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

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

<!-- language: lang-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

<!-- language: lang-c++ -->

#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.

Fix the memory allocation bug in your echo function. For example,

algo.go:

<!-- language: lang-go -->

//algo.go
package main

//#cgo LDFLAGS:
//#include <stdio.h>
//#include <stdlib.h>
//#include <string.h>
//char* echo(char* s);
import "C"
import (
	"fmt"
	"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))
	fmt.Println(C.GoString(echoOut))
}

algo.cpp:

<!-- language: lang-c++ -->

//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) {
        char* out = (char*)malloc(strlen(o)+1);
        strcpy(out, o);
        return out;
    }
}

Output:

$ cd algo
$ go build && ./algo
Hello from stdio

$ 

I'm using the following go version go version go1.8 linux/amd64 and I have no problems running your code after uncommenting your deferred C.free.

I added a loop to allow me to track memory leaks via htop. Without the deferred free it does leak, but uncommenting it fixes the problem.

The code is below.

<!-- language: lang-go -->

//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() {
	for i := 0; i < 1000000000; i++ {
		allocateAndDeallocate()
	}
}

func allocateAndDeallocate() {
	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)) // no crash here
}

<!-- language: lang-c++ -->

//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;
    }

}