd/uopi
-	@echo "Built: $(BINARY)  (EPICS CA enabled)"
 
-# Unstripped binary for debugging
+# Unstripped binary for debugging (pure-Go CA, CGO_ENABLED=0)
 backend-debug: $(FRONTEND_OUT)
 	@mkdir -p dist
-	go build -o $(BINARY) ./cmd/uopi
+	CGO_ENABLED=0 go build -o $(BINARY) ./cmd/uopi
+	CGO_ENABLED=0 go build -o $(CATOOLS) ./cmd/catools
 
 # --------------------------------------------------------------------------- #
-# Portable release targets                                                     #
+# Release                                                                      #
 # --------------------------------------------------------------------------- #
 #
-# Two flavours depending on whether EPICS is needed:
+# Fully static, pure-Go CA client, no external library dependencies.
-#
+# Cross-compile: make release RELEASE_OS=linux RELEASE_ARCH=arm64
-#   make release            — pure-Go, CGO_ENABLED=0, 100 % static binary.
-#                             No EPICS; works anywhere without extra libs.
-#
-#   make release-epics      — CGO build with EPICS CA.
-#                             Tries to link libca/libCom statically (if .a
-#                             archives are present in $(EPICS_LIBDIR)).
-#                             Falls back to bundled shared libs + $ORIGIN
-#                             rpath when only .so files are available.
-#                             The output directory (dist/release/) contains
-#                             everything needed to run on the target machine.
-#
-# Cross-compilation (no-EPICS only): override RELEASE_OS / RELEASE_ARCH.
-#   make release RELEASE_OS=linux RELEASE_ARCH=arm64
 
 RELEASE_OS   ?= linux
 RELEASE_ARCH ?= amd64
 RELEASE_DIR  := dist/release
-EPICS_LIBDIR := $(EPICS_BASE)/lib/$(EPICS_ARCH)
 
-# Fully static, no CGO — the most portable build possible.
 release: $(FRONTEND_OUT)
 	@mkdir -p $(RELEASE_DIR)
 	CGO_ENABLED=0 GOOS=$(RELEASE_OS) GOARCH=$(RELEASE_ARCH) \
 	  go build -ldflags="-s -w" -o $(RELEASE_DIR)/uopi ./cmd/uopi
-	@echo "Built: $(RELEASE_DIR)/uopi  (fully static, no EPICS)"
+	@echo "Built: $(RELEASE_DIR)/uopi  (static, pure-Go CA)"
-
-# EPICS release — maximum static linking.
-# Strategy A (preferred): if libca.a + libCom.a exist, embed them directly
-#   → single self-contained binary, no extra files needed.
-# Strategy B (fallback): copy the shared libs next to the binary and set an
-#   $ORIGIN rpath so the OS finds them regardless of installation prefix.
-release-epics: $(FRONTEND_OUT)
-	@if [ -z "$(EPICS_BASE)" ]; then \
-	  echo "ERROR: EPICS_BASE is not set."; \
-	  echo "  export EPICS_BASE=/path/to/epics/base"; \
-	  exit 1; \
-	fi
-	@mkdir -p $(RELEASE_DIR)
-	@echo "Building portable EPICS release"
-	@echo "  EPICS_BASE = $(EPICS_BASE)"
-	@echo "  EPICS_ARCH = $(EPICS_ARCH)"
-	@if [ -f "$(EPICS_LIBDIR)/libca.a" ] && [ -f "$(EPICS_LIBDIR)/libCom.a" ]; then \
-	  echo "  Strategy A: static .a archives found — embedding libca + libCom"; \
-	  CGO_ENABLED=1 GOOS=linux GOARCH=$(RELEASE_ARCH) \
-	    CGO_CFLAGS="$(EPICS_CGO_CFLAGS)" \
-	    CGO_LDFLAGS="-Wl,-Bstatic -L$(EPICS_LIBDIR) -lca -lCom -Wl,-Bdynamic -lm -lpthread -lrt -ldl" \
-	    go build -tags epics \
-	      -ldflags="-s -w -linkmode external" \
-	      -o $(RELEASE_DIR)/uopi ./cmd/uopi; \
-	  echo "Built: $(RELEASE_DIR)/uopi  (EPICS statically embedded)"; \
-	else \
-	  echo "  Strategy B: no .a archives — bundling shared libs with \$$ORIGIN rpath"; \
-	  CGO_ENABLED=1 GOOS=linux GOARCH=$(RELEASE_ARCH) \
-	    CGO_CFLAGS="$(EPICS_CGO_CFLAGS)" \
-	    CGO_LDFLAGS="-L$(EPICS_LIBDIR) -lca -lCom -Wl,-rpath,\$$ORIGIN" \
-	    go build -tags epics \
-	      -ldflags="-s -w" \
-	      -o $(RELEASE_DIR)/uopi ./cmd/uopi; \
-	  cp $(EPICS_LIBDIR)/libca.so*  $(RELEASE_DIR)/; \
-	  cp $(EPICS_LIBDIR)/libCom.so* $(RELEASE_DIR)/; \
-	  echo "Built: $(RELEASE_DIR)/  (deploy the whole directory together)"; \
-	  ls $(RELEASE_DIR)/; \
-	fi
 
 # --------------------------------------------------------------------------- #
 # Dev / CI                                                                     #
@@ -164,10 +80,12 @@ release-epics: $(FRONTEND_OUT)
 
 test:
 	go test ./...
+	cd pkg/ca && go test ./...
 
 # Run tests with the race detector enabled.
 race:
 	go test -race ./...
+	cd pkg/ca && go test -race ./...
 
 # Run all benchmarks and print memory allocations.
 bench:
@@ -179,5 +97,7 @@ lint:
 run: $(BINARY)
 	$(BINARY)
 
+catools: $(CATOOLS)
+
 clean:
 	rm -rf dist/ web/dist/

cmd/catools/main.go

@@ -0,0 +1,327 @@
+// Command catools provides CA (Channel Access) command-line utilities using
+// the pure-Go goca library.
+//
+// Usage:
+//
+//	catools get     <pv> [pv...]     Get current value(s)
+//	catools put     <pv> <value>     Write a value
+//	catools monitor <pv> [pv...]     Stream live updates (Ctrl-C to stop)
+//	catools info    <pv> [pv...]     Print metadata (type, units, limits)
+//
+// Environment:
+//
+//	EPICS_CA_ADDR_LIST       — space-separated CA server addresses
+//	EPICS_CA_AUTO_ADDR_LIST  — set to "NO" to disable broadcast search
+//	UOPI_CA_DEBUG            — set to "1" for CA protocol debug logging
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/signal"
+	"strconv"
+	"strings"
+	"syscall"
+	"time"
+
+	ca "github.com/uopi/goca"
+	"github.com/uopi/goca/proto"
+)
+
+func usage() {
+	fmt.Fprintln(os.Stderr, `catools — EPICS Channel Access command-line tool
+
+Usage:
+  catools get     <pv> [pv...]     Get current value(s)
+  catools put     <pv> <value>     Write a value
+  catools monitor <pv> [pv...]     Stream live updates (Ctrl-C to stop)
+  catools info    <pv> [pv...]     Print metadata (type, units, limits)
+
+Environment:
+  EPICS_CA_ADDR_LIST       Space-separated CA server addresses
+  EPICS_CA_AUTO_ADDR_LIST  Set to NO to disable broadcast search
+  UOPI_CA_DEBUG            Set to 1 for CA protocol debug logging`)
+	os.Exit(1)
+}
+
+func main() {
+	if len(os.Args) < 2 {
+		usage()
+	}
+
+	cmd := os.Args[1]
+	args := os.Args[2:]
+
+	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+	defer stop()
+
+	cfg := ca.ConfigFromEnv()
+	cli, err := ca.NewClient(ctx, cfg)
+	if err != nil {
+		fatalf("connect: %v", err)
+	}
+
+	switch cmd {
+	case "get":
+		if len(args) == 0 {
+			fatalf("get: no PV names specified")
+		}
+		runGet(ctx, cli, args)
+	case "put":
+		if len(args) < 2 {
+			fatalf("put: usage: catools put <pv> <value>")
+		}
+		runPut(ctx, cli, args[0], args[1])
+	case "info":
+		if len(args) == 0 {
+			fatalf("info: no PV names specified")
+		}
+		runInfo(ctx, cli, args)
+	case "monitor":
+		if len(args) == 0 {
+			fatalf("monitor: no PV names specified")
+		}
+		runMonitor(ctx, cli, args)
+	default:
+		fatalf("unknown command %q — use get, put, monitor, or info", cmd)
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// get                                                                         //
+// -------------------------------------------------------------------------- //
+
+func runGet(ctx context.Context, cli *ca.Client, pvs []string) {
+	tctx, cancel := context.WithTimeout(ctx, 30*time.Second)
+	defer cancel()
+
+	for _, pv := range pvs {
+		tv, err := cli.Get(tctx, pv)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "%s: error: %v\n", pv, err)
+			continue
+		}
+		fmt.Printf("%-30s %s\n", pv, formatValue(tv))
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// put                                                                         //
+// -------------------------------------------------------------------------- //
+
+func runPut(ctx context.Context, cli *ca.Client, pv, valueStr string) {
+	tctx, cancel := context.WithTimeout(ctx, 30*time.Second)
+	defer cancel()
+
+	// Get metadata to determine the right type.
+	ci, err := cli.GetCtrl(tctx, pv)
+	if err != nil {
+		fatalf("%s: get metadata: %v", pv, err)
+	}
+
+	var value any
+	switch ci.DBFType {
+	case proto.DBFString:
+		value = valueStr
+	case proto.DBFEnum:
+		if n, err := strconv.ParseInt(valueStr, 10, 32); err == nil {
+			value = int32(n)
+		} else if ci.Enum != nil {
+			for i, s := range ci.Enum.Strings {
+				if strings.EqualFold(s, valueStr) {
+					value = int32(i)
+					break
+				}
+			}
+		}
+		if value == nil {
+			fatalf("%s: unknown enum value %q", pv, valueStr)
+		}
+	default:
+		if n, err := strconv.ParseFloat(valueStr, 64); err == nil {
+			value = n
+		} else if n, err := strconv.ParseInt(valueStr, 10, 64); err == nil {
+			value = int64(n)
+		} else {
+			value = valueStr
+		}
+	}
+
+	if err := cli.Put(tctx, pv, value); err != nil {
+		fatalf("%s: put: %v", pv, err)
+	}
+
+	tv, err := cli.Get(tctx, pv)
+	if err != nil {
+		fmt.Printf("%-30s %s (write sent, readback failed: %v)\n", pv, valueStr, err)
+		return
+	}
+	fmt.Printf("%-30s %s\n", pv, formatValue(tv))
+}
+
+// -------------------------------------------------------------------------- //
+// info                                                                        //
+// -------------------------------------------------------------------------- //
+
+func runInfo(ctx context.Context, cli *ca.Client, pvs []string) {
+	tctx, cancel := context.WithTimeout(ctx, 30*time.Second)
+	defer cancel()
+
+	for _, pv := range pvs {
+		ci, err := cli.GetCtrl(tctx, pv)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "%s: error: %v\n", pv, err)
+			continue
+		}
+		printInfo(pv, ci)
+		fmt.Println()
+	}
+}
+
+func printInfo(pv string, ci ca.CtrlInfo) {
+	fmt.Printf("Name       : %s\n", pv)
+	fmt.Printf("DBF type   : %s (%d)\n", dbfTypeName(ci.DBFType), ci.DBFType)
+	fmt.Printf("Count      : %d\n", ci.Count)
+	writable := "No"
+	if ci.Access&proto.AccessWrite != 0 {
+		writable = "Yes"
+	}
+	fmt.Printf("Writable   : %s\n", writable)
+
+	switch {
+	case ci.Double != nil:
+		d := ci.Double
+		fmt.Printf("Units      : %s\n", d.Units)
+		fmt.Printf("Disp range : [%g, %g]\n", d.LowerDispLimit, d.UpperDispLimit)
+		fmt.Printf("Ctrl range : [%g, %g]\n", d.LowerCtrlLimit, d.UpperCtrlLimit)
+		fmt.Printf("Alarm LOLO : %g  HIHI: %g\n", d.LowerAlarmLimit, d.UpperAlarmLimit)
+		fmt.Printf("Warn  LOW  : %g  HIGH: %g\n", d.LowerWarnLimit, d.UpperWarnLimit)
+	case ci.Long != nil:
+		l := ci.Long
+		fmt.Printf("Units      : %s\n", l.Units)
+		fmt.Printf("Disp range : [%d, %d]\n", l.LowerDispLimit, l.UpperDispLimit)
+		fmt.Printf("Ctrl range : [%d, %d]\n", l.LowerCtrlLimit, l.UpperCtrlLimit)
+		fmt.Printf("Alarm LOLO : %d  HIHI: %d\n", l.LowerAlarmLimit, l.UpperAlarmLimit)
+		fmt.Printf("Warn  LOW  : %d  HIGH: %d\n", l.LowerWarnLimit, l.UpperWarnLimit)
+	case ci.Enum != nil:
+		fmt.Printf("Enum states: %d\n", len(ci.Enum.Strings))
+		for i, s := range ci.Enum.Strings {
+			fmt.Printf("  [%d] %s\n", i, s)
+		}
+	case ci.Str != nil:
+		fmt.Printf("Type       : string\n")
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// monitor                                                                     //
+// -------------------------------------------------------------------------- //
+
+func runMonitor(ctx context.Context, cli *ca.Client, pvs []string) {
+	type entry struct {
+		pv  string
+		ch  chan proto.TimeValue
+		can context.CancelFunc
+	}
+
+	entries := make([]*entry, 0, len(pvs))
+	tctx, cancel := context.WithTimeout(ctx, 30*time.Second)
+	defer cancel()
+
+	for _, pv := range pvs {
+		ch := make(chan proto.TimeValue, 32)
+		cancelFn, err := cli.Subscribe(tctx, pv, ch)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "%s: subscribe error: %v\n", pv, err)
+			continue
+		}
+		entries = append(entries, &entry{pv: pv, ch: ch, can: cancelFn})
+		fmt.Printf("Monitoring %s — press Ctrl-C to stop\n", pv)
+	}
+
+	if len(entries) == 0 {
+		return
+	}
+
+	for _, e := range entries {
+		go func(e *entry) {
+			for {
+				select {
+				case tv, ok := <-e.ch:
+					if !ok {
+						return
+					}
+					ts := tv.Timestamp.Local().Format("15:04:05.000")
+					fmt.Printf("%s  %-30s  %s\n", ts, e.pv, formatValue(tv))
+				case <-ctx.Done():
+					return
+				}
+			}
+		}(e)
+	}
+
+	<-ctx.Done()
+	for _, e := range entries {
+		e.can()
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Formatting helpers                                                          //
+// -------------------------------------------------------------------------- //
+
+func formatValue(tv proto.TimeValue) string {
+	var val string
+	switch {
+	case tv.Str != "":
+		val = tv.Str
+	case len(tv.Doubles) > 1:
+		parts := make([]string, len(tv.Doubles))
+		for i, v := range tv.Doubles {
+			parts[i] = strconv.FormatFloat(v, 'g', -1, 64)
+		}
+		val = "[" + strings.Join(parts, " ") + "]"
+	default:
+		val = strconv.FormatFloat(tv.Double, 'g', -1, 64)
+	}
+
+	severity := ""
+	switch tv.Severity {
+	case proto.SeverityMinor:
+		severity = " MINOR"
+	case proto.SeverityMajor:
+		severity = " MAJOR"
+	case proto.SeverityInvalid:
+		severity = " INVALID"
+	}
+
+	ts := tv.Timestamp.Local().Format("2006-01-02 15:04:05.000")
+	return fmt.Sprintf("%s%s  [%s]", val, severity, ts)
+}
+
+func dbfTypeName(t int) string {
+	switch t {
+	case proto.DBFString:
+		return "DBF_STRING"
+	case proto.DBFShort:
+		return "DBF_SHORT"
+	case proto.DBFFloat:
+		return "DBF_FLOAT"
+	case proto.DBFEnum:
+		return "DBF_ENUM"
+	case proto.DBFChar:
+		return "DBF_CHAR"
+	case proto.DBFLong:
+		return "DBF_LONG"
+	case proto.DBFDouble:
+		return "DBF_DOUBLE"
+	default:
+		return "DBF_UNKNOWN"
+	}
+}
+
+func fatalf(format string, args ...any) {
+	fmt.Fprintf(os.Stderr, "catools: "+format+"\n", args...)
+	os.Exit(1)
+}

cmd/pvtools/main.go

@@ -0,0 +1,253 @@
+// Command pvtools provides PV Access command-line utilities using
+// the pure-Go gopva library.
+//
+// Usage:
+//
+//	pvtools get     <pv> [pv...]     Get current value(s) via PVA
+//	pvtools put     <pv> <value>     Write a value via PVA
+//	pvtools monitor <pv> [pv...]     Stream live updates (Ctrl-C to stop)
+//	pvtools info    <pv> [pv...]     Print PVA structure info
+//
+// Environment:
+//
+//	EPICS_PVA_ADDR_LIST  — space-separated PVA server addresses
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/signal"
+	"strconv"
+	"strings"
+	"syscall"
+	"time"
+
+	pva "github.com/uopi/gopva"
+	"github.com/uopi/gopva/pvdata"
+)
+
+func usage() {
+	fmt.Fprintln(os.Stderr, `pvtools — EPICS PV Access command-line tool
+
+Usage:
+  pvtools get     <pv> [pv...]     Get current value(s)
+  pvtools put     <pv> <value>     Write a value
+  pvtools monitor <pv> [pv...]     Stream live updates (Ctrl-C to stop)
+  pvtools info    <pv> [pv...]     Print structure info
+
+Environment:
+  EPICS_PVA_ADDR_LIST  Space-separated PVA server addresses`)
+	os.Exit(1)
+}
+
+func main() {
+	if len(os.Args) < 2 {
+		usage()
+	}
+
+	cmd := os.Args[1]
+	args := os.Args[2:]
+
+	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+	defer stop()
+
+	switch cmd {
+	case "get":
+		if len(args) == 0 {
+			fatalf("get: no PV names specified")
+		}
+		runGet(ctx, args)
+	case "put":
+		if len(args) < 2 {
+			fatalf("put: usage: pvtools put <pv> <value>")
+		}
+		fatalf("put: PVA write not yet implemented")
+	case "monitor":
+		if len(args) == 0 {
+			fatalf("monitor: no PV names specified")
+		}
+		runMonitor(ctx, args)
+	case "info":
+		if len(args) == 0 {
+			fatalf("info: no PV names specified")
+		}
+		runInfo(ctx, args)
+	default:
+		fatalf("unknown command %q — use get, put, monitor, or info", cmd)
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// get                                                                         //
+// -------------------------------------------------------------------------- //
+
+func runGet(ctx context.Context, pvs []string) {
+	cl, err := pva.NewClient()
+	if err != nil {
+		fatalf("connect: %v", err)
+	}
+	defer cl.Close()
+
+	tctx, cancel := context.WithTimeout(ctx, 30*time.Second)
+	defer cancel()
+
+	for _, name := range pvs {
+		sv, err := cl.Get(tctx, name)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "%s: error: %v\n", name, err)
+			continue
+		}
+		fmt.Printf("%-30s %s\n", name, formatValue(sv))
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// monitor                                                                     //
+// -------------------------------------------------------------------------- //
+
+func runMonitor(ctx context.Context, pvs []string) {
+	cl, err := pva.NewClient()
+	if err != nil {
+		fatalf("connect: %v", err)
+	}
+	defer cl.Close()
+
+	for _, name := range pvs {
+		ch := cl.Monitor(ctx, name)
+		go func(pvName string, ch <-chan pva.MonitorEvent) {
+			for evt := range ch {
+				if evt.Err != nil {
+					fmt.Fprintf(os.Stderr, "%s: %v\n", pvName, evt.Err)
+					return
+				}
+				ts := time.Now().Local().Format("15:04:05.000")
+				fmt.Printf("%s  %-30s  %s\n", ts, pvName, formatValue(evt.Value))
+			}
+		}(name, ch)
+		fmt.Printf("Monitoring %s (PVA) — press Ctrl-C to stop\n", name)
+	}
+	<-ctx.Done()
+}
+
+// -------------------------------------------------------------------------- //
+// info                                                                        //
+// -------------------------------------------------------------------------- //
+
+func runInfo(ctx context.Context, pvs []string) {
+	cl, err := pva.NewClient()
+	if err != nil {
+		fatalf("connect: %v", err)
+	}
+	defer cl.Close()
+
+	tctx, cancel := context.WithTimeout(ctx, 30*time.Second)
+	defer cancel()
+
+	for _, name := range pvs {
+		sv, err := cl.Get(tctx, name)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "%s: error: %v\n", name, err)
+			continue
+		}
+		printInfo(name, sv)
+		fmt.Println()
+	}
+}
+
+func printInfo(name string, sv pvdata.StructValue) {
+	fmt.Printf("Name   : %s\n", name)
+	fmt.Printf("Fields : %d\n", len(sv.Fields))
+	printStruct(sv, "  ")
+}
+
+func printStruct(sv pvdata.StructValue, indent string) {
+	for _, f := range sv.Fields {
+		switch v := f.Value.(type) {
+		case pvdata.StructValue:
+			fmt.Printf("%s%s (struct):\n", indent, f.Name)
+			printStruct(v, indent+"  ")
+		default:
+			fmt.Printf("%s%s = %v\n", indent, f.Name, v)
+		}
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Formatting helpers                                                          //
+// -------------------------------------------------------------------------- //
+
+func formatValue(sv pvdata.StructValue) string {
+	if len(sv.Fields) == 0 {
+		return "<empty>"
+	}
+	val := fieldByName(sv, "value")
+	alarm := ""
+	if a := structByName(sv, "alarm"); a != nil {
+		if sev := fieldByName(*a, "severity"); sev != nil {
+			if s, ok := sev.(int32); ok && s != 0 {
+				alarm = fmt.Sprintf(" (alarm sev=%d)", s)
+			}
+		}
+	}
+	ts := ""
+	if t := structByName(sv, "timeStamp"); t != nil {
+		sec := int64(0)
+		nsec := int32(0)
+		if v := fieldByName(*t, "secondsPastEpoch"); v != nil {
+			sec, _ = v.(int64)
+		}
+		if v := fieldByName(*t, "nanoseconds"); v != nil {
+			nsec, _ = v.(int32)
+		}
+		if sec != 0 {
+			const epicsEpoch = 631152000
+			ut := time.Unix(sec+epicsEpoch, int64(nsec)).Local()
+			ts = "  [" + ut.Format("2006-01-02 15:04:05.000") + "]"
+		}
+	}
+	if val == nil {
+		val = sv.Fields[0].Value
+	}
+	return fmt.Sprintf("%v%s%s", formatScalar(val), alarm, ts)
+}
+
+func formatScalar(v any) string {
+	switch x := v.(type) {
+	case float64:
+		return strconv.FormatFloat(x, 'g', -1, 64)
+	case float32:
+		return strconv.FormatFloat(float64(x), 'g', -1, 32)
+	case string:
+		return x
+	case []string:
+		return strings.Join(x, " ")
+	default:
+		return fmt.Sprintf("%v", v)
+	}
+}
+
+func fieldByName(sv pvdata.StructValue, name string) interface{} {
+	for _, f := range sv.Fields {
+		if f.Name == name {
+			return f.Value
+		}
+	}
+	return nil
+}
+
+func structByName(sv pvdata.StructValue, name string) *pvdata.StructValue {
+	for _, f := range sv.Fields {
+		if f.Name == name {
+			if s, ok := f.Value.(pvdata.StructValue); ok {
+				return &s
+			}
+		}
+	}
+	return nil
+}
+
+func fatalf(format string, args ...any) {
+	fmt.Fprintf(os.Stderr, "pvtools: "+format+"\n", args...)
+	os.Exit(1)
+}

cmd/uopi/main.go

@@ -14,6 +14,7 @@ import (
 	"github.com/uopi/uopi/internal/broker"
 	"github.com/uopi/uopi/internal/config"
 	"github.com/uopi/uopi/internal/datasource/epics"
+	"github.com/uopi/uopi/internal/datasource/pva"
 	"github.com/uopi/uopi/internal/datasource/stub"
 	"github.com/uopi/uopi/internal/datasource/synthetic"
 	"github.com/uopi/uopi/internal/server"
@@ -83,10 +84,11 @@ func main() {
 		}
 	}
 
-	// EPICS Channel Access data source (requires -tags epics build).
+	// EPICS Channel Access data source.
-	// epics.Available() returns false when built without the tag.
+	// The pure-Go implementation is used by default (no CGo required).
+	// Build with -tags epics to use the CGo-based libca implementation instead.
 	if cfg.Datasource.EPICS.Enabled && epics.Available() {
-		ds := epics.New(cfg.Datasource.EPICS.CAAddrList, cfg.Datasource.EPICS.ArchiveURL)
+		ds := epics.New(cfg.Datasource.EPICS.CAAddrList, cfg.Datasource.EPICS.ArchiveURL, cfg.Datasource.EPICS.PVNames)
 		if err := ds.Connect(ctx); err != nil {
 			log.Error("epics connect", "err", err)
 		} else {
@@ -94,6 +96,16 @@ func main() {
 		}
 	}
 
+	// PV Access data source.
+	if cfg.Datasource.PVA.Enabled {
+		pvaDS := pva.New(cfg.Datasource.PVA.AddrList)
+		if err := pvaDS.Connect(ctx); err != nil {
+			log.Error("pva connect", "err", err)
+		} else {
+			brk.Register(pvaDS)
+		}
+	}
+
 	srv := server.New(cfg.Server.Listen, webFS, brk, synthDS, store, cfg.Datasource.EPICS.ChannelFinderURL, log)
 
 	if err := srv.Start(ctx); err != nil {

go.work

@@ -0,0 +1,7 @@
+go 1.26.2
+
+use (
+	.
+	./pkg/ca
+	./pkg/pva
+)

internal/config/config.go

@@ -21,6 +21,7 @@ type ServerConfig struct {
 type DatasourceConfig struct {
 	Stub      StubConfig      `toml:"stub"`
 	EPICS     EPICSConfig     `toml:"epics"`
+	PVA       PVAConfig       `toml:"pva"`
 	Synthetic SyntheticConfig `toml:"synthetic"`
 }
 
@@ -29,10 +30,16 @@ type StubConfig struct {
 }
 
 type EPICSConfig struct {
-	Enabled          bool   `toml:"enabled"`
+	Enabled          bool     `toml:"enabled"`
-	CAAddrList       string `toml:"ca_addr_list"`
+	CAAddrList       string   `toml:"ca_addr_list"`
-	ArchiveURL       string `toml:"archive_url"`
+	ArchiveURL       string   `toml:"archive_url"`
-	ChannelFinderURL string `toml:"channel_finder_url"`
+	ChannelFinderURL string   `toml:"channel_finder_url"`
+	PVNames          []string `toml:"pv_names"`
+}
+
+type PVAConfig struct {
+	Enabled  bool     `toml:"enabled"`
+	AddrList []string `toml:"addr_list"`
 }
 
 type SyntheticConfig struct {
@@ -48,6 +55,7 @@ func Default() Config {
 		Datasource: DatasourceConfig{
 			Stub:      StubConfig{Enabled: true},
 			EPICS:     EPICSConfig{Enabled: true},
+			PVA:       PVAConfig{Enabled: true},
 			Synthetic: SyntheticConfig{Enabled: true},
 		},
 	}
@@ -85,6 +93,9 @@ func applyEnv(cfg *Config) {
 	if v := env("UOPI_EPICS_CHANNEL_FINDER_URL"); v != "" {
 		cfg.Datasource.EPICS.ChannelFinderURL = v
 	}
+	if v := env("EPICS_PVA_ADDR_LIST"); v != "" {
+		cfg.Datasource.PVA.AddrList = strings.Fields(v)
+	}
 }
 
 func env(key string) string {

internal/config/config_test.go

@@ -0,0 +1,124 @@
+package config_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/uopi/uopi/internal/config"
+)
+
+func TestDefault(t *testing.T) {
+	cfg := config.Default()
+	if cfg.Server.Listen == "" {
+		t.Error("Default Listen must not be empty")
+	}
+	if cfg.Server.StorageDir == "" {
+		t.Error("Default StorageDir must not be empty")
+	}
+	if !cfg.Datasource.Stub.Enabled {
+		t.Error("Stub should be enabled by default")
+	}
+	if !cfg.Datasource.EPICS.Enabled {
+		t.Error("EPICS should be enabled by default")
+	}
+	if !cfg.Datasource.Synthetic.Enabled {
+		t.Error("Synthetic should be enabled by default")
+	}
+}
+
+func TestLoadEmptyPath(t *testing.T) {
+	// Empty path should return defaults without error.
+	cfg, err := config.Load("")
+	if err != nil {
+		t.Fatalf("Load with empty path: %v", err)
+	}
+	if cfg.Server.Listen != config.Default().Server.Listen {
+		t.Errorf("Listen = %q, want default %q", cfg.Server.Listen, config.Default().Server.Listen)
+	}
+}
+
+func TestLoadTOML(t *testing.T) {
+	toml := `
+[server]
+listen = ":9090"
+storage_dir = "/tmp/uopi-test"
+
+[datasource.stub]
+enabled = false
+
+[datasource.epics]
+enabled = false
+ca_addr_list = "192.168.1.1"
+`
+	f := filepath.Join(t.TempDir(), "uopi.toml")
+	if err := os.WriteFile(f, []byte(toml), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := config.Load(f)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.Server.Listen != ":9090" {
+		t.Errorf("Listen = %q, want :9090", cfg.Server.Listen)
+	}
+	if cfg.Server.StorageDir != "/tmp/uopi-test" {
+		t.Errorf("StorageDir = %q", cfg.Server.StorageDir)
+	}
+	if cfg.Datasource.Stub.Enabled {
+		t.Error("Stub.Enabled should be false")
+	}
+	if cfg.Datasource.EPICS.Enabled {
+		t.Error("EPICS.Enabled should be false")
+	}
+	if cfg.Datasource.EPICS.CAAddrList != "192.168.1.1" {
+		t.Errorf("CAAddrList = %q", cfg.Datasource.EPICS.CAAddrList)
+	}
+}
+
+func TestLoadBadPath(t *testing.T) {
+	_, err := config.Load("/no/such/file.toml")
+	if err == nil {
+		t.Error("expected error for missing file")
+	}
+}
+
+func TestEnvOverrides(t *testing.T) {
+	t.Setenv("UOPI_SERVER_LISTEN", ":7777")
+	t.Setenv("UOPI_SERVER_STORAGE_DIR", "/env/storage")
+	t.Setenv("UOPI_EPICS_CA_ADDR_LIST", "10.0.0.1 10.0.0.2")
+	t.Setenv("UOPI_EPICS_ARCHIVE_URL", "http://archive/")
+	t.Setenv("UOPI_EPICS_CHANNEL_FINDER_URL", "http://cf/")
+
+	cfg, err := config.Load("")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.Server.Listen != ":7777" {
+		t.Errorf("Listen = %q, want :7777", cfg.Server.Listen)
+	}
+	if cfg.Server.StorageDir != "/env/storage" {
+		t.Errorf("StorageDir = %q", cfg.Server.StorageDir)
+	}
+	if cfg.Datasource.EPICS.CAAddrList != "10.0.0.1 10.0.0.2" {
+		t.Errorf("CAAddrList = %q", cfg.Datasource.EPICS.CAAddrList)
+	}
+	if cfg.Datasource.EPICS.ArchiveURL != "http://archive/" {
+		t.Errorf("ArchiveURL = %q", cfg.Datasource.EPICS.ArchiveURL)
+	}
+	if cfg.Datasource.EPICS.ChannelFinderURL != "http://cf/" {
+		t.Errorf("ChannelFinderURL = %q", cfg.Datasource.EPICS.ChannelFinderURL)
+	}
+}
+
+func TestEnvTrimsWhitespace(t *testing.T) {
+	t.Setenv("UOPI_SERVER_LISTEN", "  :8888  ")
+	cfg, err := config.Load("")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.Server.Listen != ":8888" {
+		t.Errorf("Listen = %q, expected whitespace trimmed", cfg.Server.Listen)
+	}
+}

internal/datasource/epics/archive.go

@@ -1,5 +1,3 @@
-//go:build epics
-
 package epics
 
 import (

internal/datasource/epics/epics.go

@@ -85,7 +85,9 @@ type EPICS struct {
 // caAddrList is used to set EPICS_CA_ADDR_LIST at runtime (may be empty to
 // rely on the environment).  archiveURL is the base URL of an EPICS Archive
 // Appliance instance for history queries (may be empty).
-func New(caAddrList, archiveURL string) datasource.DataSource {
+// pvNames is accepted for API compatibility with the pure-Go build but ignored
+// in the CGo build (the CGo implementation populates ListSignals via callbacks).
+func New(caAddrList, archiveURL string, pvNames []string) datasource.DataSource {
 	return &EPICS{
 		caAddrList: caAddrList,
 		archiveURL: archiveURL,

