c - 释放内存的正确方法

Question

我使用优先级队列创建程序，我需要在程序结束时释放所有内存。我使用 Valgrind 查找内存泄漏，并在这段代码中收到警告:

int len;

//vlozeni prvniho prvku
len = snprintf(NULL, 0, "%d,%d", startX, startY);
char *link = malloc(len + 1);
sprintf(link,"%d,%d", startX, startY);
priq_push(queue, link, 0);

while(1)
{
    for(some_condition)
    {
        if(another_condition)
        {
            len = snprintf(NULL, 0, "%d,%d", neighbourX, neighbourY);
            link = realloc(NULL, len + 1);
            sprintf(link,"%d,%d", neighbourX, neighbourY);
            priq_push(queue, link, elementHeight + heights[neighbourX][neighbourY]);
            break;
        }
    }

    if(f == 1)
    {
        break;
    }
}

free(link);

通过这段代码，我从 Valgrind 得到了这个警告:

4 bytes in 1 blocks are definitely lost in loss record 1 of 4
==9069==    at 0x4C2C857: malloc (vg_replace_malloc.c:291)
==9069==    by 0x401233: main (main.c:229)
==9069== 
==9069== 7,628,843 bytes in 932,764 blocks are definitely lost in loss record 4 of 4
==9069==    at 0x4C2C857: malloc (vg_replace_malloc.c:291)
==9069==    by 0x4C2C9CB: realloc (vg_replace_malloc.c:687)
==9069==    by 0x401554: main (main.c:305)

第 291 行首先是 malloc:char *link = malloc(len + 1);

我做错了什么？

编辑。

void priq_push(priority_queue q, void *data, int height)
{
    q_elem_t *b;
    int n, m;

    if (q->position >= q->alloc) 
    {
        q->alloc *= 2;
        b = q->buffer = realloc(q->buffer, sizeof(q_elem_t) * q->alloc);
    } 
    else
    {
        b = q->buffer;
    }

    n = q->position++;
    /* append at end, then up heap */
    while ((m = n / 2) && height < b[m].height)
    {
        b[n] = b[m];
        n = m;
    }
    b[n].data = data;
    b[n].height = height;
}

/* remove top item. returns 0 if empty. *priority can be null. */
void * priq_pop(priority_queue q, int *height)
{
    void *out;
    if (q->position == 1)
    {
        return 0;
    }

    q_elem_t *b = q->buffer;

    out = b[1].data;
    if (height)
    {
        *height = b[1].height;
    }

    /* pull last item to top, then down heap. */
    --q->position;

    int n = 1, m;
    while ((m = n * 2) < q->position) 
    {
        if (m + 1 < q->position && b[m].height > b[m + 1].height)
        {
            m++;
        }

        if (b[q->position].height <= b[m].height)
        {
            break;
        }
        b[n] = b[m];
        n = m;
    }

    b[n] = b[q->position];
    if (q->position < q->alloc / 2 && q->position >= 16)
    q->buffer = realloc(q->buffer, (q->alloc /= 2) * sizeof(b[0]));

    return out;
}

我自己写的函数，最后会释放队列

void free_queue(priority_queue queue)
{
    free(queue->buffer);
    free(queue);
}

Answer 1

优先级队列持有你传递给它的指针。这意味着队列必须拥有内存。您需要将项目从队列中拉出的代码，以便在处理每个项目时调用 free。

我们看不到该代码，它是实际调用 priq_pop 的代码。但是泄漏报告似乎表明调用 priq_pop 的代码没有释放返回的项目。

解决此问题的步骤是:

1. 将 realloc 替换为 malloc，后者等效但读起来更好。
2. 删除推送循环末尾的 free(link)。这是不正确的，因为队列拥有内存。
3. 每当您调用 priq_pop 并完成对返回项目的处理时，添加对 free() 的调用。