reallocptr 0 is not equivalent to freeptr

realloc is conceptually equivalent to malloc + memcpy + free on the other pointer.

If the size of the space requested is zero, the behavior of realloc is implementation-defined. This is similar for all memory allocation functions that receive a size parameter of value 0. Such functions may in fact return a non-null pointer, but that must never be dereferenced.

Thus, realloc(ptr,0) is not equivalent to free(ptr). It may

• be a “lazy” implementation and just return ptr
• free(ptr), allocate a dummy element and return that
• free(ptr) and return 0
• just return 0 for failure and do nothing else.

So in particular the latter two cases are indistinguishable by application code.

This means realloc(ptr,0) may not really free/deallocate the memory, and thus it should never be used as a replacement for free.