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

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?

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=[])
$