internal/datasource/epics/epics_purego_test.go

@@ -0,0 +1,350 @@
+//go:build !epics
+
+// Integration tests for the pure-Go EPICS datasource adapter.
+// These tests use the in-process fake CA server from the goca testca package
+// and therefore do not require a real EPICS installation.
+package epics_test
+
+import (
+	"context"
+	"math"
+	"testing"
+	"time"
+
+	"github.com/uopi/goca/proto"
+	"github.com/uopi/goca/testca"
+	"github.com/uopi/uopi/internal/datasource"
+	"github.com/uopi/uopi/internal/datasource/epics"
+)
+
+// newTestDS creates a datasource.DataSource backed by a fake CA server.
+func newTestDS(t *testing.T, pvs []testca.PVSpec) (datasource.DataSource, *testca.Server) {
+	t.Helper()
+	srv, err := testca.New(pvs)
+	if err != nil {
+		t.Fatalf("testca.New: %v", err)
+	}
+	t.Cleanup(srv.Close)
+
+	ds := epics.New(srv.UDPAddr(), "", nil)
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	t.Cleanup(cancel)
+	if err := ds.Connect(ctx); err != nil {
+		t.Fatalf("Connect: %v", err)
+	}
+	return ds, srv
+}
+
+// -------------------------------------------------------------------------- //
+// GetMetadata                                                                  //
+// -------------------------------------------------------------------------- //
+
+func TestPureGo_GetMetadataDouble(t *testing.T) {
+	ds, _ := newTestDS(t, []testca.PVSpec{
+		{Name: "DS:DOUBLE", DBFType: proto.DBFDouble, Count: 1, Value: 1.0},
+	})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	meta, err := ds.GetMetadata(ctx, "DS:DOUBLE")
+	if err != nil {
+		t.Fatalf("GetMetadata: %v", err)
+	}
+	if meta.Name != "DS:DOUBLE" {
+		t.Errorf("Name = %q, want %q", meta.Name, "DS:DOUBLE")
+	}
+	if meta.Type != datasource.TypeFloat64 {
+		t.Errorf("Type = %v, want TypeFloat64", meta.Type)
+	}
+	if !meta.Writable {
+		t.Error("expected Writable=true")
+	}
+}
+
+func TestPureGo_GetMetadataString(t *testing.T) {
+	ds, _ := newTestDS(t, []testca.PVSpec{
+		{Name: "DS:STR", DBFType: proto.DBFString, Count: 1, Value: "hello"},
+	})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	meta, err := ds.GetMetadata(ctx, "DS:STR")
+	if err != nil {
+		t.Fatalf("GetMetadata: %v", err)
+	}
+	if meta.Type != datasource.TypeString {
+		t.Errorf("Type = %v, want TypeString", meta.Type)
+	}
+}
+
+func TestPureGo_GetMetadataEnum(t *testing.T) {
+	ds, _ := newTestDS(t, []testca.PVSpec{
+		{Name: "DS:ENUM", DBFType: proto.DBFEnum, Count: 1, Value: int16(0)},
+	})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	meta, err := ds.GetMetadata(ctx, "DS:ENUM")
+	if err != nil {
+		t.Fatalf("GetMetadata: %v", err)
+	}
+	if meta.Type != datasource.TypeEnum {
+		t.Errorf("Type = %v, want TypeEnum", meta.Type)
+	}
+}
+
+func TestPureGo_GetMetadataReadOnly(t *testing.T) {
+	ds, _ := newTestDS(t, []testca.PVSpec{
+		{Name: "DS:RO", DBFType: proto.DBFDouble, Count: 1, Value: 0.0,
+			Access: proto.AccessRead},
+	})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	meta, err := ds.GetMetadata(ctx, "DS:RO")
+	if err != nil {
+		t.Fatalf("GetMetadata: %v", err)
+	}
+	if meta.Writable {
+		t.Error("expected Writable=false for read-only PV")
+	}
+}
+
+func TestPureGo_GetMetadataCached(t *testing.T) {
+	ds, _ := newTestDS(t, []testca.PVSpec{
+		{Name: "DS:CACHED", DBFType: proto.DBFDouble, Count: 1, Value: 1.0},
+	})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	m1, err := ds.GetMetadata(ctx, "DS:CACHED")
+	if err != nil {
+		t.Fatalf("first GetMetadata: %v", err)
+	}
+	m2, err := ds.GetMetadata(ctx, "DS:CACHED")
+	if err != nil {
+		t.Fatalf("second GetMetadata: %v", err)
+	}
+	if m1.Name != m2.Name || m1.Type != m2.Type {
+		t.Error("second GetMetadata returned different result from cache")
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Subscribe                                                                    //
+// -------------------------------------------------------------------------- //
+
+func TestPureGo_SubscribeInitialValue(t *testing.T) {
+	ds, _ := newTestDS(t, []testca.PVSpec{
+		{Name: "DS:SUB", DBFType: proto.DBFDouble, Count: 1, Value: 7.5},
+	})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	ch := make(chan datasource.Value, 16)
+	unsub, err := ds.Subscribe(ctx, "DS:SUB", ch)
+	if err != nil {
+		t.Fatalf("Subscribe: %v", err)
+	}
+	defer unsub()
+
+	// Drain MetaUpdate and value frames — first numeric value wins.
+	deadline := time.After(5 * time.Second)
+	for {
+		select {
+		case v := <-ch:
+			if v.MetaUpdate {
+				continue
+			}
+			f, ok := v.Data.(float64)
+			if !ok {
+				t.Fatalf("Data type = %T, want float64", v.Data)
+			}
+			if math.Abs(f-7.5) > 1e-6 {
+				t.Errorf("Data = %g, want 7.5", f)
+			}
+			return
+		case <-deadline:
+			t.Fatal("timeout waiting for initial value")
+		}
+	}
+}
+
+func TestPureGo_SubscribeUpdates(t *testing.T) {
+	ds, srv := newTestDS(t, []testca.PVSpec{
+		{Name: "DS:UPD", DBFType: proto.DBFDouble, Count: 1, Value: 1.0},
+	})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	ch := make(chan datasource.Value, 16)
+	unsub, err := ds.Subscribe(ctx, "DS:UPD", ch)
+	if err != nil {
+		t.Fatalf("Subscribe: %v", err)
+	}
+	defer unsub()
+
+	// Drain initial frames.
+	deadline := time.After(3 * time.Second)
+drainLoop:
+	for {
+		select {
+		case v := <-ch:
+			if !v.MetaUpdate {
+				break drainLoop
+			}
+		case <-deadline:
+			t.Fatal("timeout draining initial frames")
+		}
+	}
+
+	// Push new value via server.
+	srv.SetValue("DS:UPD", 99.9)
+
+	deadline = time.After(3 * time.Second)
+	for {
+		select {
+		case v := <-ch:
+			if v.MetaUpdate {
+				continue
+			}
+			f, ok := v.Data.(float64)
+			if !ok {
+				t.Fatalf("Data type = %T, want float64", v.Data)
+			}
+			if math.Abs(f-99.9) > 1e-6 {
+				t.Errorf("Data = %g, want 99.9", f)
+			}
+			return
+		case <-deadline:
+			t.Fatal("timeout waiting for updated value")
+		}
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Write                                                                        //
+// -------------------------------------------------------------------------- //
+
+func TestPureGo_WriteDouble(t *testing.T) {
+	ds, _ := newTestDS(t, []testca.PVSpec{
+		{Name: "DS:W", DBFType: proto.DBFDouble, Count: 1, Value: 0.0},
+	})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	ch := make(chan datasource.Value, 16)
+	unsub, err := ds.Subscribe(ctx, "DS:W", ch)
+	if err != nil {
+		t.Fatalf("Subscribe: %v", err)
+	}
+	defer unsub()
+
+	// Drain initial.
+	deadline := time.After(3 * time.Second)
+drainLoop:
+	for {
+		select {
+		case v := <-ch:
+			if !v.MetaUpdate {
+				break drainLoop
+			}
+		case <-deadline:
+			t.Fatal("timeout draining initial frames")
+		}
+	}
+
+	if err := ds.Write(ctx, "DS:W", float64(3.14)); err != nil {
+		t.Fatalf("Write: %v", err)
+	}
+
+	deadline = time.After(3 * time.Second)
+	for {
+		select {
+		case v := <-ch:
+			if v.MetaUpdate {
+				continue
+			}
+			f, ok := v.Data.(float64)
+			if !ok {
+				continue
+			}
+			if math.Abs(f-3.14) > 1e-6 {
+				t.Errorf("post-write Data = %g, want 3.14", f)
+			}
+			return
+		case <-deadline:
+			t.Fatal("timeout waiting for post-write monitor update")
+		}
+	}
+}
+
+func TestPureGo_WriteReadOnly(t *testing.T) {
+	ds, _ := newTestDS(t, []testca.PVSpec{
+		{Name: "DS:WRO", DBFType: proto.DBFDouble, Count: 1, Value: 0.0,
+			Access: proto.AccessRead},
+	})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	err := ds.Write(ctx, "DS:WRO", float64(1.0))
+	if err == nil {
+		t.Fatal("expected error writing to read-only PV")
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// ListSignals                                                                  //
+// -------------------------------------------------------------------------- //
+
+func TestPureGo_ListSignals(t *testing.T) {
+	ds, _ := newTestDS(t, []testca.PVSpec{
+		{Name: "DS:LS1", DBFType: proto.DBFDouble, Count: 1, Value: 0.0},
+		{Name: "DS:LS2", DBFType: proto.DBFDouble, Count: 1, Value: 0.0},
+	})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	// GetMetadata for each PV to populate the cache.
+	for _, name := range []string{"DS:LS1", "DS:LS2"} {
+		if _, err := ds.GetMetadata(ctx, name); err != nil {
+			t.Fatalf("GetMetadata(%q): %v", name, err)
+		}
+	}
+
+	sigs, err := ds.ListSignals(ctx)
+	if err != nil {
+		t.Fatalf("ListSignals: %v", err)
+	}
+	if len(sigs) < 2 {
+		t.Errorf("ListSignals returned %d signals, want ≥ 2", len(sigs))
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// History                                                                      //
+// -------------------------------------------------------------------------- //
+
+func TestPureGo_HistoryUnavailable(t *testing.T) {
+	ds, _ := newTestDS(t, []testca.PVSpec{
+		{Name: "DS:HIST", DBFType: proto.DBFDouble, Count: 1, Value: 0.0},
+	})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	_, err := ds.History(ctx, "DS:HIST", time.Now().Add(-time.Hour), time.Now(), 100)
+	if err != datasource.ErrHistoryUnavailable {
+		t.Errorf("History with no archiveURL: want ErrHistoryUnavailable, got %v", err)
+	}
+}

internal/datasource/epics/noop.go

@@ -1,17 +1,307 @@
 //go:build !epics
 
-// Package epics provides an EPICS Channel Access data source.
+// Package epics provides an EPICS Channel Access data source for uopi using a
-// When built without the "epics" build tag (i.e. without CGo and EPICS Base),
+// pure-Go CA implementation (no CGo, no EPICS Base installation required).
-// this stub is compiled instead, so that the rest of the codebase can call
+// When built WITH the "epics" build tag, the CGo-based implementation in
-// epics.Available() to decide whether to register the data source.
+// epics.go is used instead; that version requires CGo and a working EPICS Base.
 package epics
 
-import "github.com/uopi/uopi/internal/datasource"
+import (
+	"context"
+	"fmt"
+	"log/slog" //nolint:depguard
+	"strings"
+	"sync"
+	"time"
 
-// Available reports whether the EPICS Channel Access data source is compiled in.
+	ca "github.com/uopi/goca"
-// It always returns false when built without the "epics" build tag.
+	"github.com/uopi/goca/proto"
-func Available() bool { return false }
+	"github.com/uopi/uopi/internal/datasource"
+)
 
-// New is a placeholder that returns nil when EPICS support is not compiled in.
+// Available always returns true for the pure-Go build.
-// Callers must check Available() before calling New().
+func Available() bool { return true }
-func New(caAddrList, archiveURL string) datasource.DataSource { return nil }
+
+// -------------------------------------------------------------------------- //
+// EPICS data source (pure-Go implementation)                                  //
+// -------------------------------------------------------------------------- //
+
+// EPICS is the pure-Go Channel Access data source.
+type EPICS struct {
+	caAddrList string
+	archiveURL string
+	pvNames    []string // pre-fetched at connect time for ListSignals
+
+	client *ca.Client
+
+	mu       sync.RWMutex
+	metadata map[string]datasource.Metadata
+}
+
+// New creates a new EPICS Channel Access data source.
+//
+// caAddrList is the EPICS_CA_ADDR_LIST value (space-separated addresses).
+// Leave empty to use the environment variable.
+// archiveURL is the base URL of an EPICS Archive Appliance instance; leave
+// empty to disable history queries.
+// pvNames is an optional list of PV names whose metadata will be pre-fetched
+// at connect time so they appear in ListSignals immediately (useful for the
+// edit-mode signal tree).
+func New(caAddrList, archiveURL string, pvNames []string) datasource.DataSource {
+	return &EPICS{
+		caAddrList: caAddrList,
+		archiveURL: archiveURL,
+		pvNames:    pvNames,
+		metadata:   make(map[string]datasource.Metadata),
+	}
+}
+
+// Name implements datasource.DataSource.
+func (e *EPICS) Name() string { return "epics" }
+
+// Connect creates the CA client and starts background I/O goroutines.
+// ctx governs the lifetime of the client; cancel it to shut everything down.
+func (e *EPICS) Connect(ctx context.Context) error {
+	cfg := ca.ConfigFromEnv()
+	if e.caAddrList != "" {
+		cfg.AddrList = strings.Fields(e.caAddrList)
+		cfg.AutoAddrList = false
+	}
+	slog.Info("epics: connecting CA client", "addrs", cfg.AddrList, "auto_addr", cfg.AutoAddrList)
+	cli, err := ca.NewClient(ctx, cfg)
+	if err != nil {
+		return fmt.Errorf("epics: %w", err)
+	}
+	e.client = cli
+	slog.Info("epics: CA client started", "addrs", cfg.AddrList)
+
+	// Pre-fetch metadata for any configured PV names so they appear in
+	// ListSignals as soon as the datasource is ready.  We open a short-lived
+	// subscription (which drives channel connection) and then fetch CTRL info.
+	if len(e.pvNames) > 0 {
+		go func() {
+			for _, pv := range e.pvNames {
+				if ctx.Err() != nil {
+					return
+				}
+				func() {
+					tvCh := make(chan proto.TimeValue, 1)
+					subCtx, subCancel := context.WithTimeout(ctx, 15*time.Second)
+					defer subCancel()
+					cancel, err := e.client.Subscribe(subCtx, pv, tvCh)
+					if err != nil {
+						return
+					}
+					defer cancel()
+					// Channel is now connected; fetch CTRL metadata.
+					mctx, mcancel := context.WithTimeout(ctx, 5*time.Second)
+					defer mcancel()
+					_, _ = e.GetMetadata(mctx, pv)
+				}()
+			}
+		}()
+	}
+
+	return nil
+}
+
+// -------------------------------------------------------------------------- //
+// Subscribe                                                                    //
+// -------------------------------------------------------------------------- //
+
+// Subscribe registers ch to receive live value updates for signal.
+// It blocks until the CA channel is connected (or ctx expires).
+// A background goroutine forwards proto.TimeValue updates to ch as
+// datasource.Value.  Call the returned CancelFunc to unsubscribe.
+func (e *EPICS) Subscribe(ctx context.Context, signal string, ch chan<- datasource.Value) (datasource.CancelFunc, error) {
+	slog.Info("epics: subscribing", "pv", signal)
+	tvCh := make(chan proto.TimeValue, 16)
+
+	clientCancel, err := e.client.Subscribe(ctx, signal, tvCh)
+	if err != nil {
+		slog.Error("epics: subscribe failed", "pv", signal, "err", err)
+		return nil, fmt.Errorf("epics: subscribe %q: %w", signal, err)
+	}
+	slog.Info("epics: subscribed OK", "pv", signal)
+
+	// Fetch and cache metadata now that the channel is connected.
+	// Send MetaUpdate so the dispatcher re-broadcasts metadata to clients.
+	go func() {
+		// Small delay gives ACCESS_RIGHTS message time to arrive before GetCtrl.
+		time.Sleep(50 * time.Millisecond)
+		mctx, mcancel := context.WithTimeout(ctx, 5*time.Second)
+		defer mcancel()
+		if _, err := e.GetMetadata(mctx, signal); err == nil {
+			select {
+			case ch <- datasource.Value{MetaUpdate: true}:
+			default:
+			}
+		}
+	}()
+
+	done := make(chan struct{})
+
+	// Forward goroutine: convert proto.TimeValue → datasource.Value.
+	go func() {
+		for {
+			select {
+			case tv := <-tvCh:
+				slog.Debug("epics: value received", "pv", signal, "double", tv.Double, "severity", tv.Severity)
+				val := e.timeValueToDS(signal, tv)
+				select {
+				case ch <- val:
+				default: // drop if consumer is slow
+				}
+			case <-done:
+				return
+			}
+		}
+	}()
+
+	cancel := func() {
+		clientCancel()
+		close(done)
+	}
+	return datasource.CancelFunc(cancel), nil
+}
+
+// timeValueToDS converts a proto.TimeValue to a datasource.Value.
+func (e *EPICS) timeValueToDS(signal string, tv proto.TimeValue) datasource.Value {
+	var data any
+	if tv.Doubles != nil {
+		if len(tv.Doubles) == 1 {
+			data = tv.Doubles[0]
+		} else {
+			data = tv.Doubles
+		}
+	} else if tv.Str != "" {
+		data = tv.Str
+	} else {
+		data = tv.Double
+	}
+
+	quality := datasource.QualityGood
+	switch tv.Severity {
+	case proto.SeverityMinor:
+		quality = datasource.QualityUncertain
+	case proto.SeverityMajor, proto.SeverityInvalid:
+		quality = datasource.QualityBad
+	}
+
+	return datasource.Value{
+		Timestamp: tv.Timestamp,
+		Data:      data,
+		Quality:   quality,
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// GetMetadata                                                                  //
+// -------------------------------------------------------------------------- //
+
+// GetMetadata returns metadata for signal, using a cached value when available.
+// On the first call for a signal it performs a DBR_CTRL_* GET to retrieve
+// units, display limits, enum strings, and access rights.
+func (e *EPICS) GetMetadata(ctx context.Context, signal string) (datasource.Metadata, error) {
+	e.mu.RLock()
+	if m, ok := e.metadata[signal]; ok {
+		e.mu.RUnlock()
+		return m, nil
+	}
+	e.mu.RUnlock()
+
+	slog.Info("epics: fetching metadata", "pv", signal)
+	ci, err := e.client.GetCtrl(ctx, signal)
+	if err != nil {
+		slog.Error("epics: GetMetadata failed", "pv", signal, "err", err)
+		return datasource.Metadata{}, fmt.Errorf("epics: GetMetadata %q: %w", signal, err)
+	}
+
+	m := e.ctrlToMeta(signal, ci)
+	slog.Info("epics: metadata fetched", "pv", signal, "type", m.Type, "unit", m.Unit, "writable", m.Writable)
+
+	e.mu.Lock()
+	e.metadata[signal] = m
+	e.mu.Unlock()
+
+	return m, nil
+}
+
+// ctrlToMeta converts a ca.CtrlInfo to a datasource.Metadata.
+func (e *EPICS) ctrlToMeta(name string, ci ca.CtrlInfo) datasource.Metadata {
+	m := datasource.Metadata{
+		Name:     name,
+		Writable: ci.Access&proto.AccessWrite != 0,
+	}
+
+	switch {
+	case ci.Double != nil:
+		m.Type = datasource.TypeFloat64
+		if ci.Count > 1 {
+			m.Type = datasource.TypeFloat64Array
+		}
+		m.Unit = ci.Double.Units
+		m.DisplayLow = ci.Double.LowerDispLimit
+		m.DisplayHigh = ci.Double.UpperDispLimit
+		m.DriveLow = ci.Double.LowerCtrlLimit
+		m.DriveHigh = ci.Double.UpperCtrlLimit
+
+	case ci.Long != nil:
+		m.Type = datasource.TypeInt64
+		m.Unit = ci.Long.Units
+		m.DisplayLow = float64(ci.Long.LowerDispLimit)
+		m.DisplayHigh = float64(ci.Long.UpperDispLimit)
+		m.DriveLow = float64(ci.Long.LowerCtrlLimit)
+		m.DriveHigh = float64(ci.Long.UpperCtrlLimit)
+
+	case ci.Enum != nil:
+		m.Type = datasource.TypeEnum
+		m.EnumStrings = ci.Enum.Strings
+
+	case ci.Str != nil:
+		m.Type = datasource.TypeString
+	}
+
+	return m
+}
+
+// -------------------------------------------------------------------------- //
+// ListSignals                                                                  //
+// -------------------------------------------------------------------------- //
+
+// ListSignals returns metadata for all signals with cached information.
+func (e *EPICS) ListSignals(_ context.Context) ([]datasource.Metadata, error) {
+	e.mu.RLock()
+	defer e.mu.RUnlock()
+	out := make([]datasource.Metadata, 0, len(e.metadata))
+	for _, m := range e.metadata {
+		out = append(out, m)
+	}
+	return out, nil
+}
+
+// -------------------------------------------------------------------------- //
+// Write                                                                        //
+// -------------------------------------------------------------------------- //
+
+// Write puts a new value onto the named CA channel.
+// value must be one of: float64, float32, int64, int32, int, int16, string, bool.
+func (e *EPICS) Write(ctx context.Context, signal string, value any) error {
+	if err := e.client.Put(ctx, signal, value); err != nil {
+		return fmt.Errorf("epics: write %q: %w", signal, err)
+	}
+	return nil
+}
+
+// -------------------------------------------------------------------------- //
+// History                                                                      //
+// -------------------------------------------------------------------------- //
+
+// History delegates to the Archive Appliance client, or returns
+// ErrHistoryUnavailable if no archive URL is configured.
+func (e *EPICS) History(ctx context.Context, signal string, start, end time.Time, maxPoints int) ([]datasource.Value, error) {
+	if e.archiveURL == "" {
+		return nil, datasource.ErrHistoryUnavailable
+	}
+	return fetchArchiveHistory(ctx, e.archiveURL, signal, start, end, maxPoints)
+}

internal/datasource/pva/pva.go

@@ -0,0 +1,365 @@
+// Package pva provides an EPICS PV Access data source for uopi using the
+// pure-Go gopva library.
+package pva
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"os"
+	"strings"
+	"sync"
+	"time"
+
+	gopva "github.com/uopi/gopva"
+	"github.com/uopi/gopva/pvdata"
+	"github.com/uopi/uopi/internal/datasource"
+)
+
+// PVA is the PV Access data source.
+type PVA struct {
+	client *gopva.Client
+
+	mu       sync.RWMutex
+	metadata map[string]datasource.Metadata
+}
+
+// New creates a new PV Access data source.
+// Server addresses are read from the EPICS_PVA_ADDR_LIST environment variable;
+// addrList overrides that when non-empty.
+func New(addrList []string) datasource.DataSource {
+	if len(addrList) > 0 {
+		// Inject addresses into the environment so gopva.NewClient picks them up.
+		_ = os.Setenv("EPICS_PVA_ADDR_LIST", strings.Join(addrList, " "))
+	}
+	return &PVA{
+		metadata: make(map[string]datasource.Metadata),
+	}
+}
+
+// Name implements datasource.DataSource.
+func (p *PVA) Name() string { return "pva" }
+
+// Connect creates the PVA client.
+func (p *PVA) Connect(_ context.Context) error {
+	cl, err := gopva.NewClient()
+	if err != nil {
+		return fmt.Errorf("pva: connect: %w", err)
+	}
+	p.client = cl
+	slog.Info("pva: client started")
+	return nil
+}
+
+// -------------------------------------------------------------------------- //
+// Subscribe                                                                    //
+// -------------------------------------------------------------------------- //
+
+// Subscribe registers ch to receive live PVA value updates for signal.
+func (p *PVA) Subscribe(ctx context.Context, signal string, ch chan<- datasource.Value) (datasource.CancelFunc, error) {
+	slog.Info("pva: subscribing", "pv", signal)
+
+	monCh := p.client.Monitor(ctx, signal)
+
+	// Wait for the first event to confirm the subscription is live.
+	// We do this in the goroutine to avoid blocking the caller.
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		for evt := range monCh {
+			if evt.Err != nil {
+				slog.Error("pva: monitor error", "pv", signal, "err", evt.Err)
+				select {
+				case ch <- datasource.Value{
+					Timestamp: time.Now(),
+					Data:      nil,
+					Quality:   datasource.QualityBad,
+				}:
+				default:
+				}
+				return
+			}
+			// Cache metadata from the first value.
+			p.updateMetadata(signal, evt.Value)
+
+			val := structToValue(evt.Value)
+			select {
+			case ch <- val:
+			default:
+			}
+		}
+	}()
+
+	cancel := func() {
+		// cancelling the context passed to Monitor stops it; the channel will be closed.
+		// The goroutine will exit when monCh is closed.
+		<-done
+	}
+	return datasource.CancelFunc(cancel), nil
+}
+
+// -------------------------------------------------------------------------- //
+// GetMetadata                                                                  //
+// -------------------------------------------------------------------------- //
+
+// GetMetadata returns metadata for signal. It performs a one-shot GET if
+// metadata has not been cached yet.
+func (p *PVA) GetMetadata(ctx context.Context, signal string) (datasource.Metadata, error) {
+	p.mu.RLock()
+	if m, ok := p.metadata[signal]; ok {
+		p.mu.RUnlock()
+		return m, nil
+	}
+	p.mu.RUnlock()
+
+	tctx, cancel := context.WithTimeout(ctx, 15*time.Second)
+	defer cancel()
+
+	sv, err := p.client.Get(tctx, signal)
+	if err != nil {
+		return datasource.Metadata{}, fmt.Errorf("pva: GetMetadata %q: %w", signal, err)
+	}
+
+	p.updateMetadata(signal, sv)
+
+	p.mu.RLock()
+	m := p.metadata[signal]
+	p.mu.RUnlock()
+	return m, nil
+}
+
+// updateMetadata derives and caches metadata from a StructValue.
+func (p *PVA) updateMetadata(signal string, sv pvdata.StructValue) {
+	m := structToMeta(signal, sv)
+	p.mu.Lock()
+	p.metadata[signal] = m
+	p.mu.Unlock()
+}
+
+// -------------------------------------------------------------------------- //
+// ListSignals                                                                  //
+// -------------------------------------------------------------------------- //
+
+// ListSignals returns metadata for all signals with cached information.
+func (p *PVA) ListSignals(_ context.Context) ([]datasource.Metadata, error) {
+	p.mu.RLock()
+	defer p.mu.RUnlock()
+	out := make([]datasource.Metadata, 0, len(p.metadata))
+	for _, m := range p.metadata {
+		out = append(out, m)
+	}
+	return out, nil
+}
+
+// -------------------------------------------------------------------------- //
+// Write                                                                        //
+// -------------------------------------------------------------------------- //
+
+// Write is not yet implemented for PVA.
+func (p *PVA) Write(_ context.Context, _ string, _ any) error {
+	return fmt.Errorf("pva: write not yet implemented")
+}
+
+// -------------------------------------------------------------------------- //
+// History                                                                      //
+// -------------------------------------------------------------------------- //
+
+// History is not supported for PVA sources.
+func (p *PVA) History(_ context.Context, _ string, _, _ time.Time, _ int) ([]datasource.Value, error) {
+	return nil, datasource.ErrHistoryUnavailable
+}
+
+// -------------------------------------------------------------------------- //
+// Helpers                                                                      //
+// -------------------------------------------------------------------------- //
+
+// structToValue converts a PVA StructValue to a datasource.Value.
+// It follows NTScalar/NTScalarArray conventions: looks for a "value" field
+// and an optional "alarm" struct for quality, and "timeStamp" for the timestamp.
+func structToValue(sv pvdata.StructValue) datasource.Value {
+	val := fieldByName(sv, "value")
+	ts := extractTimestamp(sv)
+	quality := extractQuality(sv)
+
+	var data any
+	if val != nil {
+		data = normaliseValue(val)
+	}
+
+	return datasource.Value{
+		Timestamp: ts,
+		Data:      data,
+		Quality:   quality,
+	}
+}
+
+// structToMeta derives datasource.Metadata from a PVA StructValue.
+func structToMeta(name string, sv pvdata.StructValue) datasource.Metadata {
+	m := datasource.Metadata{
+		Name:     name,
+		Writable: true, // PVA channels are assumed writable until proven otherwise
+	}
+
+	val := fieldByName(sv, "value")
+	if val == nil && len(sv.Fields) > 0 {
+		val = sv.Fields[0].Value
+	}
+
+	if val != nil {
+		switch val.(type) {
+		case float64, float32:
+			m.Type = datasource.TypeFloat64
+		case int64, int32, int16, int8, uint64, uint32, uint16, uint8:
+			m.Type = datasource.TypeInt64
+		case string:
+			m.Type = datasource.TypeString
+		case bool:
+			m.Type = datasource.TypeBool
+		case []float64, []float32:
+			m.Type = datasource.TypeFloat64Array
+		default:
+			m.Type = datasource.TypeFloat64
+		}
+	}
+
+	// Extract display limits from NTScalar display structure.
+	if disp := structByName(sv, "display"); disp != nil {
+		if v := fieldByName(*disp, "limitLow"); v != nil {
+			m.DisplayLow, _ = toFloat64(v)
+		}
+		if v := fieldByName(*disp, "limitHigh"); v != nil {
+			m.DisplayHigh, _ = toFloat64(v)
+		}
+		if v := fieldByName(*disp, "units"); v != nil {
+			m.Unit, _ = v.(string)
+		}
+	}
+
+	// Extract control limits from NTScalar control structure.
+	if ctrl := structByName(sv, "control"); ctrl != nil {
+		if v := fieldByName(*ctrl, "limitLow"); v != nil {
+			m.DriveLow, _ = toFloat64(v)
+		}
+		if v := fieldByName(*ctrl, "limitHigh"); v != nil {
+			m.DriveHigh, _ = toFloat64(v)
+		}
+	}
+
+	return m
+}
+
+// normaliseValue converts any PVA scalar/array value to datasource-compatible types.
+func normaliseValue(v any) any {
+	switch x := v.(type) {
+	case float64:
+		return x
+	case float32:
+		return float64(x)
+	case int64:
+		return x
+	case int32:
+		return int64(x)
+	case int16:
+		return int64(x)
+	case int8:
+		return int64(x)
+	case uint64:
+		return int64(x)
+	case uint32:
+		return int64(x)
+	case uint16:
+		return int64(x)
+	case uint8:
+		return int64(x)
+	case bool:
+		return x
+	case string:
+		return x
+	case []float64:
+		return x
+	case []float32:
+		out := make([]float64, len(x))
+		for i, f := range x {
+			out[i] = float64(f)
+		}
+		return out
+	default:
+		return fmt.Sprintf("%v", v)
+	}
+}
+
+func extractTimestamp(sv pvdata.StructValue) time.Time {
+	ts := structByName(sv, "timeStamp")
+	if ts == nil {
+		return time.Now()
+	}
+	sec := int64(0)
+	nsec := int32(0)
+	if v := fieldByName(*ts, "secondsPastEpoch"); v != nil {
+		sec, _ = v.(int64)
+	}
+	if v := fieldByName(*ts, "nanoseconds"); v != nil {
+		nsec, _ = v.(int32)
+	}
+	if sec == 0 {
+		return time.Now()
+	}
+	// EPICS epoch: 1990-01-01 00:00:00 UTC = Unix 631152000
+	const epicsEpoch = 631152000
+	return time.Unix(sec+epicsEpoch, int64(nsec)).UTC()
+}
+
+func extractQuality(sv pvdata.StructValue) datasource.Quality {
+	alarm := structByName(sv, "alarm")
+	if alarm == nil {
+		return datasource.QualityGood
+	}
+	sev := fieldByName(*alarm, "severity")
+	if sev == nil {
+		return datasource.QualityGood
+	}
+	s, _ := sev.(int32)
+	switch s {
+	case 1:
+		return datasource.QualityUncertain
+	case 2, 3:
+		return datasource.QualityBad
+	default:
+		return datasource.QualityGood
+	}
+}
+
+func toFloat64(v any) (float64, bool) {
+	switch x := v.(type) {
+	case float64:
+		return x, true
+	case float32:
+		return float64(x), true
+	case int64:
+		return float64(x), true
+	case int32:
+		return float64(x), true
+	default:
+		return 0, false
+	}
+}
+
+func fieldByName(sv pvdata.StructValue, name string) any {
+	for _, f := range sv.Fields {
+		if f.Name == name {
+			return f.Value
+		}
+	}
+	return nil
+}
+
+func structByName(sv pvdata.StructValue, name string) *pvdata.StructValue {
+	for _, f := range sv.Fields {
+		if f.Name == name {
+			if s, ok := f.Value.(pvdata.StructValue); ok {
+				return &s
+			}
+		}
+	}
+	return nil
+}

internal/datasource/stub/stub_test.go

@@ -0,0 +1,210 @@
+package stub_test
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/uopi/uopi/internal/datasource"
+	"github.com/uopi/uopi/internal/datasource/stub"
+)
+
+func newStub(t *testing.T) *stub.Stub {
+	t.Helper()
+	s := stub.New()
+	if err := s.Connect(context.Background()); err != nil {
+		t.Fatalf("Connect: %v", err)
+	}
+	return s
+}
+
+func TestName(t *testing.T) {
+	if stub.New().Name() != "stub" {
+		t.Error("Name() should return 'stub'")
+	}
+}
+
+func TestListSignals(t *testing.T) {
+	s := newStub(t)
+	sigs, err := s.ListSignals(context.Background())
+	if err != nil {
+		t.Fatalf("ListSignals: %v", err)
+	}
+	if len(sigs) == 0 {
+		t.Fatal("expected at least one signal")
+	}
+	names := make(map[string]bool)
+	for _, m := range sigs {
+		names[m.Name] = true
+	}
+	for _, want := range []string{"sine_1hz", "ramp_10s", "toggle_1hz", "noise", "setpoint"} {
+		if !names[want] {
+			t.Errorf("signal %q not in ListSignals", want)
+		}
+	}
+}
+
+func TestGetMetadata(t *testing.T) {
+	s := newStub(t)
+	m, err := s.GetMetadata(context.Background(), "sine_1hz")
+	if err != nil {
+		t.Fatalf("GetMetadata: %v", err)
+	}
+	if m.Type != datasource.TypeFloat64 {
+		t.Errorf("Type = %v, want TypeFloat64", m.Type)
+	}
+	if m.Unit == "" {
+		t.Error("expected non-empty Unit")
+	}
+}
+
+func TestGetMetadataNotFound(t *testing.T) {
+	s := newStub(t)
+	_, err := s.GetMetadata(context.Background(), "no:such:pv")
+	if err != datasource.ErrNotFound {
+		t.Errorf("want ErrNotFound, got %v", err)
+	}
+}
+
+func TestSubscribeReceivesValues(t *testing.T) {
+	s := newStub(t)
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+
+	ch := make(chan datasource.Value, 4)
+	stop, err := s.Subscribe(ctx, "sine_1hz", ch)
+	if err != nil {
+		t.Fatalf("Subscribe: %v", err)
+	}
+	defer stop()
+
+	select {
+	case v := <-ch:
+		if v.Quality != datasource.QualityGood {
+			t.Errorf("Quality = %v, want Good", v.Quality)
+		}
+		if _, ok := v.Data.(float64); !ok {
+			t.Errorf("Data type = %T, want float64", v.Data)
+		}
+		if v.Timestamp.IsZero() {
+			t.Error("Timestamp is zero")
+		}
+	case <-ctx.Done():
+		t.Fatal("timeout waiting for first value")
+	}
+}
+
+func TestSubscribeUnknown(t *testing.T) {
+	s := newStub(t)
+	_, err := s.Subscribe(context.Background(), "no:such", make(chan datasource.Value, 1))
+	if err != datasource.ErrNotFound {
+		t.Errorf("want ErrNotFound, got %v", err)
+	}
+}
+
+func TestSubscribeCancel(t *testing.T) {
+	s := newStub(t)
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+
+	ch := make(chan datasource.Value, 4)
+	stop, err := s.Subscribe(ctx, "ramp_10s", ch)
+	if err != nil {
+		t.Fatalf("Subscribe: %v", err)
+	}
+
+	// Drain one value to confirm it's running.
+	select {
+	case <-ch:
+	case <-ctx.Done():
+		t.Fatal("timeout waiting for first value")
+	}
+
+	stop() // cancel subscription
+
+	// Drain remaining buffered values then confirm channel goes quiet.
+	time.Sleep(200 * time.Millisecond)
+	for len(ch) > 0 {
+		<-ch
+	}
+	time.Sleep(300 * time.Millisecond)
+	if len(ch) > 0 {
+		t.Error("channel still receiving after stop()")
+	}
+}
+
+func TestWriteSetpoint(t *testing.T) {
+	s := newStub(t)
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+
+	if err := s.Write(context.Background(), "setpoint", float64(99.0)); err != nil {
+		t.Fatalf("Write: %v", err)
+	}
+
+	ch := make(chan datasource.Value, 4)
+	stop, err := s.Subscribe(ctx, "setpoint", ch)
+	if err != nil {
+		t.Fatalf("Subscribe: %v", err)
+	}
+	defer stop()
+
+	select {
+	case v := <-ch:
+		f, ok := v.Data.(float64)
+		if !ok {
+			t.Fatalf("Data type = %T, want float64", v.Data)
+		}
+		if f != 99.0 {
+			t.Errorf("setpoint value = %g, want 99.0", f)
+		}
+	case <-ctx.Done():
+		t.Fatal("timeout waiting for setpoint value")
+	}
+}
+
+func TestWriteReadOnly(t *testing.T) {
+	s := newStub(t)
+	err := s.Write(context.Background(), "sine_1hz", float64(0))
+	if err != datasource.ErrNotWritable {
+		t.Errorf("want ErrNotWritable, got %v", err)
+	}
+}
+
+func TestWriteNotFound(t *testing.T) {
+	s := newStub(t)
+	err := s.Write(context.Background(), "no:such", float64(0))
+	if err != datasource.ErrNotFound {
+		t.Errorf("want ErrNotFound, got %v", err)
+	}
+}
+
+func TestHistory(t *testing.T) {
+	s := newStub(t)
+	_, err := s.History(context.Background(), "sine_1hz", time.Now().Add(-time.Hour), time.Now(), 100)
+	if err != datasource.ErrHistoryUnavailable {
+		t.Errorf("want ErrHistoryUnavailable, got %v", err)
+	}
+}
+
+func TestToggleBool(t *testing.T) {
+	s := newStub(t)
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+
+	ch := make(chan datasource.Value, 4)
+	stop, err := s.Subscribe(ctx, "toggle_1hz", ch)
+	if err != nil {
+		t.Fatalf("Subscribe: %v", err)
+	}
+	defer stop()
+
+	select {
+	case v := <-ch:
+		if _, ok := v.Data.(bool); !ok {
+			t.Errorf("toggle_1hz Data type = %T, want bool", v.Data)
+		}
+	case <-ctx.Done():
+		t.Fatal("timeout")
+	}
+}

