c - 结构中大小的读取无效

Question

我有和这个几乎相同的问题: Getting data from pointer in struct "Invalid read/write"

但是当我尝试遵循这些建议时，我仍然遇到相同的无效读取大小。

我的结构如下:

typedef struct{
   int lenght;
   int max_lenght;
   int extract;
   int inserting;
   void** structure;
} queue_t;

我的循环缓冲区代码:

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

#include "queue.h"

 /* creates a new queue with a given size */
queue_t* create_queue(int capacity){

    queue_t* queue = malloc (sizeof(queue_t));

    queue->lenght = 0;
    queue -> max_lenght = capacity;
    queue -> extract = 0;
    queue -> inserting = 0;
    queue -> structure = malloc(sizeof(void*) * capacity);
    return queue;
}
/* deletes the queue and all allocated memory */
void delete_queue(queue_t *queue){
    free(queue->structure);
    free(queue);
}

/*
 * inserts a reference to the element into the queue
 * returns: true on success; false otherwise
 */
bool push_to_queue(queue_t* queue, void* data){
    bool succes;
    if ((queue -> max_lenght) <= (queue -> lenght)){
        succes = false;
    }
    else{
        if (queue -> inserting == queue->max_lenght){
            queue -> inserting = 0;
        }
        queue -> structure[queue -> inserting] = data;
        queue -> inserting += 1;
        queue -> lenght += 1;
        succes = true;
    }
    return succes;
}

/*
 * gets the first element from the queue and removes it from the queue
 * returns: the first element on success; NULL otherwise
 */
void* pop_from_queue(queue_t *queue){
    void* element;
    if ((queue->lenght) <= 0){
        element = NULL;
    }
    else{
        element = queue -> structure[queue-> extract];
        queue -> extract += 1;
        queue -> lenght -= 1;
        if(queue -> extract == queue -> max_lenght){
            queue -> extract = 0;
        }
    }
    return element;
}

/*
 * gets idx-th element from the queue
 * returns: the idx-th element on success; NULL otherwise
 */
void* get_from_queue(queue_t *queue, int idx){
    void* element;
    if(idx >= queue -> lenght){
        element = NULL;
    }
    else{
        if (queue -> extract + idx >= queue->max_lenght){
            element = &queue -> structure[queue->extract+idx - queue->         max_lenght];
        }
        else{
            element = &queue -> structure[queue-> extract+idx];
        }
    }
    return element;
}

/* gets number of stored elements */
int get_queue_size(queue_t *q){
    return q -> lenght;
}

当我尝试调用 pop_from_queue 时，我仍然收到来自 valgring 的消息，表明我位于数组之外。例如:

==236== Invalid read of size 4
==236==    at 0x4009C8: pop_from_queue (queue.c:53)
==236==    by 0x400721: pop (main.c:33)
==236==    by 0x400817: main (main.c:78)
==236==  Address 0x51fc040 is 0 bytes inside a block of size 24 free'd
==236==    at 0x4C2BD57: free (vg_replace_malloc.c:530)
==236==    by 0x40073D: pop (main.c:35)
==236==    by 0x400817: main (main.c:78)
==236==  Block was alloc'd at
==236==    at 0x4C2AC3D: malloc (vg_replace_malloc.c:299)
==236==    by 0x4008B8: create_queue (queue.c:10)
==236==    by 0x400798: main (main.c:57)
==236==
==236== Invalid read of size 8
==236==    at 0x4009DC: pop_from_queue (queue.c:57)
==236==    by 0x400721: pop (main.c:33)
==236==    by 0x400817: main (main.c:78)
==236==  Address 0x51fc050 is 16 bytes inside a block of size 24 free'd
==236==    at 0x4C2BD57: free (vg_replace_malloc.c:530)
==236==    by 0x40073D: pop (main.c:35)
==236==    by 0x400817: main (main.c:78)
==236==  Block was alloc'd at
==236==    at 0x4C2AC3D: malloc (vg_replace_malloc.c:299)
==236==    by 0x4008B8: create_queue (queue.c:10)
==236==    by 0x400798: main (main.c:57)
==236==
==236== Invalid read of size 4
==236==    at 0x4009E4: pop_from_queue (queue.c:57)
==236==    by 0x400721: pop (main.c:33)
==236==    by 0x400817: main (main.c:78)
==236==  Address 0x51fc048 is 8 bytes inside a block of size 24 free'd
==236==    at 0x4C2BD57: free (vg_replace_malloc.c:530)
==236==    by 0x40073D: pop (main.c:35)
==236==    by 0x400817: main (main.c:78)
==236==  Block was alloc'd at
==236==    at 0x4C2AC3D: malloc (vg_replace_malloc.c:299)
==236==    by 0x4008B8: create_queue (queue.c:10)
==236==    by 0x400798: main (main.c:57)
==236==
==236== Invalid read of size 4
==236==    at 0x4009FB: pop_from_queue (queue.c:58)
==236==    by 0x400721: pop (main.c:33)
==236==    by 0x400817: main (main.c:78)
==236==  Address 0x51fc048 is 8 bytes inside a block of size 24 free'd
==236==    at 0x4C2BD57: free (vg_replace_malloc.c:530)
==236==    by 0x40073D: pop (main.c:35)
==236==    by 0x400817: main (main.c:78)
==236==  Block was alloc'd at
==236==    at 0x4C2AC3D: malloc (vg_replace_malloc.c:299)
==236==    by 0x4008B8: create_queue (queue.c:10)
==236==    by 0x400798: main (main.c:57)
==236==
etc.

