valgrind --trace-children=yes 报告泄漏尽管 atexit 清理

Question

我正在尝试使用 valgrind 避免误报，但尽管使用了 --trace-children=yes，但我对 atexit() 和 fork() 的组合感到厌烦。我的代码：

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

static int * arr;

static void cleanup() {
    free(arr);
    printf("free arr as: %p\n", (void *)arr);
}

int main()
{
    arr = malloc(16 * sizeof(int));
    printf("allocated arr as: %p\n", (void *)arr);
    atexit(cleanup);

    pid_t pid = fork();
    if (pid == -1) {
        exit(1);
    } else if (pid == 0) {
        // child
        _exit(0);
    } else {
        // parent
        exit(0);
    }
}

命令行：

$ clang -Weverything leak.c 
$ valgrind --trace-children=yes ./a.out 
==3287== Memcheck, a memory error detector
==3287== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==3287== Using Valgrind-3.12.0 and LibVEX; rerun with -h for copyright info
==3287== Command: ./a.out
==3287== 
allocated arr as: 0x5202040
free arr as: 0x5202040
==3288== 
==3288== HEAP SUMMARY:
==3288==     in use at exit: 64 bytes in 1 blocks
==3288==   total heap usage: 2 allocs, 1 frees, 1,088 bytes allocated
==3288== 
==3288== LEAK SUMMARY:
==3288==    definitely lost: 0 bytes in 0 blocks
==3288==    indirectly lost: 0 bytes in 0 blocks
==3288==      possibly lost: 0 bytes in 0 blocks
==3288==    still reachable: 64 bytes in 1 blocks
==3288==         suppressed: 0 bytes in 0 blocks
==3288== Rerun with --leak-check=full to see details of leaked memory
==3288== 
==3288== For counts of detected and suppressed errors, rerun with: -v
==3288== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
==3287== 
==3287== HEAP SUMMARY:
==3287==     in use at exit: 0 bytes in 0 blocks
==3287==   total heap usage: 2 allocs, 2 frees, 1,088 bytes allocated
==3287== 
==3287== All heap blocks were freed -- no leaks are possible
==3287== 
==3287== For counts of detected and suppressed errors, rerun with: -v
==3287== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

根据 printf() 输出，看起来没有泄漏。我可以让 valgrind 相信这一点，还是应该将其添加到我的 valgrind 抑制文件中？

Answer 1

Based on the printf() output, it looks like there's no leaks. Can I convince valgrind of this, or should I just add this to my valgrind suppression file?

看来 valgrind 是对的。如果您将 printf() 输出解释为表明没有泄漏，那么您就不会欣赏 fork().

的效果

当您分叉一个 child 时，它会得到其 parent 地址 space 的完整副本。这通常通过 copy-on-write 页实现，但它仍然构成属于 child 的内存。在您的情况下，这包括 dynamically-allocated 数组 arr.

的副本

child 通过调用 _exit() 退出，因此虽然它继承了其 parent 的 exit-handler 注册，但在该进程中不会调用已注册的退出处理程序。你可以这么说，因为你只看到一次 cleanup() 的输出。因此，属于 child 的 arr 副本永远不会被释放，正如 valgrind 告诉您的那样。

不过，将此称为内存泄漏有点迂腐。当程序终止时，有问题的内存仍然可以访问，此时它会被系统回收。它根本没有在终止前显式释放。

Answer 2

您正在子进程中使用 _exit()。根据 _exit 的手册页：

The function _exit() is like exit(3), but does not call any functions registered with atexit(3) or on_exit(3).

改为exit(0)。应该可以。

valgrind --trace-children=yes 报告泄漏尽管 atexit 清理

valgrind --trace-children=yes reports leak despite atexit cleanup

c

static

valgrind

memory-leaks

atexit