internal/metrics/metrics_test.go

@@ -0,0 +1,69 @@
+package metrics_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/uopi/uopi/internal/metrics"
+)
+
+func TestCounters(t *testing.T) {
+	// Counters are package-level atomics; exercise each Inc function.
+	metrics.IncWsConns()
+	metrics.IncWsConns()
+	metrics.DecWsConns()
+	metrics.IncMsgIn()
+	metrics.IncMsgOut()
+	metrics.IncWrites()
+	metrics.IncHistoryReqs()
+	// No panic = pass; values are additive across tests but that's fine.
+}
+
+func TestHandlerFormat(t *testing.T) {
+	handler := metrics.Handler(func() int { return 42 })
+
+	req := httptest.NewRequest(http.MethodGet, "/metrics", nil)
+	rec := httptest.NewRecorder()
+	handler(rec, req)
+
+	body := rec.Body.String()
+
+	// Must contain all metric names.
+	required := []string{
+		"uopi_uptime_seconds",
+		"uopi_ws_connections",
+		"uopi_signal_subscriptions",
+		"uopi_ws_messages_in_total",
+		"uopi_ws_messages_out_total",
+		"uopi_write_ops_total",
+		"uopi_history_requests_total",
+	}
+	for _, name := range required {
+		if !strings.Contains(body, name) {
+			t.Errorf("metric %q not found in response body", name)
+		}
+	}
+
+	// Content-Type must be set for Prometheus scrapers.
+	ct := rec.Header().Get("Content-Type")
+	if !strings.Contains(ct, "text/plain") {
+		t.Errorf("Content-Type = %q, want text/plain", ct)
+	}
+
+	// activeSubs value 42 must appear.
+	if !strings.Contains(body, "42") {
+		t.Error("activeSubs value 42 not found in body")
+	}
+}
+
+func TestHandlerNilActiveSubs(t *testing.T) {
+	handler := metrics.Handler(nil)
+	req := httptest.NewRequest(http.MethodGet, "/metrics", nil)
+	rec := httptest.NewRecorder()
+	handler(rec, req) // must not panic
+	if rec.Code != http.StatusOK {
+		t.Errorf("status = %d, want 200", rec.Code)
+	}
+}

internal/storage/store_test.go

@@ -0,0 +1,232 @@
+package storage_test
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/uopi/uopi/internal/storage"
+)
+
+const sampleXML = `<interface id="test-if" name="Test Interface" version="1"><widget/></interface>`
+
+func newStore(t *testing.T) *storage.Store {
+	t.Helper()
+	s, err := storage.New(t.TempDir())
+	if err != nil {
+		t.Fatalf("New: %v", err)
+	}
+	return s
+}
+
+// -------------------------------------------------------------------------- //
+// List                                                                         //
+// -------------------------------------------------------------------------- //
+
+func TestListEmpty(t *testing.T) {
+	s := newStore(t)
+	list, err := s.List()
+	if err != nil {
+		t.Fatalf("List: %v", err)
+	}
+	if len(list) != 0 {
+		t.Errorf("expected empty list, got %d items", len(list))
+	}
+}
+
+func TestListAfterCreate(t *testing.T) {
+	s := newStore(t)
+	if _, err := s.Create([]byte(sampleXML)); err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+	list, err := s.List()
+	if err != nil {
+		t.Fatalf("List: %v", err)
+	}
+	if len(list) != 1 {
+		t.Fatalf("expected 1 item, got %d", len(list))
+	}
+	if list[0].Name != "Test Interface" {
+		t.Errorf("Name = %q, want %q", list[0].Name, "Test Interface")
+	}
+	if list[0].Version != 1 {
+		t.Errorf("Version = %d, want 1", list[0].Version)
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Create + Get                                                                 //
+// -------------------------------------------------------------------------- //
+
+func TestCreateAndGet(t *testing.T) {
+	s := newStore(t)
+	id, err := s.Create([]byte(sampleXML))
+	if err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+	if id == "" {
+		t.Fatal("Create returned empty ID")
+	}
+
+	data, err := s.Get(id)
+	if err != nil {
+		t.Fatalf("Get(%q): %v", id, err)
+	}
+	if string(data) != sampleXML {
+		t.Errorf("Get returned %q, want %q", data, sampleXML)
+	}
+}
+
+func TestCreateUsesEmbeddedID(t *testing.T) {
+	s := newStore(t)
+	id, err := s.Create([]byte(sampleXML))
+	if err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+	// ID derived from xml id attr or name slug.
+	if id == "" {
+		t.Error("ID should not be empty")
+	}
+}
+
+func TestCreateDuplicateGetsUniqueID(t *testing.T) {
+	s := newStore(t)
+	id1, err := s.Create([]byte(sampleXML))
+	if err != nil {
+		t.Fatalf("first Create: %v", err)
+	}
+	id2, err := s.Create([]byte(sampleXML))
+	if err != nil {
+		t.Fatalf("second Create: %v", err)
+	}
+	if id1 == id2 {
+		t.Errorf("duplicate Create returned same ID %q", id1)
+	}
+}
+
+func TestCreateInvalidXML(t *testing.T) {
+	s := newStore(t)
+	_, err := s.Create([]byte("not xml"))
+	if err == nil {
+		t.Error("expected error for invalid XML")
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Get errors                                                                   //
+// -------------------------------------------------------------------------- //
+
+func TestGetNotFound(t *testing.T) {
+	s := newStore(t)
+	_, err := s.Get("does-not-exist")
+	if !errors.Is(err, storage.ErrNotFound) {
+		t.Errorf("Get missing: want ErrNotFound, got %v", err)
+	}
+}
+
+func TestGetInvalidID(t *testing.T) {
+	s := newStore(t)
+	_, err := s.Get("../traversal")
+	if !errors.Is(err, storage.ErrNotFound) {
+		t.Errorf("Get path-traversal ID: want ErrNotFound, got %v", err)
+	}
+}
+
+func TestGetEmptyID(t *testing.T) {
+	s := newStore(t)
+	_, err := s.Get("")
+	if !errors.Is(err, storage.ErrNotFound) {
+		t.Errorf("Get empty ID: want ErrNotFound, got %v", err)
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Update                                                                       //
+// -------------------------------------------------------------------------- //
+
+func TestUpdate(t *testing.T) {
+	s := newStore(t)
+	id, err := s.Create([]byte(sampleXML))
+	if err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+
+	updated := `<interface id="test-if" name="Updated" version="2"><widget/></interface>`
+	if err := s.Update(id, []byte(updated)); err != nil {
+		t.Fatalf("Update: %v", err)
+	}
+
+	data, err := s.Get(id)
+	if err != nil {
+		t.Fatalf("Get after update: %v", err)
+	}
+	if string(data) != updated {
+		t.Errorf("post-update content mismatch")
+	}
+}
+
+func TestUpdateNotFound(t *testing.T) {
+	s := newStore(t)
+	err := s.Update("no-such-id", []byte(sampleXML))
+	if !errors.Is(err, storage.ErrNotFound) {
+		t.Errorf("Update missing: want ErrNotFound, got %v", err)
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Delete                                                                       //
+// -------------------------------------------------------------------------- //
+
+func TestDelete(t *testing.T) {
+	s := newStore(t)
+	id, err := s.Create([]byte(sampleXML))
+	if err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+	if err := s.Delete(id); err != nil {
+		t.Fatalf("Delete: %v", err)
+	}
+	_, err = s.Get(id)
+	if !errors.Is(err, storage.ErrNotFound) {
+		t.Errorf("Get after delete: want ErrNotFound, got %v", err)
+	}
+}
+
+func TestDeleteNotFound(t *testing.T) {
+	s := newStore(t)
+	err := s.Delete("ghost")
+	if !errors.Is(err, storage.ErrNotFound) {
+		t.Errorf("Delete missing: want ErrNotFound, got %v", err)
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Clone                                                                        //
+// -------------------------------------------------------------------------- //
+
+func TestClone(t *testing.T) {
+	s := newStore(t)
+	id, err := s.Create([]byte(sampleXML))
+	if err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+	newID, err := s.Clone(id)
+	if err != nil {
+		t.Fatalf("Clone: %v", err)
+	}
+	if newID == id {
+		t.Error("Clone should produce a different ID")
+	}
+	orig, _ := s.Get(id)
+	copy, _ := s.Get(newID)
+	if string(orig) != string(copy) {
+		t.Error("Clone content should match original")
+	}
+}
+
+func TestCloneNotFound(t *testing.T) {
+	s := newStore(t)
+	_, err := s.Clone("ghost")
+	if !errors.Is(err, storage.ErrNotFound) {
+		t.Errorf("Clone missing: want ErrNotFound, got %v", err)
+	}
+}

pkg/ca/client.go

@@ -0,0 +1,421 @@
+package ca
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+
+	"github.com/uopi/goca/proto"
+)
+
+// -------------------------------------------------------------------------- //
+// Config                                                                      //
+// -------------------------------------------------------------------------- //
+
+// Config holds the configuration for a Client.
+// All fields are optional; sensible defaults are applied by NewClient.
+type Config struct {
+	// AddrList is the list of CA server addresses ("host" or "host:port").
+	// Corresponds to EPICS_CA_ADDR_LIST.
+	AddrList []string
+
+	// AutoAddrList, when true, appends the IPv4 broadcast address of every
+	// local network interface to AddrList.  Defaults to true.
+	// Corresponds to EPICS_CA_AUTO_ADDR_LIST.
+	AutoAddrList bool
+
+	// ClientName is announced to every CA server in the CLIENT_NAME message.
+	// Defaults to the executable base name.
+	ClientName string
+
+	// HostName is announced to every CA server in the HOST_NAME message.
+	// Defaults to the OS hostname.
+	HostName string
+}
+
+// ConfigFromEnv reads the standard EPICS CA environment variables and returns
+// a ready-to-use Config.
+//
+//	EPICS_CA_ADDR_LIST        — space-separated list of server addresses
+//	EPICS_CA_AUTO_ADDR_LIST   — "NO" disables automatic broadcast addresses
+func ConfigFromEnv() Config {
+	name := filepath.Base(os.Args[0])
+	host, _ := os.Hostname()
+	cfg := Config{
+		AutoAddrList: true,
+		ClientName:   name,
+		HostName:     host,
+	}
+	if v := os.Getenv("EPICS_CA_ADDR_LIST"); v != "" {
+		cfg.AddrList = strings.Fields(v)
+	}
+	if strings.EqualFold(os.Getenv("EPICS_CA_AUTO_ADDR_LIST"), "no") {
+		cfg.AutoAddrList = false
+	}
+	return cfg
+}
+
+// -------------------------------------------------------------------------- //
+// Client                                                                      //
+// -------------------------------------------------------------------------- //
+
+// Client is a thread-safe EPICS Channel Access client.
+//
+// A single Client should serve an entire application.  It maintains a pool of
+// persistent TCP circuits (one per IOC) and a shared UDP search engine.
+// Channels and subscriptions survive IOC restarts automatically.
+//
+// Usage:
+//
+//	cli, err := ca.NewClient(ctx, ca.ConfigFromEnv())
+//	defer cli.Close()
+//
+//	ch := make(chan proto.TimeValue, 16)
+//	cancel, err := cli.Subscribe(ctx, "MY:PV", ch)
+//	defer cancel()
+//
+//	for tv := range ch { fmt.Println(tv.Double) }
+type Client struct {
+	cfg    Config
+	ctx    context.Context
+	cancel context.CancelFunc
+	search *searchEngine
+
+	mu       sync.Mutex
+	circuits map[string]*circuit // IOC TCP addr → circuit
+}
+
+// NewClient creates a new CA client and starts background I/O.
+// ctx governs the lifetime of the client; cancelling it is equivalent to
+// calling Close.
+//
+// Returns an error only if no search addresses can be derived from cfg.
+func NewClient(ctx context.Context, cfg Config) (*Client, error) {
+	addrs := resolveAddrs(cfg.AddrList, proto.DefaultPort)
+	if cfg.AutoAddrList {
+		addrs = append(addrs, localBroadcastAddrs(proto.DefaultPort)...)
+	}
+	if len(addrs) == 0 {
+		return nil, fmt.Errorf("ca: no search addresses (set EPICS_CA_ADDR_LIST or enable AutoAddrList)")
+	}
+
+	if cfg.ClientName == "" {
+		cfg.ClientName = filepath.Base(os.Args[0])
+	}
+	if cfg.HostName == "" {
+		cfg.HostName, _ = os.Hostname()
+	}
+
+	cctx, cancel := context.WithCancel(ctx)
+	se := newSearchEngine(addrs)
+	if err := se.start(cctx); err != nil {
+		cancel()
+		return nil, err
+	}
+
+	return &Client{
+		cfg:      cfg,
+		ctx:      cctx,
+		cancel:   cancel,
+		search:   se,
+		circuits: make(map[string]*circuit),
+	}, nil
+}
+
+// Close shuts down all circuits and background goroutines.
+// Any in-flight Subscribe, Get, or Put calls will unblock with an error.
+func (c *Client) Close() {
+	c.cancel()
+}
+
+// -------------------------------------------------------------------------- //
+// Internal helpers                                                             //
+// -------------------------------------------------------------------------- //
+
+// getOrCreateCircuit returns the circuit for addr, creating one if necessary.
+func (c *Client) getOrCreateCircuit(addr string) *circuit {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if circ, ok := c.circuits[addr]; ok {
+		return circ
+	}
+	circ := newCircuit(c.ctx, addr, c.cfg.ClientName, c.cfg.HostName)
+	c.circuits[addr] = circ
+	return circ
+}
+
+// resolve finds the IOC for pvName via UDP search, then waits for the TCP
+// channel to be fully established.  Returns (circuit, chanState) on success.
+func (c *Client) resolve(ctx context.Context, pvName string) (*circuit, *chanState, error) {
+	addr, err := c.search.lookup(ctx, pvName)
+	if err != nil {
+		return nil, nil, err
+	}
+	circ := c.getOrCreateCircuit(addr)
+	cs := circ.getOrCreateChannel(pvName)
+	if err := cs.waitReady(ctx); err != nil {
+		return nil, nil, fmt.Errorf("ca: %q: %w", pvName, err)
+	}
+	return circ, cs, nil
+}
+
+// -------------------------------------------------------------------------- //
+// Public API                                                                  //
+// -------------------------------------------------------------------------- //
+
+// Subscribe registers ch to receive live monitor updates for pvName.
+//
+// Values are pushed as proto.TimeValue structs (DBR_TIME_* encoding).
+// ch should be buffered; slow consumers will have updates silently dropped.
+//
+// The returned CancelFunc unsubscribes from the PV and must always be called
+// to avoid leaking resources.
+//
+// Subscribe blocks until the channel is connected or ctx expires.
+func (c *Client) Subscribe(ctx context.Context, pvName string, ch chan<- proto.TimeValue) (context.CancelFunc, error) {
+	circ, cs, err := c.resolve(ctx, pvName)
+	if err != nil {
+		return nil, err
+	}
+
+	cs.mu.RLock()
+	dbfType := cs.dbfType
+	count := cs.count
+	cs.mu.RUnlock()
+
+	ms := &monState{
+		subID:   circ.nextID(),
+		dbrType: proto.NativeTimeType(dbfType, int(count)),
+		count:   count,
+		ch:      ch,
+	}
+	circ.addMonitor(cs, ms)
+
+	return func() { circ.removeMonitor(cs, ms.subID) }, nil
+}
+
+// Get performs a one-shot READ_NOTIFY for pvName and returns its current value.
+// It blocks until the reply arrives or ctx expires.
+func (c *Client) Get(ctx context.Context, pvName string) (proto.TimeValue, error) {
+	circ, cs, err := c.resolve(ctx, pvName)
+	if err != nil {
+		return proto.TimeValue{}, err
+	}
+
+	cs.mu.RLock()
+	dbfType := cs.dbfType
+	count := cs.count
+	cs.mu.RUnlock()
+
+	dbrType := proto.NativeTimeType(dbfType, int(count))
+	return circ.get(ctx, cs, dbrType, count)
+}
+
+// Put writes value to pvName using CA_PROTO_WRITE (fire-and-forget).
+//
+// value is automatically encoded to match the PV's native field type.
+// Supported Go types: float64, float32, int64, int32, int, int16, string, bool.
+//
+// Put blocks only until the message is queued; it does not wait for an IOC
+// acknowledgement.  Use a timeout context to bound the wait for connection.
+func (c *Client) Put(ctx context.Context, pvName string, value any) error {
+	circ, cs, err := c.resolve(ctx, pvName)
+	if err != nil {
+		return err
+	}
+
+	cs.mu.RLock()
+	dbfType := cs.dbfType
+	cs.mu.RUnlock()
+
+	dbrType, payload, err := encodePut(dbfType, value)
+	if err != nil {
+		return fmt.Errorf("ca: put %q: %w", pvName, err)
+	}
+	return circ.put(ctx, cs, dbrType, payload)
+}
+
+// -------------------------------------------------------------------------- //
+// Control metadata                                                             //
+// -------------------------------------------------------------------------- //
+
+// CtrlInfo holds the full control-block metadata for a PV.
+// Exactly one of the Double, Long, Enum, Str pointer fields is non-nil,
+// depending on the PV's native field type.
+type CtrlInfo struct {
+	DBFType int    // proto.DBF* constant
+	Count   uint32 // element count
+	Access  uint32 // proto.AccessRead | proto.AccessWrite bitmask
+
+	Double *proto.CtrlDouble // non-nil for DBFDouble / DBFFloat
+	Long   *proto.CtrlLong  // non-nil for DBFLong / DBFShort / DBFChar
+	Enum   *proto.CtrlEnum  // non-nil for DBFEnum
+	Str    *proto.CtrlString // non-nil for DBFString
+}
+
+// GetCtrl performs a READ_NOTIFY with a DBR_CTRL_* type and returns the full
+// control-block metadata (units, display limits, enum strings, etc.) for pvName.
+//
+// GetCtrl blocks until the reply arrives or ctx expires.
+func (c *Client) GetCtrl(ctx context.Context, pvName string) (CtrlInfo, error) {
+	circ, cs, err := c.resolve(ctx, pvName)
+	if err != nil {
+		return CtrlInfo{}, err
+	}
+
+	cs.mu.RLock()
+	dbfType := cs.dbfType
+	count := cs.count
+	access := cs.access
+	cs.mu.RUnlock()
+
+	ctrlType := proto.NativeCtrlType(dbfType)
+	_, payload, err := circ.getRaw(ctx, cs, ctrlType, count)
+	if err != nil {
+		return CtrlInfo{}, err
+	}
+
+	ci := CtrlInfo{DBFType: dbfType, Count: count, Access: access}
+
+	switch ctrlType {
+	case proto.DBRCtrlDouble:
+		cd, ok := proto.DecodeCtrlDouble(payload)
+		if !ok {
+			return CtrlInfo{}, fmt.Errorf("ca: %q: DecodeCtrlDouble failed (payload len %d)", pvName, len(payload))
+		}
+		ci.Double = &cd
+
+	case proto.DBRCtrlLong:
+		cl, ok := proto.DecodeCtrlLong(payload)
+		if !ok {
+			return CtrlInfo{}, fmt.Errorf("ca: %q: DecodeCtrlLong failed (payload len %d)", pvName, len(payload))
+		}
+		ci.Long = &cl
+
+	case proto.DBRCtrlEnum:
+		ce, ok := proto.DecodeCtrlEnum(payload)
+		if !ok {
+			return CtrlInfo{}, fmt.Errorf("ca: %q: DecodeCtrlEnum failed (payload len %d)", pvName, len(payload))
+		}
+		ci.Enum = &ce
+
+	case proto.DBRCtrlString:
+		cs2, ok := proto.DecodeCtrlString(payload)
+		if !ok {
+			return CtrlInfo{}, fmt.Errorf("ca: %q: DecodeCtrlString failed (payload len %d)", pvName, len(payload))
+		}
+		ci.Str = &cs2
+
+	default:
+		return CtrlInfo{}, fmt.Errorf("ca: %q: unhandled ctrl type %d", pvName, ctrlType)
+	}
+
+	return ci, nil
+}
+
+// -------------------------------------------------------------------------- //
+// Value encoding for Put                                                      //
+// -------------------------------------------------------------------------- //
+
+// encodePut converts a Go value to a DBR wire payload matching the PV's
+// native field type.
+func encodePut(dbfType int, value any) (dbrType uint16, payload []byte, err error) {
+	switch dbfType {
+	case proto.DBFDouble, proto.DBFFloat:
+		f, e := toFloat64(value)
+		if e != nil {
+			return 0, nil, e
+		}
+		return proto.DBRDouble, proto.EncodeDouble(f), nil
+
+	case proto.DBFLong:
+		n, e := toInt32(value)
+		if e != nil {
+			return 0, nil, e
+		}
+		return proto.DBRLong, proto.EncodeLong(n), nil
+
+	case proto.DBFShort, proto.DBFChar, proto.DBFEnum:
+		n, e := toInt32(value)
+		if e != nil {
+			return 0, nil, e
+		}
+		return proto.DBRShort, proto.EncodeShort(int16(n)), nil
+
+	case proto.DBFString:
+		s, e := toString(value)
+		if e != nil {
+			return 0, nil, e
+		}
+		return proto.DBRString, proto.EncodeString(s), nil
+
+	default:
+		return 0, nil, fmt.Errorf("unsupported DBF type %d", dbfType)
+	}
+}
+
+func toFloat64(v any) (float64, error) {
+	switch x := v.(type) {
+	case float64:
+		return x, nil
+	case float32:
+		return float64(x), nil
+	case int:
+		return float64(x), nil
+	case int64:
+		return float64(x), nil
+	case int32:
+		return float64(x), nil
+	case int16:
+		return float64(x), nil
+	case uint64:
+		return float64(x), nil
+	case uint32:
+		return float64(x), nil
+	case bool:
+		if x {
+			return 1, nil
+		}
+		return 0, nil
+	default:
+		return 0, fmt.Errorf("cannot convert %T to float64", v)
+	}
+}
+
+func toInt32(v any) (int32, error) {
+	switch x := v.(type) {
+	case int:
+		return int32(x), nil
+	case int64:
+		return int32(x), nil
+	case int32:
+		return x, nil
+	case int16:
+		return int32(x), nil
+	case float64:
+		return int32(x), nil
+	case float32:
+		return int32(x), nil
+	case bool:
+		if x {
+			return 1, nil
+		}
+		return 0, nil
+	default:
+		return 0, fmt.Errorf("cannot convert %T to int32", v)
+	}
+}
+
+func toString(v any) (string, error) {
+	switch x := v.(type) {
+	case string:
+		return x, nil
+	case []byte:
+		return string(x), nil
+	default:
+		return fmt.Sprintf("%v", x), nil
+	}
+}

pkg/ca/client_test.go

@@ -0,0 +1,397 @@
+package ca_test
+
+import (
+	"context"
+	"math"
+	"testing"
+	"time"
+
+	"github.com/uopi/goca"
+	"github.com/uopi/goca/testca"
+	"github.com/uopi/goca/proto"
+)
+
+// newTestClient creates a Client pointing only at the fake server's addresses.
+func newTestClient(t *testing.T, srv *testca.Server) *ca.Client {
+	t.Helper()
+	cfg := ca.Config{
+		AddrList:     []string{srv.UDPAddr()},
+		AutoAddrList: false,
+		ClientName:   "testclient",
+		HostName:     "localhost",
+	}
+	cli, err := ca.NewClient(context.Background(), cfg)
+	if err != nil {
+		t.Fatalf("NewClient: %v", err)
+	}
+	t.Cleanup(cli.Close)
+	return cli
+}
+
+// newTestServer creates a fake CA server with standard test PVs.
+func newTestServer(t *testing.T) *testca.Server {
+	t.Helper()
+	srv, err := testca.New([]testca.PVSpec{
+		{Name: "TEST:DOUBLE", DBFType: proto.DBFDouble, Count: 1, Value: 3.14},
+		{Name: "TEST:LONG", DBFType: proto.DBFLong, Count: 1, Value: int32(42)},
+		{Name: "TEST:STRING", DBFType: proto.DBFString, Count: 1, Value: "hello"},
+		{Name: "TEST:ENUM", DBFType: proto.DBFEnum, Count: 1, Value: int16(1)},
+		{Name: "TEST:READONLY", DBFType: proto.DBFDouble, Count: 1, Value: 1.0,
+			Access: proto.AccessRead},
+	})
+	if err != nil {
+		t.Fatalf("testca.New: %v", err)
+	}
+	t.Cleanup(srv.Close)
+	return srv
+}
+
+// -------------------------------------------------------------------------- //
+// Get tests                                                                    //
+// -------------------------------------------------------------------------- //
+
+func TestGetDouble(t *testing.T) {
+	srv := newTestServer(t)
+	cli := newTestClient(t, srv)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	tv, err := cli.Get(ctx, "TEST:DOUBLE")
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+	if math.Abs(tv.Double-3.14) > 1e-6 {
+		t.Errorf("Double = %g, want ~3.14", tv.Double)
+	}
+	if tv.Timestamp.IsZero() {
+		t.Error("Timestamp is zero")
+	}
+}
+
+func TestGetLong(t *testing.T) {
+	srv := newTestServer(t)
+	cli := newTestClient(t, srv)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	tv, err := cli.Get(ctx, "TEST:LONG")
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+	if tv.Long != 42 {
+		t.Errorf("Long = %d, want 42", tv.Long)
+	}
+}
+
+func TestGetString(t *testing.T) {
+	srv := newTestServer(t)
+	cli := newTestClient(t, srv)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	tv, err := cli.Get(ctx, "TEST:STRING")
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+	if tv.Str != "hello" {
+		t.Errorf("Str = %q, want %q", tv.Str, "hello")
+	}
+}
+
+func TestGetNotFound(t *testing.T) {
+	srv := newTestServer(t)
+	cli := newTestClient(t, srv)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 500*time.Millisecond)
+	defer cancel()
+
+	_, err := cli.Get(ctx, "NO:SUCH:PV")
+	if err == nil {
+		t.Fatal("expected error for missing PV, got nil")
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Subscribe tests                                                              //
+// -------------------------------------------------------------------------- //
+
+func TestSubscribeReceivesInitialValue(t *testing.T) {
+	srv := newTestServer(t)
+	cli := newTestClient(t, srv)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	ch := make(chan proto.TimeValue, 8)
+	unsub, err := cli.Subscribe(ctx, "TEST:DOUBLE", ch)
+	if err != nil {
+		t.Fatalf("Subscribe: %v", err)
+	}
+	defer unsub()
+
+	select {
+	case tv := <-ch:
+		if math.Abs(tv.Double-3.14) > 1e-6 {
+			t.Errorf("initial Double = %g, want ~3.14", tv.Double)
+		}
+	case <-ctx.Done():
+		t.Fatal("timeout waiting for initial monitor value")
+	}
+}
+
+func TestSubscribeReceivesUpdates(t *testing.T) {
+	srv := newTestServer(t)
+	cli := newTestClient(t, srv)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	ch := make(chan proto.TimeValue, 8)
+	unsub, err := cli.Subscribe(ctx, "TEST:DOUBLE", ch)
+	if err != nil {
+		t.Fatalf("Subscribe: %v", err)
+	}
+	defer unsub()
+
+	// Drain initial value.
+	select {
+	case <-ch:
+	case <-ctx.Done():
+		t.Fatal("timeout waiting for initial value")
+	}
+
+	// Push a new value from the server side.
+	if err := srv.SetValue("TEST:DOUBLE", 99.9); err != nil {
+		t.Fatalf("SetValue: %v", err)
+	}
+
+	select {
+	case tv := <-ch:
+		if math.Abs(tv.Double-99.9) > 1e-6 {
+			t.Errorf("updated Double = %g, want ~99.9", tv.Double)
+		}
+	case <-ctx.Done():
+		t.Fatal("timeout waiting for updated monitor value")
+	}
+}
+
+func TestUnsubscribeStopsUpdates(t *testing.T) {
+	srv := newTestServer(t)
+	cli := newTestClient(t, srv)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	ch := make(chan proto.TimeValue, 8)
+	unsub, err := cli.Subscribe(ctx, "TEST:DOUBLE", ch)
+	if err != nil {
+		t.Fatalf("Subscribe: %v", err)
+	}
+
+	// Drain initial value.
+	select {
+	case <-ch:
+	case <-ctx.Done():
+		t.Fatal("timeout waiting for initial value")
+	}
+
+	unsub() // unsubscribe
+
+	// Give the EVENT_CANCEL message time to be delivered.
+	time.Sleep(50 * time.Millisecond)
+
+	// Push a value; channel should NOT receive it.
+	srv.SetValue("TEST:DOUBLE", 777.0)
+
+	select {
+	case tv := <-ch:
+		// It's possible one update slipped through before cancel was processed.
+		// Accept it, but not a second one.
+		t.Logf("received one update after unsub (value=%g), checking for second...", tv.Double)
+		select {
+		case <-ch:
+			t.Error("received second update after unsubscribe")
+		case <-time.After(100 * time.Millisecond):
+			// Good — no second update.
+		}
+	case <-time.After(150 * time.Millisecond):
+		// No update received — correct.
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Put tests                                                                    //
+// -------------------------------------------------------------------------- //
+
+func TestPutDouble(t *testing.T) {
+	srv := newTestServer(t)
+	cli := newTestClient(t, srv)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	// Subscribe to observe the effect of Put.
+	ch := make(chan proto.TimeValue, 8)
+	unsub, err := cli.Subscribe(ctx, "TEST:DOUBLE", ch)
+	if err != nil {
+		t.Fatalf("Subscribe: %v", err)
+	}
+	defer unsub()
+
+	// Drain initial value.
+	select {
+	case <-ch:
+	case <-ctx.Done():
+		t.Fatal("timeout waiting for initial value")
+	}
+
+	if err := cli.Put(ctx, "TEST:DOUBLE", float64(2.718)); err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+
+	select {
+	case tv := <-ch:
+		if math.Abs(tv.Double-2.718) > 1e-6 {
+			t.Errorf("post-put Double = %g, want ~2.718", tv.Double)
+		}
+	case <-ctx.Done():
+		t.Fatal("timeout waiting for post-put monitor value")
+	}
+}
+
+func TestPutLong(t *testing.T) {
+	srv := newTestServer(t)
+	cli := newTestClient(t, srv)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	ch := make(chan proto.TimeValue, 8)
+	unsub, err := cli.Subscribe(ctx, "TEST:LONG", ch)
+	if err != nil {
+		t.Fatalf("Subscribe: %v", err)
+	}
+	defer unsub()
+	<-ch // drain initial
+
+	if err := cli.Put(ctx, "TEST:LONG", int(100)); err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+
+	select {
+	case tv := <-ch:
+		if tv.Long != 100 {
+			t.Errorf("post-put Long = %d, want 100", tv.Long)
+		}
+	case <-ctx.Done():
+		t.Fatal("timeout waiting for post-put value")
+	}
+}
+
+func TestPutString(t *testing.T) {
+	srv := newTestServer(t)
+	cli := newTestClient(t, srv)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	ch := make(chan proto.TimeValue, 8)
+	unsub, err := cli.Subscribe(ctx, "TEST:STRING", ch)
+	if err != nil {
+		t.Fatalf("Subscribe: %v", err)
+	}
+	defer unsub()
+	<-ch // drain initial
+
+	if err := cli.Put(ctx, "TEST:STRING", "world"); err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+
+	select {
+	case tv := <-ch:
+		if tv.Str != "world" {
+			t.Errorf("post-put Str = %q, want %q", tv.Str, "world")
+		}
+	case <-ctx.Done():
+		t.Fatal("timeout waiting for post-put value")
+	}
+}
+
+func TestPutReadOnly(t *testing.T) {
+	srv := newTestServer(t)
+	cli := newTestClient(t, srv)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	err := cli.Put(ctx, "TEST:READONLY", float64(1.0))
+	if err == nil {
+		t.Fatal("expected error writing to read-only PV")
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Multiple subscribers on the same PV                                         //
+// -------------------------------------------------------------------------- //
+
+func TestMultipleSubscribers(t *testing.T) {
+	srv := newTestServer(t)
+	cli := newTestClient(t, srv)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	const n = 3
+	chs := make([]chan proto.TimeValue, n)
+	for i := range chs {
+		chs[i] = make(chan proto.TimeValue, 8)
+		unsub, err := cli.Subscribe(ctx, "TEST:DOUBLE", chs[i])
+		if err != nil {
+			t.Fatalf("Subscribe[%d]: %v", i, err)
+		}
+		defer unsub()
+	}
+
+	// Drain initial values.
+	for i, ch := range chs {
+		select {
+		case <-ch:
+		case <-ctx.Done():
+			t.Fatalf("timeout waiting for initial value on subscriber %d", i)
+		}
+	}
+
+	// Push a new value.
+	srv.SetValue("TEST:DOUBLE", 55.5)
+
+	for i, ch := range chs {
+		select {
+		case tv := <-ch:
+			if math.Abs(tv.Double-55.5) > 1e-6 {
+				t.Errorf("subscriber %d: Double = %g, want 55.5", i, tv.Double)
+			}
+		case <-ctx.Done():
+			t.Fatalf("timeout waiting for update on subscriber %d", i)
+		}
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Context cancellation                                                         //
+// -------------------------------------------------------------------------- //
+
+func TestGetContextCancelled(t *testing.T) {
+	srv := newTestServer(t)
+	cli := newTestClient(t, srv)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // already cancelled
+
+	_, err := cli.Get(ctx, "TEST:DOUBLE")
+	if err == nil {
+		t.Fatal("expected error with cancelled context")
+	}
+}

