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malloc(3C) manual page Table of Contents

Name

malloc, calloc, free, memalign, realloc, valloc, alloca - memory allocator

Synopsis

#include <stdlib.h>

void *malloc(size_t size);

void *calloc(size_t nelem, size_t elsize);

void free(void *ptr);

void *memalign(size_t alignment, size_t size);

void *realloc(void *ptr, size_t size);

void *valloc(size_t size);

#include <alloca.h>
void *alloca(size_t size);

Description

malloc() and free() provide a simple general-purpose memory allocation package. malloc() 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(). After free() is performed this space is made available for further allocation. 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().

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

memalign() allocates size bytes on a specified alignment boundary, and returns a pointer to the allocated block. The value of the returned address is guaranteed to be an even multiple of alignment. Note: the value of alignment must be a power of two, and must be greater than or equal to the size of a word.

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 the lesser of the new and old sizes. If ptr is NULL , realloc() behaves like malloc() for the specified size. If size is zero and ptr is not a null pointer, the object pointed to is freed.

valloc() is equivalent to memalign(sysconf(_SC_PAGESIZE),size).

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

malloc(), realloc(), memalign(), and valloc() will fail if there is not enough available memory.

alloca() allocates size bytes of space in the stack frame of the caller, and returns a pointer to the allocated block. This temporary space is automatically freed when the caller returns. Note: if the allocated block is beyond the current stack limit, the resulting behavior is undefined.

Return Values

If there is no available memory, malloc(), realloc(), memalign(), valloc(), 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.

Errors

If malloc(), calloc(), or realloc() returns unsuccessfully, errno will be set to indicate the following:

ENOMEM: size bytes of memory exceeds the physical limits of your system, and cannot be allocated.

EAGAIN: There is not enough memory available AT THIS POINT IN TIME to allocate size bytes of memory; but the application could try again later.

Warnings

Undefined results will occur if the size requested for a block of memory exceeds the maximum size of a process’s heap, which may be obtained with getrlimit().

alloca() is machine-, compiler-, and most of all, system-dependent. Its use is strongly discouraged.

Notes

Comparative Features of malloc(3C) , bsdmalloc(3X) , and malloc(3X) :

The bsdmalloc(3X) routines afford better performance, but are space-inefficient.
The malloc(3X) routines are space-efficient, but have slower performance.
The standard, fully SCD-compliant malloc routines are a trade-off between performance and space-efficiency.

free() does not set errno.

Table of Contents

Name
Synopsis
MT-Level
Description
Return Values
Errors
See Also
Warnings
Notes