c - 内存未释放但仍可访问，是否泄漏？

Question

通过检查 valgrind，我看到在终止我的程序后没有释放 5 个内存块，但它们仍然可以访问。我需要被它打扰吗？

它是如何发生的？

zhanwu@gelata:~/sandbox$ valgrind ./a.out
==2430== Memcheck, a memory error detector
==2430== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==2430== Using Valgrind-3.6.1 and LibVEX; rerun with -h for copyright info
==2430== Command: ./a.out
==2430== 
Hello world!
Thread1 returns 1
Thread2 returns 10
Thread3 returns 10
==2430== 
==2430== HEAP SUMMARY:
==2430==     in use at exit: 1,590 bytes in 5 blocks
==2430==   total heap usage: 14 allocs, 9 frees, 2,442 bytes allocated
==2430== 
==2430== LEAK SUMMARY:
==2430==    definitely lost: 0 bytes in 0 blocks
==2430==    indirectly lost: 0 bytes in 0 blocks
==2430==      possibly lost: 0 bytes in 0 blocks
==2430==    still reachable: 1,590 bytes in 5 blocks
==2430==         suppressed: 0 bytes in 0 blocks
==2430== Rerun with --leak-check=full to see details of leaked memory
==2430== 
==2430== For counts of detected and suppressed errors, rerun with: -v
==2430== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 4 from 4)

---

下面是我的代码，如果我打算释放这 5 个 block ，我该怎么做？

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

pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;

void* myfunction(void *ptr)
{
    static int n_call = 0;
    int *retval = malloc(sizeof(int));

    pthread_mutex_lock( &mutex1 );
    n_call++;
    *retval = n_call;
    pthread_mutex_unlock( &mutex1 );

    if(n_call < 2)
    {
        char *msg;
        msg = (char *)ptr;
        printf("%s\n", msg);

        return retval;
    }
    else
    {
        *retval = 10;
        pthread_exit(retval);
    }
}

int main(int argc, char *argv[])
{
    pthread_t t1, t2, t3;

    char *msg = "Hello world!";
    pthread_create(&t1, NULL, myfunction, (void *)msg);
    pthread_create(&t2, NULL, myfunction, (void *)msg);
    pthread_create(&t3, NULL, myfunction, (void *)msg);

    int **s1 = malloc(sizeof(int*));
    int **s2 = malloc(sizeof(int*));
    int **s3 = malloc(sizeof(int*));

    pthread_join(t1, (void **)s1);
    pthread_join(t2, (void **)s2);
    pthread_join(t3, (void **)s3);

    printf("Thread1 returns %d\n", **s1);
    printf("Thread2 returns %d\n", **s2);
    printf("Thread3 returns %d\n", **s3);

    free(*s1);
    free(*s2);
    free(*s3);

    free(s1);
    free(s2);
    free(s3);

    return 0;
}

Answer 1

不，这不是内存泄漏。
这意味着您的程序仍然引用稍后将释放的内存。

Valgrind FAQ 区分不同的消息如下:

With Memcheck's memory leak detector, what's the difference between "definitely lost", "indirectly lost", "possibly lost", "still reachable", and "suppressed"?

The details are in the Memcheck section of the user manual.

In short:

• definitely lost means your program is leaking memory -- fix those leaks!

• indirectly lost means your program is leaking memory in a pointer-based structure. (E.g. if the root node of a binary tree is "definitely lost", all the children will be "indirectly lost".) If you fix the definitely lost leaks, the indirectly lost leaks should go away.

• possibly lost means your program is leaking memory, unless you're doing funny things with pointers. This is sometimes reasonable.
  Use --show-possibly-lost=no if you don't want to see these reports.

• still reachable means your program is probably ok -- it didn't free some memory it could have. This is quite common and often reasonable.
  Don't use --show-reachable=yes if you don't want to see these reports.

• suppressed means that a leak error has been suppressed. There are some suppressions in the default suppression files. You can ignore suppressed errors.