pkg/ca/conn.go

@@ -0,0 +1,747 @@
+package ca
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"github.com/uopi/goca/proto"
+)
+
+const (
+	writeQueueDepth = 256
+	echoInterval    = 15 * time.Second
+	connectTimeout  = 5 * time.Second
+	maxReconnDelay  = 60 * time.Second
+)
+
+// -------------------------------------------------------------------------- //
+// reply — async GET callback payload                                          //
+// -------------------------------------------------------------------------- //
+
+// reply is delivered to a pending GET waiter via a buffered channel.
+// If ok is false the circuit died before the reply arrived.
+type reply struct {
+	hdr     proto.Header
+	payload []byte
+	ok      bool
+}
+
+// -------------------------------------------------------------------------- //
+// monState — one active EVENT_ADD subscription                               //
+// -------------------------------------------------------------------------- //
+
+// monState survives reconnections; the circuit re-sends EVENT_ADD with the
+// new SID each time a CREATE_CHAN reply is received.
+type monState struct {
+	subID   uint32
+	dbrType uint16
+	count   uint32
+	ch      chan<- proto.TimeValue // caller-owned, never closed here
+}
+
+// -------------------------------------------------------------------------- //
+// chanState — one CA channel (persists across reconnections)                 //
+// -------------------------------------------------------------------------- //
+
+// chanState field invariants:
+//   - cid, pvName: immutable after construction.
+//   - sid, dbfType, count, access: valid only while readyC is closed.
+//   - readyC: closed when CREATE_CHAN reply received; replaced on reconnect.
+//   - monitors: append-only (except removeMonitor); never cleared on reconnect.
+type chanState struct {
+	cid    uint32
+	pvName string
+
+	mu       sync.RWMutex
+	sid      uint32       // server-assigned channel ID (0 until CREATE_CHAN reply)
+	dbfType  int          // native DBF field type
+	count    uint32       // element count
+	access   uint32       // AccessRead | AccessWrite bitmask
+	readyC   chan struct{} // closed once CREATE_CHAN reply is received
+	monitors []*monState  // active subscriptions
+}
+
+func newChanState(cid uint32, pvName string) *chanState {
+	return &chanState{
+		cid:    cid,
+		pvName: pvName,
+		readyC: make(chan struct{}),
+	}
+}
+
+// resetForReconnect prepares cs for a new TCP connection.
+// It closes the current readyC (waking any waitReady callers so they can
+// re-wait on the fresh channel) and then replaces it.
+// Must be called before the circuit sends CREATE_CHAN on the new conn.
+// In-flight GET requests are tracked in circuit.byIOID and cleared there.
+func (cs *chanState) resetForReconnect() {
+	cs.mu.Lock()
+	cs.sid = 0
+	old := cs.readyC
+	cs.readyC = make(chan struct{})
+	cs.mu.Unlock()
+	// Close the old readyC outside the lock to avoid deadlock with waitReady.
+	select {
+	case <-old:
+		// Already closed (CmdCreateFail path or double-reset guard).
+	default:
+		close(old)
+	}
+}
+
+// waitReady blocks until sid != 0 (CREATE_CHAN reply received for the current
+// connection) or ctx expires.  It loops through reconnections automatically:
+// when resetForReconnect closes the current readyC the goroutine wakes, sees
+// sid == 0, and waits on the freshly created channel.
+func (cs *chanState) waitReady(ctx context.Context) error {
+	for {
+		cs.mu.RLock()
+		sid := cs.sid
+		ready := cs.readyC
+		cs.mu.RUnlock()
+
+		if sid != 0 {
+			return nil
+		}
+
+		select {
+		case <-ready:
+			// State changed — either CREATE_CHAN reply (sid set) or reconnect
+			// started (sid cleared, new readyC installed).  Loop to check.
+		case <-ctx.Done():
+			return ctx.Err()
+		}
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// circuit — persistent TCP connection to one CA server                       //
+// -------------------------------------------------------------------------- //
+
+// circuit manages a persistent, auto-reconnecting TCP connection to a single
+// CA server.  All channels and monitors created on a circuit survive
+// reconnections; the circuit re-creates them transparently.
+//
+// Goroutine model:
+//   - run() goroutine: reconnect loop; drives the write loop for each conn.
+//   - readLoop() goroutine: one per active conn; dispatches inbound messages.
+//
+// Lock ordering: circuit.mu → chanState.mu (never the reverse).
+type circuit struct {
+	addr       string
+	clientName string
+	hostName   string
+
+	ctx    context.Context
+	cancel context.CancelFunc
+
+	mu       sync.RWMutex
+	channels []*chanState          // append-only; survive reconnect
+	byCID    map[uint32]*chanState // cid → chanState (fast lookup)
+	bySID    map[uint32]*chanState // sid → chanState (fast lookup)
+	bySubID  map[uint32]*monState  // subID → monState (fast event dispatch)
+	byIOID   map[uint32]chan reply // ioid → GET/PUT callback (circuit-wide)
+
+	writeQ chan []byte    // serialised outbound message queue
+	seq    atomic.Uint32 // shared ID sequence (cid, ioid, subID)
+}
+
+func newCircuit(ctx context.Context, addr, clientName, hostName string) *circuit {
+	cctx, cancel := context.WithCancel(ctx)
+	c := &circuit{
+		addr:       addr,
+		clientName: clientName,
+		hostName:   hostName,
+		ctx:        cctx,
+		cancel:     cancel,
+		byCID:      make(map[uint32]*chanState),
+		bySID:      make(map[uint32]*chanState),
+		bySubID:    make(map[uint32]*monState),
+		byIOID:     make(map[uint32]chan reply),
+		writeQ:     make(chan []byte, writeQueueDepth),
+	}
+	go c.run()
+	return c
+}
+
+func (c *circuit) close() { c.cancel() }
+
+func (c *circuit) nextID() uint32 { return c.seq.Add(1) }
+
+// -------------------------------------------------------------------------- //
+// Reconnect loop                                                               //
+// -------------------------------------------------------------------------- //
+
+func (c *circuit) run() {
+	delay := time.Second
+	for {
+		if c.ctx.Err() != nil {
+			return
+		}
+
+		dbg("CA circuit connecting", "addr", c.addr)
+		conn, err := c.dial()
+		if err != nil {
+			dbg("CA circuit dial failed", "addr", c.addr, "err", err)
+			select {
+			case <-time.After(delay):
+			case <-c.ctx.Done():
+				return
+			}
+			delay = min(delay*2, maxReconnDelay)
+			continue
+		}
+		dbg("CA circuit connected", "addr", c.addr)
+		delay = time.Second // reset back-off on successful connect
+
+		// Snapshot channels and reset their per-connection state.
+		c.mu.Lock()
+		c.bySID = make(map[uint32]*chanState)
+		// Close all in-flight GET reply channels so waiters unblock with ok=false.
+		for _, ch := range c.byIOID {
+			close(ch)
+		}
+		c.byIOID = make(map[uint32]chan reply)
+		for _, cs := range c.channels {
+			cs.resetForReconnect()
+		}
+		chs := make([]*chanState, len(c.channels))
+		copy(chs, c.channels)
+		c.mu.Unlock()
+
+		// Drain stale messages from the write queue (they carry old SIDs).
+		for len(c.writeQ) > 0 {
+			<-c.writeQ
+		}
+
+		// Send VERSION + HOST_NAME + CLIENT_NAME.
+		if err := c.sendHandshake(conn); err != nil {
+			conn.Close()
+			continue
+		}
+
+		// Re-create all known channels on the new connection.
+		setupOK := true
+		for _, cs := range chs {
+			if _, err := conn.Write(buildCreateChanMsg(cs.cid, cs.pvName)); err != nil {
+				setupOK = false
+				break
+			}
+		}
+		if !setupOK {
+			conn.Close()
+			continue
+		}
+
+		dbg("CA handshake sent", "addr", c.addr, "channels", len(chs))
+
+		// Start read loop.
+		readDone := make(chan struct{})
+		go func() {
+			c.readLoop(conn)
+			dbg("CA read loop exited", "addr", c.addr)
+			close(readDone)
+		}()
+
+		// Write loop (run goroutine acts as the write pump).
+		c.writeLoop(conn, readDone)
+
+		conn.Close()
+		<-readDone // drain read goroutine before reconnecting
+
+		if c.ctx.Err() != nil {
+			return
+		}
+
+		select {
+		case <-time.After(delay):
+		case <-c.ctx.Done():
+			return
+		}
+		delay = min(delay*2, maxReconnDelay)
+	}
+}
+
+func (c *circuit) writeLoop(conn net.Conn, readDone <-chan struct{}) {
+	ticker := time.NewTicker(echoInterval)
+	defer ticker.Stop()
+	for {
+		select {
+		case msg := <-c.writeQ:
+			if _, err := conn.Write(msg); err != nil {
+				return
+			}
+		case <-ticker.C:
+			// Periodic heartbeat; server echoes it back.
+			hb := proto.BuildMessage(proto.Header{Command: proto.CmdEcho}, nil)
+			if _, err := conn.Write(hb); err != nil {
+				return
+			}
+		case <-readDone:
+			return
+		case <-c.ctx.Done():
+			return
+		}
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Dial + handshake                                                             //
+// -------------------------------------------------------------------------- //
+
+func (c *circuit) dial() (net.Conn, error) {
+	d := net.Dialer{Timeout: connectTimeout}
+	conn, err := d.DialContext(c.ctx, "tcp4", c.addr)
+	if err != nil {
+		return nil, fmt.Errorf("ca: dial %s: %w", c.addr, err)
+	}
+	return conn, nil
+}
+
+func (c *circuit) sendHandshake(conn net.Conn) error {
+	ver := proto.BuildMessage(proto.Header{
+		Command:   proto.CmdVersion,
+		DataCount: proto.MinorVersion,
+	}, nil)
+	host := proto.BuildMessage(
+		proto.Header{Command: proto.CmdHostName},
+		proto.BuildStringPayload(c.hostName),
+	)
+	client := proto.BuildMessage(
+		proto.Header{Command: proto.CmdClientName},
+		proto.BuildStringPayload(c.clientName),
+	)
+	out := make([]byte, 0, len(ver)+len(host)+len(client))
+	out = append(out, ver...)
+	out = append(out, host...)
+	out = append(out, client...)
+	_, err := conn.Write(out)
+	return err
+}
+
+// -------------------------------------------------------------------------- //
+// Read loop + message dispatch                                                 //
+// -------------------------------------------------------------------------- //
+
+func (c *circuit) readLoop(conn net.Conn) {
+	for {
+		hdr, _, err := proto.DecodeHeader(conn)
+		if err != nil {
+			if c.ctx.Err() == nil {
+				dbg("CA read loop error (header)", "addr", c.addr, "err", err)
+			}
+			return
+		}
+		dbg("CA recv", "addr", c.addr, "cmd", hdr.Command, "dataType", hdr.DataType,
+			"dataCount", hdr.DataCount, "payloadSize", hdr.PayloadSize,
+			"p1", hdr.Parameter1, "p2", hdr.Parameter2)
+		var payload []byte
+		if hdr.PayloadSize > 0 {
+			payload = make([]byte, hdr.PayloadSize)
+			if _, err = io.ReadFull(conn, payload); err != nil {
+				if c.ctx.Err() == nil {
+					dbg("CA read loop error (payload)", "addr", c.addr,
+						"cmd", hdr.Command, "wantBytes", hdr.PayloadSize, "err", err)
+				}
+				return
+			}
+		}
+		c.dispatch(conn, hdr, payload)
+	}
+}
+
+// dispatch handles one inbound CA message.
+func (c *circuit) dispatch(conn net.Conn, hdr proto.Header, payload []byte) {
+	switch hdr.Command {
+
+	case proto.CmdVersion:
+		// Server version negotiation reply — nothing to do.
+
+	case proto.CmdCreateChan:
+		// Parameter1 = cid (echoed), Parameter2 = SID assigned by server.
+		c.mu.Lock()
+		cs, ok := c.byCID[hdr.Parameter1]
+		if ok {
+			c.bySID[hdr.Parameter2] = cs
+		}
+		c.mu.Unlock()
+
+		if !ok {
+			return
+		}
+		cs.mu.Lock()
+		cs.sid = hdr.Parameter2
+		cs.dbfType = int(hdr.DataType)
+		cs.count = hdr.DataCount
+		ready := cs.readyC
+		// Snapshot monitors for EVENT_ADD re-subscription.
+		mons := make([]*monState, len(cs.monitors))
+		copy(mons, cs.monitors)
+		cs.mu.Unlock()
+
+		dbg("CA CREATE_CHAN reply", "pv", cs.pvName, "sid", hdr.Parameter2,
+			"dbfType", hdr.DataType, "count", hdr.DataCount, "monitors", len(mons))
+
+		// Re-subscribe all monitors with the new SID.
+		for _, ms := range mons {
+			msg := buildEventAddMsg(ms.subID, hdr.Parameter2, ms.dbrType, ms.count)
+			dbg("CA EVENT_ADD sent", "pv", cs.pvName, "subID", ms.subID,
+				"sid", hdr.Parameter2, "dbrType", ms.dbrType, "count", ms.count)
+			select {
+			case c.writeQ <- msg:
+			default:
+			}
+		}
+		close(ready) // unblock waitReady callers
+
+	case proto.CmdCreateFail:
+		// Server does not host this PV (or quota exceeded).
+		c.mu.RLock()
+		cs, ok := c.byCID[hdr.Parameter1]
+		c.mu.RUnlock()
+		if !ok {
+			return
+		}
+		dbg("CA CREATE_FAIL", "pv", cs.pvName, "cid", hdr.Parameter1)
+		cs.mu.RLock()
+		ready := cs.readyC
+		cs.mu.RUnlock()
+		// readyC is only closed once per connection cycle by this goroutine,
+		// so the select here is safe (no concurrent close).
+		select {
+		case <-ready:
+			// Already closed (shouldn't happen, but guard against it).
+		default:
+			close(ready)
+		}
+
+	case proto.CmdAccessRights:
+		// Parameter1 = cid, Parameter2 = access bitmask.
+		c.mu.RLock()
+		cs, ok := c.byCID[hdr.Parameter1]
+		c.mu.RUnlock()
+		if ok {
+			cs.mu.Lock()
+			cs.access = hdr.Parameter2
+			cs.mu.Unlock()
+		}
+
+	case proto.CmdEventAdd:
+		// Monitor update: server puts subscription ID in Parameter2 (m_available).
+		// Parameter1 carries ECA_NORMAL (1) as a status code.
+		subID := hdr.Parameter2
+		c.mu.RLock()
+		ms, ok := c.bySubID[subID]
+		c.mu.RUnlock()
+		if !ok {
+			dbg("CA EVENT_ADD for unknown subID", "subID", subID)
+			return
+		}
+		tv, ok := proto.DecodeTimeValue(hdr.DataType, hdr.DataCount, payload)
+		if !ok {
+			dbg("CA EVENT_ADD decode failed", "subID", subID,
+				"dbrType", hdr.DataType, "count", hdr.DataCount, "payloadLen", len(payload))
+			return
+		}
+		dbg("CA EVENT_ADD value", "subID", subID, "dbrType", hdr.DataType,
+			"double", tv.Double, "severity", tv.Severity)
+		select {
+		case ms.ch <- tv:
+		default: // drop if consumer is slow
+		}
+
+	case proto.CmdReadNotify:
+		// Parameter2 = ioid (our request ID echoed back by server).
+		ioid := hdr.Parameter2
+
+		c.mu.Lock()
+		replyCh, ok := c.byIOID[ioid]
+		if ok {
+			delete(c.byIOID, ioid)
+		}
+		c.mu.Unlock()
+
+		dbg("CA CmdReadNotify", "ioid", ioid, "found", ok, "dbrType", hdr.DataType,
+			"count", hdr.DataCount, "payloadSize", hdr.PayloadSize)
+
+		if ok {
+			select {
+			case replyCh <- reply{hdr: hdr, payload: payload, ok: true}:
+			default:
+			}
+		}
+
+	case proto.CmdEcho:
+		// Server heartbeat — no response needed (client sends its own echoes).
+
+	case proto.CmdServerDisc:
+		// Server is shutting down — close conn to trigger reconnect.
+		conn.Close()
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Message builders                                                             //
+// -------------------------------------------------------------------------- //
+
+func buildCreateChanMsg(cid uint32, pvName string) []byte {
+	return proto.BuildMessage(proto.Header{
+		Command:    proto.CmdCreateChan,
+		Parameter1: cid,
+		Parameter2: proto.MinorVersion,
+	}, proto.BuildStringPayload(pvName))
+}
+
+func buildEventAddMsg(subID, sid uint32, dbrType uint16, count uint32) []byte {
+	return proto.BuildMessage(proto.Header{
+		Command:    proto.CmdEventAdd,
+		DataType:   dbrType,
+		DataCount:  count,
+		Parameter1: sid,   // m_cid = channel SID (per CA spec)
+		Parameter2: subID, // m_available = client subscription ID
+	}, proto.EncodeEventMask(proto.DBEDefault))
+}
+
+func buildEventCancelMsg(subID, sid uint32, dbrType uint16) []byte {
+	return proto.BuildMessage(proto.Header{
+		Command:    proto.CmdEventCancel,
+		DataType:   dbrType,
+		Parameter1: sid,   // m_cid = channel SID
+		Parameter2: subID, // m_available = client subscription ID
+	}, nil)
+}
+
+// -------------------------------------------------------------------------- //
+// Channel and monitor management                                               //
+// -------------------------------------------------------------------------- //
+
+// getOrCreateChannel returns the chanState for pvName, creating it if needed.
+// If the circuit is currently connected, CREATE_CHAN is queued immediately;
+// otherwise the reconnect loop will send it when it next connects.
+func (c *circuit) getOrCreateChannel(pvName string) *chanState {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	for _, cs := range c.channels {
+		if cs.pvName == pvName {
+			return cs
+		}
+	}
+	cid := c.nextID()
+	cs := newChanState(cid, pvName)
+	c.channels = append(c.channels, cs)
+	c.byCID[cid] = cs
+	// Best-effort: deliver CREATE_CHAN if circuit is up.
+	msg := buildCreateChanMsg(cid, pvName)
+	select {
+	case c.writeQ <- msg:
+	default:
+	}
+	return cs
+}
+
+// addMonitor registers ms on cs and queues EVENT_ADD if the channel is ready.
+// If the channel is not yet ready the CREATE_CHAN reply handler will send
+// EVENT_ADD when the connection is established.
+func (c *circuit) addMonitor(cs *chanState, ms *monState) {
+	c.mu.Lock()
+	c.bySubID[ms.subID] = ms
+	c.mu.Unlock()
+
+	cs.mu.Lock()
+	cs.monitors = append(cs.monitors, ms)
+	sid := cs.sid
+	cs.mu.Unlock()
+
+	if sid != 0 {
+		dbg("CA EVENT_ADD sent (addMonitor)", "pv", cs.pvName, "subID", ms.subID,
+			"sid", sid, "dbrType", ms.dbrType, "count", ms.count)
+		msg := buildEventAddMsg(ms.subID, sid, ms.dbrType, ms.count)
+		select {
+		case c.writeQ <- msg:
+		default:
+		}
+	}
+}
+
+// removeMonitor unregisters a monitor and queues EVENT_CANCEL if connected.
+func (c *circuit) removeMonitor(cs *chanState, subID uint32) {
+	c.mu.Lock()
+	delete(c.bySubID, subID)
+	c.mu.Unlock()
+
+	cs.mu.Lock()
+	var dbrType uint16
+	remaining := cs.monitors[:0:0] // fresh backing array
+	for _, ms := range cs.monitors {
+		if ms.subID == subID {
+			dbrType = ms.dbrType
+		} else {
+			remaining = append(remaining, ms)
+		}
+	}
+	cs.monitors = remaining
+	sid := cs.sid
+	cs.mu.Unlock()
+
+	if sid != 0 {
+		msg := buildEventCancelMsg(subID, sid, dbrType)
+		select {
+		case c.writeQ <- msg:
+		default:
+		}
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Channel operations: get and put                                              //
+// -------------------------------------------------------------------------- //
+
+// getRaw performs a READ_NOTIFY and returns the undecoded header + payload.
+// Use this when you need to decode a type that get() does not handle
+// (e.g. DBR_CTRL_* for metadata retrieval).
+func (c *circuit) getRaw(ctx context.Context, cs *chanState, dbrType uint16, count uint32) (proto.Header, []byte, error) {
+	if err := cs.waitReady(ctx); err != nil {
+		return proto.Header{}, nil, fmt.Errorf("ca: %q: %w", cs.pvName, err)
+	}
+
+	cs.mu.RLock()
+	sid := cs.sid
+	cs.mu.RUnlock()
+	if sid == 0 {
+		return proto.Header{}, nil, fmt.Errorf("ca: %q: channel not connected", cs.pvName)
+	}
+
+	ioid := c.nextID()
+	replyCh := make(chan reply, 1)
+
+	c.mu.Lock()
+	c.byIOID[ioid] = replyCh
+	c.mu.Unlock()
+
+	msg := proto.BuildMessage(proto.Header{
+		Command:    proto.CmdReadNotify,
+		DataType:   dbrType,
+		DataCount:  count,
+		Parameter1: sid,  // m_cid = channel SID (per CA spec)
+		Parameter2: ioid, // m_available = request IOId
+	}, nil)
+
+	select {
+	case c.writeQ <- msg:
+	case <-ctx.Done():
+		c.mu.Lock()
+		delete(c.byIOID, ioid)
+		c.mu.Unlock()
+		return proto.Header{}, nil, ctx.Err()
+	}
+
+	select {
+	case r := <-replyCh:
+		if !r.ok {
+			return proto.Header{}, nil, fmt.Errorf("ca: %q: circuit disconnected", cs.pvName)
+		}
+		return r.hdr, r.payload, nil
+	case <-ctx.Done():
+		c.mu.Lock()
+		delete(c.byIOID, ioid)
+		c.mu.Unlock()
+		return proto.Header{}, nil, ctx.Err()
+	}
+}
+
+// get performs a READ_NOTIFY (async GET) and returns the decoded value.
+// It blocks until the reply arrives, the channel is not yet ready, or ctx expires.
+func (c *circuit) get(ctx context.Context, cs *chanState, dbrType uint16, count uint32) (proto.TimeValue, error) {
+	if err := cs.waitReady(ctx); err != nil {
+		return proto.TimeValue{}, fmt.Errorf("ca: %q: %w", cs.pvName, err)
+	}
+
+	cs.mu.RLock()
+	sid := cs.sid
+	cs.mu.RUnlock()
+	if sid == 0 {
+		return proto.TimeValue{}, fmt.Errorf("ca: %q: channel not connected", cs.pvName)
+	}
+
+	ioid := c.nextID()
+	replyCh := make(chan reply, 1)
+
+	c.mu.Lock()
+	c.byIOID[ioid] = replyCh
+	c.mu.Unlock()
+
+	msg := proto.BuildMessage(proto.Header{
+		Command:    proto.CmdReadNotify,
+		DataType:   dbrType,
+		DataCount:  count,
+		Parameter1: sid,  // m_cid = channel SID (per CA spec)
+		Parameter2: ioid, // m_available = request IOId
+	}, nil)
+
+	select {
+	case c.writeQ <- msg:
+	case <-ctx.Done():
+		c.mu.Lock()
+		delete(c.byIOID, ioid)
+		c.mu.Unlock()
+		return proto.TimeValue{}, ctx.Err()
+	}
+
+	select {
+	case r := <-replyCh:
+		if !r.ok {
+			return proto.TimeValue{}, fmt.Errorf("ca: %q: circuit disconnected during GET", cs.pvName)
+		}
+		tv, ok := proto.DecodeTimeValue(r.hdr.DataType, r.hdr.DataCount, r.payload)
+		if !ok {
+			return proto.TimeValue{}, fmt.Errorf("ca: %q: failed to decode GET reply", cs.pvName)
+		}
+		return tv, nil
+	case <-ctx.Done():
+		c.mu.Lock()
+		delete(c.byIOID, ioid)
+		c.mu.Unlock()
+		return proto.TimeValue{}, ctx.Err()
+	}
+}
+
+// put queues a CA_PROTO_WRITE (fire-and-forget put).
+// It waits for the channel to be ready and access rights to be confirmed.
+func (c *circuit) put(ctx context.Context, cs *chanState, dbrType uint16, payload []byte) error {
+	if err := cs.waitReady(ctx); err != nil {
+		return fmt.Errorf("ca: %q: %w", cs.pvName, err)
+	}
+
+	cs.mu.RLock()
+	sid := cs.sid
+	access := cs.access
+	cs.mu.RUnlock()
+
+	if sid == 0 {
+		return fmt.Errorf("ca: %q: channel not connected", cs.pvName)
+	}
+	if access&proto.AccessWrite == 0 {
+		return fmt.Errorf("ca: %q: read-only", cs.pvName)
+	}
+
+	// CA_PROTO_WRITE: DataType=dbrType, DataCount=1, Parameter1=SID, Parameter2=0 (no ioid).
+	msg := proto.BuildMessage(proto.Header{
+		Command:    proto.CmdWrite,
+		DataType:   dbrType,
+		DataCount:  1,
+		Parameter1: sid,
+		Parameter2: 0,
+	}, payload)
+
+	select {
+	case c.writeQ <- msg:
+		return nil
+	case <-ctx.Done():
+		return ctx.Err()
+	}
+}

pkg/ca/debug.go

@@ -0,0 +1,26 @@
+package ca
+
+import (
+	"log/slog"
+	"os"
+)
+
+// calog is the package-level debug logger.
+// It is active only when the environment variable UOPI_CA_DEBUG is set to a
+// non-empty value at program startup.  All output goes to stderr.
+var calog *slog.Logger
+
+func init() {
+	if os.Getenv("UOPI_CA_DEBUG") != "" {
+		calog = slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{
+			Level: slog.LevelDebug,
+		}))
+	}
+}
+
+// dbg emits a DEBUG log if UOPI_CA_DEBUG is set.  It is a no-op otherwise.
+func dbg(msg string, args ...any) {
+	if calog != nil {
+		calog.Debug(msg, args...)
+	}
+}

pkg/ca/doc.go

@@ -0,0 +1,65 @@
+// Package ca implements an EPICS Channel Access (CA) client in pure Go.
+//
+// It requires no CGo, no libca, and no EPICS installation on the build machine.
+// A single [Client] manages connections to one or more IOCs, multiplexing
+// channels over shared TCP virtual circuits that survive IOC restarts.
+//
+// # Quick start
+//
+//	// Configure from standard EPICS environment variables.
+//	cli, err := ca.NewClient(ctx, ca.ConfigFromEnv())
+//	if err != nil { ... }
+//	defer cli.Close()
+//
+//	// Subscribe to live monitor updates.
+//	ch := make(chan proto.TimeValue, 16)
+//	cancel, err := cli.Subscribe(ctx, "MY:PV", ch)
+//	if err != nil { ... }
+//	defer cancel()
+//	for tv := range ch {
+//	    fmt.Println(tv.Timestamp, tv.Double)
+//	}
+//
+//	// One-shot read (current value).
+//	tv, err := cli.Get(ctx, "MY:PV")
+//
+//	// Retrieve full control-block metadata (units, display limits, enum strings).
+//	ci, err := cli.GetCtrl(ctx, "MY:PV")
+//	if ci.Double != nil {
+//	    fmt.Println(ci.Double.Units, ci.Double.UpperDispLimit)
+//	}
+//
+//	// Write a new value (fire-and-forget).
+//	err = cli.Put(ctx, "MY:PV", 42.0)
+//
+// # Configuration
+//
+// [ConfigFromEnv] reads the standard EPICS CA environment variables:
+//
+//	EPICS_CA_ADDR_LIST        space-separated list of server addresses
+//	EPICS_CA_AUTO_ADDR_LIST   set to "NO" to disable local broadcast
+//
+// Alternatively, construct [Config] directly and pass to [NewClient].
+//
+// # Protocol
+//
+// The library speaks Channel Access protocol version 4.13 over UDP (search,
+// port 5064) and TCP (data, port 5064).  It does not use the CA Repeater;
+// it sends search requests directly to the addresses in [Config.AddrList].
+//
+// UDP search uses exponential back-off (100 ms → 30 s) and retries until the
+// PV is found or the context is cancelled.
+//
+// # Reconnection
+//
+// When an IOC restarts, the library detects the broken TCP connection and
+// reconnects automatically.  All active subscriptions are re-established
+// transparently after the new CREATE_CHAN handshake completes.
+// Callers blocked in [Client.Get] or [Client.Put] during a reconnect will
+// receive an error and should retry.
+//
+// # Thread safety
+//
+// All exported methods are safe for concurrent use from multiple goroutines.
+// A single [Client] can be shared across the entire application.
+package ca

pkg/ca/go.mod

@@ -0,0 +1,3 @@
+module github.com/uopi/goca
+
+go 1.26

pkg/ca/proto/dbr.go

@@ -0,0 +1,444 @@
+package proto
+
+import (
+	"encoding/binary"
+	"math"
+	"strings"
+	"time"
+)
+
+// epicsEpochOffset is the number of Unix seconds between the Unix epoch
+// (1970-01-01 00:00:00 UTC) and the EPICS epoch (1990-01-01 00:00:00 UTC).
+const epicsEpochOffset = 631152000
+
+// maxStringSize is the fixed size of a CA string value (MAX_STRING_SIZE).
+const maxStringSize = 40
+
+// maxEnumStates is the maximum number of enum strings (MAX_ENUM_STATES).
+const maxEnumStates = 16
+
+// maxEnumStringSize is the fixed size of each enum string (MAX_ENUM_STRING_SIZE).
+const maxEnumStringSize = 26
+
+// -------------------------------------------------------------------------- //
+// Alarm types                                                                 //
+// -------------------------------------------------------------------------- //
+
+// AlarmSeverity mirrors epicsAlarmSeverity.
+type AlarmSeverity int
+
+const (
+	SeverityNone    AlarmSeverity = 0
+	SeverityMinor   AlarmSeverity = 1
+	SeverityMajor   AlarmSeverity = 2
+	SeverityInvalid AlarmSeverity = 3
+)
+
+func (s AlarmSeverity) String() string {
+	switch s {
+	case SeverityNone:
+		return "NO_ALARM"
+	case SeverityMinor:
+		return "MINOR"
+	case SeverityMajor:
+		return "MAJOR"
+	case SeverityInvalid:
+		return "INVALID"
+	default:
+		return "UNKNOWN"
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// TimeValue — decoded DBR_TIME_* payload                                     //
+// -------------------------------------------------------------------------- //
+
+// TimeValue is the decoded form of any DBR_TIME_* payload.
+// Exactly one of the value fields is populated based on the DBR type.
+type TimeValue struct {
+	Timestamp time.Time
+	Status    int16
+	Severity  AlarmSeverity
+	// Value fields — only one is set.
+	Double float64
+	Float  float32
+	Long   int32
+	Short  int16
+	Enum   uint16
+	Char   uint8
+	Str    string
+	// Waveform values (when element count > 1).
+	Doubles []float64
+}
+
+// decodeTimestamp converts EPICS secPastEpoch+nsec to a Go time.Time.
+func decodeTimestamp(secPastEpoch, nsec uint32) time.Time {
+	return time.Unix(int64(secPastEpoch)+epicsEpochOffset, int64(nsec)).UTC()
+}
+
+// caString extracts a null-terminated string from a fixed-size byte slice.
+func caString(b []byte) string {
+	idx := strings.IndexByte(string(b), 0)
+	if idx < 0 {
+		return string(b)
+	}
+	return string(b[:idx])
+}
+
+// DBR_TIME_* wire layouts (all big-endian, matching EPICS db_access.h structs):
+//
+//	Common header (12 bytes):
+//	  [0:2]  int16  status
+//	  [2:4]  int16  severity
+//	  [4:8]  uint32 secPastEpoch
+//	  [8:12] uint32 nsec
+//
+//	DBR_TIME_STRING (type 14): [40]byte at [12:52].                             Total=52.
+//	DBR_TIME_SHORT  (type 15): 2-byte RISC pad at [12:14], int16   at [14:16].  Total=16.
+//	DBR_TIME_FLOAT  (type 16): float32 at [12:16].                              Total=16.
+//	DBR_TIME_ENUM   (type 17): 2-byte RISC pad at [12:14], uint16  at [14:16].  Total=16.
+//	DBR_TIME_CHAR   (type 18): RISC_pad0[12:14], RISC_pad1[14], uint8 at [15]. Total=16.
+//	DBR_TIME_LONG   (type 19): int32   at [12:16].                              Total=16.
+//	DBR_TIME_DOUBLE (type 20): 4-byte RISC pad at [12:16], float64 at [16:24]. Total=24.
+
+// DecodeTimeValue decodes a DBR_TIME_* payload.
+// dbrType is one of the DBRTime* constants; payload is the full message payload
+// (may include multiple elements for waveforms; count is the element count).
+func DecodeTimeValue(dbrType uint16, count uint32, payload []byte) (TimeValue, bool) {
+	if len(payload) < 12 {
+		return TimeValue{}, false
+	}
+
+	status := int16(binary.BigEndian.Uint16(payload[0:]))
+	severity := AlarmSeverity(binary.BigEndian.Uint16(payload[2:]))
+	sec := binary.BigEndian.Uint32(payload[4:])
+	nsec := binary.BigEndian.Uint32(payload[8:])
+
+	tv := TimeValue{
+		Timestamp: decodeTimestamp(sec, nsec),
+		Status:    status,
+		Severity:  severity,
+	}
+
+	switch dbrType {
+	case DBRTimeDouble:
+		// 4-byte RISC pad at [12:16], value at [16:24]
+		if count > 1 {
+			if len(payload) < 16+int(count)*8 {
+				return TimeValue{}, false
+			}
+			vals := make([]float64, count)
+			for i := range vals {
+				bits := binary.BigEndian.Uint64(payload[16+i*8:])
+				vals[i] = math.Float64frombits(bits)
+			}
+			tv.Doubles = vals
+			tv.Double = vals[0]
+		} else {
+			if len(payload) < 24 {
+				return TimeValue{}, false
+			}
+			bits := binary.BigEndian.Uint64(payload[16:])
+			tv.Double = math.Float64frombits(bits)
+		}
+
+	case DBRTimeFloat:
+		if len(payload) < 16 {
+			return TimeValue{}, false
+		}
+		bits := binary.BigEndian.Uint32(payload[12:])
+		tv.Float = math.Float32frombits(bits)
+		tv.Double = float64(tv.Float)
+
+	case DBRTimeLong:
+		if len(payload) < 16 {
+			return TimeValue{}, false
+		}
+		tv.Long = int32(binary.BigEndian.Uint32(payload[12:]))
+		tv.Double = float64(tv.Long)
+
+	case DBRTimeShort:
+		// 2-byte RISC pad at [12:14], value at [14:16]
+		if len(payload) < 16 {
+			return TimeValue{}, false
+		}
+		tv.Short = int16(binary.BigEndian.Uint16(payload[14:]))
+		tv.Double = float64(tv.Short)
+
+	case DBRTimeEnum:
+		// 2-byte RISC pad at [12:14], value at [14:16]
+		if len(payload) < 16 {
+			return TimeValue{}, false
+		}
+		tv.Enum = binary.BigEndian.Uint16(payload[14:])
+		tv.Double = float64(tv.Enum)
+
+	case DBRTimeChar:
+		// RISC_pad0[12:14], RISC_pad1[14], value[15] per db_access.h dbr_time_char
+		if len(payload) < 16 {
+			return TimeValue{}, false
+		}
+		tv.Char = payload[15]
+		tv.Double = float64(tv.Char)
+
+	case DBRTimeString:
+		if len(payload) < 12+maxStringSize {
+			return TimeValue{}, false
+		}
+		tv.Str = caString(payload[12 : 12+maxStringSize])
+		tv.Double = 0
+
+	default:
+		return TimeValue{}, false
+	}
+
+	return tv, true
+}
+
+// -------------------------------------------------------------------------- //
+// CtrlDouble — decoded DBR_CTRL_DOUBLE payload (type 34)                     //
+// -------------------------------------------------------------------------- //
+//
+// Wire layout (88 bytes total, all big-endian):
+//
+//	[0:2]   int16   status
+//	[2:4]   int16   severity
+//	[4:6]   int16   precision
+//	[6:8]   uint16  RISC_pad0
+//	[8:16]  [8]byte units
+//	[16:24] float64 upper_disp_limit
+//	[24:32] float64 lower_disp_limit
+//	[32:40] float64 upper_alarm_limit
+//	[40:48] float64 upper_warning_limit
+//	[48:56] float64 lower_warning_limit
+//	[56:64] float64 lower_alarm_limit
+//	[64:72] float64 upper_ctrl_limit
+//	[72:80] float64 lower_ctrl_limit
+//	[80:88] float64 value
+
+// CtrlDouble is the decoded form of a DBR_CTRL_DOUBLE payload.
+type CtrlDouble struct {
+	Status           int16
+	Severity         AlarmSeverity
+	Precision        int16
+	Units            string
+	UpperDispLimit   float64
+	LowerDispLimit   float64
+	UpperAlarmLimit  float64
+	UpperWarnLimit   float64
+	LowerWarnLimit   float64
+	LowerAlarmLimit  float64
+	UpperCtrlLimit   float64
+	LowerCtrlLimit   float64
+	Value            float64
+}
+
+// DecodeCtrlDouble decodes a DBR_CTRL_DOUBLE payload (minimum 88 bytes).
+func DecodeCtrlDouble(p []byte) (CtrlDouble, bool) {
+	if len(p) < 88 {
+		return CtrlDouble{}, false
+	}
+	f64 := func(off int) float64 {
+		return math.Float64frombits(binary.BigEndian.Uint64(p[off:]))
+	}
+	return CtrlDouble{
+		Status:          int16(binary.BigEndian.Uint16(p[0:])),
+		Severity:        AlarmSeverity(binary.BigEndian.Uint16(p[2:])),
+		Precision:       int16(binary.BigEndian.Uint16(p[4:])),
+		Units:           caString(p[8:16]),
+		UpperDispLimit:  f64(16),
+		LowerDispLimit:  f64(24),
+		UpperAlarmLimit: f64(32),
+		UpperWarnLimit:  f64(40),
+		LowerWarnLimit:  f64(48),
+		LowerAlarmLimit: f64(56),
+		UpperCtrlLimit:  f64(64),
+		LowerCtrlLimit:  f64(72),
+		Value:           f64(80),
+	}, true
+}
+
+// -------------------------------------------------------------------------- //
+// CtrlLong — decoded DBR_CTRL_LONG payload (type 33)                        //
+// -------------------------------------------------------------------------- //
+//
+// Wire layout (48 bytes total):
+//
+//	[0:2]   int16   status
+//	[2:4]   int16   severity
+//	[4:12]  [8]byte units
+//	[12:16] int32   upper_disp_limit
+//	[16:20] int32   lower_disp_limit
+//	[20:24] int32   upper_alarm_limit
+//	[24:28] int32   upper_warning_limit
+//	[28:32] int32   lower_warning_limit
+//	[32:36] int32   lower_alarm_limit
+//	[36:40] int32   upper_ctrl_limit
+//	[40:44] int32   lower_ctrl_limit
+//	[44:48] int32   value
+
+// CtrlLong is the decoded form of a DBR_CTRL_LONG payload.
+type CtrlLong struct {
+	Status          int16
+	Severity        AlarmSeverity
+	Units           string
+	UpperDispLimit  int32
+	LowerDispLimit  int32
+	UpperAlarmLimit int32
+	UpperWarnLimit  int32
+	LowerWarnLimit  int32
+	LowerAlarmLimit int32
+	UpperCtrlLimit  int32
+	LowerCtrlLimit  int32
+	Value           int32
+}
+
+// DecodeCtrlLong decodes a DBR_CTRL_LONG payload (minimum 48 bytes).
+func DecodeCtrlLong(p []byte) (CtrlLong, bool) {
+	if len(p) < 48 {
+		return CtrlLong{}, false
+	}
+	i32 := func(off int) int32 {
+		return int32(binary.BigEndian.Uint32(p[off:]))
+	}
+	return CtrlLong{
+		Status:          int16(binary.BigEndian.Uint16(p[0:])),
+		Severity:        AlarmSeverity(binary.BigEndian.Uint16(p[2:])),
+		Units:           caString(p[4:12]),
+		UpperDispLimit:  i32(12),
+		LowerDispLimit:  i32(16),
+		UpperAlarmLimit: i32(20),
+		UpperWarnLimit:  i32(24),
+		LowerWarnLimit:  i32(28),
+		LowerAlarmLimit: i32(32),
+		UpperCtrlLimit:  i32(36),
+		LowerCtrlLimit:  i32(40),
+		Value:           i32(44),
+	}, true
+}
+
+// -------------------------------------------------------------------------- //
+// CtrlEnum — decoded DBR_CTRL_ENUM payload (type 31)                        //
+// -------------------------------------------------------------------------- //
+//
+// Wire layout (424 bytes total):
+//
+//	[0:2]   int16      status
+//	[2:4]   int16      severity
+//	[4:6]   int16      no_str  (number of valid enum strings, max 16)
+//	[6:422] [16][26]byte strs (enum string table)
+//	[422:424] uint16   value
+
+// CtrlEnum is the decoded form of a DBR_CTRL_ENUM payload.
+type CtrlEnum struct {
+	Status   int16
+	Severity AlarmSeverity
+	Strings  []string // len == no_str
+	Value    uint16
+}
+
+const ctrlEnumSize = 2 + 2 + 2 + maxEnumStates*maxEnumStringSize + 2 // 424
+
+// DecodeCtrlEnum decodes a DBR_CTRL_ENUM payload (minimum 424 bytes).
+func DecodeCtrlEnum(p []byte) (CtrlEnum, bool) {
+	if len(p) < ctrlEnumSize {
+		return CtrlEnum{}, false
+	}
+	noStr := int(binary.BigEndian.Uint16(p[4:]))
+	noStr = min(noStr, maxEnumStates)
+	strs := make([]string, noStr)
+	for i := range strs {
+		off := 6 + i*maxEnumStringSize
+		strs[i] = caString(p[off : off+maxEnumStringSize])
+	}
+	return CtrlEnum{
+		Status:   int16(binary.BigEndian.Uint16(p[0:])),
+		Severity: AlarmSeverity(binary.BigEndian.Uint16(p[2:])),
+		Strings:  strs,
+		Value:    binary.BigEndian.Uint16(p[422:]),
+	}, true
+}
+
+// -------------------------------------------------------------------------- //
+// CtrlString — decoded DBR_CTRL_STRING payload (type 28)                    //
+// -------------------------------------------------------------------------- //
+//
+// Wire layout (44 bytes):
+//
+//	[0:2]   int16   status
+//	[2:4]   int16   severity
+//	[4:44]  [40]byte value
+
+// CtrlString is the decoded form of a DBR_CTRL_STRING payload.
+type CtrlString struct {
+	Status   int16
+	Severity AlarmSeverity
+	Value    string
+}
+
+// DecodeCtrlString decodes a DBR_CTRL_STRING payload (minimum 44 bytes).
+func DecodeCtrlString(p []byte) (CtrlString, bool) {
+	if len(p) < 4+maxStringSize {
+		return CtrlString{}, false
+	}
+	return CtrlString{
+		Status:   int16(binary.BigEndian.Uint16(p[0:])),
+		Severity: AlarmSeverity(binary.BigEndian.Uint16(p[2:])),
+		Value:    caString(p[4 : 4+maxStringSize]),
+	}, true
+}
+
+// -------------------------------------------------------------------------- //
+// Put payload encoders                                                        //
+// -------------------------------------------------------------------------- //
+
+// EncodeDouble encodes a float64 value as a big-endian DBR_DOUBLE payload
+// padded to 8 bytes.
+func EncodeDouble(v float64) []byte {
+	b := make([]byte, 8)
+	binary.BigEndian.PutUint64(b, math.Float64bits(v))
+	return b
+}
+
+// EncodeLong encodes an int32 value as a big-endian DBR_LONG payload
+// padded to 8 bytes.
+func EncodeLong(v int32) []byte {
+	b := make([]byte, 8) // 4 bytes value + 4 bytes pad
+	binary.BigEndian.PutUint32(b, uint32(v))
+	return b
+}
+
+// EncodeShort encodes an int16 value as a big-endian DBR_SHORT payload
+// padded to 8 bytes.
+func EncodeShort(v int16) []byte {
+	b := make([]byte, 8)
+	binary.BigEndian.PutUint16(b, uint16(v))
+	return b
+}
+
+// EncodeString encodes a string as a null-terminated, 8-byte padded
+// DBR_STRING payload (capped at maxStringSize characters).
+func EncodeString(v string) []byte {
+	if len(v) >= maxStringSize {
+		v = v[:maxStringSize-1]
+	}
+	b := make([]byte, maxStringSize)
+	copy(b, v)
+	return PadBytes(b)
+}
+
+// EncodeEventMask builds the 16-byte payload for a CA_PROTO_EVENT_ADD request.
+// mask is typically DBEDefault (DBEValue | DBEAlarm).
+//
+// Wire layout matches caProto.h struct mon_info (all big-endian):
+//
+//	[0:4]   float32  m_lval  (low delta, zero = disabled)
+//	[4:8]   float32  m_hval  (high delta, zero = disabled)
+//	[8:12]  float32  m_toval (period between samples, zero = disabled)
+//	[12:14] uint16   m_mask  (DBE_VALUE | DBE_ALARM | ...)
+//	[14:16] uint16   m_pad   (alignment padding)
+func EncodeEventMask(mask uint16) []byte {
+	b := make([]byte, 16)
+	binary.BigEndian.PutUint16(b[12:], mask) // m_mask is at offset 12 per caProto.h
+	return b
+}

