CGO: how to free memory allocated in C using malloc from go to avoid memory leak

[hiepkh...@gmail.com]

Hi,

I am trying to use CGO to call an optimized C++ CPU-bound implementation of a complex algorithms from golang. Basically, it will pass a string into c++ function and get a string back. A simplified version of the code can be seen in the below:

    //algo.go

    package main

//#cgo LDFLAGS:

//#include <stdio.h>

//#include <stdlib.h>

//#include <string.h>

//char* echo(char* s);

import "C"

import "unsafe"

func main() {

cs := C.CString("Hello from stdio\n")

defer C.free(unsafe.Pointer(cs))

var echoOut *C.char = C.echo(cs)

//defer C.free(unsafe.Pointer(echoOut)); -> using this will crash the code

fmt.Println(C.GoString(echoOut));

}

//algo.cpp

#include <stdlib.h>

#include <stdio.h>

#include <string.h>

#include <iostream>

#include <math.h>

using namespace std;

extern "C" {

   char* echo(char* o) {

       int len = sizeof(o) / sizeof(char);

       char* out = (char*)malloc(len * sizeof(char));

       strcpy(out, o);

       return out;

   }

}    

In this link, ppl mentions that C++ code should call "free" by itself to free the allocated memory: http://grokbase.com/t/gg/golang-nuts/149hxezftf/go-nuts-cgo-is-it-safe-to-malloc-and-free-in-seperate-c-functions. But then it's very tricky because my c++ function return an allocated pointer so that golang can get the result. I cannot call free in the c++ code? What should be the correct way to handle this? I have a webserver will call the c++ code per each request and want to make sure it doesn't introduce any memory leak.

Thanks.

PS: another side questions, my actual c++ function is something like: char* lsh_hash(char* text, char* vectors_key); whereas vectors_key is a big object. I am current using: cVectorsKey := C.CString(string(byte_arr)); and then get the data like: C.GoString(C.lsh_hash(cText, cVectorsKey)). I assume that it will only pass pointer into c++ function but want to double confirm if it's best way to handle long vector_key or I should somehow manage to set vector key inside c++ code as global variable to avoid passing data from go function into c++ function. 

 

[mjy]

Code looks OK for my superficial understanding, except you allocate space for a pointer and not for the string:

       int len = sizeof(o) / sizeof(char);

Should probably be strlen(o) ...

-mjy 

[auxdx]

Hi,

Also, I just realize that my code are actually leaking memory. Seems that i would have to somehow free the allocated (malloc) memory within C code or finding a workaround solution, for example, initialize string in gocode and pass the pointer into c++ code in which c++ will use to copy the result into?

Is this a better way to handle?

[auxdx]

Thanks,

Seems that the reason it crash before is because I allocated memory within c++ wrongly.

But i am still wondering if I should free the memory in go code using: defer C.free(unsafe.Pointer(echoOut)) for each cgo function call. Or have to free the memory somehow in the c++ code? (for example, having a static library instead of allocated it using malloc).

It was mentioned overhere that: go code has no idea how to release c++ dynamically allocated memory. http://grokbase.com/t/gg/golang-nuts/149hxezftf/go-nuts-cgo-is-it-safe-to-malloc-and-free-in-seperate-c-functions. Are they correct?

On Friday, 19 February 2016 22:47:09 UTC+8, mjy wrote:

[Tamás Gulácsi]

C.free should work, if it is not, then another error is on the C side.