2.13.3.4 realloc
Declaration:
void *realloc(void *ptr, size_tsize);
Attempts to resize the memory block pointed to by ptr that was previously allocated with a call to malloc or calloc. The contents pointed to by ptr are unchanged. If the value of size is greater than the previous size of the block, then the additional bytes have an undeterminate value. If the value of size is less than the previous size of the block, then the difference of bytes at the end of the block are freed. If ptr is null, then it behaves like malloc. If ptr points to a memory block that was not allocated with calloc or malloc, or is a space that has been deallocated, then the result is undefined. If the new space cannot be allocated, then the contents pointed to by ptr are unchanged. If size is zero, then the memory block is completely freed.
On success a pointer to the memory block is returned (which may be in a different location as before). On failure or if size is zero, a null pointer is returned.