cmd/cgo: add C.CBytes

Add a C.CBytes function to copy a Go byte slice into C memory. This
returns an unsafe.Pointer, since that is what needs to be passed to
C.free, and the data is often opaque bytes anyway.

Fixes #14838

Change-Id: Ic7bc29637eb6f1f5ee409b3898c702a59833a85a
Reviewed-on: https://go-review.googlesource.com/20762
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/test/cgo_test.go

@@ -69,5 +69,6 @@ func Test11925(t *testing.T)                 { test11925(t) }
 func Test12030(t *testing.T)                 { test12030(t) }
 func TestGCC68255(t *testing.T)              { testGCC68255(t) }
 func TestCallGoWithString(t *testing.T)      { testCallGoWithString(t) }
+func Test14838(t *testing.T)                 { test14838(t) }
 
 func BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }

misc/cgo/test/issue14838.go

@@ -0,0 +1,37 @@
+// Copyright 2016 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.
+
+// Issue 14838. add CBytes function
+
+package cgotest
+
+/*
+#include <stdlib.h>
+
+int check_cbytes(char *b, size_t l) {
+	int i;
+	for (i = 0; i < l; i++) {
+		if (b[i] != i) {
+			return 0;
+		}
+	}
+	return 1;
+}
+*/
+import "C"
+
+import (
+	"testing"
+	"unsafe"
+)
+
+func test14838(t *testing.T) {
+	data := []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+	cData := C.CBytes(data)
+	defer C.free(cData)
+
+	if C.check_cbytes((*C.char)(cData), C.size_t(len(data))) == 0 {
+		t.Fatalf("mismatched data: expected %v, got %v", data, (*(*[10]byte)(unsafe.Pointer(cData)))[:])
+	}
+}

src/cmd/cgo/doc.go

@@ -198,6 +198,13 @@ by making copies of the data.  In pseudo-Go definitions:
 	// if C.free is needed).
 	func C.CString(string) *C.char
 
+	// Go []byte slice to C array
+	// The C array is allocated in the C heap using malloc.
+	// It is the caller's responsibility to arrange for it to be
+	// freed, such as by calling C.free (be sure to include stdlib.h
+	// if C.free is needed).
+	func C.CBytes([]byte) unsafe.Pointer
+
 	// C string to Go string
 	func C.GoString(*C.char) string
 

src/cmd/cgo/out.go

@@ -533,6 +533,7 @@ func fixGo(name string) string {
 
 var isBuiltin = map[string]bool{
 	"_Cfunc_CString":   true,
+	"_Cfunc_CBytes":    true,
 	"_Cfunc_GoString":  true,
 	"_Cfunc_GoStringN": true,
 	"_Cfunc_GoBytes":   true,
@@ -1324,6 +1325,7 @@ _GoString_ GoString(char *p);
 _GoString_ GoStringN(char *p, int l);
 _GoBytes_ GoBytes(void *p, int n);
 char *CString(_GoString_);
+void *CBytes(_GoBytes_);
 void *_CMalloc(size_t);
 `
 
@@ -1389,6 +1391,15 @@ func _Cfunc_CString(s string) *_Ctype_char {
 }
 `
 
+const cBytesDef = `
+func _Cfunc_CBytes(b []byte) unsafe.Pointer {
+	p := _cgo_runtime_cmalloc(uintptr(len(b)))
+	pp := (*[1<<30]byte)(p)
+	copy(pp[:], b)
+	return p
+}
+`
+
 const cMallocDef = `
 func _Cfunc__CMalloc(n _Ctype_size_t) unsafe.Pointer {
 	return _cgo_runtime_cmalloc(uintptr(n))
@@ -1400,6 +1411,7 @@ var builtinDefs = map[string]string{
 	"GoStringN": goStringNDef,
 	"GoBytes":   goBytesDef,
 	"CString":   cStringDef,
+	"CBytes":    cBytesDef,
 	"_CMalloc":  cMallocDef,
 }
 
@@ -1437,6 +1449,12 @@ const char *_cgoPREFIX_Cfunc_CString(struct __go_string s) {
 	return p;
 }
 
+void *_cgoPREFIX_Cfunc_CBytes(struct __go_open_array b) {
+	char *p = malloc(b.__count);
+	memmove(p, b.__data, b.__count);
+	return p;
+}
+
 struct __go_string _cgoPREFIX_Cfunc_GoString(char *p) {
 	intgo len = (p != NULL) ? strlen(p) : 0;
 	return __go_byte_array_to_string(p, len);