我是结构初学者，因此欢迎任何帮助。

编辑:这些错误位于以下行:

53 if ((queue->lenght) <= 0){
57 element = queue -> structure[queue-> extract];
58 queue -> extract += 1;
59 queue -> lenght -= 1;
60 if(queue -> extract == queue -> max_lenght){
89 return q -> lenght;

我的程序主要方法:

#include "stdio.h"
#include "stdlib.h"
#include "string.h"

#include "queue.h"


/* allocate new integer with value a and add it to the queue */
void add(int a, queue_t *queue)
{
   int *p = (int*)malloc(sizeof(int));
   *p = a;
   bool ret = push_to_queue(queue, (void*)p);
   if (!ret) {
      // free memory on failure
      free(p);
   }
}

/* print the int value on pointer p */
void print_int(void *p)
{
   if(p != NULL){
      printf("%d\n", *((int*)p));
   } else {
      printf("NULL\n");
   }
}

/* pop from the queue, print and free the element */
void pop(queue_t *queue)
{
   void *p = pop_from_queue(queue);
   print_int(p);
   free(queue);
}

/* get i-th element and print it (do not remove them) */
void get(int idx, queue_t *queue)
{
   print_int(get_from_queue(queue, idx));
}

/* 
 * TEST PROGRAM
 * - reads commands from stdin and executes them in the queue
 */
int main(int argc, char *argv[])
{
   int n;
   /* the tested queue */
   queue_t *queue;

   // read the size of the queue
   scanf("%d", &n);
   // create queue
   queue = create_queue(n);

   while (true) {
      char s[2];
      // read one command
      int ret = scanf("%1s", s);
      if (ret != 1) {
     break;
      }

      // add command
      if (s[0] == 'a') {
     int a;
     // read the argument of the command
     ret = scanf("%d", &a);
     if (ret != 1) {
        break;
     }
     add(a, queue);
     // remove command   
      } else if (s[0] == 'r') {
     pop(queue);
     // get command  
      } else if (s[0] == 'g') {
     int a;
     // read the argument of the command
     ret = scanf("%d", &a);
     if (ret != 1) {
        break;
     }
     get(a, queue);
      }
   }

   // remove rest of the elements in the queue
   while (get_queue_size(queue)) {
      void *p = pop_from_queue(queue);
      free(p);
   }

   // free memory
   delete_queue(queue);
   queue = NULL;

   // return 0 on succes
   return 0;
}

Answer 1

通过肉眼查看代码，我发现了一些直接的问题。

/* pop from the queue, print and free the element */
void pop(queue_t *queue)
{
    void *p = pop_from_queue(queue);
    print_int(p);
    free(queue);
}

我不认为你的意思是释放整个 queue在这里，而是p .

<小时/>

void delete_queue(queue_t *queue){
    free(queue->structure);
    free(queue);
}

自 queue->structure是一个指针列表，这只释放列表的内存。它指向的内存仍然需要释放。这可能是调用者的责任，但也可以将其转移到队列上。

对于这样的通用结构，您通常会为该结构提供一个函数指针，该指针知道如何释放队列中的内存。一个很好的例子，看看 the initializer of GLib's pointer arrays take a destroy function .

<小时/>

bool push_to_queue(queue_t* queue, void* data){
    bool succes;
    if ((queue -> max_lenght) <= (queue -> lenght)){
        succes = false;
    }
    else{
        if (queue -> inserting == queue->max_lenght){
            queue -> inserting = 0;
        }
        queue -> structure[queue -> inserting] = data;
        queue -> inserting += 1;
        queue -> lenght += 1;
        succes = true;
    }
    return succes;
}

if ((queue -> max_lenght) <= (queue -> lenght))包含无效状态，其中 queue -> max_lenght 小于 queue->lenght 。这绝对不应该发生。

最好使用 assert 明确检查所有内容是否有效。 。这是一个调试语句，断言必须为真，如 assert( queue->length <= queue->max_length ) 。如果不是，程序将崩溃并通知您断言失败。否则，您的代码会将尝试插入太多元素和已经拥有太多元素视为相同。

在push_to_queue开头处放置该断言您可以查看if( queue->max_length == queue->length ) .

<小时/>

我建议您在尝试在更大的程序中使用队列库之前对其进行单元测试。使用正常情况和边缘情况测试每种方法。例如...

void test_delete_queue() {
    queue_t *q = create_queue(3);

    int nums[3] = {4,5,6};
    for( int i = 0; i < 3; i++ ) {
        push_to_queue(q, &num);
    }

    delete_queue(q);
}

虽然这似乎不包含任何测试，但它可以让您知道 delete_queue不会出现段错误，并且使用 valgrind 运行它会检测到任何泄漏。

再举个例子，在阅读你的代码时，我对 queue->inserting 高度怀疑。和queue->extracting 。在我看来，如果你推和弹出足够多，它们就会不同步。所以我测试了它。而且，令我惊讶的是，它有效!现在我们确信这不是问题。

void test_push_pop() {
    queue_t *q = create_queue(3);

    int nums[4] = {10, 20, 30, 40};

    /* Push twice then pop once */
    assert( push_to_queue(q, &nums[0]) );
    assert( push_to_queue(q, &nums[1]) );
    assert( (int*)pop_from_queue(q) == &nums[0] );

    /* Push and pop again */
    assert( push_to_queue(q, &nums[2]) );
    assert( (int*)pop_from_queue(q) == &nums[1] );

    /* Push one more than the max length. This should be ok
       as we've already popped twice */
    assert( push_to_queue(q, &nums[3] ) );
    assert( (int*)pop_from_queue(q) == &nums[2] );
    assert( (int*)pop_from_queue(q) == &nums[3] );

    assert( get_queue_size(q) == 0 );

    delete_queue(q);
}

但是对 pop 进行了类似的测试行不通，因为它会释放整个队列。