CGO: how to free memory allocated in C using malloc from go to avoid memory leak
Solution 1
Fix the memory allocation bug in your echo
function. For example,
algo.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
:
//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
$
Solution 2
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.
//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
}
//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;
}
}
auxdx
Updated on June 04, 2022Comments
-
auxdx almost 2 years
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.
-
auxdx about 8 yearsThanks. It works. But I am still wondering if i should release the memory in go using defer C.free(unsafe.Pointer(echoOut)) or it has to be somewhere within c++ code ?
-
auxdx about 8 yearsBtw, I just realized that my code is leaking memory. Seems that i would have to free c++ malloc'ed memory or pass go pointer to c++ code instead. Any suggestions?