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

huangapple
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英文:

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

问题

我正在尝试使用CGO从golang调用一个经过优化的C++ CPU密集型算法实现。基本上，它会将一个字符串传递给C++函数，并获得一个字符串作为返回值。下面是代码的简化版本：

algo.go

  1. package main
  3. //#cgo LDFLAGS:
  4. //#include <stdio.h>
  5. //#include <stdlib.h>
  6. //#include <string.h>
  7. //char* echo(char* s);
  8. import "C"
  9. import "unsafe"
  11. func main() {
  12.     cs := C.CString("Hello from stdio\n")
  13.     defer C.free(unsafe.Pointer(cs))
  14.     var echoOut *C.char = C.echo(cs)
  15.     //defer C.free(unsafe.Pointer(echoOut)); -> using this will crash the code
  16.     fmt.Println(C.GoString(echoOut))
  17. }

algo.cpp

  1. #include <stdlib.h>
  2. #include <stdio.h>
  3. #include <string.h>
  4. #include <iostream>
  5. #include <math.h>
  7. using namespace std;
  9. extern "C" {
  10.     char* echo(char* o) {
  11.         int len = sizeof(o) / sizeof(char);
  12.         char* out = (char*)malloc(len * sizeof(char));
  13.         strcpy(out, o);
  14.         return out;
  15.     }
  16. }

在这个链接中，有人提到C++代码应该自己调用"free"来释放分配的内存：http://grokbase.com/t/gg/golang-nuts/149hxezftf/go-nuts-cgo-is-it-safe-to-malloc-and-free-in-seperate-c-functions。但是这样做非常棘手，因为我的C++函数返回一个已分配的指针，以便golang可以获得结果。我不能在C++代码中调用free吗？应该如何正确处理这个问题？我有一个Web服务器，每个请求都会调用C++代码，并希望确保它不会引入任何内存泄漏。

谢谢。

英文:

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 -->

  1. package main
  3. //#cgo LDFLAGS:
  4. //#include &lt;stdio.h&gt;
  5. //#include &lt;stdlib.h&gt;
  6. //#include &lt;string.h&gt;
  7. //char* echo(char* s);
  8. import &quot;C&quot;
  9. import &quot;unsafe&quot;
  11. func main() {
  12. 	cs := C.CString(&quot;Hello from stdio\n&quot;)
  13. 	defer C.free(unsafe.Pointer(cs))
  14. 	var echoOut *C.char = C.echo(cs)
  15. 	//defer C.free(unsafe.Pointer(echoOut)); -&gt; using this will crash the code
  16. 	fmt.Println(C.GoString(echoOut));
  17. }

algo.cpp

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

  1. #include &lt;stdlib.h&gt;
  2. #include &lt;stdio.h&gt;
  3. #include &lt;string.h&gt;
  4. #include &lt;iostream&gt;
  5. #include &lt;math.h&gt;
  7. using namespace std;
  9. extern &quot;C&quot; {
  10.     char* echo(char* o) {
  11.         int len = sizeof(o) / sizeof(char);
  12.         char* out = (char*)malloc(len * sizeof(char));
  13.         strcpy(out, o);
  14.         return out;
  15.     }
  16. }

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.

答案1

得分: 2

修复你的echo函数中的内存分配错误。例如，

algo.go:

  1. //algo.go
  2. package main
  4. //#cgo LDFLAGS:
  5. //#include <stdio.h>
  6. //#include <stdlib.h>
  7. //#include <string.h>
  8. //char* echo(char* s);
  9. import "C"
  10. import (
  11. 	"fmt"
  12. 	"unsafe"
  13. )
  15. func main() {
  16. 	cs := C.CString("Hello from stdio\n")
  17. 	defer C.free(unsafe.Pointer(cs))
  18. 	var echoOut *C.char = C.echo(cs)
  19. 	defer C.free(unsafe.Pointer(echoOut))
  20. 	fmt.Println(C.GoString(echoOut))
  21. }

algo.cpp:

  1. //algo.cpp
  2. #include <stdlib.h>
  3. #include <stdio.h>
  4. #include <string.h>
  5. #include <iostream>
  6. #include <math.h>
  8. using namespace std;
  10. extern "C" {
  11.     char* echo(char* o) {
  12.         char* out = (char*)malloc(strlen(o)+1);
  13.         strcpy(out, o);
  14.         return out;
  15.     }
  16. }

输出:

  1. $ cd algo
  2. $ go build && ./algo
  3. Hello from stdio
  5. $
英文:

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

algo.go:

