How do I release the memory allocated by C.CString? - go

Question

Here is my code:
helloworld.go:
package main
import "C"
import "unsafe"
//export HelloWorld
func HelloWorld() *C.char {
cs := C.CString("Hello World!")
C.free(unsafe.Pointer(cs))
return cs
}
func main() {}
One of the errors I got:
src/helloworld.go:9:2: could not determine kind of name for C.free
Based on this article: https://blog.golang.org/c-go-cgo
I also found out that I need to add #include <stdlib.h> at top of my C file.
helloworld.h:
#include <stdlib.h>
/* Created by "go tool cgo" - DO NOT EDIT. */
/* package command-line-arguments */
/* Start of preamble from import "C" comments. */
/* End of preamble from import "C" comments. */
/* Start of boilerplate cgo prologue. */
#line 1 "cgo-gcc-export-header-prolog"
#ifndef GO_CGO_PROLOGUE_H
#define GO_CGO_PROLOGUE_H
typedef signed char GoInt8;
typedef unsigned char GoUint8;
typedef short GoInt16;
typedef unsigned short GoUint16;
typedef int GoInt32;
typedef unsigned int GoUint32;
typedef long long GoInt64;
typedef unsigned long long GoUint64;
typedef GoInt64 GoInt;
typedef GoUint64 GoUint;
typedef __SIZE_TYPE__ GoUintptr;
typedef float GoFloat32;
typedef double GoFloat64;
typedef float _Complex GoComplex64;
typedef double _Complex GoComplex128;
/*
static assertion to make sure the file is being used on architecture
at least with matching size of GoInt.
*/
typedef char _check_for_64_bit_pointer_matching_GoInt[sizeof(void*)==64/8 ? 1:-1];
typedef struct { const char *p; GoInt n; } GoString;
typedef void *GoMap;
typedef void *GoChan;
typedef struct { void *t; void *v; } GoInterface;
typedef struct { void *data; GoInt len; GoInt cap; } GoSlice;
#endif
/* End of boilerplate cgo prologue. */
#ifdef __cplusplus
extern "C" {
#endif
extern char* HelloWorld();
#ifdef __cplusplus
}
#endif
I tried it to no success.
I need to free the memory.
How do I do this? Because in there example it prints out the string. I want to return it. Without C.free. I am able to do it. I am just worried of causing a memory leak or some other issue.

Answer 1

Command cgo
// Go string to C string
// The C string 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.CString(string) *C.char
If the import of "C" is immediately preceded by a comment, that
comment, called the preamble, is used as a header when compiling the C
parts of the package.
For example,
package main
/*
#include <stdlib.h>
*/
import "C"
import "unsafe"
//export HelloWorld
func HelloWorld() *C.char {
cs := C.CString("Hello World!")
C.free(unsafe.Pointer(cs))
return cs
}
func main() {}