man pages section 3: Basic Library Functions

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Updated: July 2014



mapmalloc - memory allocator


cc [ flag ... ] file ... –lmapmalloc [ library ... ]
#include <stdlib.h>

void *malloc(size_t size);
void *calloc(size_t nelem, size_t elsize);
void free(void * ptr);
void *realloc(void *ptr, size_t size);


The collection of malloc functions in this library use mmap (2) instead of sbrk(2) for acquiring new heap space. The functions in this library are intended to be used only if necessary, when applications must call sbrk(), but need to call other library routines that might call malloc. The algorithms used by these functions are not sophisticated. There is no reclaiming of memory.

The malloc() and free() functions provide a simple general-purpose memory allocation package.

The malloc() function returns a pointer to a block of at least size bytes suitably aligned for any use.

The argument to free() is a pointer to a block previously allocated by malloc(), calloc() or realloc(). If ptr is a NULL pointer, no action occurs.

Undefined results will occur if the space assigned by malloc() is overrun or if some random number is handed to free().

The calloc() function allocates space for an array of nelem elements of size elsize. The space is initialized to zeros.

The realloc() function 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 the lesser of the new and old sizes. If the new size of the block requires movement of the block, the space for the previous instantiation of the block is freed. If the new size is larger, the contents of the newly allocated portion of the block are unspecified. If ptr is NULL, realloc() behaves like malloc() for the specified size. If size is 0 and ptr is not a null pointer, the space pointed to is freed.

Each of the allocation functions returns a pointer to space suitably aligned (after possible pointer coercion) for storage of any type of object.

The malloc() and realloc() functions will fail if there is not enough available memory.

Entry points for malloc_debug(), mallocmap(), mallopt(), mallinfo(), memalign(), and valloc() are empty routines, and are provided only to protect the user from mixing malloc () functions from different implementations.

Return Values

If there is no available memory, malloc(), realloc(), and calloc() return a null pointer. When realloc() returns NULL, the block pointed to by ptr is left intact. If size, nelem, or elsize is 0, a unique pointer to the arena is returned.


See attributes(5) for descriptions of the following attributes:


See Also

brk(2), getrlimit(2), mmap(2), realloc(3C), malloc(3MALLOC), attributes(5)