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>

2 files changed, 25 insertions, 0 deletions

diff --git a/src/cmd/cgo/doc.go b/src/cmd/cgo/doc.go
index 61e480c585..6e0bfa58c6 100644
--- a/src/cmd/cgo/doc.go
+++ b/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
 
diff --git a/src/cmd/cgo/out.go b/src/cmd/cgo/out.go
index 40c76b52e4..621c41c6b2 100644
--- a/src/cmd/cgo/out.go
+++ b/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);