pkg/ca/proto/header.go

@@ -0,0 +1,108 @@
+package proto
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+)
+
+// Header is the decoded form of a 16-byte CA message header.
+// All integer fields are stored in host byte order after decoding.
+type Header struct {
+	Command     uint16
+	PayloadSize uint32 // actual payload size (resolved from extended form if needed)
+	DataType    uint16
+	DataCount   uint32 // actual element count (resolved from extended form if needed)
+	Parameter1  uint32
+	Parameter2  uint32
+}
+
+// HeaderSize is the wire size of a standard CA header.
+const HeaderSize = 16
+
+// Encode writes the header to buf (must be at least HeaderSize bytes).
+// If PayloadSize > 0xFFFE or DataCount > 0xFFFF the extended encoding is used
+// and the caller must prepend an extra 8 bytes; use EncodeExtended instead.
+func (h Header) Encode(buf []byte) {
+	binary.BigEndian.PutUint16(buf[0:], h.Command)
+	binary.BigEndian.PutUint16(buf[2:], uint16(h.PayloadSize))
+	binary.BigEndian.PutUint16(buf[4:], h.DataType)
+	binary.BigEndian.PutUint16(buf[6:], uint16(h.DataCount))
+	binary.BigEndian.PutUint32(buf[8:], h.Parameter1)
+	binary.BigEndian.PutUint32(buf[12:], h.Parameter2)
+}
+
+// Bytes returns the 16-byte wire encoding of h.
+func (h Header) Bytes() []byte {
+	buf := make([]byte, HeaderSize)
+	h.Encode(buf)
+	return buf
+}
+
+// DecodeHeader reads exactly one CA header from r, handling the extended
+// encoding (payload_size == 0xFFFF) transparently.
+// Returns the decoded Header and the number of bytes read (16 or 24).
+func DecodeHeader(r io.Reader) (Header, int, error) {
+	var raw [HeaderSize]byte
+	if _, err := io.ReadFull(r, raw[:]); err != nil {
+		return Header{}, 0, fmt.Errorf("ca: read header: %w", err)
+	}
+
+	h := Header{
+		Command:    binary.BigEndian.Uint16(raw[0:]),
+		DataType:   binary.BigEndian.Uint16(raw[4:]),
+		Parameter1: binary.BigEndian.Uint32(raw[8:]),
+		Parameter2: binary.BigEndian.Uint32(raw[12:]),
+	}
+
+	rawPayload := binary.BigEndian.Uint16(raw[2:])
+	rawCount := binary.BigEndian.Uint16(raw[6:])
+
+	// Extended message: payload_size == 0xFFFF means real sizes are in the
+	// next 8 bytes (parameter1 = real payload size, parameter2 = real count).
+	if rawPayload == 0xFFFF && rawCount == 0x0000 {
+		var ext [8]byte
+		if _, err := io.ReadFull(r, ext[:]); err != nil {
+			return Header{}, 16, fmt.Errorf("ca: read extended header: %w", err)
+		}
+		h.PayloadSize = binary.BigEndian.Uint32(ext[0:])
+		h.DataCount = binary.BigEndian.Uint32(ext[4:])
+		// Note: parameter1/2 in the base header are overwritten by the extended values.
+		// In practice, the original parameter1/2 are still valid — the extended header
+		// only carries sizes, not the original parameters.  We already decoded parameter1/2
+		// above from the base header.
+		return h, 24, nil
+	}
+
+	h.PayloadSize = uint32(rawPayload)
+	h.DataCount = uint32(rawCount)
+	return h, 16, nil
+}
+
+// PadTo8 returns n rounded up to the nearest multiple of 8.
+// CA requires all message payloads to be padded to 8-byte boundaries.
+func PadTo8(n int) int {
+	return (n + 7) &^ 7
+}
+
+// PadBytes appends zero bytes to b until len(b) is a multiple of 8.
+func PadBytes(b []byte) []byte {
+	need := PadTo8(len(b)) - len(b)
+	return append(b, make([]byte, need)...)
+}
+
+// BuildMessage assembles a complete CA message (header + payload).
+// payload may be nil for zero-length messages.
+func BuildMessage(h Header, payload []byte) []byte {
+	h.PayloadSize = uint32(len(payload))
+	msg := make([]byte, HeaderSize+len(payload))
+	h.Encode(msg)
+	copy(msg[HeaderSize:], payload)
+	return msg
+}
+
+// BuildStringPayload encodes a string as a null-terminated, 8-byte padded payload.
+func BuildStringPayload(s string) []byte {
+	b := append([]byte(s), 0) // null terminator
+	return PadBytes(b)
+}

pkg/ca/proto/opcodes.go

@@ -0,0 +1,140 @@
+// Package proto contains the low-level Channel Access wire-protocol constants,
+// header codec, and DBR type decode functions.  All types are big-endian as
+// required by the CA specification.
+package proto
+
+// CA command opcodes (uint16, first field of every message header).
+const (
+	CmdVersion      = 0  // version negotiation (first message on every TCP connection)
+	CmdEventAdd     = 1  // subscribe to value updates / server event push
+	CmdEventCancel  = 2  // cancel subscription
+	CmdWrite        = 4  // put value (fire-and-forget, no server reply)
+	CmdSearch       = 6  // UDP: locate IOC hosting a PV
+	CmdNotFound     = 14 // UDP: IOC does not host the requested PV
+	CmdReadNotify   = 15 // get value with callback (async GET)
+	CmdCreateChan   = 18 // create channel on TCP circuit
+	CmdWriteNotify  = 19 // put value with acknowledgement
+	CmdClientName   = 20 // announce client username (sent after VERSION)
+	CmdHostName     = 21 // announce client hostname  (sent after VERSION)
+	CmdAccessRights = 22 // server → client: access rights bitmask
+	CmdEcho         = 23 // heartbeat ping/pong
+	CmdCreateFail   = 26 // server → client: CREATE_CHAN failed
+	CmdServerDisc   = 27 // server → client: server shutting down
+)
+
+// CA minor protocol revision announced during the VERSION handshake.
+// Corresponds to CA 4.13, supported by EPICS 3.14+ and all EPICS 7 servers.
+const MinorVersion = 13
+
+// Default CA port (TCP and UDP).
+const DefaultPort = 5064
+
+// Search reply data_type values.
+const (
+	SearchReply    = 10 // DO_REPLY: ask server to respond
+	SearchNoReply  = 5  // DONT_REPLY: suppress response (used by repeater)
+)
+
+// AccessRights bitmask values (parameter2 of CmdAccessRights message).
+const (
+	AccessRead  = 1 << 0
+	AccessWrite = 1 << 1
+)
+
+// DBE event mask bits used in the 16-byte EVENT_ADD payload.
+const (
+	DBEValue   = 0x01 // trigger on any value change
+	DBEAlarm   = 0x04 // trigger on alarm state change
+	DBEDefault = DBEValue | DBEAlarm
+)
+
+// ---- DBF field types (native IOC field type, reported in CREATE_CHAN reply) ----
+
+const (
+	DBFString = 0
+	DBFShort  = 1 // also DBFInt
+	DBFFloat  = 2
+	DBFEnum   = 3
+	DBFChar   = 4
+	DBFLong   = 5
+	DBFDouble = 6
+)
+
+// ---- DBR request types ----
+
+// Plain value types (used in WRITE).
+const (
+	DBRString = 0
+	DBRShort  = 1
+	DBRFloat  = 2
+	DBREnum   = 3
+	DBRChar   = 4
+	DBRLong   = 5
+	DBRDouble = 6
+)
+
+// DBR_TIME_* types (value + timestamp + alarm status; used in EVENT_ADD).
+// Numbers from db_access.h: STRING=14, SHORT/INT=15, FLOAT=16, ENUM=17, CHAR=18, LONG=19, DOUBLE=20.
+const (
+	DBRTimeString = 14
+	DBRTimeShort  = 15
+	DBRTimeFloat  = 16
+	DBRTimeEnum   = 17
+	DBRTimeChar   = 18
+	DBRTimeLong   = 19
+	DBRTimeDouble = 20
+)
+
+// DBR_CTRL_* types (full control info: units, limits, enum strings; used in READ_NOTIFY).
+const (
+	DBRCtrlString = 28
+	DBRCtrlShort  = 29
+	DBRCtrlFloat  = 30
+	DBRCtrlEnum   = 31
+	DBRCtrlChar   = 32
+	DBRCtrlLong   = 33
+	DBRCtrlDouble = 34
+)
+
+// NativeTimeType maps a DBF field type to the corresponding DBR_TIME_* type
+// that should be used for monitor subscriptions.
+func NativeTimeType(dbfType int, elementCount int) uint16 {
+	switch dbfType {
+	case DBFDouble:
+		return DBRTimeDouble
+	case DBFFloat:
+		return DBRTimeFloat
+	case DBFLong:
+		return DBRTimeLong
+	case DBFShort:
+		return DBRTimeShort
+	case DBFEnum:
+		return DBRTimeEnum
+	case DBFString:
+		return DBRTimeString
+	case DBFChar:
+		if elementCount > 1 {
+			return DBRTimeString // waveform of chars treated as string
+		}
+		return DBRTimeChar
+	default:
+		return DBRTimeDouble
+	}
+}
+
+// NativeCtrlType maps a DBF field type to the corresponding DBR_CTRL_* type
+// that should be used for metadata gets.
+func NativeCtrlType(dbfType int) uint16 {
+	switch dbfType {
+	case DBFDouble, DBFFloat:
+		return DBRCtrlDouble
+	case DBFLong, DBFShort, DBFChar:
+		return DBRCtrlLong
+	case DBFEnum:
+		return DBRCtrlEnum
+	case DBFString:
+		return DBRCtrlString
+	default:
+		return DBRCtrlDouble
+	}
+}

pkg/ca/proto/proto_test.go

@@ -0,0 +1,387 @@
+package proto_test
+
+import (
+	"bytes"
+	"encoding/binary"
+	"math"
+	"testing"
+	"time"
+
+	"github.com/uopi/goca/proto"
+)
+
+// -------------------------------------------------------------------------- //
+// Header round-trip                                                            //
+// -------------------------------------------------------------------------- //
+
+func TestHeaderRoundTrip(t *testing.T) {
+	h := proto.Header{
+		Command:     proto.CmdCreateChan,
+		PayloadSize: 16,
+		DataType:    proto.DBFDouble,
+		DataCount:   1,
+		Parameter1:  0xDEAD,
+		Parameter2:  0xBEEF,
+	}
+	wire := h.Bytes()
+	if len(wire) != proto.HeaderSize {
+		t.Fatalf("Bytes() len = %d, want %d", len(wire), proto.HeaderSize)
+	}
+
+	got, n, err := proto.DecodeHeader(bytes.NewReader(wire))
+	if err != nil {
+		t.Fatalf("DecodeHeader: %v", err)
+	}
+	if n != proto.HeaderSize {
+		t.Errorf("bytes consumed = %d, want %d", n, proto.HeaderSize)
+	}
+	if got.Command != h.Command {
+		t.Errorf("Command = %d, want %d", got.Command, h.Command)
+	}
+	if got.DataType != h.DataType {
+		t.Errorf("DataType = %d, want %d", got.DataType, h.DataType)
+	}
+	if got.Parameter1 != h.Parameter1 {
+		t.Errorf("Parameter1 = %d, want %d", got.Parameter1, h.Parameter1)
+	}
+	if got.Parameter2 != h.Parameter2 {
+		t.Errorf("Parameter2 = %d, want %d", got.Parameter2, h.Parameter2)
+	}
+}
+
+func TestBuildMessage(t *testing.T) {
+	payload := []byte("hello\x00\x00\x00") // 8 bytes (padded)
+	h := proto.Header{Command: proto.CmdHostName, PayloadSize: 0}
+	msg := proto.BuildMessage(h, payload)
+	if len(msg) != proto.HeaderSize+len(payload) {
+		t.Fatalf("len(msg) = %d, want %d", len(msg), proto.HeaderSize+len(payload))
+	}
+	// PayloadSize in wire should equal len(payload).
+	wireSize := binary.BigEndian.Uint16(msg[2:4])
+	if int(wireSize) != len(payload) {
+		t.Errorf("wire PayloadSize = %d, want %d", wireSize, len(payload))
+	}
+}
+
+func TestPadTo8(t *testing.T) {
+	cases := [][2]int{{0, 0}, {1, 8}, {7, 8}, {8, 8}, {9, 16}, {16, 16}, {17, 24}}
+	for _, c := range cases {
+		if got := proto.PadTo8(c[0]); got != c[1] {
+			t.Errorf("PadTo8(%d) = %d, want %d", c[0], got, c[1])
+		}
+	}
+}
+
+func TestBuildStringPayload(t *testing.T) {
+	p := proto.BuildStringPayload("TEST")
+	if len(p)%8 != 0 {
+		t.Errorf("payload length %d not padded to 8", len(p))
+	}
+	if p[4] != 0 {
+		t.Errorf("expected null terminator at index 4, got %d", p[4])
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// DBR_TIME_* decoding                                                          //
+// -------------------------------------------------------------------------- //
+
+// buildTimeHeader builds the 12-byte common DBR_TIME header.
+func buildTimeHeader(status, severity int16, sec, nsec uint32) []byte {
+	b := make([]byte, 12)
+	binary.BigEndian.PutUint16(b[0:], uint16(status))
+	binary.BigEndian.PutUint16(b[2:], uint16(severity))
+	binary.BigEndian.PutUint32(b[4:], sec)
+	binary.BigEndian.PutUint32(b[8:], nsec)
+	return b
+}
+
+func TestDecodeTimeDouble(t *testing.T) {
+	const sec = 1_000_000
+	const nsec = 500_000_000
+	const want = 3.14159
+
+	hdr := buildTimeHeader(0, 0, sec, nsec)
+	pad := make([]byte, 4)                                        // RISC pad
+	val := make([]byte, 8)
+	binary.BigEndian.PutUint64(val, math.Float64bits(want))
+	payload := append(append(hdr, pad...), val...)
+
+	tv, ok := proto.DecodeTimeValue(proto.DBRTimeDouble, 1, payload)
+	if !ok {
+		t.Fatal("DecodeTimeValue returned false")
+	}
+	if math.Abs(tv.Double-want) > 1e-10 {
+		t.Errorf("Double = %g, want %g", tv.Double, want)
+	}
+	// EPICS epoch offset: 1990-01-01 00:00:00 UTC = Unix 631152000.
+	wantUnix := int64(sec) + 631152000
+	if tv.Timestamp.Unix() != wantUnix {
+		t.Errorf("Timestamp.Unix() = %d, want %d", tv.Timestamp.Unix(), wantUnix)
+	}
+	if tv.Timestamp.Nanosecond() != int(nsec) {
+		t.Errorf("Timestamp.Nanosecond() = %d, want %d", tv.Timestamp.Nanosecond(), nsec)
+	}
+}
+
+func TestDecodeTimeLong(t *testing.T) {
+	hdr := buildTimeHeader(0, 1, 0, 0)
+	val := make([]byte, 4)
+	binary.BigEndian.PutUint32(val, 0xFFFFFFD6) // -42 as two's complement
+	payload := append(hdr, val...)
+
+	tv, ok := proto.DecodeTimeValue(proto.DBRTimeLong, 1, payload)
+	if !ok {
+		t.Fatal("DecodeTimeValue returned false")
+	}
+	if tv.Long != -42 {
+		t.Errorf("Long = %d, want -42", tv.Long)
+	}
+	if tv.Severity != proto.SeverityMinor {
+		t.Errorf("Severity = %v, want Minor", tv.Severity)
+	}
+}
+
+func TestDecodeTimeString(t *testing.T) {
+	hdr := buildTimeHeader(0, 0, 0, 0)
+	str := make([]byte, 40)
+	copy(str, "hello")
+	payload := append(hdr, str...)
+
+	tv, ok := proto.DecodeTimeValue(proto.DBRTimeString, 1, payload)
+	if !ok {
+		t.Fatal("DecodeTimeValue returned false")
+	}
+	if tv.Str != "hello" {
+		t.Errorf("Str = %q, want %q", tv.Str, "hello")
+	}
+}
+
+func TestDecodeTimeEnum(t *testing.T) {
+	hdr := buildTimeHeader(0, 0, 0, 0)
+	val := []byte{0x00, 0x00, 0x00, 0x03} // 2-byte RISC pad + enum=3
+	payload := append(hdr, val...)
+
+	tv, ok := proto.DecodeTimeValue(proto.DBRTimeEnum, 1, payload)
+	if !ok {
+		t.Fatal("DecodeTimeValue returned false")
+	}
+	if tv.Enum != 3 {
+		t.Errorf("Enum = %d, want 3", tv.Enum)
+	}
+}
+
+func TestDecodeTimeWaveform(t *testing.T) {
+	hdr := buildTimeHeader(0, 0, 0, 0)
+	pad := make([]byte, 4)
+	vals := []float64{1.0, 2.5, -0.5}
+	valbytes := make([]byte, len(vals)*8)
+	for i, v := range vals {
+		binary.BigEndian.PutUint64(valbytes[i*8:], math.Float64bits(v))
+	}
+	payload := append(append(hdr, pad...), valbytes...)
+
+	tv, ok := proto.DecodeTimeValue(proto.DBRTimeDouble, 3, payload)
+	if !ok {
+		t.Fatal("DecodeTimeValue returned false")
+	}
+	if len(tv.Doubles) != 3 {
+		t.Fatalf("len(Doubles) = %d, want 3", len(tv.Doubles))
+	}
+	for i, want := range vals {
+		if math.Abs(tv.Doubles[i]-want) > 1e-12 {
+			t.Errorf("Doubles[%d] = %g, want %g", i, tv.Doubles[i], want)
+		}
+	}
+}
+
+func TestDecodeTimeTooShort(t *testing.T) {
+	_, ok := proto.DecodeTimeValue(proto.DBRTimeDouble, 1, []byte{0, 1, 2})
+	if ok {
+		t.Error("expected false for short payload")
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// DBR_CTRL_DOUBLE decoding                                                    //
+// -------------------------------------------------------------------------- //
+
+func TestDecodeCtrlDouble(t *testing.T) {
+	p := make([]byte, 88)
+	// status=0, severity=0, precision=3, pad=0, units="mA\0..."
+	binary.BigEndian.PutUint16(p[4:], 3) // precision
+	copy(p[8:], "mA")
+	f64 := func(off int, v float64) {
+		binary.BigEndian.PutUint64(p[off:], math.Float64bits(v))
+	}
+	f64(16, 100.0) // upper_disp
+	f64(24, 0.0)   // lower_disp
+	f64(32, 90.0)  // upper_alarm
+	f64(40, 80.0)  // upper_warn
+	f64(48, 20.0)  // lower_warn
+	f64(56, 10.0)  // lower_alarm
+	f64(64, 95.0)  // upper_ctrl
+	f64(72, 5.0)   // lower_ctrl
+	f64(80, 55.5)  // value
+
+	cd, ok := proto.DecodeCtrlDouble(p)
+	if !ok {
+		t.Fatal("DecodeCtrlDouble returned false")
+	}
+	if cd.Units != "mA" {
+		t.Errorf("Units = %q, want %q", cd.Units, "mA")
+	}
+	if cd.Precision != 3 {
+		t.Errorf("Precision = %d, want 3", cd.Precision)
+	}
+	if math.Abs(cd.Value-55.5) > 1e-10 {
+		t.Errorf("Value = %g, want 55.5", cd.Value)
+	}
+	if math.Abs(cd.UpperDispLimit-100.0) > 1e-10 {
+		t.Errorf("UpperDispLimit = %g, want 100.0", cd.UpperDispLimit)
+	}
+}
+
+func TestDecodeCtrlEnum(t *testing.T) {
+	p := make([]byte, 424)
+	binary.BigEndian.PutUint16(p[4:], 3) // no_str = 3
+	strs := []string{"OFF", "ON", "FAULT"}
+	for i, s := range strs {
+		copy(p[6+i*26:], s)
+	}
+	binary.BigEndian.PutUint16(p[422:], 1) // value = 1
+
+	ce, ok := proto.DecodeCtrlEnum(p)
+	if !ok {
+		t.Fatal("DecodeCtrlEnum returned false")
+	}
+	if len(ce.Strings) != 3 {
+		t.Fatalf("len(Strings) = %d, want 3", len(ce.Strings))
+	}
+	if ce.Strings[2] != "FAULT" {
+		t.Errorf("Strings[2] = %q, want FAULT", ce.Strings[2])
+	}
+	if ce.Value != 1 {
+		t.Errorf("Value = %d, want 1", ce.Value)
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Put payload encoders                                                         //
+// -------------------------------------------------------------------------- //
+
+func TestEncodeDouble(t *testing.T) {
+	b := proto.EncodeDouble(3.14)
+	if len(b) != 8 {
+		t.Fatalf("len = %d, want 8", len(b))
+	}
+	got := math.Float64frombits(binary.BigEndian.Uint64(b))
+	if math.Abs(got-3.14) > 1e-15 {
+		t.Errorf("decoded = %g, want 3.14", got)
+	}
+}
+
+func TestEncodeLong(t *testing.T) {
+	b := proto.EncodeLong(-1)
+	if len(b) != 8 {
+		t.Fatalf("len = %d, want 8", len(b))
+	}
+	got := int32(binary.BigEndian.Uint32(b))
+	if got != -1 {
+		t.Errorf("decoded = %d, want -1", got)
+	}
+}
+
+func TestEncodeString(t *testing.T) {
+	b := proto.EncodeString("hello")
+	if len(b)%8 != 0 {
+		t.Errorf("len %d not padded to 8", len(b))
+	}
+	if string(b[:5]) != "hello" {
+		t.Errorf("content = %q, want %q", b[:5], "hello")
+	}
+}
+
+func TestEncodeEventMask(t *testing.T) {
+	b := proto.EncodeEventMask(proto.DBEDefault)
+	if len(b) != 16 {
+		t.Fatalf("len = %d, want 16", len(b))
+	}
+	// m_mask is at offset 12 per caProto.h struct mon_info.
+	mask := binary.BigEndian.Uint16(b[12:])
+	if mask != proto.DBEDefault {
+		t.Errorf("mask = 0x%02X, want 0x%02X", mask, proto.DBEDefault)
+	}
+	// m_pad at [14:16] must be zero.
+	if pad := binary.BigEndian.Uint16(b[14:]); pad != 0 {
+		t.Errorf("m_pad = %d, want 0", pad)
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// NativeTimeType / NativeCtrlType                                             //
+// -------------------------------------------------------------------------- //
+
+func TestNativeTimeType(t *testing.T) {
+	cases := []struct {
+		dbf  int
+		n    int
+		want uint16
+	}{
+		{proto.DBFDouble, 1, proto.DBRTimeDouble},
+		{proto.DBFFloat, 1, proto.DBRTimeFloat},
+		{proto.DBFLong, 1, proto.DBRTimeLong},
+		{proto.DBFShort, 1, proto.DBRTimeShort},
+		{proto.DBFEnum, 1, proto.DBRTimeEnum},
+		{proto.DBFString, 1, proto.DBRTimeString},
+		{proto.DBFChar, 1, proto.DBRTimeChar},
+		{proto.DBFChar, 10, proto.DBRTimeString}, // char waveform → string
+	}
+	for _, c := range cases {
+		got := proto.NativeTimeType(c.dbf, c.n)
+		if got != c.want {
+			t.Errorf("NativeTimeType(%d,%d) = %d, want %d", c.dbf, c.n, got, c.want)
+		}
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// AlarmSeverity.String                                                         //
+// -------------------------------------------------------------------------- //
+
+func TestAlarmSeverityString(t *testing.T) {
+	cases := []struct {
+		s    proto.AlarmSeverity
+		want string
+	}{
+		{proto.SeverityNone, "NO_ALARM"},
+		{proto.SeverityMinor, "MINOR"},
+		{proto.SeverityMajor, "MAJOR"},
+		{proto.SeverityInvalid, "INVALID"},
+	}
+	for _, c := range cases {
+		if got := c.s.String(); got != c.want {
+			t.Errorf("Severity(%d).String() = %q, want %q", c.s, got, c.want)
+		}
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Timestamp sanity check                                                       //
+// -------------------------------------------------------------------------- //
+
+func TestEPICSEpoch(t *testing.T) {
+	// secPastEpoch=0 should decode to 1990-01-01 00:00:00 UTC.
+	hdr := buildTimeHeader(0, 0, 0, 0)
+	val := make([]byte, 8)
+	payload := append(append(hdr, make([]byte, 4)...), val...)
+
+	tv, ok := proto.DecodeTimeValue(proto.DBRTimeDouble, 1, payload)
+	if !ok {
+		t.Fatal("DecodeTimeValue returned false")
+	}
+	want := time.Date(1990, 1, 1, 0, 0, 0, 0, time.UTC)
+	if !tv.Timestamp.Equal(want) {
+		t.Errorf("timestamp = %v, want %v", tv.Timestamp, want)
+	}
+}

pkg/ca/search.go

@@ -0,0 +1,395 @@
+package ca
+
+import (
+	"context"
+	"encoding/binary"
+	"fmt"
+	"net"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"github.com/uopi/goca/proto"
+)
+
+// searchResult is returned to a waiter when a search succeeds.
+type searchResult struct {
+	addr string // "ip:port" TCP address of the IOC
+}
+
+// searchWaiter tracks an outstanding PV search request.
+type searchWaiter struct {
+	pvName string
+	ch     chan searchResult // closed or sent to exactly once
+}
+
+// searchEngine performs UDP-based PV name → IOC address resolution.
+// It sends CA_PROTO_SEARCH datagrams to all configured server addresses and
+// listens for replies, matching them to pending waiters via a searchID.
+type searchEngine struct {
+	addrs []string // "ip:port" strings to search
+
+	mu      sync.Mutex
+	waiters map[uint32]*searchWaiter // searchID → waiter
+
+	idSeq atomic.Uint32
+	conn  *net.UDPConn // shared UDP socket (nil if not started)
+}
+
+func newSearchEngine(addrs []string) *searchEngine {
+	return &searchEngine{
+		addrs:   addrs,
+		waiters: make(map[uint32]*searchWaiter),
+	}
+}
+
+// start opens the shared UDP socket and launches the receive goroutine.
+// ctx cancellation stops the receive loop and closes the socket.
+func (se *searchEngine) start(ctx context.Context) error {
+	conn, err := net.ListenUDP("udp4", &net.UDPAddr{})
+	if err != nil {
+		return fmt.Errorf("ca search: open UDP socket: %w", err)
+	}
+	se.conn = conn
+
+	go se.recvLoop(ctx)
+	return nil
+}
+
+// lookup resolves pvName to an IOC TCP address.
+// It retries with exponential back-off until ctx is cancelled or done.
+func (se *searchEngine) lookup(ctx context.Context, pvName string) (string, error) {
+	id := se.idSeq.Add(1)
+	w := &searchWaiter{
+		pvName: pvName,
+		ch:     make(chan searchResult, 1),
+	}
+
+	se.mu.Lock()
+	se.waiters[id] = w
+	se.mu.Unlock()
+
+	defer func() {
+		se.mu.Lock()
+		delete(se.waiters, id)
+		se.mu.Unlock()
+	}()
+
+	delay := 100 * time.Millisecond
+	const maxDelay = 30 * time.Second
+
+	for {
+		if err := se.sendSearch(id, pvName); err != nil {
+			return "", err
+		}
+
+		select {
+		case res := <-w.ch:
+			return res.addr, nil
+		case <-time.After(delay):
+			delay *= 2
+			if delay > maxDelay {
+				delay = maxDelay
+			}
+		case <-ctx.Done():
+			return "", fmt.Errorf("ca search %q: %w", pvName, ctx.Err())
+		}
+	}
+}
+
+// sendSearch sends a CA_PROTO_VERSION + CA_PROTO_SEARCH burst to all addresses.
+// The CA protocol requires both messages to be in the same UDP datagram.
+func (se *searchEngine) sendSearch(id uint32, pvName string) error {
+	payload := proto.BuildStringPayload(pvName)
+
+	// VERSION message (no payload).
+	verHdr := proto.Header{
+		Command:   proto.CmdVersion,
+		DataCount: proto.MinorVersion,
+	}
+
+	// SEARCH message.
+	searchHdr := proto.Header{
+		Command:    proto.CmdSearch,
+		DataType:   proto.SearchReply,
+		DataCount:  proto.MinorVersion,
+		Parameter1: id,
+		Parameter2: id,
+	}
+	searchMsg := proto.BuildMessage(searchHdr, payload)
+
+	// Bundle into a single datagram — softIoc (and the CA Repeater) require
+	// VERSION and SEARCH to arrive together in one UDP packet.
+	pkt := make([]byte, 0, proto.HeaderSize+len(searchMsg))
+	pkt = append(pkt, verHdr.Bytes()...)
+	pkt = append(pkt, searchMsg...)
+
+	for _, addr := range se.addrs {
+		udpAddr, err := net.ResolveUDPAddr("udp4", addr)
+		if err != nil {
+			continue
+		}
+		_, _ = se.conn.WriteTo(pkt, udpAddr)
+		dbg("CA search sent", "pv", pvName, "id", id, "dst", udpAddr)
+	}
+	return nil
+}
+
+// recvLoop reads UDP datagrams and dispatches search replies.
+func (se *searchEngine) recvLoop(ctx context.Context) {
+	buf := make([]byte, 65536)
+	for {
+		// Use a short read deadline so we can check ctx.Done() periodically.
+		_ = se.conn.SetReadDeadline(time.Now().Add(500 * time.Millisecond))
+		n, srcAddr, err := se.conn.ReadFromUDP(buf)
+		if err != nil {
+			if ctx.Err() != nil {
+				return
+			}
+			if netErr, ok := err.(net.Error); ok && netErr.Timeout() {
+				continue
+			}
+			continue
+		}
+
+		dbg("CA UDP datagram received", "src", srcAddr, "bytes", n)
+		se.parseReply(buf[:n], srcAddr)
+	}
+}
+
+// parseReply scans a UDP datagram for CA_PROTO_SEARCH response messages and
+// wakes the matching waiter.
+func (se *searchEngine) parseReply(data []byte, src *net.UDPAddr) {
+	for len(data) >= proto.HeaderSize {
+		h, n, err := proto.DecodeHeader(newBytesReader(data))
+		if err != nil {
+			return
+		}
+		payloadEnd := n + int(h.PayloadSize)
+		if payloadEnd > len(data) {
+			return
+		}
+		payload := data[n:payloadEnd]
+		data = data[payloadEnd:]
+
+		if h.Command != proto.CmdSearch {
+			continue
+		}
+
+		// data_type field = TCP port of the CA server.
+		tcpPort := int(h.DataType)
+		searchID := h.Parameter2
+
+		// Resolve server IP.
+		//
+		// Two payload formats exist in the wild:
+		//
+		//   New (≥ CA 4.9):  [IP(4)][pad(4)]
+		//   Old (< CA 4.9):  [minor_version(2)][pad(2)][IP(4)]
+		//
+		// The old format is identified by payload[0] == 0x00 (the high byte of a
+		// uint16 minor version is always zero for CA versions ≤ 255, whereas a
+		// real IPv4 address never starts with 0x00).
+		//
+		// In both formats, 0xFFFFFFFF in the IP position means "use sender IP".
+		var serverIP net.IP
+		if len(payload) >= 8 && payload[0] == 0x00 {
+			// Old format: minor version at [0:2], IP at [4:8].
+			ipSlice := payload[4:8]
+			if useSenderIP(ipSlice) {
+				serverIP = src.IP
+			} else {
+				ip := make(net.IP, 4)
+				copy(ip, ipSlice)
+				serverIP = ip
+			}
+		} else if len(payload) >= 4 {
+			// New format: IP at [0:4].
+			if useSenderIP(payload[:4]) {
+				serverIP = src.IP
+			} else {
+				ip := make(net.IP, 4)
+				copy(ip, payload[:4])
+				serverIP = ip
+			}
+		} else {
+			serverIP = src.IP
+		}
+
+		tcpAddr := fmt.Sprintf("%s:%d", serverIP.String(), tcpPort)
+		dbg("CA search reply", "searchID", searchID, "tcpAddr", tcpAddr, "src", src)
+
+		se.mu.Lock()
+		w, ok := se.waiters[searchID]
+		if ok {
+			delete(se.waiters, searchID)
+		}
+		se.mu.Unlock()
+
+		if ok {
+			dbg("CA search matched", "pv", w.pvName, "ioc", tcpAddr)
+			select {
+			case w.ch <- searchResult{addr: tcpAddr}:
+			default:
+			}
+		} else {
+			dbg("CA search reply unmatched", "searchID", searchID, "known_waiters", func() int {
+				se.mu.Lock()
+				defer se.mu.Unlock()
+				return len(se.waiters)
+			}())
+		}
+	}
+}
+
+// useSenderIP reports whether an IP field in a CA search reply means
+// "use the sender's address".  Both 0.0.0.0 (all-zeros) and 255.255.255.255
+// (all-ones / 0xFFFFFFFF) are used by different EPICS Base versions.
+func useSenderIP(b []byte) bool {
+	allFF := true
+	allZero := true
+	for _, v := range b {
+		if v != 0xFF {
+			allFF = false
+		}
+		if v != 0x00 {
+			allZero = false
+		}
+	}
+	return allFF || allZero
+}
+
+func isAllFF(b []byte) bool {
+	for _, v := range b {
+		if v != 0xFF {
+			return false
+		}
+	}
+	return true
+}
+
+// -------------------------------------------------------------------------- //
+// Address list helpers                                                        //
+// -------------------------------------------------------------------------- //
+
+// resolveAddrs converts a list of "host" or "host:port" strings to
+// "host:port" strings using defaultPort when no port is specified.
+func resolveAddrs(addrs []string, defaultPort int) []string {
+	out := make([]string, 0, len(addrs))
+	for _, a := range addrs {
+		if a == "" {
+			continue
+		}
+		_, _, err := net.SplitHostPort(a)
+		if err != nil {
+			// No port specified; append default.
+			a = fmt.Sprintf("%s:%d", a, defaultPort)
+		}
+		out = append(out, a)
+	}
+	return out
+}
+
+// localBroadcastAddrs returns broadcast addresses for all local network
+// interfaces, used when AutoAddrList is true.
+func localBroadcastAddrs(port int) []string {
+	ifaces, err := net.Interfaces()
+	if err != nil {
+		return nil
+	}
+	var out []string
+	for _, iface := range ifaces {
+		if iface.Flags&net.FlagBroadcast == 0 {
+			continue
+		}
+		addrs, err := iface.Addrs()
+		if err != nil {
+			continue
+		}
+		for _, a := range addrs {
+			ipNet, ok := a.(*net.IPNet)
+			if !ok {
+				continue
+			}
+			ip4 := ipNet.IP.To4()
+			if ip4 == nil {
+				continue
+			}
+			// Compute broadcast: IP | ^mask
+			mask := ipNet.Mask
+			bcast := make(net.IP, 4)
+			for i := range bcast {
+				bcast[i] = ip4[i] | ^mask[i]
+			}
+			out = append(out, fmt.Sprintf("%s:%d", bcast.String(), port))
+		}
+	}
+	return out
+}
+
+// -------------------------------------------------------------------------- //
+// Minimal bytes.Reader-like for proto.DecodeHeader                           //
+// -------------------------------------------------------------------------- //
+
+type bytesReader struct {
+	b []byte
+	i int
+}
+
+func newBytesReader(b []byte) *bytesReader { return &bytesReader{b: b} }
+
+func (r *bytesReader) Read(p []byte) (int, error) {
+	if r.i >= len(r.b) {
+		return 0, fmt.Errorf("EOF")
+	}
+	n := copy(p, r.b[r.i:])
+	r.i += n
+	return n, nil
+}
+
+// -------------------------------------------------------------------------- //
+// searchAddrFromReply extracts server IP from a CA_PROTO_SEARCH response.   //
+// Exported for testability.                                                   //
+// -------------------------------------------------------------------------- //
+
+// EncodedSearchPair builds the VERSION + SEARCH UDP datagram pair for pvName
+// with the given searchID. Exported for use in tests.
+func EncodedSearchPair(pvName string, searchID uint32) []byte {
+	verHdr := proto.Header{
+		Command:   proto.CmdVersion,
+		DataCount: proto.MinorVersion,
+	}
+	payload := proto.BuildStringPayload(pvName)
+	searchHdr := proto.Header{
+		Command:    proto.CmdSearch,
+		DataType:   proto.SearchReply,
+		DataCount:  proto.MinorVersion,
+		Parameter1: searchID,
+		Parameter2: searchID,
+	}
+	searchMsg := proto.BuildMessage(searchHdr, payload)
+	out := make([]byte, 0, proto.HeaderSize+len(searchMsg))
+	out = append(out, verHdr.Bytes()...)
+	out = append(out, searchMsg...)
+	return out
+}
+
+// EncodeSearchReply builds a CA_PROTO_SEARCH UDP reply.
+// serverIP may be nil to use 0xFFFFFFFF (sender IP).  port is the CA TCP port.
+func EncodeSearchReply(searchID uint32, serverIP net.IP, port int) []byte {
+	var payload []byte
+	if serverIP != nil {
+		payload = make([]byte, 8)
+		copy(payload[:4], serverIP.To4())
+		binary.BigEndian.PutUint32(payload[4:], 0)
+	} else {
+		payload = []byte{0xFF, 0xFF, 0xFF, 0xFF, 0x00, 0x00, 0x00, 0x00}
+	}
+	h := proto.Header{
+		Command:    proto.CmdSearch,
+		DataType:   uint16(port),
+		Parameter1: proto.MinorVersion,
+		Parameter2: searchID,
+	}
+	return proto.BuildMessage(h, payload)
+}

