void *realloc(void *p, size_t size);
realloc deallocates the old object pointed to by ptr and returns a pointer to a new object that has the size specified by size. The contents of the new object is identical to that of the old object prior to deallocation, up to the lesser of the new and old sizes. Any bytes in the new object beyond the size of the old object have indeterminate values.
If ptr is a null pointer, realloc behaves like realloc for the specified size. If memory for the new object cannot be allocated, the old object is not deallocated and its value is unchanged.
realloc returns a pointer to the new object (which may have the same value as a pointer to the old object), or a null pointer if the new object could not be allocated.
If ptr does not match a pointer earlier returned by calloc, malloc, or realloc, or if the space has been deallocated by a call to free or realloc, the behavior is undefined.