char *realloc (ptr, size)
char *calloc(nelem, eisize)
unsigned elem, eisize;
The argument to free is a pointer to a block previously allocated by malloc; this space is made available for further allocation, but its contents are left undisturbed.
Grave disorder will result if the space assigned by malloc is overrun or if some random number is handed to free.
malloc allocates the first big enough contiguous reach of free space found in a circular search from the last block allocated or freed, coalescing adjacent free blocks as it searches. It calls sbrk (see brk(2P) ) to get more memory from the system when there is no suitable space already free.
realloc changes the size of the block pointed to by ptr to size bytes and returns a pointer to the (possibly moved) block. The contents will be unchanged up to lesser of the new and old sizes.
realloc also works if ptr points to a block freed since the last call of malloc, realloc, or calloc; thus sequences of free, malloc, and realloc can exploit the search strategy of malloc to do storage compaction.
calloc allocates space for an array of nelem elements of size eisize. The space is initialized to zeros.
Each of the allocation routines return a pointer to space suitably aligned (after possible pointer coercion) for storage of any type of object.