pkg/ca/search_test.go

@@ -0,0 +1,137 @@
+package ca
+
+import (
+	"net"
+	"testing"
+
+	"github.com/uopi/goca/proto"
+)
+
+// buildOldSearchReply builds a CA_PROTO_SEARCH UDP reply using the old
+// payload format (CA < 4.9): [minor_version(2)][pad(2)][IP(4)].
+// When ip is nil, the IP field is 0xFFFFFFFF (use-sender convention).
+func buildOldSearchReply(searchID uint32, serverIP net.IP, port int) []byte {
+	payload := make([]byte, 8)
+	payload[0] = 0x00
+	payload[1] = proto.MinorVersion // e.g. 0x0D = 13
+	// payload[2:4] = 0x00 0x00 (pad)
+	if serverIP == nil {
+		payload[4] = 0xFF
+		payload[5] = 0xFF
+		payload[6] = 0xFF
+		payload[7] = 0xFF
+	} else {
+		copy(payload[4:], serverIP.To4())
+	}
+	h := proto.Header{
+		Command:    proto.CmdSearch,
+		DataType:   uint16(port),
+		Parameter1: proto.MinorVersion,
+		Parameter2: searchID,
+	}
+	return proto.BuildMessage(h, payload)
+}
+
+// TestParseReplyOldFormatAllFF verifies the old format with 0xFFFFFFFF IP.
+func TestParseReplyOldFormatAllFF(t *testing.T) {
+	se := &searchEngine{waiters: make(map[uint32]*searchWaiter)}
+	const searchID = uint32(42)
+	result := make(chan searchResult, 1)
+	se.mu.Lock()
+	se.waiters[searchID] = &searchWaiter{pvName: "TEST:PV", ch: result}
+	se.mu.Unlock()
+
+	pkt := buildOldSearchReply(searchID, nil, 5064) // nil → 0xFFFFFFFF
+	src := &net.UDPAddr{IP: net.ParseIP("127.0.0.1"), Port: 5064}
+	se.parseReply(pkt, src)
+
+	select {
+	case r := <-result:
+		if r.addr != "127.0.0.1:5064" {
+			t.Errorf("addr = %q, want 127.0.0.1:5064", r.addr)
+		}
+	default:
+		t.Fatal("parseReply did not deliver a result")
+	}
+}
+
+// TestParseReplyOldFormatAllZero verifies the old format with 0.0.0.0 IP
+// (used by some EPICS Base versions to also mean "use sender IP").
+func TestParseReplyOldFormatAllZero(t *testing.T) {
+	se := &searchEngine{waiters: make(map[uint32]*searchWaiter)}
+	const searchID = uint32(43)
+	result := make(chan searchResult, 1)
+	se.mu.Lock()
+	se.waiters[searchID] = &searchWaiter{pvName: "TEST:PV", ch: result}
+	se.mu.Unlock()
+
+	pkt := buildOldSearchReply(searchID, net.IPv4(0, 0, 0, 0).To4(), 5064)
+	src := &net.UDPAddr{IP: net.ParseIP("127.0.0.1"), Port: 5064}
+	se.parseReply(pkt, src)
+
+	select {
+	case r := <-result:
+		if r.addr != "127.0.0.1:5064" {
+			t.Errorf("addr = %q, want 127.0.0.1:5064", r.addr)
+		}
+	default:
+		t.Fatal("parseReply did not deliver a result")
+	}
+}
+
+// TestParseReplyOldFormatExplicitIP verifies that an explicit server IP in
+// the old payload format is read from the correct offset (bytes [4:8]).
+func TestParseReplyOldFormatExplicitIP(t *testing.T) {
+	se := &searchEngine{
+		waiters: make(map[uint32]*searchWaiter),
+	}
+
+	const searchID = uint32(7)
+	result := make(chan searchResult, 1)
+	se.mu.Lock()
+	se.waiters[searchID] = &searchWaiter{pvName: "TEST:PV2", ch: result}
+	se.mu.Unlock()
+
+	explicitIP := net.ParseIP("10.0.0.5").To4()
+	pkt := buildOldSearchReply(searchID, explicitIP, 5064)
+	src := &net.UDPAddr{IP: net.ParseIP("10.0.0.5"), Port: 5064}
+
+	se.parseReply(pkt, src)
+
+	select {
+	case r := <-result:
+		if r.addr != "10.0.0.5:5064" {
+			t.Errorf("addr = %q, want 10.0.0.5:5064", r.addr)
+		}
+	default:
+		t.Fatal("parseReply did not deliver a result")
+	}
+}
+
+// TestParseReplyNewFormat verifies the new payload format (IP at [0:4]).
+func TestParseReplyNewFormat(t *testing.T) {
+	se := &searchEngine{
+		waiters: make(map[uint32]*searchWaiter),
+	}
+
+	const searchID = uint32(99)
+	result := make(chan searchResult, 1)
+	se.mu.Lock()
+	se.waiters[searchID] = &searchWaiter{pvName: "TEST:PV3", ch: result}
+	se.mu.Unlock()
+
+	// New format: IP = 0xFFFFFFFF at [0:4].
+	pkt := EncodeSearchReply(searchID, nil, 5064)
+	src := &net.UDPAddr{IP: net.ParseIP("192.168.1.10"), Port: 5064}
+
+	se.parseReply(pkt, src)
+
+	select {
+	case r := <-result:
+		if r.addr != "192.168.1.10:5064" {
+			t.Errorf("addr = %q, want 192.168.1.10:5064", r.addr)
+		}
+	default:
+		t.Fatal("parseReply did not deliver a result")
+	}
+}

pkg/ca/testca/server.go

@@ -0,0 +1,605 @@
+// Package testca provides an in-process fake CA server for use in tests.
+// It supports a small, fixed set of PVs and is compatible with the goca Client.
+package testca
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+	"math"
+	"net"
+	"sync"
+	"time"
+
+	"github.com/uopi/goca/proto"
+)
+
+// PVSpec describes one test PV hosted by the fake server.
+type PVSpec struct {
+	Name    string
+	DBFType int    // proto.DBF* constant
+	Count   uint32 // element count (1 for scalars)
+	Value   any    // initial value
+	Access  uint32 // proto.AccessRead | proto.AccessWrite
+}
+
+// Server is an in-process fake CA server.  It listens on a random TCP port and
+// an ephemeral UDP port.  Use Addr() to get the addresses for client config.
+type Server struct {
+	tcpLn net.Listener
+	udpCn *net.UDPConn
+
+	mu   sync.RWMutex
+	pvs  map[string]*serverPV
+	subs map[uint32]*serverSub // subID → sub
+
+	done chan struct{}
+}
+
+type serverPV struct {
+	spec PVSpec
+	mu   sync.RWMutex
+	val  any
+}
+
+type serverSub struct {
+	pvName string
+	subID  uint32
+	cid    uint32
+	sid    uint32
+	conn   net.Conn
+	dbrType uint16
+	count   uint32
+}
+
+// New creates and starts a fake CA server hosting the given PVs.
+func New(pvs []PVSpec) (*Server, error) {
+	tcpLn, err := net.Listen("tcp4", "127.0.0.1:0")
+	if err != nil {
+		return nil, fmt.Errorf("testca: tcp listen: %w", err)
+	}
+	udpCn, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.ParseIP("127.0.0.1")})
+	if err != nil {
+		tcpLn.Close()
+		return nil, fmt.Errorf("testca: udp listen: %w", err)
+	}
+
+	s := &Server{
+		tcpLn: tcpLn,
+		udpCn: udpCn,
+		pvs:   make(map[string]*serverPV, len(pvs)),
+		subs:  make(map[uint32]*serverSub),
+		done:  make(chan struct{}),
+	}
+	for _, spec := range pvs {
+		s.pvs[spec.Name] = &serverPV{spec: spec, val: spec.Value}
+	}
+
+	go s.acceptLoop()
+	go s.udpLoop()
+	return s, nil
+}
+
+// TCPAddr returns the TCP "host:port" address clients should connect to.
+func (s *Server) TCPAddr() string { return s.tcpLn.Addr().String() }
+
+// UDPAddr returns the UDP "host:port" address clients should search against.
+func (s *Server) UDPAddr() string { return s.udpCn.LocalAddr().String() }
+
+// Close shuts down the server.
+func (s *Server) Close() {
+	close(s.done)
+	s.tcpLn.Close()
+	s.udpCn.Close()
+}
+
+// SetValue updates a PV's value and pushes a monitor event to all subscribers.
+func (s *Server) SetValue(pvName string, val any) error {
+	s.mu.RLock()
+	pv, ok := s.pvs[pvName]
+	s.mu.RUnlock()
+	if !ok {
+		return fmt.Errorf("testca: unknown PV %q", pvName)
+	}
+	pv.mu.Lock()
+	pv.val = val
+	pv.mu.Unlock()
+
+	// Push to all subscribers of this PV.
+	s.mu.RLock()
+	for _, sub := range s.subs {
+		if sub.pvName == pvName {
+			if msg := s.buildEventMsg(sub, pv); msg != nil {
+				_, _ = sub.conn.Write(msg)
+			}
+		}
+	}
+	s.mu.RUnlock()
+	return nil
+}
+
+// -------------------------------------------------------------------------- //
+// UDP search loop                                                               //
+// -------------------------------------------------------------------------- //
+
+func (s *Server) udpLoop() {
+	buf := make([]byte, 65536)
+	for {
+		_ = s.udpCn.SetReadDeadline(time.Now().Add(200 * time.Millisecond))
+		n, src, err := s.udpCn.ReadFromUDP(buf)
+		if err != nil {
+			select {
+			case <-s.done:
+				return
+			default:
+				continue
+			}
+		}
+		s.handleUDP(buf[:n], src)
+	}
+}
+
+func (s *Server) handleUDP(data []byte, src *net.UDPAddr) {
+	for len(data) >= proto.HeaderSize {
+		hdr, n, err := proto.DecodeHeader(newBytesReader(data))
+		if err != nil {
+			return
+		}
+		payEnd := n + int(hdr.PayloadSize)
+		if payEnd > len(data) {
+			return
+		}
+		payload := data[n:payEnd]
+		data = data[payEnd:]
+
+		if hdr.Command != proto.CmdSearch {
+			continue
+		}
+		// Extract PV name from payload (null-terminated).
+		pvName := nullStr(payload)
+
+		s.mu.RLock()
+		_, ok := s.pvs[pvName]
+		s.mu.RUnlock()
+		if !ok {
+			continue
+		}
+
+		// Reply: data_type = TCP port, parameter2 = searchID.
+		tcpPort := s.tcpLn.Addr().(*net.TCPAddr).Port
+		// Use 0xFFFFFFFF IP to tell client to use sender's IP.
+		reply := buildSearchReply(hdr.Parameter2, tcpPort)
+		_, _ = s.udpCn.WriteToUDP(reply, src)
+	}
+}
+
+func buildSearchReply(searchID uint32, port int) []byte {
+	payload := []byte{0xFF, 0xFF, 0xFF, 0xFF, 0x00, 0x00, 0x00, 0x00}
+	h := proto.Header{
+		Command:    proto.CmdSearch,
+		DataType:   uint16(port),
+		Parameter1: proto.MinorVersion,
+		Parameter2: searchID,
+	}
+	return proto.BuildMessage(h, payload)
+}
+
+// -------------------------------------------------------------------------- //
+// TCP accept + per-connection handler                                          //
+// -------------------------------------------------------------------------- //
+
+func (s *Server) acceptLoop() {
+	for {
+		conn, err := s.tcpLn.Accept()
+		if err != nil {
+			select {
+			case <-s.done:
+				return
+			default:
+				continue
+			}
+		}
+		go s.handleConn(conn)
+	}
+}
+
+func (s *Server) handleConn(conn net.Conn) {
+	defer conn.Close()
+
+	// Track CID → (pvName, SID).
+	type chanInfo struct {
+		pvName string
+		sid    uint32
+	}
+	sidSeq := uint32(1000)
+	channels := make(map[uint32]*chanInfo) // cid → info
+
+	for {
+		hdr, _, err := proto.DecodeHeader(conn)
+		if err != nil {
+			return
+		}
+		var payload []byte
+		if hdr.PayloadSize > 0 {
+			payload = make([]byte, hdr.PayloadSize)
+			if _, err = io.ReadFull(conn, payload); err != nil {
+				return
+			}
+		}
+
+		switch hdr.Command {
+		case proto.CmdVersion:
+			// Respond with VERSION.
+			reply := proto.BuildMessage(proto.Header{
+				Command:   proto.CmdVersion,
+				DataCount: proto.MinorVersion,
+			}, nil)
+			conn.Write(reply)
+
+		case proto.CmdHostName, proto.CmdClientName:
+			// Ignore client identity.
+
+		case proto.CmdCreateChan:
+			cid := hdr.Parameter1
+			pvName := nullStr(payload)
+
+			s.mu.RLock()
+			pv, ok := s.pvs[pvName]
+			s.mu.RUnlock()
+
+			if !ok {
+				// CREATE_FAIL
+				fail := proto.BuildMessage(proto.Header{
+					Command:    proto.CmdCreateFail,
+					Parameter1: cid,
+				}, nil)
+				conn.Write(fail)
+				continue
+			}
+
+			sid := sidSeq
+			sidSeq++
+			channels[cid] = &chanInfo{pvName: pvName, sid: sid}
+
+			// CREATE_CHAN reply: DataType=dbfType, DataCount=count, p1=cid, p2=sid.
+			pv.mu.RLock()
+			dbfType := pv.spec.DBFType
+			count := pv.spec.Count
+			pv.mu.RUnlock()
+
+			reply := proto.BuildMessage(proto.Header{
+				Command:    proto.CmdCreateChan,
+				DataType:   uint16(dbfType),
+				DataCount:  count,
+				Parameter1: cid,
+				Parameter2: sid,
+			}, nil)
+			conn.Write(reply)
+
+			// ACCESS_RIGHTS: p1=cid, p2=access.
+			access := pv.spec.Access
+			if access == 0 {
+				access = proto.AccessRead | proto.AccessWrite
+			}
+			ar := proto.BuildMessage(proto.Header{
+				Command:    proto.CmdAccessRights,
+				Parameter1: cid,
+				Parameter2: access,
+			}, nil)
+			conn.Write(ar)
+
+		case proto.CmdEventAdd:
+			// p1=SID (channel), p2=subscriptionID (client-assigned).
+			sid := hdr.Parameter1
+			subID := hdr.Parameter2
+
+			// Find channel by SID.
+			var pvName string
+			for _, info := range channels {
+				if info.sid == sid {
+					pvName = info.pvName
+					break
+				}
+			}
+			if pvName == "" {
+				continue
+			}
+
+			s.mu.Lock()
+			sub := &serverSub{
+				pvName:  pvName,
+				subID:   subID,
+				sid:     sid,
+				conn:    conn,
+				dbrType: hdr.DataType,
+				count:   hdr.DataCount,
+			}
+			s.subs[subID] = sub
+			s.mu.Unlock()
+
+			// Send initial value.
+			s.mu.RLock()
+			pv := s.pvs[pvName]
+			s.mu.RUnlock()
+			if msg := s.buildEventMsg(sub, pv); msg != nil {
+				conn.Write(msg)
+			}
+
+		case proto.CmdEventCancel:
+			// p1=SID, p2=subscriptionID.
+			subID := hdr.Parameter2
+			s.mu.Lock()
+			delete(s.subs, subID)
+			s.mu.Unlock()
+
+		case proto.CmdReadNotify:
+			// p1=SID, p2=ioid (per CA spec).
+			sid := hdr.Parameter1
+			ioid := hdr.Parameter2
+
+			var pvName string
+			var cid uint32
+			for c, info := range channels {
+				if info.sid == sid {
+					pvName = info.pvName
+					cid = c
+					break
+				}
+			}
+			if pvName == "" {
+				continue
+			}
+
+			s.mu.RLock()
+			pv := s.pvs[pvName]
+			s.mu.RUnlock()
+
+			pv.mu.RLock()
+			val := pv.val
+			pv.mu.RUnlock()
+
+			var replyPayload []byte
+			switch hdr.DataType {
+			case proto.DBRCtrlDouble, proto.DBRCtrlLong, proto.DBRCtrlEnum, proto.DBRCtrlString,
+				proto.DBRCtrlFloat, proto.DBRCtrlShort, proto.DBRCtrlChar:
+				replyPayload = s.encodeCtrlValue(hdr.DataType, pv)
+			default:
+				replyPayload = s.encodeTimeValue(hdr.DataType, hdr.DataCount, val)
+			}
+			reply := proto.BuildMessage(proto.Header{
+				Command:    proto.CmdReadNotify,
+				DataType:   hdr.DataType,
+				DataCount:  hdr.DataCount,
+				Parameter1: cid,  // echo client channel ID (matches real EPICS IOC)
+				Parameter2: ioid, // echo ioid so client can match the reply
+			}, replyPayload)
+			conn.Write(reply)
+
+		case proto.CmdWrite:
+			// p1=SID, p2=0.
+			sid := hdr.Parameter1
+			var pvName string
+			for _, info := range channels {
+				if info.sid == sid {
+					pvName = info.pvName
+					break
+				}
+			}
+			if pvName == "" {
+				continue
+			}
+			s.mu.RLock()
+			pv := s.pvs[pvName]
+			s.mu.RUnlock()
+
+			val := decodeWritePayload(hdr.DataType, payload)
+			if val == nil {
+				continue
+			}
+			pv.mu.Lock()
+			pv.val = val
+			pv.mu.Unlock()
+
+			// Push update to all subscribers.
+			s.mu.RLock()
+			for _, sub := range s.subs {
+				if sub.pvName == pvName {
+					if msg := s.buildEventMsg(sub, pv); msg != nil {
+						sub.conn.Write(msg)
+					}
+				}
+			}
+			s.mu.RUnlock()
+
+		case proto.CmdEcho:
+			// No response needed (server echoes are optional).
+		}
+	}
+}
+
+// -------------------------------------------------------------------------- //
+// Value encoding helpers                                                       //
+// -------------------------------------------------------------------------- //
+
+func (s *Server) buildEventMsg(sub *serverSub, pv *serverPV) []byte {
+	pv.mu.RLock()
+	val := pv.val
+	pv.mu.RUnlock()
+	payload := s.encodeTimeValue(sub.dbrType, sub.count, val)
+	if payload == nil {
+		return nil
+	}
+	return proto.BuildMessage(proto.Header{
+		Command:    proto.CmdEventAdd,
+		DataType:   sub.dbrType,
+		DataCount:  sub.count,
+		Parameter1: 1, // ECA_NORMAL
+		Parameter2: sub.subID,
+	}, payload)
+}
+
+// encodeTimeValue builds a DBR_TIME_* payload for val.
+func (s *Server) encodeTimeValue(dbrType uint16, _ uint32, val any) []byte {
+	now := time.Now().UTC()
+	sec := uint32(now.Unix() - 631152000) // EPICS epoch offset
+	nsec := uint32(now.Nanosecond())
+
+	hdr := make([]byte, 12)
+	// status=0, severity=0
+	binary.BigEndian.PutUint32(hdr[4:], sec)
+	binary.BigEndian.PutUint32(hdr[8:], nsec)
+
+	var body []byte
+	switch dbrType {
+	case proto.DBRTimeDouble:
+		body = make([]byte, 12) // 4 pad + 8 value
+		f := toF64(val)
+		binary.BigEndian.PutUint64(body[4:], math.Float64bits(f))
+
+	case proto.DBRTimeFloat:
+		body = make([]byte, 4)
+		binary.BigEndian.PutUint32(body, math.Float32bits(float32(toF64(val))))
+
+	case proto.DBRTimeLong:
+		body = make([]byte, 4)
+		binary.BigEndian.PutUint32(body, uint32(int32(toF64(val))))
+
+	case proto.DBRTimeShort, proto.DBRTimeEnum:
+		body = make([]byte, 4) // 2-byte RISC pad + 2-byte value
+		binary.BigEndian.PutUint16(body[2:], uint16(int16(toF64(val))))
+
+	case proto.DBRTimeChar:
+		body = make([]byte, 4) // RISC_pad0[0:2] + RISC_pad1[2] + value[3]
+		body[3] = byte(uint8(toF64(val)))
+
+	case proto.DBRTimeString:
+		body = make([]byte, 40)
+		if str, ok := val.(string); ok {
+			copy(body, str)
+		}
+
+	default:
+		return nil
+	}
+
+	return append(hdr, body...)
+}
+
+// encodeCtrlValue builds a DBR_CTRL_* payload for a GET reply.
+// The fake server returns minimal but structurally correct payloads so that
+// the client-side decode functions succeed.
+func (s *Server) encodeCtrlValue(dbrType uint16, pv *serverPV) []byte {
+	pv.mu.RLock()
+	val := pv.val
+	access := pv.spec.Access
+	pv.mu.RUnlock()
+
+	switch dbrType {
+	case proto.DBRCtrlDouble: // 88 bytes
+		p := make([]byte, 88)
+		if access == proto.AccessRead {
+			// status=0, severity=0 (read-only signalled via ACCESS_RIGHTS, not here)
+		}
+		// units at [8:16] (empty string = "")
+		// value at [80:88]
+		binary.BigEndian.PutUint64(p[80:], math.Float64bits(toF64(val)))
+		return p
+
+	case proto.DBRCtrlFloat, proto.DBRCtrlShort, proto.DBRCtrlChar: // map to Long-style
+		// Treat as CtrlLong (48 bytes); caller asked for these types but our
+		// NativeCtrlType maps them to Double/Long anyway.  Return a valid stub.
+		p := make([]byte, 48)
+		binary.BigEndian.PutUint32(p[44:], uint32(int32(toF64(val))))
+		return p
+
+	case proto.DBRCtrlLong: // 48 bytes
+		p := make([]byte, 48)
+		binary.BigEndian.PutUint32(p[44:], uint32(int32(toF64(val))))
+		return p
+
+	case proto.DBRCtrlEnum: // 424 bytes
+		p := make([]byte, 424)
+		// no_str at [4:6] = 0 (no enum strings in fake server)
+		binary.BigEndian.PutUint16(p[422:], uint16(int16(toF64(val))))
+		return p
+
+	case proto.DBRCtrlString: // 44 bytes: status(2)+severity(2)+value[40]
+		p := make([]byte, 44)
+		if str, ok := val.(string); ok {
+			copy(p[4:], str)
+		}
+		return p
+
+	default:
+		return nil
+	}
+}
+
+func decodeWritePayload(dbrType uint16, payload []byte) any {
+	switch dbrType {
+	case proto.DBRDouble:
+		if len(payload) < 8 {
+			return nil
+		}
+		return math.Float64frombits(binary.BigEndian.Uint64(payload))
+	case proto.DBRLong:
+		if len(payload) < 4 {
+			return nil
+		}
+		return int32(binary.BigEndian.Uint32(payload))
+	case proto.DBRShort, proto.DBREnum:
+		if len(payload) < 2 {
+			return nil
+		}
+		return int16(binary.BigEndian.Uint16(payload))
+	case proto.DBRString:
+		return nullStr(payload)
+	default:
+		return nil
+	}
+}
+
+func toF64(v any) float64 {
+	switch x := v.(type) {
+	case float64:
+		return x
+	case float32:
+		return float64(x)
+	case int:
+		return float64(x)
+	case int32:
+		return float64(x)
+	case int16:
+		return float64(x)
+	case int64:
+		return float64(x)
+	default:
+		return 0
+	}
+}
+
+func nullStr(b []byte) string {
+	for i, c := range b {
+		if c == 0 {
+			return string(b[:i])
+		}
+	}
+	return string(b)
+}
+
+// -------------------------------------------------------------------------- //
+// Minimal io.Reader for proto.DecodeHeader                                   //
+// -------------------------------------------------------------------------- //
+
+type bytesReader struct{ b []byte; i int }
+
+func newBytesReader(b []byte) *bytesReader { return &bytesReader{b: b} }
+
+func (r *bytesReader) Read(p []byte) (int, error) {
+	if r.i >= len(r.b) {
+		return 0, io.EOF
+	}
+	n := copy(p, r.b[r.i:])
+	r.i += n
+	return n, nil
+}

pkg/pva/client.go

@@ -0,0 +1,340 @@
+package pva
+
+import (
+	"bytes"
+	"context"
+	"encoding/binary"
+	"fmt"
+	"log/slog"
+	"net"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"github.com/uopi/gopva/pvdata"
+)
+
+// DefaultTCPPort is the standard PVA TCP port for channel connections.
+const DefaultTCPPort = 5075
+
+// DefaultUDPPort is the PVA broadcast/search port (distinct from TCP).
+const DefaultUDPPort = 5076
+
+// Client is a PVA client that manages connections to one or more servers.
+// It performs UDP search to locate PVs, then opens TCP connections on demand.
+type Client struct {
+	mu      sync.Mutex
+	conns   map[string]*conn    // server addr → connection
+	pending map[string][]waiter // pvName → waiters for address
+
+	searchSeq atomic.Uint32
+	udp       *net.UDPConn
+
+	ctx    context.Context
+	cancel context.CancelFunc
+}
+
+type waiter struct {
+	pvName string
+	cb     func(addr string, err error)
+}
+
+// NewClient creates a PVA client.  Call Close when done.
+func NewClient() (*Client, error) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cl := &Client{
+		conns:   make(map[string]*conn),
+		pending: make(map[string][]waiter),
+		ctx:     ctx,
+		cancel:  cancel,
+	}
+	if err := cl.startUDP(); err != nil {
+		cancel()
+		return nil, err
+	}
+	return cl, nil
+}
+
+// Close shuts down the client and all connections.
+func (cl *Client) Close() {
+	cl.cancel()
+	if cl.udp != nil {
+		cl.udp.Close()
+	}
+	cl.mu.Lock()
+	defer cl.mu.Unlock()
+	for _, c := range cl.conns {
+		c.close()
+	}
+}
+
+// Get issues a one-shot GET for pvName and returns the decoded structure.
+func (cl *Client) Get(ctx context.Context, pvName string) (pvdata.StructValue, error) {
+	ch := make(chan struct {
+		v   pvdata.StructValue
+		err error
+	}, 1)
+	cl.withConn(ctx, pvName, func(c *conn, err error) {
+		if err != nil {
+			ch <- struct {
+				v   pvdata.StructValue
+				err error
+			}{err: err}
+			return
+		}
+		c.get(pvName, func(v pvdata.StructValue, e error) {
+			ch <- struct {
+				v   pvdata.StructValue
+				err error
+			}{v, e}
+		})
+	})
+	select {
+	case res := <-ch:
+		return res.v, res.err
+	case <-ctx.Done():
+		return pvdata.StructValue{}, ctx.Err()
+	}
+}
+
+// Monitor subscribes to pvName and returns a channel that receives updates.
+// The channel is closed when ctx is cancelled or a fatal error occurs.
+func (cl *Client) Monitor(ctx context.Context, pvName string) <-chan MonitorEvent {
+	ch := make(chan MonitorEvent, 16)
+	cl.withConn(ctx, pvName, func(c *conn, err error) {
+		if err != nil {
+			ch <- MonitorEvent{Err: err}
+			close(ch)
+			return
+		}
+		c.subscribe(ctx, pvName, ch)
+	})
+	return ch
+}
+
+// ---- Internal --------------------------------------------------------
+
+// withConn locates (or creates) a connection to the server hosting pvName
+// and calls cb on it.  cb may be called from a goroutine.
+func (cl *Client) withConn(ctx context.Context, pvName string, cb func(*conn, error)) {
+	cl.search(ctx, pvName, func(addr string, err error) {
+		if err != nil {
+			cb(nil, err)
+			return
+		}
+		cl.mu.Lock()
+		c, ok := cl.conns[addr]
+		cl.mu.Unlock()
+		if ok {
+			cb(c, nil)
+			return
+		}
+		// open new TCP connection
+		tc, err := dial(ctx, addr)
+		if err != nil {
+			cb(nil, fmt.Errorf("pva: connect %s: %w", addr, err))
+			return
+		}
+		cl.mu.Lock()
+		cl.conns[addr] = tc
+		cl.mu.Unlock()
+		cb(tc, nil)
+	})
+}
+
+// ---- UDP search -------------------------------------------------------
+
+func (cl *Client) startUDP() error {
+	conn, err := net.ListenUDP("udp4", &net.UDPAddr{Port: 0})
+	if err != nil {
+		return fmt.Errorf("pva: UDP listen: %w", err)
+	}
+	cl.udp = conn
+	go cl.udpReadLoop()
+	return nil
+}
+
+// search sends a PVA search request and calls cb when a response arrives.
+// Currently does broadcast + localhost; production code would also check
+// EPICS_PVA_ADDR_LIST environment variable.
+func (cl *Client) search(ctx context.Context, pvName string, cb func(string, error)) {
+	seq := cl.searchSeq.Add(1)
+
+	cl.mu.Lock()
+	cl.pending[pvName] = append(cl.pending[pvName], waiter{pvName: pvName, cb: cb})
+	cl.mu.Unlock()
+
+	go func() {
+		for attempt := 0; attempt < 10; attempt++ {
+			select {
+			case <-ctx.Done():
+				cl.mu.Lock()
+				cl.removeWaiter(pvName, cb)
+				cl.mu.Unlock()
+				cb("", ctx.Err())
+				return
+			default:
+			}
+			cl.sendSearchRequest(seq, pvName)
+			// exponential backoff: 100ms, 200ms, 400ms … up to 2s
+			delay := time.Duration(100<<min(attempt, 4)) * time.Millisecond
+			select {
+			case <-time.After(delay):
+			case <-ctx.Done():
+				cl.mu.Lock()
+				cl.removeWaiter(pvName, cb)
+				cl.mu.Unlock()
+				cb("", ctx.Err())
+				return
+			}
+		}
+		// give up
+		cl.mu.Lock()
+		cl.removeWaiter(pvName, cb)
+		cl.mu.Unlock()
+		cb("", fmt.Errorf("pva: search timeout for %q", pvName))
+	}()
+}
+
+func (cl *Client) removeWaiter(pvName string, cb func(string, error)) {
+	ws := cl.pending[pvName]
+	for i, w := range ws {
+		// compare by pointer (closure identity via fmt trick)
+		if fmt.Sprintf("%p", w.cb) == fmt.Sprintf("%p", cb) {
+			cl.pending[pvName] = append(ws[:i], ws[i+1:]...)
+			return
+		}
+	}
+}
+
+// sendSearchRequest broadcasts a PVA SEARCH request.
+func (cl *Client) sendSearchRequest(seq uint32, pvName string) {
+	// SEARCH payload (spec §6.3):
+	// searchSeqID(4) + flags(1) + reserved(3) + responseAddress(16) + responsePort(2)
+	// + transportCount(1) + transports[](1=TCP) + pvCount(2) + pvs[](channelID(4)+name(str))
+	var buf bytes.Buffer
+	binary.Write(&buf, binary.LittleEndian, seq) // searchSeqID
+	buf.WriteByte(0x81)                          // flags: unicast=0, reply=1, mustReply=1
+	buf.Write([]byte{0, 0, 0})                   // reserved
+	// responseAddress: 16 bytes (IPv4-mapped IPv6: ::ffff:0.0.0.0 = no specific address)
+	buf.Write(make([]byte, 12))
+	buf.Write([]byte{0, 0, 0, 0}) // 0.0.0.0 → server will reply to source addr
+
+	// responsePort: our UDP listening port
+	laddr := cl.udp.LocalAddr().(*net.UDPAddr)
+	binary.Write(&buf, binary.LittleEndian, uint16(laddr.Port))
+
+	buf.WriteByte(1)        // transportCount = 1
+	buf.WriteByte(0x01)     // transport[0]: TCP
+
+	binary.Write(&buf, binary.LittleEndian, uint16(1)) // pvCount = 1
+	binary.Write(&buf, binary.LittleEndian, uint32(seq)) // channelID (reuse seq)
+	pvdata.WriteString(&buf, pvName)
+
+	msg := BuildMessage(CmdSearchRequest, flagApp, buf.Bytes())
+
+	// Send to localhost and broadcast on the PVA UDP search port (5076).
+	targets := []string{
+		fmt.Sprintf("127.0.0.1:%d", DefaultUDPPort),
+		fmt.Sprintf("255.255.255.255:%d", DefaultUDPPort),
+	}
+	// Also check EPICS_PVA_ADDR_LIST if set (common in labs).
+	// Entries without an explicit port default to DefaultTCPPort for TCP
+	// connections; for the search broadcast we still use the UDP port.
+	for _, addr := range addrsFromEnv() {
+		targets = append(targets, fmt.Sprintf("%s:%d", addr, DefaultUDPPort))
+	}
+	for _, addr := range targets {
+		dst, err := net.ResolveUDPAddr("udp4", addr)
+		if err != nil {
+			continue
+		}
+		if _, err := cl.udp.WriteTo(msg, dst); err != nil {
+			slog.Debug("pva search send", "dst", addr, "err", err)
+		}
+	}
+}
+
+// udpReadLoop reads SEARCH_RESPONSE (and BEACON) UDP messages.
+func (cl *Client) udpReadLoop() {
+	buf := make([]byte, 65536)
+	for {
+		n, src, err := cl.udp.ReadFromUDP(buf)
+		if err != nil {
+			return
+		}
+		cl.handleUDP(buf[:n], src)
+	}
+}
+
+func (cl *Client) handleUDP(data []byte, src *net.UDPAddr) {
+	if len(data) < headerSize {
+		return
+	}
+	r := bytes.NewReader(data)
+	hdr, err := ReadHeader(r)
+	if err != nil || hdr.isControl() {
+		return
+	}
+	if hdr.Command != CmdSearchResponse {
+		return
+	}
+
+	payload := make([]byte, hdr.Size)
+	if _, err := r.Read(payload); err != nil {
+		return
+	}
+	pr := bytes.NewReader(payload)
+
+	// SEARCH_RESPONSE:
+	// guid(12) + searchSeqID(4) + serverAddress(16) + serverPort(2)
+	// + protocol(str) + found(1) + pvCount(2) + pvIDs[](4)
+	var guid [12]byte
+	pr.Read(guid[:])
+	var seq uint32
+	binary.Read(pr, binary.LittleEndian, &seq)
+	var addr [16]byte
+	pr.Read(addr[:])
+	var port uint16
+	binary.Read(pr, binary.LittleEndian, &port)
+	proto, _ := readPVAString(pr, binary.LittleEndian)
+	if proto != "tcp" {
+		return
+	}
+	found, _ := pr.ReadByte()
+	if found == 0 {
+		return
+	}
+	var pvCount uint16
+	binary.Read(pr, binary.LittleEndian, &pvCount)
+	// pvIDs — we match by searching for waiters by name (seq used as channelID)
+
+	// Determine server address: use src if serverAddress is 0.0.0.0
+	ip := net.IP(addr[12:16]) // last 4 bytes = IPv4
+	if ip.Equal(net.IPv4zero) {
+		ip = src.IP
+	}
+	serverAddr := fmt.Sprintf("%s:%d", ip.String(), port)
+
+	// Notify all pending waiters (simple: we got a response so all PVs are on this server)
+	cl.mu.Lock()
+	var toNotify []func(string, error)
+	for pvName, ws := range cl.pending {
+		for _, w := range ws {
+			toNotify = append(toNotify, w.cb)
+		}
+		delete(cl.pending, pvName)
+	}
+	cl.mu.Unlock()
+
+	for _, cb := range toNotify {
+		go cb(serverAddr, nil)
+	}
+}
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}

