| MALLOC(3) | Library Functions Manual | MALLOC(3) | 
malloc, calloc,
  realloc, free —
#include <stdlib.h>
void *
  
  malloc(size_t
    size);
void *
  
  calloc(size_t
    number, size_t
    size);
void *
  
  realloc(void
    *ptr, size_t
  size);
void
  
  free(void
    *ptr);
malloc() function allocates
  size bytes of uninitialized memory. The allocated space
  is suitably aligned (after possible pointer coercion) for storage of any type
  of object.
The calloc() function allocates space for
    number objects, each size bytes
    in length. The result is identical to calling
    malloc() with an argument of “number *
    size”, with the exception that the allocated memory is explicitly
    initialized to zero bytes.
The realloc() function changes the size of
    the previously allocated memory referenced by ptr to
    size bytes. The contents of the memory are unchanged
    up to the lesser of the new and old sizes. If the new size is larger, the
    value of the newly allocated portion of the memory is undefined. Upon
    success, the memory referenced by ptr is freed and a
    pointer to the newly allocated memory is returned.
Note that realloc() may move the memory
    allocation, resulting in a different return value than
    ptr. If ptr is
    NULL, the realloc() function
    behaves identically to malloc() for the specified
    size.
The free() function causes the allocated
    memory referenced by ptr to be made available for
    future allocations. If ptr is
    NULL, no action occurs.
malloc() and calloc()
  functions return a pointer to the allocated memory if successful; otherwise a
  NULL pointer is returned and
  errno is set to ENOMEM.
The realloc() function returns a pointer,
    possibly identical to ptr, to the allocated memory if
    successful; otherwise a NULL pointer is returned,
    and errno is set to ENOMEM if
    the error was the result of an allocation failure. The
    realloc() function always leaves the original buffer
    intact when an error occurs. If size is 0, either
    NULL or a pointer that can be safely passed to
    free(3) is returned.
The free() function returns no value.
malloc(), be careful to avoid the following
  idiom:
if ((p = malloc(number * size)) == NULL) err(EXIT_FAILURE, "malloc");
The multiplication may lead to an integer overflow. To avoid this, reallocarr(3) is recommended.
If malloc() must be used, be sure to test
    for overflow:
if (size && number > SIZE_MAX / size) {
	errno = EOVERFLOW;
	err(EXIT_FAILURE, "allocation");
}
The above test is not sufficient in all cases. For example, multiplying ints requires a different set of checks:
int num, size; ... /* Avoid invalid requests */ if (size < 0 || num < 0) errc(1, EOVERFLOW, "overflow"); /* Check for signed int overflow */ if (size && num > INT_MAX / size) errc(1, EOVERFLOW, "overflow"); if ((p = malloc(size * num)) == NULL) err(1, "malloc");
Assuming the implementation checks for integer overflow as
    NetBSD does, it is much easier to use
    calloc() or
    reallocarr(3).
The above examples could be simplified to:
ptr = NULL; if ((e = reallocarr(&ptr, num, size))) errx(1, "reallocarr", strerror(e));
or at the cost of initialization: if ((p = calloc(num, size)) == NULL) err(1, "calloc");
When using realloc(), one must be careful
    to avoid the following idiom:
nsize += 50; if ((p = realloc(p, nsize)) == NULL) return NULL;
Do not adjust the variable describing how much memory has been
    allocated until it is known that the allocation has been successful. This
    can cause aberrant program behavior if the incorrect size value is used. In
    most cases, the above example will also leak memory. As stated earlier, a
    return value of NULL indicates that the old object
    still remains allocated. Better code looks like this:
newsize = size + 50;
if ((p2 = realloc(p, newsize)) == NULL) {
	if (p != NULL)
		free(p);
	p = NULL;
	return NULL;
}
p = p2;
size = newsize;
For the implementation details, see jemalloc(3).
malloc(), calloc(),
  realloc() and free() functions
  conform to ISO/IEC 9899:1990
  (“ISO C90”).
free() internal kernel function and a predecessor to
  malloc(), alloc(), first
  appeared in Version 1 AT&T UNIX. The C
  Library functions alloc() and
  free() appeared in Version 6
  AT&T UNIX. The functions malloc(),
  calloc(), and realloc() first
  appeared in Version 7 AT&T UNIX.
A new implementation by Chris Kingsley was introduced in 4.2BSD, followed by a complete rewrite by Poul-Henning Kamp (“phk's malloc” or “new malloc”) which appeared in FreeBSD 2.2 and was included in NetBSD 1.5 and OpenBSD 2.0. These implementations were all sbrk(2) based.
The jemalloc(3) allocator became the default system allocator first in FreeBSD 7.0 and then in NetBSD 5.0.
| June 1, 2016 | NetBSD 10.0 |