Getting double free or corruption (out) when using free in cgo, golang

Question

I am trying to understand and learn cgo, and as a part of that, I wrote a function that checks for filepermission using C.stat.

import (
    "fmt"
    "unsafe"
    "os"
)
//#include <sys/stat.h>
//#include <stdlib.h>
import "C"

func CheckPerm(filename string) {

    statt := C.stat //stat struct from C
    path := C.CString(filename)
    st := *(*C.struct_stat)(unsafe.Pointer(statt)) //Casting unsafe pointer to C.struct_stat
    defer C.free(unsafe.Pointer(path)) //free
    defer C.free(unsafe.Pointer(statt))
    C.stat(path, &st)
    if st.st_mode & C.S_IRGRP > 0 || st.st_mode & C.S_IWGRP > 0 || st.st_mode & C.S_IXGRP > 0 || st.st_mode & C.S_IROTH > 0 || st.st_mode & C.S_IWOTH > 0 || st.st_mode & C.S_IXOTH > 0 {
        fmt.Println("file permission too broad, make it non-readable to groups and others.")
        os.Exit(1)
    }
    fmt.Println("File permission looks fine")

}

Upon passing a filepath to the function, it errors out saying

*** Error in `/tmp/Build go-sync.go and run77go': double free or corruption (out): 0x0000000000609480 ***
SIGABRT: abort
PC=0x7fe4302f3267 m=0
signal arrived during cgo execution

goroutine 1 [syscall, locked to thread]:
runtime.cgocall(0x401930, 0xc820097808, 0xc800000000)
/usr/lib/go/src/runtime/cgocall.go:120 +0x11b fp=0xc8200977d8 sp=0xc8200977a8
gosyncmodules._Cfunc_free(0x609480)
==========snip============

Because of the last line pasted here gosyncmodules._Cfunc_free(0x609480), I removed the defer C.free(unsafe.Pointer(statt)) and it works fine now.

• Why is it throwing error when I am trying to free the struct that was created, whereas it can be used to free the path variable?
• Will a similar scenario cause a memory leak, if not in this case?

Answer 1

From https://golang.org/cmd/cgo/#hdr-Go_references_to_C :

// 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

Your

path := C.CString(filename)

makes a copy and you have to free it while statt is a normal Go-memory-managed variable.