Storage Management

The functions malloc and calloc obtain blocks of memory dynamically.

void *malloc(size_t n)

returns a pointer to n bytes of uninitialized storage, or NULL if the request cannot be satisfied.

void *calloc(size_t n, size_t size)

returns a pointer to enough free space for an array of n objects of the specified size, or NULL if the request cannot be satisfied. The storage is initialized to zero.

The pointer returned by malloc or calloc has the proper alignment for the object in question, but it must be cast into the appropriate type, as in:

int *ip;
ip = (int *) calloc(n, sizeof(int));

free(p) frees the space pointed to by p, where p was originally obtained by a call to malloc or calloc. There are no restrictions on the order in which space is freed, but it is a ghastly error to free something not obtained by calling malloc or calloc.

It is also an error to use something after it has been freed. A typical but incorrect piece of code is this loop that frees items from a list:

for (p = head; p != NULL; p = p->next) /* WRONG */ 
    free(p);

The right way is to save whatever is needed before freeing:

for (p = head; p != NULL; p = q) {
    q = p->next;
    free(p);
}

Section 8.7 shows the implementation of a storage allocator like malloc, in which allocated blocks may be freed in any order.