c - 避免 POSIX 线程中的内存泄漏

Question

我开始了解 POSIX 线程以及从通过 pthread_create 调用的函数中释放内存，该函数内部有一个 malloc 调用。所以我来到了以下程序:

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

void *foo( void *vargp );

int main( void )
{
    pthread_t thread_id[5];
    const char *msg = "Hello\n";
    char **ptr = NULL;
    printf("Before Thread\n");

    for( int i = 0 ; i < 5 ; i++ )
    {
        pthread_create(&thread_id[i], NULL, foo, (void*)msg);
    }

    for( int i = 0 ; i < 5 ; i++ )
    {
        pthread_join(thread_id[i], ( void* )&ptr );
        free( ptr );
    }

    printf("After Thread\n");
}

void *foo( void *vargp )
{
    char *ptr = malloc( 256 * sizeof( * ptr ) );
    strcpy( ptr, (char*)vargp );
    printf("The message is: %s \n", ptr );
    pthread_exit( ptr );
}

在我运行 valgrind 之后:

valgrind --leak-check=full --track-origins=yes --show-leak-kinds=all ./Demo

我得到以下输出，这似乎对内存没问题，valgrind 没有报告泄漏:

==5255== Memcheck, a memory error detector
==5255== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==5255== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==5255== Command: ./Demo
==5255== 
Before Thread
The message is: Hello

The message is: Hello

The message is: Hello

The message is: Hello

The message is: Hello

After Thread
==5255== 
==5255== HEAP SUMMARY:
==5255==     in use at exit: 0 bytes in 0 blocks
==5255==   total heap usage: 16 allocs, 16 frees, 5,358 bytes allocated
==5255== 
==5255== All heap blocks were freed -- no leaks are possible
==5255== 
==5255== For counts of detected and suppressed errors, rerun with: -v
==5255== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

如果我从这里删除 de \n，现在这里是我不明白的地方:

const char *msg = "Hello\n";

然后我再次编译并运行 valgrind，我得到:

