if new_size is zero, the behavior is implementation defined (null pointer may be returned (in which case the old memory block may or may not be freed), or some non-null pointer may be returned that may not be used to access storage). Such usage is deprecated (via C DR 400).(since C17) (until C23) if new_size is zero, the behavior is undefined. (since C23) realloc is thread-safe: it behaves as though only accessing the memory locations visible through its argument, and not any static storage. A previous call to free or realloc that deallocates a region of memory synchronizes-with a call to any allocation function, including realloc that allocates the same or a part of the same region of memory. This synchronization occurs after any access to the memory by the deallocating function and before any access to the memory by realloc. There is a single total order of all allocation and deallocation functions operating on each particular region of memory.