calloc() allocates memory for an array of nmemb elements of size bytes each and returns a pointer to the allocated memory. The memory is set to zero.
malloc() allocates size bytes and returns a pointer to the allocated memory. The memory is not cleared.
free() frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc() or realloc(). Otherwise, or if "free(""ptr") has already been called before, undefined behaviour occurs. If ptr is NULL, no operation is performed.
realloc() changes the size of the memory block pointed to by ptr to size bytes. The contents will be unchanged to the minimum of the old and new sizes; newly allocated memory will be uninitialized. If ptr is NULL, the call is equivalent to malloc(size); if size is equal to zero, the call is equivalent to "free(""ptr"). Unless ptr is NULL, it must have been returned by an earlier call to malloc(), calloc() or realloc().
For calloc()" and "malloc(), the value returned is a pointer to the allocated memory, which is suitably aligned for any kind of variable, or NULL if the request fails.
free() returns no value.
realloc() returns a pointer to the newly allocated memory, which is suitably aligned for any kind of variable and may be different from ptr, or NULL if the request fails. If size was equal to 0, either NULL or a pointer suitable to be passed to free() is returned. If realloc() fails the original block is left untouched - it is not freed or moved.
malloc(), calloc(), and realloc() to set malloc(), free() or realloc() are almost always related to heap corruption, such as overflowing MALLOC_CHECK_ is set, a special (less efficient) implementation is used which free() with the same argument, or overruns of a single byte (off-by-one abort() is called immediately. This can be useful because otherwise malloc() returns non-NULL there is no guarantee that the memory really