Freeing C variables in Golang?

Question

I'm confused about which variables need to be freed if I'm using C variables in Go.

For example, if I do this:

    s := C.CString(`something`)

Is that memory now allocated until I call C.free(unsafe.Pointer(s)), or is that OK to be garbage collected by Go when the function ends?

Or is it only variables that are created from the imported C code that need to be freed, and these C variables created from the Go code will be garbage collected?

Answer 1

The documentation does mention:

// 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

The wiki shows an example:

package cgoexample

/*
#include <stdio.h>
#include <stdlib.h>

void myprint(char* s) {
        printf("%s", s);
}
*/
import "C"

import "unsafe"

func Example() {
        cs := C.CString("Hello from stdio\n")
        C.myprint(cs)
        C.free(unsafe.Pointer(cs))
}

---

The article "C? Go? Cgo!" shows that you don't need to free C numeric types:

func Random() int {
    var r C.long = C.random()
    return int(r)
}

But you would for string:

import "C"
import "unsafe"

func Print(s string) {
    cs := C.CString(s)
    C.fputs(cs, (*C.FILE)(C.stdout))
    C.free(unsafe.Pointer(cs))
}