It's clear from the C standards that NULL is an acceptable input to free(), with the result being no operation performed. However, this seems to violate the idea that free() must only be called on memory that was explicitly allocated using malloc(), calloc(), or realloc().

What's the reasoning behind this exception? The only reason I can figure is so that callers do not have to explicitly check for NULL before calling free().

CodePudding user response：

The malloc function (as well as calloc and realloc) may return a NULL pointer if the requested size is 0. Therefore, it makes sense that NULL pointer can also be passed to free since it was returned from a successful call to malloc:

Section 7.22.3p1 of the C standard specifies this behavior:

If the space cannot be allocated, a null pointer is returned. If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.