在 C 语言中释放一个 NULL 指针是一个好的做法吗？

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Ptr 为 NULL 的 free (ptr)是否损坏内存?

我正在编写一个 C 函数，它释放一个指针，如果它是 malloc()ed。指针可以是 NULL (在发生错误并且代码没有得到分配任何东西的机会的情况下) ，也可以用 malloc()分配。用 free(ptr);代替 if (ptr != NULL) free(ptr);安全吗？

gcc完全不抱怨，即使是 -Wall -Wextra -ansi -pedantic，但这是好的做法吗？

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最佳答案

Quoting the C standard, 7.20.3.2/2 from ISO-IEC 9899:

void free(void *ptr);

If ptr is a null pointer, no action occurs.

Don't check for NULL, it only adds more dummy code to read and is thus a bad practice.

However, you must always check for NULL pointers when using malloc & co. In that case NULL mean that something went wrong, most likely that no memory was available.

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See http://linux.die.net/man/3/free which states:

If ptr is NULL, no operation is performed.

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In my opinion, no, at least not in your case.

If you couldn't allocate memory, you should have checked that WAY before the call of free.

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It is good practice to not bother checking for NULL before calling free. Checking just adds unnecessary clutter to your code, and free(NULL) is guaranteed to be safe. From section 7.20.3.2/2 of the C99 standard:

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.

As noted in the comments, some people sometimes wonder if checking for NULL is more efficient than making a possibly unnecessary function call. However, this:

  • Is a premature micro-optimization.
  • Shouldn't matter. Checking for NULL first even might be a pessimization. For example, if 99% of the time your pointers aren't NULL, then there would be a redundant NULL check 99% of the time to avoid an extra function call 1% of the time.

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