How to free unsafe.Pointer using a wrapper in cgo?

Question

How would I go about creating a wrapper that frees the unsafe.Pointer in my code? Here's my cgo code:

//export clientpy
func clientpy(url *C.char, headersfrompy *C.char, proxy *C.char) unsafe.Pointer {
    s := C.GoString(url)
    headers := C.GoString(headersfrompy)
    p := C.GoString(proxy)

    request := UrlGet(s, headers, p)

    length := make([]byte, 8)

    binary.LittleEndian.PutUint64(length, uint64(len(request)))
    return C.CBytes(append(length, request...))
}

//export FreeCByte
func FreeCByte(b *unsafe.Pointer) {
    C.free(unsafe.Pointer(b))
}

It seems like I cannot free the memory in my python code. I am creating a wrapper so that I can free the memory in python instead of inside of go so that I won't have a dangling pointer.

Here's my python code:

from ctypes import cdll
import ctypes, cchardet, json
from bs4 import BeautifulSoup

lib = cdll.LoadLibrary("./test.so")
lib.cclientpy.argtypes = [ctypes.c_char_p, ctypes.c_char_p, ctypes.c_char_p]
lib.cclientpy.restype = ctypes.POINTER(ctypes.c_ubyte * 8)
""" THERE IS A BIG MEMORY LEAK, BEWARE """

free = lib.free
free.argtypes = [ctypes.POINTER(ctypes.c_ubyte * 8)]

def newrequest(path, lister={}, proxy=[]):
    try:
        print(f"proxy: {proxy}")
        ptr = lib.cclientpy(path.encode("utf-8"), str(lister).encode("utf-8"), str(proxy).encode("utf-8"))
        length = int.from_bytes(ptr.contents, byteorder="little")
        data = bytes(ctypes.cast(ptr,
                                 ctypes.POINTER(ctypes.c_ubyte * (8 + length))
                                 ).contents[8:])
        #free(ptr)
        lib.FreeCByte(ptr)
        print(f'bytes: {bytes(ctypes.cast(ptr,ctypes.POINTER(ctypes.c_ubyte * (8 + length))).contents[8:])}')

        return data
    except:
        pass

How can I free the unsafe pointer in go?

Answer 1

Free the unsafe.Pointer in Go, not a pointer to the unsafe.Pointer.

//export FreeCByte
func FreeCByte(b unsafe.Pointer) {
    C.free(b)
}

---

Comment: I am still getting memory leaks. – JJ Cauton

The code in your question does not compile and run. I fixed your code to compile and run, and added some debugging code. Clearly, memory is being freed.

$ go version
go version devel go1.18-8214257347 Wed Sep 8 14:51:40 2021 +0000 linux/amd64
$ go build -o test.so -buildmode=c-shared test.go
$ python3 --version
Python 3.9.5
$ python3 test.py
proxy: []
Go: cclientpy: C.CBytes: 0x1a7e420
Go: FreeCByte: 0x1a7e420
Go: FreeCByte: double free 0x1a7e420
free(): double free detected in tcache 2
Aborted (core dumped)
$ 

---

$ cat test.go

package main

import (
    "encoding/binary"
    "fmt"
    "os"
    "unsafe"
)

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

func UrlGet(s, headers, p string) []byte {
    return nil
}

const debug = true

//export cclientpy
func cclientpy(url *C.char, headersfrompy *C.char, proxy *C.char) unsafe.Pointer {
    s := C.GoString(url)
    headers := C.GoString(headersfrompy)
    p := C.GoString(proxy)

    request := UrlGet(s, headers, p)

    length := make([]byte, 8)

    binary.LittleEndian.PutUint64(length, uint64(len(request)))
    cbytes := C.CBytes(append(length, request...))

    if debug {
        fmt.Fprintln(os.Stderr, "Go: cclientpy: C.CBytes:", cbytes)
    }

    return cbytes
}

//export FreeCByte
func FreeCByte(b unsafe.Pointer) {
    if debug {
        fmt.Fprintln(os.Stderr, "Go: FreeCByte:", b)
    }

    C.free(b)

    if debug {
        // tests to see if already freed, should fail
        // free(): double free detected in tcache 2
        // Aborted (core dumped)
        fmt.Fprintln(os.Stderr, "Go: FreeCByte: double free", b)
        C.free(b)
    }
}

func main() {}

$ 

---

$ cat test.py

from ctypes import cdll
import ctypes, chardet as cchardet, json
from bs4 import BeautifulSoup

lib = cdll.LoadLibrary("./test.so")
lib.cclientpy.argtypes = [ctypes.c_char_p, ctypes.c_char_p, ctypes.c_char_p]
lib.cclientpy.restype = ctypes.POINTER(ctypes.c_ubyte * 8)
""" THERE IS A BIG MEMORY LEAK, BEWARE """

free = lib.free
free.argtypes = [ctypes.POINTER(ctypes.c_ubyte * 8)]

def newrequest(path, lister={}, proxy=[]):
    try:
        print(f"proxy: {proxy}")
        ptr = lib.cclientpy(path.encode("utf-8"), str(lister).encode("utf-8"), str(proxy).encode("utf-8"))
        length = int.from_bytes(ptr.contents, byteorder="little")
        data = bytes(ctypes.cast(ptr,
                                 ctypes.POINTER(ctypes.c_ubyte * (8 + length))
                                 ).contents[8:])
        #free(ptr)
        lib.FreeCByte(ptr)
        print(f'bytes: {bytes(ctypes.cast(ptr,ctypes.POINTER(ctypes.c_ubyte * (8 + length))).contents[8:])}')

        return data
    except:
        pass
    
newrequest(path='argpath', lister={}, proxy=[])

$