The  malloc()  function allocates size bytes and returns a pointer
to the allocated memory.  The memory is not initialized.  If  size
is 0, then malloc() returns either NULL, or a unique pointer value
that can later be successfully passed to free().

The free() function frees the memory  space  pointed  to  by  ptr,
which must have been returned by a previous call to malloc(), cal‐
loc(), or realloc().  Otherwise, or if free(ptr) has already  been
called  before, undefined behavior occurs.  If ptr is NULL, no op‐
eration is performed.

The calloc() function allocates memory for an array of nmemb  ele‐
ments  of  size  bytes each and returns a pointer to the allocated
memory.  The memory is set to zero.  If nmemb or size is  0,  then
calloc()  returns  either NULL, or a unique pointer value that can
later be successfully passed to free().  If the multiplication  of
nmemb and size would result in integer overflow, then calloc() re‐
turns an error.  By contrast, an integer overflow would not be de‐
tected  in the following call to malloc(), with the result that an
incorrectly sized block of memory would be allocated:

   malloc(nmemb * size);