Add example, update notes, gofmt

unix/example_sysvshm_test.go

@@ -0,0 +1,57 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build (darwin && amd64) || linux
+// +build darwin,amd64 linux
+
+package unix_test
+
+import (
+	"log"
+	"runtime"
+
+	"golang.org/x/sys/unix"
+)
+
+func ExampleSysvShmGet() {
+	// sysv shared memory isn't implemented on Android
+	if runtime.GOOS == "android" {
+		return
+	}
+
+	// create shared memory region of 1024 bytes
+	id, err := unix.SysvShmGet(unix.IPC_PRIVATE, 1024, unix.IPC_CREAT|unix.IPC_EXCL|0o600)
+	if err != nil {
+		log.Fatal("sysv shm create failed:", err)
+	}
+
+	// warning: sysv shared memory segments persist even after after a process
+	// is destroyed, so it's very important to explicitly delete it when you
+	// don't need it anymore.
+	defer func() {
+		_, err := unix.SysvShmCtl(id, unix.IPC_RMID, nil)
+		if err != nil {
+			log.Fatal(err)
+		}
+	}()
+
+	// to use a shared memory region you must attach to it
+	b, err := unix.SysvShmAttach(id, 0, 0)
+	if err != nil {
+		log.Fatal("sysv attach failed:", err)
+	}
+
+	// you should detach from the segment when finished with it. The byte
+	// slice is no longer valid after detaching
+	defer func() {
+		if err = unix.SysvShmDetach(b); err != nil {
+			log.Fatal("sysv detach failed:", err)
+		}
+	}()
+
+	// Changes to the contents of the byte slice are reflected in other
+	// mappings of the shared memory identifer in this and other processes
+	b[42] = 'h'
+	b[43] = 'i'
+}

unix/sysvshm_unix.go

@@ -42,8 +42,6 @@ func SysvShmAttach(id int, addr uintptr, flag int) ([]byte, error) {
 	return b, nil
 }
 
-
-
 // SysvShmDetach unmaps the shared memory slice returned from SysvShmAttach.
 //
 // It is not safe to use the slice after calling this function.
@@ -56,6 +54,7 @@ func SysvShmDetach(data []byte) error {
 }
 
 // SysvShmGet returns the Sysv shared memory identifier associated with key.
+// If the IPC_CREAT flag is specified a new segment is created.
 func SysvShmGet(key, size, flag int) (id int, err error) {
 	return shmget(key, size, flag)
 }

unix/sysvshm_unix_test.go

@@ -23,7 +23,7 @@ func TestSysvSharedMemory(t *testing.T) {
 		if err != unix.ENOSYS {
 			t.Fatalf("expected android to fail, but it didn't")
 		}
-		return	
+		return
 	}
 
 	if err != nil {