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

  1. //algo.go
  2. package main
  4. //#cgo LDFLAGS:
  5. //#include &lt;stdio.h&gt;
  6. //#include &lt;stdlib.h&gt;
  7. //#include &lt;string.h&gt;
  8. //char* echo(char* s);
  9. import &quot;C&quot;
  10. import (
  11. 	&quot;fmt&quot;
  12. 	&quot;unsafe&quot;
  13. )
  15. func main() {
  16. 	cs := C.CString(&quot;Hello from stdio\n&quot;)
  17. 	defer C.free(unsafe.Pointer(cs))
  18. 	var echoOut *C.char = C.echo(cs)
  19. 	defer C.free(unsafe.Pointer(echoOut))
  20. 	fmt.Println(C.GoString(echoOut))
  21. }

algo.cpp:

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

  1. //algo.cpp
  2. #include &lt;stdlib.h&gt;
  3. #include &lt;stdio.h&gt;
  4. #include &lt;string.h&gt;
  5. #include &lt;iostream&gt;
  6. #include &lt;math.h&gt;
  8. using namespace std;
  10. extern &quot;C&quot; {
  11.     char* echo(char* o) {
  12.         char* out = (char*)malloc(strlen(o)+1);
  13.         strcpy(out, o);
  14.         return out;
  15.     }
  16. }

Output:

  1. $ cd algo
  2. $ go build &amp;&amp; ./algo
  3. Hello from stdio
  5. $

答案2

得分: 1

我正在使用以下go版本 go version go1.8 linux/amd64，在取消注释你的deferred C.free后，运行你的代码没有任何问题。

我添加了一个循环，以便通过htop跟踪内存泄漏。如果不取消注释，它确实会泄漏，但取消注释后问题就解决了。

以下是代码。

  1. //algo.go
  2. package main
  4. //#cgo LDFLAGS:
  5. //#include <stdio.h>
  6. //#include <stdlib.h>
  7. //#include <string.h>
  8. //char* echo(char* s);
  9. import "C"
  10. import "unsafe"
  12. func main() {
  13.     for i := 0; i < 1000000000; i++ {
  14.         allocateAndDeallocate()
  15.     }
  16. }
  18. func allocateAndDeallocate() {
  19.     cs := C.CString("Hello from stdio\n")
  20.     defer C.free(unsafe.Pointer(cs))
  21.     var echoOut *C.char = C.echo(cs)
  22.     defer C.free(unsafe.Pointer(echoOut)) // no crash here
  23. }
  1. //algo.cpp
  2. #include <stdlib.h>
  3. #include <stdio.h>
  4. #include <string.h>
  5. #include <iostream>
  6. #include <math.h>
  8. using namespace std;
  10. extern "C" {
  12.     char* echo(char* o) {
  13.         int len = sizeof(o) / sizeof(char);
  14.         char* out = (char*)malloc(len * sizeof(char));
  15.         strcpy(out, o);
  16.         return out;
  17.     }
  19. }

希望对你有帮助！

英文:

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 -->

  1. //algo.go
  2. package main
  4. //#cgo LDFLAGS:
  5. //#include &lt;stdio.h&gt;
  6. //#include &lt;stdlib.h&gt;
  7. //#include &lt;string.h&gt;
  8. //char* echo(char* s);
  9. import &quot;C&quot;
  10. import &quot;unsafe&quot;
  12. func main() {
  13. 	for i := 0; i &lt; 1000000000; i++ {
  14. 		allocateAndDeallocate()
  15. 	}
  16. }
  18. func allocateAndDeallocate() {
  19. 	cs := C.CString(&quot;Hello from stdio\n&quot;)
  20. 	defer C.free(unsafe.Pointer(cs))
  21. 	var echoOut *C.char = C.echo(cs)
  22. 	defer C.free(unsafe.Pointer(echoOut)) // no crash here
  23. }

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

  1. //algo.cpp
  2. #include &lt;stdlib.h&gt;
  3. #include &lt;stdio.h&gt;
  4. #include &lt;string.h&gt;
  5. #include &lt;iostream&gt;
  6. #include &lt;math.h&gt;
  8. using namespace std;
  10. extern &quot;C&quot; {
  12. 	char* echo(char* o) {
  13.         int len = sizeof(o) / sizeof(char);
  14.         char* out = (char*)malloc(len * sizeof(char));
  15.         strcpy(out, o);
  16.         return out;
  17.     }
  19. }

huangapple
  • 本文由 发表于 2016年2月19日 22:18:45
  • 转载请务必保留本文链接：https://go.coder-hub.com/35507467.html
  • cgo
  • go
  • malloc
  • memory-leaks
  • memory-management
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