What is realloc in C?

In C, the most fundamental dynamic memory functions are malloc and free, provided in stdlib.h. The malloc (“memory allocate”) function allocates a block of memory on the heap; the free function frees it again. Their signatures are:

void* malloc(size_t size);
void free(void* ptr);

But there are a couple more: calloc and realloc.

void* calloc(size_t how_many, size_t num_elements);
void* realloc(void* original, size_t new_size);

The calloc (“clear alloc”) function behaves like malloc, with two differences:

The realloc (“reallocate”) function takes a pointer to an allocated block, and attempts to expand/contract that block to the new size. If it cannot, it allocates the requested size elsewhere, and copies the old version to it. If the new size is larger than the old, then the trailing memory is uninitialized.