如果在 C 中更改指针会释放多少内存? [英] How much memory would be freed if pointer is changed in C?

问题描述

假设有 20 个内存块，并且一个指针 p 指向第一个块.现在，当我这样做时:

Let's say there are 20 blocks of memory and a pointer p is pointing to the first block. Now, when I do:

p++;
free(p);

将释放多少内存块以及为什么?

How many blocks of memory would be freed and why?

推荐答案

不确定.不能说.您需要提供由 malloc() 返回的确切指针.

Un-deterministic. Cannot say. You need to supply the exact pointer which was returned by malloc().

将一个指向 free() 的指针传递给 malloc() [和 family] 没有返回的指针是 未定义的行为.

Passing a pointer to free() which is not returned by malloc() [and family] is undefined behaviour.

根据您的问题，如果 malloc()-ed 指针存储在 p 中，并且您执行 p++ 并调用 free(p);，那么 p 不再是 calloc 返回的指针>、malloc 或 realloc 函数.

As per your question, if the malloc()-ed pointer is stored in p, and you do p++ and call free(p);, then there p is not anymore a pointer returned by the calloc, malloc, or realloc function.

根据第 7.20.3.2 章，c99 标准，第 2 段

As per Chapter 7.20.3.2, c99 standard, 2nd paragraph

free 函数导致 ptr 指向的空间被释放，即可供进一步分配.如果 ptr 是空指针，则不会发生任何操作.否则，如果参数与之前由 calloc、malloc 或 realloc 函数返回的指针不匹配，或者如果空间已通过调用 free 或 realloc 释放，则行为未定义.

The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation. If ptr is a null pointer, no action occurs. Otherwise, if the argument does not match a pointer earlier returned by the calloc, malloc, or realloc function, or if the space has been deallocated by a call to free or realloc, the behavior is undefined.

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