pkg/pva/conn.go

@@ -0,0 +1,576 @@
+package pva
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"log/slog"
+	"net"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"github.com/uopi/gopva/pvdata"
+)
+
+// ---- Channel/Request state machine -----------------------------------
+
+type chanState int
+
+const (
+	chanPending chanState = iota // CREATE_CHANNEL sent
+	chanCreated                  // server ack'd
+	chanFailed                   // CREATE_CHANNEL failed
+)
+
+type serverChannel struct {
+	pvName string
+	cid    uint32 // client channel ID
+	sid    uint32 // server channel ID (assigned on creation)
+	state  chanState
+
+	// pending callbacks waiting for channel creation
+	onCreate []func(sid uint32, err error)
+}
+
+type monitorSub struct {
+	ioid    uint32
+	sid     uint32           // server channel ID — needed for pipeline ACKs and DESTROY
+	cid     uint32
+	desc    pvdata.FieldDesc // structure descriptor (received on INIT response)
+	updates chan MonitorEvent
+}
+
+// MonitorEvent carries a decoded monitor update.
+type MonitorEvent struct {
+	// Changed is the set of top-level field indices that changed.
+	Changed pvdata.BitSet
+	// Value is the full decoded structure value.
+	Value pvdata.StructValue
+	// Err is non-nil if this is a terminal error event.
+	Err error
+}
+
+// conn is a single PVA TCP connection to one server.
+type conn struct {
+	nc   net.Conn
+	br   *bufio.Reader // shared reader — used by handshake then readLoop
+	bw   *bufio.Writer
+	mu   sync.Mutex // protects writes + state maps
+
+	// auto-incrementing IDs
+	nextCID  atomic.Uint32
+	nextIOID atomic.Uint32
+
+	// state
+	chans    map[uint32]*serverChannel // keyed by client CID
+	byPV     map[string]*serverChannel // keyed by PV name
+	monitors map[uint32]*monitorSub   // keyed by IOID
+
+	done chan struct{}
+}
+
+func dial(ctx context.Context, addr string) (*conn, error) {
+	nc, err := (&net.Dialer{}).DialContext(ctx, "tcp", addr)
+	if err != nil {
+		return nil, err
+	}
+	c := &conn{
+		nc:       nc,
+		br:       bufio.NewReader(nc),
+		bw:       bufio.NewWriter(nc),
+		chans:    make(map[uint32]*serverChannel),
+		byPV:     make(map[string]*serverChannel),
+		monitors: make(map[uint32]*monitorSub),
+		done:     make(chan struct{}),
+	}
+	if err := c.handshake(); err != nil {
+		nc.Close()
+		return nil, err
+	}
+	go c.readLoop()
+	return c, nil
+}
+
+// ---- Handshake --------------------------------------------------------
+
+// handshake completes the PVA connection setup.
+//
+// Protocol sequence (spec §4.2):
+//  1. Client → server: SET_BYTE_ORDER control (announces little-endian)
+//  2. Server → client: SET_BYTE_ORDER control (may be omitted by some servers)
+//  3. Server → client: CONNECTION_VALIDATION (0x01) — server's auth options
+//  4. Client → server: CONNECTION_VALIDATION (0x01) — client selects auth
+//
+// The client MUST NOT send step 4 before receiving step 3.
+func (c *conn) handshake() error {
+	// Step 1: announce our byte order.
+	if _, err := c.nc.Write(BuildMessage(CtrlSetByteOrder, flagControl, nil)); err != nil {
+		return fmt.Errorf("pva handshake: send byte-order: %w", err)
+	}
+
+	// Step 2+3: read server messages until CONNECTION_VALIDATION arrives.
+	// Servers typically send SET_BYTE_ORDER first, then CONNECTION_VALIDATION.
+	for {
+		hdr, err := ReadHeader(c.br)
+		if err != nil {
+			return fmt.Errorf("pva handshake: read server message: %w", err)
+		}
+		payload := make([]byte, hdr.Size)
+		if _, err := io.ReadFull(c.br, payload); err != nil {
+			return fmt.Errorf("pva handshake: read server payload: %w", err)
+		}
+		if hdr.isControl() {
+			// SET_BYTE_ORDER from server — we always use LE, ignore.
+			continue
+		}
+		if hdr.Command == CmdConnectionValid {
+			break // server is ready for our reply
+		}
+		// Ignore unexpected messages (e.g. BEACON on same port).
+	}
+
+	// Step 4: send CONNECTION_VALIDATION response.
+	// payload: clientReceiveBufferSize(4) + clientIntrospectionRegistryMaxSize(4) + authNZ(str)
+	var buf bytes.Buffer
+	binary.Write(&buf, binary.LittleEndian, uint32(0x00800000)) // 8 MiB receive buffer
+	binary.Write(&buf, binary.LittleEndian, uint32(0x00000000)) // introspection registry size
+	buf.WriteByte(0)                                            // authNZ = "" (anonymous)
+	if _, err := c.nc.Write(BuildMessage(CmdConnectionValid, flagApp, buf.Bytes())); err != nil {
+		return fmt.Errorf("pva handshake: send connection_valid: %w", err)
+	}
+	return nil
+}
+
+// ---- Write helpers ----------------------------------------------------
+
+func (c *conn) send(msg []byte) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	_, err := c.nc.Write(msg)
+	return err
+}
+
+// ---- Channel creation -------------------------------------------------
+
+// openChannel ensures a channel for pvName exists and calls cb when ready.
+func (c *conn) openChannel(pvName string, cb func(sid uint32, err error)) {
+	c.mu.Lock()
+	if ch, ok := c.byPV[pvName]; ok {
+		switch ch.state {
+		case chanCreated:
+			sid := ch.sid
+			c.mu.Unlock()
+			cb(sid, nil)
+			return
+		case chanFailed:
+			c.mu.Unlock()
+			cb(0, fmt.Errorf("pva: channel %q creation failed", pvName))
+			return
+		default:
+			ch.onCreate = append(ch.onCreate, cb)
+			c.mu.Unlock()
+			return
+		}
+	}
+	cid := c.nextCID.Add(1)
+	ch := &serverChannel{pvName: pvName, cid: cid, state: chanPending, onCreate: []func(uint32, error){cb}}
+	c.chans[cid] = ch
+	c.byPV[pvName] = ch
+	c.mu.Unlock()
+
+	// send CREATE_CHANNEL
+	// payload: count(2) + cid(4) + pvName(string)
+	var payload bytes.Buffer
+	binary.Write(&payload, binary.LittleEndian, uint16(1)) // channel count = 1
+	binary.Write(&payload, binary.LittleEndian, cid)
+	pvdata.WriteString(&payload, pvName)
+	if err := c.send(BuildMessage(CmdCreateChannel, flagApp, payload.Bytes())); err != nil {
+		c.mu.Lock()
+		delete(c.chans, cid)
+		delete(c.byPV, pvName)
+		c.mu.Unlock()
+		cb(0, err)
+	}
+}
+
+// ---- Monitor ----------------------------------------------------------
+
+// subscribe sends an EVENT_ADD equivalent (MONITOR INIT) for pvName.
+func (c *conn) subscribe(ctx context.Context, pvName string, ch chan MonitorEvent) {
+	c.openChannel(pvName, func(sid uint32, err error) {
+		if err != nil {
+			ch <- MonitorEvent{Err: err}
+			return
+		}
+		ioid := c.nextIOID.Add(1)
+
+		// Build MONITOR INIT payload:
+		// sid(4) + ioid(4) + subCmd(1=INIT) + pvRequest(FieldDesc+Value)
+		var payload bytes.Buffer
+		binary.Write(&payload, binary.LittleEndian, sid)
+		binary.Write(&payload, binary.LittleEndian, ioid)
+		payload.WriteByte(SubCmdInit) // subCmd = INIT
+
+		// pvRequest: empty structure (field "field" selecting all)
+		// Encoded as: TypeCodeStruct + typeID("") + nfields(0)
+		// This selects the whole top-level structure.
+		payload.WriteByte(pvdata.TypeCodeStruct) // FieldDesc type
+		pvdata.WriteString(&payload, "")         // typeID
+		pvdata.WriteSize(&payload, 0)            // no sub-fields = select all
+
+		c.mu.Lock()
+		var cid uint32
+		for _, sch := range c.chans {
+			if sch.sid == sid {
+				cid = sch.cid
+				break
+			}
+		}
+		ms := &monitorSub{ioid: ioid, sid: sid, cid: cid, updates: ch}
+		c.monitors[ioid] = ms
+		c.mu.Unlock()
+
+		if err := c.send(BuildMessage(CmdMonitor, flagApp, payload.Bytes())); err != nil {
+			ch <- MonitorEvent{Err: err}
+		}
+
+		// watch context cancellation → send MONITOR DESTROY
+		go func() {
+			<-ctx.Done()
+			c.cancelMonitor(ioid, sid)
+		}()
+	})
+}
+
+func (c *conn) cancelMonitor(ioid, sid uint32) {
+	var payload bytes.Buffer
+	binary.Write(&payload, binary.LittleEndian, sid)
+	binary.Write(&payload, binary.LittleEndian, ioid)
+	payload.WriteByte(SubCmdDEstroy)
+	_ = c.send(BuildMessage(CmdMonitor, flagApp, payload.Bytes()))
+}
+
+// ---- GET (one-shot) ---------------------------------------------------
+
+// get issues a GET for pvName and calls cb when the response arrives.
+func (c *conn) get(pvName string, cb func(v pvdata.StructValue, err error)) {
+	c.openChannel(pvName, func(sid uint32, err error) {
+		if err != nil {
+			cb(pvdata.StructValue{}, err)
+			return
+		}
+		ioid := c.nextIOID.Add(1)
+
+		// INIT phase: GET with subCmd=INIT only (spec §7.2 — INIT and GET are separate messages)
+		var payload bytes.Buffer
+		binary.Write(&payload, binary.LittleEndian, sid)
+		binary.Write(&payload, binary.LittleEndian, ioid)
+		payload.WriteByte(SubCmdInit) // INIT only — server returns FieldDesc
+		// pvRequest: empty struct = select all fields
+		payload.WriteByte(pvdata.TypeCodeStruct)
+		pvdata.WriteString(&payload, "")
+		pvdata.WriteSize(&payload, 0)
+
+		c.mu.Lock()
+		ms := &monitorSub{ioid: ioid, sid: sid, updates: make(chan MonitorEvent, 1)}
+		c.monitors[ioid] = ms
+		c.mu.Unlock()
+
+		if err := c.send(BuildMessage(CmdGet, flagApp, payload.Bytes())); err != nil {
+			cb(pvdata.StructValue{}, err)
+			return
+		}
+
+		// wait for INIT response, then issue GET
+		go func() {
+			evt := <-ms.updates
+			if evt.Err != nil {
+				cb(pvdata.StructValue{}, evt.Err)
+				return
+			}
+			// INIT done — send GET sub-command
+			var p2 bytes.Buffer
+			binary.Write(&p2, binary.LittleEndian, sid)
+			binary.Write(&p2, binary.LittleEndian, ioid)
+			p2.WriteByte(SubCmdGet)
+			if err := c.send(BuildMessage(CmdGet, flagApp, p2.Bytes())); err != nil {
+				cb(pvdata.StructValue{}, err)
+				return
+			}
+			// wait for data response
+			evt = <-ms.updates
+			cb(evt.Value, evt.Err)
+
+			c.mu.Lock()
+			delete(c.monitors, ioid)
+			c.mu.Unlock()
+		}()
+	})
+}
+
+// ---- Read loop --------------------------------------------------------
+
+func (c *conn) readLoop() {
+	defer close(c.done)
+	for {
+		hdr, err := ReadHeader(c.br)
+		if err != nil {
+			if err != io.EOF {
+				slog.Debug("pva readLoop", "err", err)
+			}
+			c.closeAllWithError(err)
+			return
+		}
+		payload := make([]byte, hdr.Size)
+		if _, err := io.ReadFull(c.br, payload); err != nil {
+			c.closeAllWithError(err)
+			return
+		}
+		r := bytes.NewReader(payload)
+		bo := hdr.byteOrder()
+
+		if hdr.isControl() {
+			c.handleControl(hdr, payload)
+			continue
+		}
+		switch hdr.Command {
+		case CmdCreateChannel:
+			c.handleCreateChannel(r, bo)
+		case CmdGet:
+			c.handleGet(r, bo)
+		case CmdMonitor:
+			c.handleMonitor(r, bo)
+		case CmdDestroyChannel:
+			// server killed the channel; clean up
+		case CmdMessage:
+			handleServerMessage(r, bo)
+		default:
+			// ignore unknown commands
+		}
+	}
+}
+
+func (c *conn) handleControl(hdr PVAHeader, payload []byte) {
+	switch hdr.Command {
+	case CtrlSetByteOrder:
+		// server's byte-order announcement — we always use LE so ignore
+	case CtrlEchoRequest:
+		// respond with ECHO_RESPONSE
+		_ = c.send(BuildMessage(CtrlEchoResponse, flagControl, payload))
+	}
+}
+
+// handleCreateChannel decodes a CREATE_CHANNEL response.
+// Wire: count(2) × { cid(4) + status(1+…) + sid(4) }
+func (c *conn) handleCreateChannel(r io.Reader, bo binary.ByteOrder) {
+	var count uint16
+	binary.Read(r, bo, &count)
+	for i := 0; i < int(count); i++ {
+		var cid uint32
+		binary.Read(r, bo, &cid)
+		st, err := ReadStatus(r, bo)
+		if err != nil {
+			return
+		}
+		var sid uint32
+		if st.OK() {
+			binary.Read(r, bo, &sid)
+		}
+
+		c.mu.Lock()
+		ch, ok := c.chans[cid]
+		if !ok {
+			c.mu.Unlock()
+			continue
+		}
+		cbs := ch.onCreate
+		ch.onCreate = nil
+		if st.OK() {
+			ch.sid = sid
+			ch.state = chanCreated
+		} else {
+			ch.state = chanFailed
+		}
+		c.mu.Unlock()
+
+		var cbErr error
+		if !st.OK() {
+			cbErr = fmt.Errorf("pva: %s", st.Error())
+		}
+		for _, cb := range cbs {
+			cb(sid, cbErr)
+		}
+	}
+}
+
+// handleGet decodes a GET response (both INIT and data phases).
+func (c *conn) handleGet(r io.Reader, bo binary.ByteOrder) {
+	var ioid uint32
+	binary.Read(r, bo, &ioid)
+	var subCmdBuf [1]byte
+	io.ReadFull(r, subCmdBuf[:])
+	subCmd := subCmdBuf[0]
+
+	st, err := ReadStatus(r, bo)
+	if err != nil {
+		return
+	}
+
+	c.mu.Lock()
+	ms, ok := c.monitors[ioid]
+	c.mu.Unlock()
+	if !ok {
+		return
+	}
+
+	if subCmd&SubCmdInit != 0 {
+		// INIT response — read FieldDesc
+		if st.OK() {
+			desc, err := pvdata.ReadFieldDesc(r)
+			if err == nil {
+				ms.desc = desc
+			}
+		}
+		ms.updates <- MonitorEvent{Err: asError(st)}
+		return
+	}
+
+	// GET data response
+	if !st.OK() {
+		ms.updates <- MonitorEvent{Err: fmt.Errorf("pva GET: %s", st.Error())}
+		return
+	}
+	// read bitSet (changed mask) then value
+	changed, err := pvdata.ReadBitSet(r)
+	if err != nil {
+		ms.updates <- MonitorEvent{Err: err}
+		return
+	}
+	v, err := pvdata.ReadValue(r, ms.desc)
+	if err != nil {
+		ms.updates <- MonitorEvent{Err: err}
+		return
+	}
+	sv, _ := v.(pvdata.StructValue)
+	ms.updates <- MonitorEvent{Changed: changed, Value: sv}
+}
+
+// handleMonitor decodes a MONITOR response.
+// Sub-commands: INIT (0x08) → FieldDesc, pipeline data.
+func (c *conn) handleMonitor(r io.Reader, bo binary.ByteOrder) {
+	var ioid uint32
+	binary.Read(r, bo, &ioid)
+	var subCmdBuf2 [1]byte
+	io.ReadFull(r, subCmdBuf2[:])
+	subCmd := subCmdBuf2[0]
+
+	c.mu.Lock()
+	ms, ok := c.monitors[ioid]
+	c.mu.Unlock()
+	if !ok {
+		return
+	}
+
+	if subCmd&SubCmdInit != 0 {
+		st, err := ReadStatus(r, bo)
+		if err != nil || !st.OK() {
+			ms.updates <- MonitorEvent{Err: asError(st)}
+			return
+		}
+		desc, err := pvdata.ReadFieldDesc(r)
+		if err != nil {
+			ms.updates <- MonitorEvent{Err: err}
+			return
+		}
+		ms.desc = desc
+		// Acknowledge pipeline
+		c.sendMonitorAck(ioid)
+		return
+	}
+
+	if subCmd&SubCmdDEstroy != 0 {
+		close(ms.updates)
+		c.mu.Lock()
+		delete(c.monitors, ioid)
+		c.mu.Unlock()
+		return
+	}
+
+	// Data update: changed BitSet + overrun BitSet + value
+	changed, err := pvdata.ReadBitSet(r)
+	if err != nil {
+		return
+	}
+	_, err = pvdata.ReadBitSet(r) // overrun — discard for now
+	if err != nil {
+		return
+	}
+	v, err := pvdata.ReadValue(r, ms.desc)
+	if err != nil {
+		return
+	}
+	sv, _ := v.(pvdata.StructValue)
+	select {
+	case ms.updates <- MonitorEvent{Changed: changed, Value: sv}:
+	default:
+		// slow consumer — drop (overrun)
+	}
+	// Acknowledge to allow more updates (pipeline)
+	c.sendMonitorAck(ioid)
+}
+
+func (c *conn) sendMonitorAck(ioid uint32) {
+	// MONITOR pipeline ACK: sid(4) + ioid(4) + subCmd(0x80)
+	c.mu.Lock()
+	ms, ok := c.monitors[ioid]
+	sid := uint32(0)
+	if ok {
+		sid = ms.sid
+	}
+	c.mu.Unlock()
+
+	var p bytes.Buffer
+	binary.Write(&p, binary.LittleEndian, sid)
+	binary.Write(&p, binary.LittleEndian, ioid)
+	p.WriteByte(SubCmdPipeline)
+	_ = c.send(BuildMessage(CmdMonitor, flagApp, p.Bytes()))
+}
+
+func handleServerMessage(r io.Reader, bo binary.ByteOrder) {
+	// MESSAGE: type(1) + message(string) — log and ignore
+	var t [1]byte
+	r.Read(t[:])
+	msg, _ := readPVAString(r, bo)
+	slog.Debug("pva server message", "type", t[0], "msg", msg)
+}
+
+func (c *conn) closeAllWithError(err error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	for _, ms := range c.monitors {
+		select {
+		case ms.updates <- MonitorEvent{Err: err}:
+		default:
+		}
+	}
+}
+
+func (c *conn) close() {
+	c.nc.Close()
+	select {
+	case <-c.done:
+	case <-time.After(2 * time.Second):
+	}
+}
+
+func asError(s Status) error {
+	if s.OK() {
+		return nil
+	}
+	return fmt.Errorf("%s", s.Error())
+}
+

pkg/pva/env.go

@@ -0,0 +1,21 @@
+package pva
+
+import (
+	"os"
+	"strings"
+)
+
+// addrsFromEnv returns addresses from EPICS_PVA_ADDR_LIST, if set.
+func addrsFromEnv() []string {
+	v := os.Getenv("EPICS_PVA_ADDR_LIST")
+	if v == "" {
+		return nil
+	}
+	var addrs []string
+	for _, a := range strings.Fields(v) {
+		if a != "" {
+			addrs = append(addrs, a)
+		}
+	}
+	return addrs
+}

pkg/pva/go.mod

@@ -0,0 +1,3 @@
+module github.com/uopi/gopva
+
+go 1.22

pkg/pva/proto.go

@@ -0,0 +1,232 @@
+// Package pva implements an EPICS PV Access (PVA) client in pure Go.
+//
+// PVA is the modern EPICS network protocol introduced in EPICS 7, replacing
+// Channel Access (CA) for structured/typed data.  This package implements
+// the client side of the PVA TCP protocol sufficient for GET and MONITOR
+// operations on NTScalar and NTScalarArray normative types.
+//
+// Reference: EPICS PVAccess Protocol Specification r1.1
+// https://epics-pvdata.sourceforge.net/pvAccess.html
+package pva
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"io"
+)
+
+// ---- Message flags ----------------------------------------------------
+
+const (
+	flagApp        byte = 0x00 // application message (vs control)
+	flagControl    byte = 0x01 // control message
+	flagSegFirst   byte = 0x08 // first segment
+	flagSegLast    byte = 0x10 // last segment
+	flagSegMid     byte = 0x18 // middle segment
+	flagFromServer byte = 0x40 // direction: server→client
+	flagBigEndian  byte = 0x80 // byte order: big-endian (we use little-endian)
+)
+
+// ---- Application message command codes --------------------------------
+
+const (
+	CmdBeacon          byte = 0x00
+	CmdConnectionValid byte = 0x01
+	CmdEcho            byte = 0x02
+	CmdSearchRequest   byte = 0x03
+	CmdSearchResponse  byte = 0x04
+	CmdAuthNZ          byte = 0x05 // authentication/authorisation
+	CmdAclChange       byte = 0x06
+	CmdCreateChannel   byte = 0x07
+	CmdDestroyChannel  byte = 0x08
+	CmdConnectionReq   byte = 0x09
+	CmdGet             byte = 0x0A
+	CmdPut             byte = 0x0B
+	CmdPutGet          byte = 0x0C
+	CmdMonitor         byte = 0x0D
+	CmdArray           byte = 0x0E
+	CmdDestroyReq      byte = 0x0F
+	CmdProcess         byte = 0x10
+	CmdGetField        byte = 0x11
+	CmdMessage         byte = 0x12 // server status/warning
+	CmdMultipleData    byte = 0x13
+	CmdRpcCall         byte = 0x14
+	CmdCancelRequest   byte = 0x15
+	CmdOriginTag       byte = 0x16
+)
+
+// ---- Control message sub-commands -------------------------------------
+
+const (
+	CtrlSetMarker       byte = 0x00
+	CtrlAckMarker       byte = 0x01
+	CtrlSetByteOrder    byte = 0x02
+	CtrlEchoRequest     byte = 0x03
+	CtrlEchoResponse    byte = 0x04
+)
+
+// ---- Status codes -----------------------------------------------------
+
+const (
+	StatusOK      byte = 0xFF // special "OK" short encoding
+	StatusOKFull  byte = 0x00 // full OK message (type=OK, msg="")
+	StatusWarn    byte = 0x01
+	StatusError   byte = 0x02
+	StatusFatal   byte = 0x03
+)
+
+// ---- Request sub-command bits -----------------------------------------
+
+const (
+	SubCmdInit     byte = 0x08 // initialise request (send FieldDesc)
+	SubCmdGet      byte = 0x40 // GET sub-command
+	SubCmdPipeline byte = 0x80 // pipeline (for monitor)
+	SubCmdDEstroy  byte = 0x10 // destroy request
+	SubCmdProcess  byte = 0x04
+)
+
+// ---- Header -----------------------------------------------------------
+
+// PVAHeader is the 8-byte fixed header on every PVA message.
+//
+//   byte  0   : magic 0xCA
+//   byte  1   : protocol version (0x01)
+//   byte  2   : flags (see flag* constants)
+//   byte  3   : command code
+//   bytes 4–7 : payload size (uint32, matches byte-order flag)
+type PVAHeader struct {
+	Version byte
+	Flags   byte
+	Command byte
+	Size    uint32
+}
+
+const magic byte = 0xCA
+const headerSize = 8
+
+func (h PVAHeader) isLittleEndian() bool { return h.Flags&flagBigEndian == 0 }
+func (h PVAHeader) isFromServer() bool   { return h.Flags&flagFromServer != 0 }
+func (h PVAHeader) isControl() bool      { return h.Flags&flagControl != 0 }
+
+// byteOrder returns the binary.ByteOrder matching the header flags.
+func (h PVAHeader) byteOrder() binary.ByteOrder {
+	if h.isLittleEndian() {
+		return binary.LittleEndian
+	}
+	return binary.BigEndian
+}
+
+// ReadHeader reads an 8-byte PVA message header from r.
+func ReadHeader(r io.Reader) (PVAHeader, error) {
+	var raw [headerSize]byte
+	if _, err := io.ReadFull(r, raw[:]); err != nil {
+		return PVAHeader{}, err
+	}
+	if raw[0] != magic {
+		return PVAHeader{}, fmt.Errorf("pva: bad magic 0x%02X (expected 0xCA)", raw[0])
+	}
+	h := PVAHeader{
+		Version: raw[1],
+		Flags:   raw[2],
+		Command: raw[3],
+	}
+	bo := h.byteOrder()
+	h.Size = bo.Uint32(raw[4:8])
+	return h, nil
+}
+
+// WriteHeader encodes h and writes the 8-byte header to w.
+// Always writes in little-endian (client sends LE after SetByteOrder).
+func WriteHeader(w io.Writer, h PVAHeader) error {
+	raw := [headerSize]byte{
+		magic,
+		h.Version,
+		h.Flags,
+		h.Command,
+	}
+	binary.LittleEndian.PutUint32(raw[4:8], h.Size)
+	_, err := w.Write(raw[:])
+	return err
+}
+
+// BuildMessage assembles a complete PVA message (header + payload).
+func BuildMessage(cmd byte, flags byte, payload []byte) []byte {
+	var buf bytes.Buffer
+	_ = WriteHeader(&buf, PVAHeader{
+		Version: 1,
+		Flags:   flags & ^flagFromServer, // clear server bit — this is client
+		Command: cmd,
+		Size:    uint32(len(payload)),
+	})
+	buf.Write(payload)
+	return buf.Bytes()
+}
+
+// ---- Status decoding ---------------------------------------------------
+
+// Status is a decoded PVA status message embedded in many response types.
+type Status struct {
+	Type    byte
+	Message string
+	Stack   string
+}
+
+// OK reports whether the status is success.
+func (s Status) OK() bool { return s.Type == StatusOKFull || s.Type == StatusOK }
+
+// Error implements the error interface so Status can be returned as error.
+func (s Status) Error() string {
+	if s.OK() {
+		return ""
+	}
+	return fmt.Sprintf("pva status %d: %s", s.Type, s.Message)
+}
+
+// ReadStatus reads a PVA status from r.
+// The short form (0xFF = OK) is handled transparently.
+func ReadStatus(r io.Reader, bo binary.ByteOrder) (Status, error) {
+	var b [1]byte
+	if _, err := io.ReadFull(r, b[:]); err != nil {
+		return Status{}, err
+	}
+	if b[0] == StatusOK {
+		return Status{Type: StatusOKFull}, nil
+	}
+	typ := b[0]
+	msg, err := readPVAString(r, bo)
+	if err != nil {
+		return Status{}, err
+	}
+	stack, err := readPVAString(r, bo)
+	if err != nil {
+		return Status{}, err
+	}
+	return Status{Type: typ, Message: msg, Stack: stack}, nil
+}
+
+// readPVAString reads a PVA length-prefixed string.
+// PVA uses compact size encoding (same as pvdata package).
+func readPVAString(r io.Reader, _ binary.ByteOrder) (string, error) {
+	// delegate to pvdata compact-size reader
+	var b [1]byte
+	if _, err := io.ReadFull(r, b[:]); err != nil {
+		return "", err
+	}
+	n := int(b[0])
+	if n == 0xFF {
+		var v int32
+		if err := binary.Read(r, binary.LittleEndian, &v); err != nil {
+			return "", err
+		}
+		n = int(v)
+	}
+	if n <= 0 {
+		return "", nil
+	}
+	buf := make([]byte, n)
+	if _, err := io.ReadFull(r, buf); err != nil {
+		return "", err
+	}
+	return string(buf), nil
+}

pkg/pva/pvdata/bitset.go

@@ -0,0 +1,73 @@
+package pvdata
+
+import "io"
+
+// BitSet is a variable-length set of bit indices, encoded on the wire as:
+//   compact-size(n_bytes) + n_bytes of little-endian bits.
+// Bit 0 of byte 0 is field index 0, bit 1 of byte 0 is field index 1, etc.
+// Used in pvData Monitor responses for "changed" and "overrun" masks.
+type BitSet struct {
+	bits []byte
+}
+
+// NewBitSet returns an empty BitSet.
+func NewBitSet() BitSet { return BitSet{} }
+
+// Set sets bit i in the BitSet.
+func (bs *BitSet) Set(i int) {
+	byteIdx := i / 8
+	for len(bs.bits) <= byteIdx {
+		bs.bits = append(bs.bits, 0)
+	}
+	bs.bits[byteIdx] |= 1 << (uint(i) % 8)
+}
+
+// Has reports whether bit i is set.
+func (bs *BitSet) Has(i int) bool {
+	byteIdx := i / 8
+	if byteIdx >= len(bs.bits) {
+		return false
+	}
+	return bs.bits[byteIdx]&(1<<(uint(i)%8)) != 0
+}
+
+// ReadBitSet reads a BitSet from r.
+func ReadBitSet(r io.Reader) (BitSet, error) {
+	n, err := ReadSize(r)
+	if err != nil {
+		return BitSet{}, err
+	}
+	if n == 0 {
+		return BitSet{}, nil
+	}
+	buf := make([]byte, n)
+	if _, err := io.ReadFull(r, buf); err != nil {
+		return BitSet{}, err
+	}
+	return BitSet{bits: buf}, nil
+}
+
+// WriteBitSet writes bs to w.
+func WriteBitSet(w io.Writer, bs BitSet) error {
+	if err := WriteSize(w, int64(len(bs.bits))); err != nil {
+		return err
+	}
+	if len(bs.bits) == 0 {
+		return nil
+	}
+	_, err := w.Write(bs.bits)
+	return err
+}
+
+// FieldIndices returns all set bit indices in ascending order.
+func (bs *BitSet) FieldIndices() []int {
+	var out []int
+	for bi, b := range bs.bits {
+		for bit := 0; bit < 8; bit++ {
+			if b&(1<<uint(bit)) != 0 {
+				out = append(out, bi*8+bit)
+			}
+		}
+	}
+	return out
+}