==5279== Memcheck, a memory error detector
==5279== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==5279== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==5279== Command: ./Demo
==5279== 
Before Thread
The message is: Hello 
The message is: Hello 
The message is: Hello 
The message is: Hello 
The message is: Hello 
After Thread
==5279== 
==5279== HEAP SUMMARY:
==5279==     in use at exit: 1,638 bytes in 4 blocks
==5279==   total heap usage: 16 allocs, 12 frees, 5,358 bytes allocated
==5279== 
==5279== 36 bytes in 1 blocks are still reachable in loss record 1 of 4
==5279==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5279==    by 0x401D329: strdup (strdup.c:42)
==5279==    by 0x4018656: _dl_load_cache_lookup (dl-cache.c:315)
==5279==    by 0x4009382: _dl_map_object (dl-load.c:2255)
==5279==    by 0x4014EE3: dl_open_worker (dl-open.c:235)
==5279==    by 0x53C92DE: _dl_catch_exception (dl-error-skeleton.c:196)
==5279==    by 0x40147C9: _dl_open (dl-open.c:605)
==5279==    by 0x53C83AC: do_dlopen (dl-libc.c:96)
==5279==    by 0x53C92DE: _dl_catch_exception (dl-error-skeleton.c:196)
==5279==    by 0x53C936E: _dl_catch_error (dl-error-skeleton.c:215)
==5279==    by 0x53C84D8: dlerror_run (dl-libc.c:46)
==5279==    by 0x53C84D8: __libc_dlopen_mode (dl-libc.c:195)
==5279==    by 0x5055DEA: pthread_cancel_init (unwind-forcedunwind.c:52)
==5279== 
==5279== 36 bytes in 1 blocks are still reachable in loss record 2 of 4
==5279==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5279==    by 0x400C3E7: _dl_new_object (dl-object.c:163)
==5279==    by 0x40069A4: _dl_map_object_from_fd (dl-load.c:943)
==5279==    by 0x4008FFB: _dl_map_object (dl-load.c:2389)
==5279==    by 0x4014EE3: dl_open_worker (dl-open.c:235)
==5279==    by 0x53C92DE: _dl_catch_exception (dl-error-skeleton.c:196)
==5279==    by 0x40147C9: _dl_open (dl-open.c:605)
==5279==    by 0x53C83AC: do_dlopen (dl-libc.c:96)
==5279==    by 0x53C92DE: _dl_catch_exception (dl-error-skeleton.c:196)
==5279==    by 0x53C936E: _dl_catch_error (dl-error-skeleton.c:215)
==5279==    by 0x53C84D8: dlerror_run (dl-libc.c:46)
==5279==    by 0x53C84D8: __libc_dlopen_mode (dl-libc.c:195)
==5279==    by 0x5055DEA: pthread_cancel_init (unwind-forcedunwind.c:52)
==5279== 
==5279== 384 bytes in 1 blocks are still reachable in loss record 3 of 4
==5279==    at 0x4C31B25: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5279==    by 0x4011E85: _dl_check_map_versions (dl-version.c:274)
==5279==    by 0x401524B: dl_open_worker (dl-open.c:284)
==5279==    by 0x53C92DE: _dl_catch_exception (dl-error-skeleton.c:196)
==5279==    by 0x40147C9: _dl_open (dl-open.c:605)
==5279==    by 0x53C83AC: do_dlopen (dl-libc.c:96)
==5279==    by 0x53C92DE: _dl_catch_exception (dl-error-skeleton.c:196)
==5279==    by 0x53C936E: _dl_catch_error (dl-error-skeleton.c:215)
==5279==    by 0x53C84D8: dlerror_run (dl-libc.c:46)
==5279==    by 0x53C84D8: __libc_dlopen_mode (dl-libc.c:195)
==5279==    by 0x5055DEA: pthread_cancel_init (unwind-forcedunwind.c:52)
==5279==    by 0x5055FD3: _Unwind_ForcedUnwind (unwind-forcedunwind.c:126)
==5279==    by 0x5053F0F: __pthread_unwind (unwind.c:121)
==5279== 
==5279== 1,182 bytes in 1 blocks are still reachable in loss record 4 of 4
==5279==    at 0x4C31B25: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==5279==    by 0x400C100: _dl_new_object (dl-object.c:73)
==5279==    by 0x40069A4: _dl_map_object_from_fd (dl-load.c:943)
==5279==    by 0x4008FFB: _dl_map_object (dl-load.c:2389)
==5279==    by 0x4014EE3: dl_open_worker (dl-open.c:235)
==5279==    by 0x53C92DE: _dl_catch_exception (dl-error-skeleton.c:196)
==5279==    by 0x40147C9: _dl_open (dl-open.c:605)
==5279==    by 0x53C83AC: do_dlopen (dl-libc.c:96)
==5279==    by 0x53C92DE: _dl_catch_exception (dl-error-skeleton.c:196)
==5279==    by 0x53C936E: _dl_catch_error (dl-error-skeleton.c:215)
==5279==    by 0x53C84D8: dlerror_run (dl-libc.c:46)
==5279==    by 0x53C84D8: __libc_dlopen_mode (dl-libc.c:195)
==5279==    by 0x5055DEA: pthread_cancel_init (unwind-forcedunwind.c:52)
==5279== 
==5279== LEAK SUMMARY:
==5279==    definitely lost: 0 bytes in 0 blocks
==5279==    indirectly lost: 0 bytes in 0 blocks
==5279==      possibly lost: 0 bytes in 0 blocks
==5279==    still reachable: 1,638 bytes in 4 blocks
==5279==         suppressed: 0 bytes in 0 blocks
==5279== 
==5279== For counts of detected and suppressed errors, rerun with: -v
==5279== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

所以我有 3 个问题:

1) - 我是否正确地 free() malloc

2) - 如果删除 \n

为什么会得到该输出

3) - 我是否(必须)free() foo 中的内存？

对我来说，我的程序(不知何故)是 UB。

Answer 1

1) 是的，你已经正确地释放了内存

2) 操作系统内部有时会做一些 valgrind 不理解的奇怪事情。如果您不想看到这些消息，那么您应该禁止规则 (http://valgrind.org/docs/manual/manual-core.html#manual-core.suppress)

3)不是，只要你在进程结束前释放内存，就不是内存泄漏