cgo and memory

Learn to free memory allocated with C.CString.

cgo and memory rules

There are two memory worlds: the GC manages Go memory and you manage C memory. The core rules:

Everything allocated in C (C.CString, C.malloc) is released with C.free.
You may pass a Go pointer to C, but C must not keep it after the call returns.
Go memory passed to C must not contain other Go pointers.

The easiest way to avoid leaks is to write defer C.free right below the allocation:

buf := C.malloc(1024)
defer C.free(buf)

GODEBUG=cgocheck=1 (the default) checks some pointer rules at run time and panics on a violation.

Add #include <stdlib.h> in the preamble. Allocate memory with p := C.malloc(16), write defer C.free(p) right after it and print p != nil.

Show example solution

package main

/*
#include <stdlib.h>
*/
import "C"

import "fmt"

func main() {
    p := C.malloc(16)
    defer C.free(p)
    fmt.Println(p != nil)
}

Release the memory

p := C.malloc(64)
____ C.free(p)

Show answer

p := C.malloc(64)
defer C.free(p)