pkg/pva/pvdata/types.go

@@ -0,0 +1,679 @@
+// Package pvdata implements the EPICS pvData serialisation layer.
+//
+// pvData defines a rich, self-describing type system used by PV Access (PVA).
+// Every value that flows over a PVA connection is encoded according to a
+// FieldDesc (field description / introspection interface), which is itself
+// serialised on the wire before the payload.
+//
+// Reference: EPICS pvData Specification r1.1
+// https://epics-pvdata.sourceforge.net/pvData.html
+package pvdata
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+	"math"
+)
+
+// ---- Type codes -------------------------------------------------------
+//
+// pvData uses a one-byte "type code" to identify scalar and array element
+// types.  Values from the spec §3.1.
+
+const (
+	TypeCodeBoolean  byte = 0x00
+	TypeCodeByte     byte = 0x20 // int8
+	TypeCodeShort    byte = 0x21 // int16
+	TypeCodeInt      byte = 0x22 // int32
+	TypeCodeLong     byte = 0x23 // int64
+	TypeCodeUByte    byte = 0x24 // uint8
+	TypeCodeUShort   byte = 0x25 // uint16
+	TypeCodeUInt     byte = 0x26 // uint32
+	TypeCodeULong    byte = 0x27 // uint64
+	TypeCodeFloat    byte = 0x42 // float32
+	TypeCodeDouble   byte = 0x43 // float64
+	TypeCodeString   byte = 0x60
+	TypeCodeStruct   byte = 0x80
+	TypeCodeUnion    byte = 0x81
+	TypeCodeBoolArr  byte = 0x08
+	TypeCodeByteArr  byte = 0x28
+	TypeCodeShortArr byte = 0x29
+	TypeCodeIntArr   byte = 0x2A
+	TypeCodeLongArr  byte = 0x2B
+	TypeCodeUByteArr byte = 0x2C
+	TypeCodeUShortArr byte = 0x2D
+	TypeCodeUIntArr  byte = 0x2E
+	TypeCodeULongArr byte = 0x2F
+	TypeCodeFloatArr byte = 0x4A
+	TypeCodeDoubleArr byte = 0x4B
+	TypeCodeStringArr byte = 0x68
+	TypeCodeStructArr byte = 0x88
+	TypeCodeUnionArr  byte = 0x89
+	TypeCodeVariant   byte = 0x82 // any
+	TypeCodeNull      byte = 0xFF
+)
+
+// ---- Compact size encoding --------------------------------------------
+//
+// pvData encodes array/string lengths as a "size" using 1, 4, or 8 bytes
+// (spec §3.2).
+
+// ReadSize reads a compact-format size from r.
+// 0x00–0xFE → 1-byte value
+// 0xFF       → read 4-byte int32 (negative = 8-byte int64 follows)
+func ReadSize(r io.Reader) (int64, error) {
+	var b [1]byte
+	if _, err := io.ReadFull(r, b[:]); err != nil {
+		return 0, err
+	}
+	if b[0] != 0xFF {
+		return int64(b[0]), nil
+	}
+	var v int32
+	if err := binary.Read(r, binary.LittleEndian, &v); err != nil {
+		return 0, err
+	}
+	if v >= 0 {
+		return int64(v), nil
+	}
+	// extended 8-byte size
+	var v8 int64
+	if err := binary.Read(r, binary.LittleEndian, &v8); err != nil {
+		return 0, err
+	}
+	return v8, nil
+}
+
+// WriteSize writes n as a compact-format size to w.
+func WriteSize(w io.Writer, n int64) error {
+	if n < 0xFF {
+		_, err := w.Write([]byte{byte(n)})
+		return err
+	}
+	if n <= math.MaxInt32 {
+		if _, err := w.Write([]byte{0xFF}); err != nil {
+			return err
+		}
+		return binary.Write(w, binary.LittleEndian, int32(n))
+	}
+	// rare: >2 GiB array
+	if _, err := w.Write([]byte{0xFF}); err != nil {
+		return err
+	}
+	if err := binary.Write(w, binary.LittleEndian, int32(-1)); err != nil {
+		return err
+	}
+	return binary.Write(w, binary.LittleEndian, n)
+}
+
+// ReadString reads a pvData string (compact size + UTF-8 bytes).
+func ReadString(r io.Reader) (string, error) {
+	n, err := ReadSize(r)
+	if err != nil {
+		return "", err
+	}
+	if n == 0 {
+		return "", nil
+	}
+	buf := make([]byte, n)
+	if _, err := io.ReadFull(r, buf); err != nil {
+		return "", err
+	}
+	return string(buf), nil
+}
+
+// WriteString writes a pvData string to w.
+func WriteString(w io.Writer, s string) error {
+	if err := WriteSize(w, int64(len(s))); err != nil {
+		return err
+	}
+	_, err := io.WriteString(w, s)
+	return err
+}
+
+// ---- Field description (introspection) --------------------------------
+
+// Kind classifies a FieldDesc at a high level.
+type Kind byte
+
+const (
+	KindScalar    Kind = iota // single scalar value
+	KindScalarArr             // variable-length array of scalars
+	KindString                // pvData string (treated specially)
+	KindStringArr
+	KindStruct    // structure
+	KindStructArr
+	KindUnion     // discriminated union
+	KindUnionArr
+	KindVariant   // any (variant)
+)
+
+// FieldDesc describes the type of a single field in a pvData structure.
+type FieldDesc struct {
+	Kind    Kind
+	TypeCode byte     // scalar/array type code; 0 for struct/union
+	TypeID  string   // optional structure/union type ID string
+	Fields  []Field  // child fields for struct/union
+}
+
+// Field is a named FieldDesc (one member of a struct or union).
+type Field struct {
+	Name string
+	Desc FieldDesc
+}
+
+// ---- FieldDesc codec --------------------------------------------------
+
+// ReadFieldDesc decodes a FieldDesc from r.
+// This follows the pvData introspection encoding (spec §4).
+func ReadFieldDesc(r io.Reader) (FieldDesc, error) {
+	var tc [1]byte
+	if _, err := io.ReadFull(r, tc[:]); err != nil {
+		return FieldDesc{}, err
+	}
+	return readFieldDescFromTypeCode(r, tc[0])
+}
+
+func readFieldDescFromTypeCode(r io.Reader, tc byte) (FieldDesc, error) {
+	switch {
+	case tc == TypeCodeNull:
+		return FieldDesc{TypeCode: TypeCodeNull}, nil
+
+	case tc == TypeCodeString:
+		return FieldDesc{Kind: KindString, TypeCode: tc}, nil
+
+	case tc == TypeCodeStringArr:
+		return FieldDesc{Kind: KindStringArr, TypeCode: tc}, nil
+
+	case tc == TypeCodeVariant:
+		return FieldDesc{Kind: KindVariant, TypeCode: tc}, nil
+
+	case tc == TypeCodeStruct || tc == TypeCodeUnion:
+		kind := KindStruct
+		if tc == TypeCodeUnion {
+			kind = KindUnion
+		}
+		typeID, err := ReadString(r)
+		if err != nil {
+			return FieldDesc{}, err
+		}
+		nfields, err := ReadSize(r)
+		if err != nil {
+			return FieldDesc{}, err
+		}
+		fields := make([]Field, nfields)
+		for i := range fields {
+			name, err := ReadString(r)
+			if err != nil {
+				return FieldDesc{}, err
+			}
+			desc, err := ReadFieldDesc(r)
+			if err != nil {
+				return FieldDesc{}, err
+			}
+			fields[i] = Field{Name: name, Desc: desc}
+		}
+		return FieldDesc{Kind: kind, TypeCode: tc, TypeID: typeID, Fields: fields}, nil
+
+	case tc == TypeCodeStructArr || tc == TypeCodeUnionArr:
+		kind := KindStructArr
+		if tc == TypeCodeUnionArr {
+			kind = KindUnionArr
+		}
+		// element descriptor follows
+		elem, err := ReadFieldDesc(r)
+		if err != nil {
+			return FieldDesc{}, err
+		}
+		return FieldDesc{Kind: kind, TypeCode: tc, TypeID: elem.TypeID, Fields: elem.Fields}, nil
+
+	// scalar array types: 0x28–0x2F, 0x4A–0x4B, 0x08, 0x68
+	case isScalarArrayCode(tc):
+		return FieldDesc{Kind: KindScalarArr, TypeCode: tc}, nil
+
+	// plain scalars and boolean
+	default:
+		return FieldDesc{Kind: KindScalar, TypeCode: tc}, nil
+	}
+}
+
+func isScalarArrayCode(tc byte) bool {
+	switch tc {
+	case TypeCodeBoolArr,
+		TypeCodeByteArr, TypeCodeShortArr, TypeCodeIntArr, TypeCodeLongArr,
+		TypeCodeUByteArr, TypeCodeUShortArr, TypeCodeUIntArr, TypeCodeULongArr,
+		TypeCodeFloatArr, TypeCodeDoubleArr,
+		TypeCodeStringArr:
+		return true
+	}
+	return false
+}
+
+// WriteFieldDesc encodes fd into w.
+func WriteFieldDesc(w io.Writer, fd FieldDesc) error {
+	if _, err := w.Write([]byte{fd.TypeCode}); err != nil {
+		return err
+	}
+	switch fd.Kind {
+	case KindStruct, KindUnion:
+		if err := WriteString(w, fd.TypeID); err != nil {
+			return err
+		}
+		if err := WriteSize(w, int64(len(fd.Fields))); err != nil {
+			return err
+		}
+		for _, f := range fd.Fields {
+			if err := WriteString(w, f.Name); err != nil {
+				return err
+			}
+			if err := WriteFieldDesc(w, f.Desc); err != nil {
+				return err
+			}
+		}
+	case KindStructArr, KindUnionArr:
+		// write element descriptor
+		elemTC := TypeCodeStruct
+		if fd.Kind == KindUnionArr {
+			elemTC = TypeCodeUnion
+		}
+		if err := WriteFieldDesc(w, FieldDesc{
+			Kind: KindStruct, TypeCode: byte(elemTC),
+			TypeID: fd.TypeID, Fields: fd.Fields,
+		}); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// ---- Value codec -------------------------------------------------------
+
+// Value is a decoded pvData value.  We use interface{} for flexibility;
+// callers type-assert to concrete types listed below.
+//
+// Scalar types:
+//   bool, int8, int16, int32, int64,
+//   uint8, uint16, uint32, uint64,
+//   float32, float64, string
+//
+// Array types: []T for each scalar T above; []string.
+// Struct:      StructValue
+// Union:       UnionValue
+// Variant:     VariantValue
+// Null:        nil
+type Value = interface{}
+
+// StructValue holds the fields of a decoded pvData structure.
+type StructValue struct {
+	TypeID string
+	Fields []FieldValue
+}
+
+// FieldValue is a named value within a StructValue.
+type FieldValue struct {
+	Name  string
+	Value Value
+}
+
+// UnionValue holds a selected field from a discriminated union.
+type UnionValue struct {
+	Selected int    // index into union's Fields slice
+	Name     string // field name
+	Value    Value
+}
+
+// VariantValue wraps an any-typed value together with its runtime descriptor.
+type VariantValue struct {
+	Desc  FieldDesc
+	Value Value
+}
+
+// ReadValue decodes a value conforming to desc from r.
+func ReadValue(r io.Reader, desc FieldDesc) (Value, error) {
+	switch desc.Kind {
+	case KindScalar:
+		return readScalar(r, desc.TypeCode)
+	case KindScalarArr:
+		return readScalarArray(r, desc.TypeCode)
+	case KindString:
+		return ReadString(r)
+	case KindStringArr:
+		n, err := ReadSize(r)
+		if err != nil {
+			return nil, err
+		}
+		ss := make([]string, n)
+		for i := range ss {
+			ss[i], err = ReadString(r)
+			if err != nil {
+				return nil, err
+			}
+		}
+		return ss, nil
+	case KindStruct:
+		return readStruct(r, desc)
+	case KindStructArr:
+		return readStructArr(r, desc)
+	case KindUnion:
+		return readUnion(r, desc)
+	case KindUnionArr:
+		return readUnionArr(r, desc)
+	case KindVariant:
+		return readVariant(r)
+	default:
+		return nil, fmt.Errorf("pvdata: unknown kind %d", desc.Kind)
+	}
+}
+
+func readScalar(r io.Reader, tc byte) (Value, error) {
+	switch tc {
+	case TypeCodeBoolean:
+		var b [1]byte
+		_, err := io.ReadFull(r, b[:])
+		return b[0] != 0, err
+	case TypeCodeByte:
+		var v int8
+		return v, binary.Read(r, binary.LittleEndian, &v)
+	case TypeCodeShort:
+		var v int16
+		return v, binary.Read(r, binary.LittleEndian, &v)
+	case TypeCodeInt:
+		var v int32
+		return v, binary.Read(r, binary.LittleEndian, &v)
+	case TypeCodeLong:
+		var v int64
+		return v, binary.Read(r, binary.LittleEndian, &v)
+	case TypeCodeUByte:
+		var v uint8
+		return v, binary.Read(r, binary.LittleEndian, &v)
+	case TypeCodeUShort:
+		var v uint16
+		return v, binary.Read(r, binary.LittleEndian, &v)
+	case TypeCodeUInt:
+		var v uint32
+		return v, binary.Read(r, binary.LittleEndian, &v)
+	case TypeCodeULong:
+		var v uint64
+		return v, binary.Read(r, binary.LittleEndian, &v)
+	case TypeCodeFloat:
+		var v uint32
+		if err := binary.Read(r, binary.LittleEndian, &v); err != nil {
+			return nil, err
+		}
+		return math.Float32frombits(v), nil
+	case TypeCodeDouble:
+		var v uint64
+		if err := binary.Read(r, binary.LittleEndian, &v); err != nil {
+			return nil, err
+		}
+		return math.Float64frombits(v), nil
+	default:
+		return nil, fmt.Errorf("pvdata: unknown scalar type code 0x%02X", tc)
+	}
+}
+
+func readScalarArray(r io.Reader, tc byte) (Value, error) {
+	n, err := ReadSize(r)
+	if err != nil {
+		return nil, err
+	}
+	count := int(n)
+	switch tc {
+	case TypeCodeBoolArr:
+		v := make([]bool, count)
+		for i := range v {
+			b := [1]byte{}
+			if _, err := io.ReadFull(r, b[:]); err != nil {
+				return nil, err
+			}
+			v[i] = b[0] != 0
+		}
+		return v, nil
+	case TypeCodeByteArr:
+		v := make([]int8, count)
+		return v, binary.Read(r, binary.LittleEndian, v)
+	case TypeCodeShortArr:
+		v := make([]int16, count)
+		return v, binary.Read(r, binary.LittleEndian, v)
+	case TypeCodeIntArr:
+		v := make([]int32, count)
+		return v, binary.Read(r, binary.LittleEndian, v)
+	case TypeCodeLongArr:
+		v := make([]int64, count)
+		return v, binary.Read(r, binary.LittleEndian, v)
+	case TypeCodeUByteArr:
+		v := make([]uint8, count)
+		return v, binary.Read(r, binary.LittleEndian, v)
+	case TypeCodeUShortArr:
+		v := make([]uint16, count)
+		return v, binary.Read(r, binary.LittleEndian, v)
+	case TypeCodeUIntArr:
+		v := make([]uint32, count)
+		return v, binary.Read(r, binary.LittleEndian, v)
+	case TypeCodeULongArr:
+		v := make([]uint64, count)
+		return v, binary.Read(r, binary.LittleEndian, v)
+	case TypeCodeFloatArr:
+		bits := make([]uint32, count)
+		if err := binary.Read(r, binary.LittleEndian, bits); err != nil {
+			return nil, err
+		}
+		v := make([]float32, count)
+		for i, b := range bits {
+			v[i] = math.Float32frombits(b)
+		}
+		return v, nil
+	case TypeCodeDoubleArr:
+		bits := make([]uint64, count)
+		if err := binary.Read(r, binary.LittleEndian, bits); err != nil {
+			return nil, err
+		}
+		v := make([]float64, count)
+		for i, b := range bits {
+			v[i] = math.Float64frombits(b)
+		}
+		return v, nil
+	default:
+		return nil, fmt.Errorf("pvdata: unknown array type code 0x%02X", tc)
+	}
+}
+
+func readStruct(r io.Reader, desc FieldDesc) (StructValue, error) {
+	sv := StructValue{TypeID: desc.TypeID, Fields: make([]FieldValue, len(desc.Fields))}
+	for i, f := range desc.Fields {
+		v, err := ReadValue(r, f.Desc)
+		if err != nil {
+			return StructValue{}, fmt.Errorf("field %q: %w", f.Name, err)
+		}
+		sv.Fields[i] = FieldValue{Name: f.Name, Value: v}
+	}
+	return sv, nil
+}
+
+func readStructArr(r io.Reader, desc FieldDesc) ([]StructValue, error) {
+	n, err := ReadSize(r)
+	if err != nil {
+		return nil, err
+	}
+	arr := make([]StructValue, n)
+	for i := range arr {
+		sv, err := readStruct(r, desc)
+		if err != nil {
+			return nil, err
+		}
+		arr[i] = sv
+	}
+	return arr, nil
+}
+
+func readUnion(r io.Reader, desc FieldDesc) (UnionValue, error) {
+	idx, err := ReadSize(r)
+	if err != nil {
+		return UnionValue{}, err
+	}
+	if idx < 0 || int(idx) >= len(desc.Fields) {
+		return UnionValue{Selected: int(idx)}, nil // null selector
+	}
+	f := desc.Fields[idx]
+	v, err := ReadValue(r, f.Desc)
+	if err != nil {
+		return UnionValue{}, err
+	}
+	return UnionValue{Selected: int(idx), Name: f.Name, Value: v}, nil
+}
+
+func readUnionArr(r io.Reader, desc FieldDesc) ([]UnionValue, error) {
+	n, err := ReadSize(r)
+	if err != nil {
+		return nil, err
+	}
+	arr := make([]UnionValue, n)
+	for i := range arr {
+		uv, err := readUnion(r, desc)
+		if err != nil {
+			return nil, err
+		}
+		arr[i] = uv
+	}
+	return arr, nil
+}
+
+func readVariant(r io.Reader) (VariantValue, error) {
+	desc, err := ReadFieldDesc(r)
+	if err != nil {
+		return VariantValue{}, err
+	}
+	v, err := ReadValue(r, desc)
+	if err != nil {
+		return VariantValue{}, err
+	}
+	return VariantValue{Desc: desc, Value: v}, nil
+}
+
+// WriteValue encodes v (which must match desc) into w.
+func WriteValue(w io.Writer, desc FieldDesc, v Value) error {
+	switch desc.Kind {
+	case KindScalar:
+		return writeScalar(w, desc.TypeCode, v)
+	case KindScalarArr:
+		return writeScalarArray(w, desc.TypeCode, v)
+	case KindString:
+		return WriteString(w, v.(string))
+	case KindStringArr:
+		ss := v.([]string)
+		if err := WriteSize(w, int64(len(ss))); err != nil {
+			return err
+		}
+		for _, s := range ss {
+			if err := WriteString(w, s); err != nil {
+				return err
+			}
+		}
+		return nil
+	case KindStruct:
+		return writeStruct(w, desc, v.(StructValue))
+	case KindUnion:
+		return writeUnion(w, v.(UnionValue))
+	case KindVariant:
+		vv := v.(VariantValue)
+		if err := WriteFieldDesc(w, vv.Desc); err != nil {
+			return err
+		}
+		return WriteValue(w, vv.Desc, vv.Value)
+	default:
+		return fmt.Errorf("pvdata: WriteValue unsupported kind %d", desc.Kind)
+	}
+}
+
+func writeScalar(w io.Writer, tc byte, v Value) error {
+	switch tc {
+	case TypeCodeBoolean:
+		b := byte(0)
+		if v.(bool) {
+			b = 1
+		}
+		_, err := w.Write([]byte{b})
+		return err
+	case TypeCodeFloat:
+		return binary.Write(w, binary.LittleEndian, math.Float32bits(v.(float32)))
+	case TypeCodeDouble:
+		return binary.Write(w, binary.LittleEndian, math.Float64bits(v.(float64)))
+	default:
+		return binary.Write(w, binary.LittleEndian, v)
+	}
+}
+
+func writeScalarArray(w io.Writer, tc byte, v Value) error {
+	switch tc {
+	case TypeCodeDoubleArr:
+		arr := v.([]float64)
+		if err := WriteSize(w, int64(len(arr))); err != nil {
+			return err
+		}
+		bits := make([]uint64, len(arr))
+		for i, f := range arr {
+			bits[i] = math.Float64bits(f)
+		}
+		return binary.Write(w, binary.LittleEndian, bits)
+	case TypeCodeFloatArr:
+		arr := v.([]float32)
+		if err := WriteSize(w, int64(len(arr))); err != nil {
+			return err
+		}
+		bits := make([]uint32, len(arr))
+		for i, f := range arr {
+			bits[i] = math.Float32bits(f)
+		}
+		return binary.Write(w, binary.LittleEndian, bits)
+	default:
+		// for integer arrays, v is already []intN or []uintN
+		if err := WriteSize(w, int64(arrayLen(v))); err != nil {
+			return err
+		}
+		return binary.Write(w, binary.LittleEndian, v)
+	}
+}
+
+func writeStruct(w io.Writer, desc FieldDesc, sv StructValue) error {
+	for i, fv := range sv.Fields {
+		if err := WriteValue(w, desc.Fields[i].Desc, fv.Value); err != nil {
+			return fmt.Errorf("field %q: %w", fv.Name, err)
+		}
+	}
+	return nil
+}
+
+func writeUnion(w io.Writer, uv UnionValue) error {
+	if err := WriteSize(w, int64(uv.Selected)); err != nil {
+		return err
+	}
+	// caller must supply desc to encode value; not needed if Selected is null
+	return nil
+}
+
+// arrayLen returns reflect-free length for the supported array types.
+func arrayLen(v Value) int {
+	switch x := v.(type) {
+	case []bool:
+		return len(x)
+	case []int8:
+		return len(x)
+	case []int16:
+		return len(x)
+	case []int32:
+		return len(x)
+	case []int64:
+		return len(x)
+	case []uint8:
+		return len(x)
+	case []uint16:
+		return len(x)
+	case []uint32:
+		return len(x)
+	case []uint64:
+		return len(x)
+	default:
+		return 0
+	}
+}

pkg/pva/pvdata/types_test.go

@@ -0,0 +1,220 @@
+package pvdata_test
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/uopi/gopva/pvdata"
+)
+
+// ---- Compact size encoding -------------------------------------------
+
+func TestCompactSize(t *testing.T) {
+	cases := []int64{0, 1, 10, 254, 255, 256, 1000, 65535, 70000, 1<<31 - 1}
+	for _, n := range cases {
+		var buf bytes.Buffer
+		if err := pvdata.WriteSize(&buf, n); err != nil {
+			t.Fatalf("WriteSize(%d): %v", n, err)
+		}
+		got, err := pvdata.ReadSize(&buf)
+		if err != nil {
+			t.Fatalf("ReadSize for %d: %v", n, err)
+		}
+		if got != n {
+			t.Errorf("round-trip size %d → %d", n, got)
+		}
+	}
+}
+
+// ---- String encoding ------------------------------------------------
+
+func TestString(t *testing.T) {
+	cases := []string{"", "hello", "UOPI:TICK", "a string with spaces and unicode: é"}
+	for _, s := range cases {
+		var buf bytes.Buffer
+		if err := pvdata.WriteString(&buf, s); err != nil {
+			t.Fatalf("WriteString(%q): %v", s, err)
+		}
+		got, err := pvdata.ReadString(&buf)
+		if err != nil {
+			t.Fatalf("ReadString for %q: %v", s, err)
+		}
+		if got != s {
+			t.Errorf("round-trip string %q → %q", s, got)
+		}
+	}
+}
+
+// ---- FieldDesc (introspection) round-trip ---------------------------
+
+func TestFieldDescScalar(t *testing.T) {
+	desc := pvdata.FieldDesc{Kind: pvdata.KindScalar, TypeCode: pvdata.TypeCodeDouble}
+	var buf bytes.Buffer
+	if err := pvdata.WriteFieldDesc(&buf, desc); err != nil {
+		t.Fatal(err)
+	}
+	got, err := pvdata.ReadFieldDesc(&buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.TypeCode != desc.TypeCode || got.Kind != desc.Kind {
+		t.Errorf("scalar round-trip: got %+v, want %+v", got, desc)
+	}
+}
+
+func TestFieldDescStruct(t *testing.T) {
+	// Minimal NTScalar-like struct: value(double) + timeStamp(struct{…})
+	desc := pvdata.FieldDesc{
+		Kind:     pvdata.KindStruct,
+		TypeCode: pvdata.TypeCodeStruct,
+		TypeID:   "epics:nt/NTScalar:1.0",
+		Fields: []pvdata.Field{
+			{Name: "value", Desc: pvdata.FieldDesc{Kind: pvdata.KindScalar, TypeCode: pvdata.TypeCodeDouble}},
+			{Name: "alarm", Desc: pvdata.FieldDesc{
+				Kind: pvdata.KindStruct, TypeCode: pvdata.TypeCodeStruct, TypeID: "alarm_t",
+				Fields: []pvdata.Field{
+					{Name: "severity", Desc: pvdata.FieldDesc{Kind: pvdata.KindScalar, TypeCode: pvdata.TypeCodeInt}},
+					{Name: "status", Desc: pvdata.FieldDesc{Kind: pvdata.KindScalar, TypeCode: pvdata.TypeCodeInt}},
+					{Name: "message", Desc: pvdata.FieldDesc{Kind: pvdata.KindString, TypeCode: pvdata.TypeCodeString}},
+				},
+			}},
+			{Name: "timeStamp", Desc: pvdata.FieldDesc{
+				Kind: pvdata.KindStruct, TypeCode: pvdata.TypeCodeStruct, TypeID: "time_t",
+				Fields: []pvdata.Field{
+					{Name: "secondsPastEpoch", Desc: pvdata.FieldDesc{Kind: pvdata.KindScalar, TypeCode: pvdata.TypeCodeLong}},
+					{Name: "nanoseconds", Desc: pvdata.FieldDesc{Kind: pvdata.KindScalar, TypeCode: pvdata.TypeCodeInt}},
+					{Name: "userTag", Desc: pvdata.FieldDesc{Kind: pvdata.KindScalar, TypeCode: pvdata.TypeCodeInt}},
+				},
+			}},
+		},
+	}
+
+	var buf bytes.Buffer
+	if err := pvdata.WriteFieldDesc(&buf, desc); err != nil {
+		t.Fatal(err)
+	}
+	got, err := pvdata.ReadFieldDesc(&buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.TypeID != desc.TypeID {
+		t.Errorf("typeID: got %q want %q", got.TypeID, desc.TypeID)
+	}
+	if len(got.Fields) != len(desc.Fields) {
+		t.Fatalf("field count: got %d want %d", len(got.Fields), len(desc.Fields))
+	}
+	for i, f := range desc.Fields {
+		if got.Fields[i].Name != f.Name {
+			t.Errorf("field[%d] name: got %q want %q", i, got.Fields[i].Name, f.Name)
+		}
+	}
+}
+
+// ---- Value round-trip -----------------------------------------------
+
+func TestValueDouble(t *testing.T) {
+	desc := pvdata.FieldDesc{Kind: pvdata.KindScalar, TypeCode: pvdata.TypeCodeDouble}
+	var v pvdata.Value = float64(3.14159)
+	var buf bytes.Buffer
+	if err := pvdata.WriteValue(&buf, desc, v); err != nil {
+		t.Fatal(err)
+	}
+	got, err := pvdata.ReadValue(&buf, desc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.(float64) != v.(float64) {
+		t.Errorf("double round-trip: got %v want %v", got, v)
+	}
+}
+
+func TestValueIntArray(t *testing.T) {
+	desc := pvdata.FieldDesc{Kind: pvdata.KindScalarArr, TypeCode: pvdata.TypeCodeIntArr}
+	var v pvdata.Value = []int32{1, 2, 3, -7, 1<<30}
+	var buf bytes.Buffer
+	if err := pvdata.WriteValue(&buf, desc, v); err != nil {
+		t.Fatal(err)
+	}
+	got, err := pvdata.ReadValue(&buf, desc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	arr := got.([]int32)
+	src := v.([]int32)
+	if len(arr) != len(src) {
+		t.Fatalf("int32 array length: got %d want %d", len(arr), len(src))
+	}
+	for i := range src {
+		if arr[i] != src[i] {
+			t.Errorf("int32[%d]: got %d want %d", i, arr[i], src[i])
+		}
+	}
+}
+
+func TestValueStruct(t *testing.T) {
+	desc := pvdata.FieldDesc{
+		Kind:     pvdata.KindStruct,
+		TypeCode: pvdata.TypeCodeStruct,
+		TypeID:   "test_t",
+		Fields: []pvdata.Field{
+			{Name: "x", Desc: pvdata.FieldDesc{Kind: pvdata.KindScalar, TypeCode: pvdata.TypeCodeDouble}},
+			{Name: "n", Desc: pvdata.FieldDesc{Kind: pvdata.KindScalar, TypeCode: pvdata.TypeCodeInt}},
+			{Name: "s", Desc: pvdata.FieldDesc{Kind: pvdata.KindString, TypeCode: pvdata.TypeCodeString}},
+		},
+	}
+	sv := pvdata.StructValue{
+		TypeID: "test_t",
+		Fields: []pvdata.FieldValue{
+			{Name: "x", Value: float64(2.718)},
+			{Name: "n", Value: int32(42)},
+			{Name: "s", Value: "hello"},
+		},
+	}
+	var buf bytes.Buffer
+	if err := pvdata.WriteValue(&buf, desc, sv); err != nil {
+		t.Fatal(err)
+	}
+	got, err := pvdata.ReadValue(&buf, desc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	gsv := got.(pvdata.StructValue)
+	if gsv.Fields[0].Value.(float64) != sv.Fields[0].Value.(float64) {
+		t.Errorf("x field mismatch")
+	}
+	if gsv.Fields[1].Value.(int32) != sv.Fields[1].Value.(int32) {
+		t.Errorf("n field mismatch")
+	}
+	if gsv.Fields[2].Value.(string) != sv.Fields[2].Value.(string) {
+		t.Errorf("s field mismatch")
+	}
+}
+
+// ---- BitSet ---------------------------------------------------------
+
+func TestBitSet(t *testing.T) {
+	var bs pvdata.BitSet
+	bs.Set(0)
+	bs.Set(3)
+	bs.Set(15)
+	bs.Set(16)
+
+	var buf bytes.Buffer
+	if err := pvdata.WriteBitSet(&buf, bs); err != nil {
+		t.Fatal(err)
+	}
+	got, err := pvdata.ReadBitSet(&buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, idx := range []int{0, 3, 15, 16} {
+		if !got.Has(idx) {
+			t.Errorf("BitSet missing index %d after round-trip", idx)
+		}
+	}
+	for _, idx := range []int{1, 2, 4, 14, 17} {
+		if got.Has(idx) {
+			t.Errorf("BitSet has unexpected index %d", idx)
+		}
+	}
+}

workspace/run.sh

@@ -5,9 +5,11 @@
 #   bash workspace/run.sh
 #
 # Requirements:
-#   • EPICS Base installed (EPICS_BASE set, or auto-detected below)
+#   • EPICS Base installed (softIoc binary available in PATH or EPICS_BASE)
-#   • softIoc binary available
+#   • uopi built (script builds it if missing)
-#   • uopi built with EPICS support (script builds it if missing)
+#
+# uopi connects to EPICS via its built-in pure-Go CA client — no libca needed.
+# Set UOPI_CA_DEBUG=1 (default below) to enable CA-level debug logging.
 
 set -euo pipefail
 
@@ -15,32 +17,24 @@ PROJECT_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
 WORKSPACE="$PROJECT_ROOT/workspace"
 BINARY="$PROJECT_ROOT/dist/uopi"
 
-# ── Locate EPICS Base ─────────────────────────────────────────────────────────
+# ── Locate EPICS tools ────────────────────────────────────────────────────────
 
-if [ -z "${EPICS_BASE:-}" ]; then
+# If EPICS_BASE is set, add its bin directory to PATH automatically.
-    for candidate in \
+if [ -n "${EPICS_BASE:-}" ]; then
-        /usr/lib/epics \
+    if [ -z "${EPICS_ARCH:-}" ]; then
-        /usr/local/epics \
+        EPICS_ARCH="$(ls "$EPICS_BASE/bin" 2>/dev/null | grep -i linux | head -1 || true)"
-        /opt/epics/base \
+        EPICS_ARCH="${EPICS_ARCH:-linux-x86_64}"
-        /opt/epics; do
+    fi
-        if [ -f "$candidate/include/cadef.h" ]; then
+    export PATH="$EPICS_BASE/bin/$EPICS_ARCH:$PATH"
-            export EPICS_BASE="$candidate"
+    echo "EPICS_BASE=$EPICS_BASE  EPICS_ARCH=$EPICS_ARCH"
-            break
-        fi
-    done
 fi
 
-if [ -z "${EPICS_BASE:-}" ]; then
+if ! command -v softIoc &>/dev/null; then
-    echo "ERROR: EPICS_BASE is not set and could not be auto-detected."
+    echo "ERROR: softIoc not found in PATH."
-    echo "  export EPICS_BASE=/path/to/epics/base"
+    echo "  Set EPICS_BASE=/path/to/epics/base, or add softIoc to your PATH."
     exit 1
 fi
-
+echo "softIoc: $(command -v softIoc)"
-# Detect architecture
-EPICS_ARCH="${EPICS_ARCH:-$(ls "$EPICS_BASE/lib" | grep linux | head -1)}"
-if [ -z "$EPICS_ARCH" ]; then
-    EPICS_ARCH="linux-x86_64"
-fi
 
 # ── Environment ───────────────────────────────────────────────────────────────
 
@@ -48,24 +42,17 @@ fi
 export EPICS_CA_ADDR_LIST="127.0.0.1"
 export EPICS_CA_AUTO_ADDR_LIST="NO"
 
-# Ensure the EPICS shared libraries are found at runtime.
+# Enable CA-level debug logging in uopi (set to "" to disable).
-export LD_LIBRARY_PATH="$EPICS_BASE/lib/$EPICS_ARCH${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
+export UOPI_CA_DEBUG="${UOPI_CA_DEBUG:-1}"
-
-# Make EPICS tools (softIoc, caget, caput…) available.
-export PATH="$EPICS_BASE/bin/$EPICS_ARCH:$PATH"
-
-# ── Locate softIoc ────────────────────────────────────────────────────────────
-
-if ! command -v softIoc &>/dev/null; then
-    echo "ERROR: softIoc not found in $EPICS_BASE/bin/$EPICS_ARCH or PATH."
-    exit 1
-fi
 
 # ── Build uopi ────────────────────────────────────────────────────────────────
 
-echo "Building uopi with EPICS support..."
+echo "Building uopi + catools..."
 cd "$PROJECT_ROOT"
-make backend-epics 2>&1
+make backend 2>&1
+echo ""
+echo "  catools: $PROJECT_ROOT/dist/catools"
+echo "  Usage:   dist/catools caget UOPI:TICK"
 echo ""
 
 # ── Cleanup handler ───────────────────────────────────────────────────────────
@@ -85,39 +72,66 @@ trap cleanup EXIT INT TERM
 
 pkill -x softIoc 2>/dev/null || true
 # Give the OS a moment to release the CA port (5064/tcp+udp).
-sleep 0.3
+sleep 0.5
 
 # ── Start softIOC ─────────────────────────────────────────────────────────────
 
 echo "Starting softIOC..."
+echo "  DB: $WORKSPACE/softioc/uopi_test.db"
+echo "  Log: $WORKSPACE/softioc.log"
 softIoc -S -d "$WORKSPACE/softioc/uopi_test.db" \
     >"$WORKSPACE/softioc.log" 2>&1 &
 IOC_PID=$!
 PIDS+=("$IOC_PID")
 
-# Wait for the IOC to accept Channel Access connections (up to 10 s).
+# Wait for the IOC to start listening on TCP 5064 (up to 10 s).
-echo -n "Waiting for IOC to be ready"
+# We probe with nc (netcat) first — no EPICS tools required.
+echo -n "Waiting for IOC port 5064"
+IOC_READY=0
 for i in $(seq 1 20); do
-    if caget -w 1 UOPI:TICK >/dev/null 2>&1; then
+    if nc -z 127.0.0.1 5064 2>/dev/null; then
-        echo " OK"
+        echo " OK (TCP 5064 open)"
+        IOC_READY=1
         break
     fi
     echo -n "."
     sleep 0.5
-    if [ "$i" -eq 20 ]; then
-        echo ""
-        echo "ERROR: IOC did not become ready within 10 seconds."
-        echo "Check $WORKSPACE/softioc.log for details."
-        exit 1
-    fi
 done
 
+if [ "$IOC_READY" -eq 0 ]; then
+    echo ""
+    echo "ERROR: IOC port 5064 not open after 10 seconds."
+    echo "softIoc log:"
+    cat "$WORKSPACE/softioc.log" || true
+    exit 1
+fi
+
+# Optional deeper check with caget if it's available.
+if command -v caget &>/dev/null; then
+    echo -n "Verifying UOPI:TICK with caget"
+    for i in $(seq 1 10); do
+        if caget -w 1 UOPI:TICK >/dev/null 2>&1; then
+            echo " OK"
+            break
+        fi
+        echo -n "."
+        sleep 0.5
+        if [ "$i" -eq 10 ]; then
+            echo " (timeout — continuing anyway)"
+        fi
+    done
+else
+    echo "caget not in PATH — skipping PV check (softIoc assumed ready)"
+    sleep 1
+fi
+
 # ── Start uopi ────────────────────────────────────────────────────────────────
 
+echo ""
 echo "Starting uopi (http://localhost:8080)..."
-echo "  Open the browser, switch to Edit mode, load workspace/data/epics_test.xml"
+echo "  UOPI_CA_DEBUG=$UOPI_CA_DEBUG"
-echo "  or select it from the interface list in View mode."
+echo "  Config: $WORKSPACE/uopi.toml"
-echo "  Press Ctrl-C to stop."
+echo "  Log will appear below. Press Ctrl-C to stop."
 echo ""
 
 cd "$PROJECT_ROOT"