man pages section 3: Basic Library Functions

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Updated: July 2014
 
 

realloc(3C)

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

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. If the space assigned by malloc() is overrun, the results are undefined.

The argument to free() is a pointer to a block previously allocated by malloc(), calloc(), or realloc(). After free() is executed, this space is made available for further allocation by the application, though not returned to the system. Memory is returned to the system only upon termination of the application. If ptr is a null pointer, no action occurs. If a random number is passed to free(), the results are undefined.

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

The memalign() function 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. The value of alignment must be a power of two and must be greater than or equal to the size of a word.

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 size is 0, the space pointed to by ptr is freed and a magic pointer is returned, which is considered as a NULL pointer if passed to free(). If ptr is NULL, realloc() behaves like malloc() for the specified size.

The valloc() function has the same effect as malloc(), except that the allocated memory will be aligned to a multiple of the value returned by sysconf(_SC_PAGESIZE).

The alloca() function 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. If the allocated block is beyond the current stack limit, the resulting behavior is undefined.

Return Values

Upon successful completion, each of the allocation functions returns a pointer to space suitably aligned (after possible pointer coercion) for storage of any type of object.

If there is no available memory, malloc(), realloc(), memalign(), valloc(), and calloc() return a null pointer. When realloc() is called with size > 0 and returns NULL, the block pointed to by ptr is left intact. If size, nelem, or elsize is 0, either a null pointer or a unique pointer that can be passed to free() is returned.

If malloc(), calloc(), or realloc() returns unsuccessfully, errno will be set to indicate the error. The free() function does not set errno.

Errors

The malloc(), calloc(), and realloc() functions will fail if:

ENOMEM

The physical limits of the system are exceeded by size bytes of memory which cannot be allocated.

EAGAIN

There is not enough memory available to allocate size bytes of memory; but the application could try again later.

Usage

Portable applications should avoid using valloc() but should instead use malloc() or mmap(2). On systems with a large page size, the number of successful valloc() operations might be 0.

These default memory allocation routines are safe for use in multithreaded applications but are not scalable. Concurrent accesses by multiple threads are single-threaded through the use of a single lock. Multithreaded applications that make heavy use of dynamic memory allocation should be linked with allocation libraries designed for concurrent access, such as libumem(3LIB) or libmtmalloc(3LIB). Applications that want to avoid using heap allocations (with brk(2)) can do so by using either libumem or libmapmalloc(3LIB). The allocation libraries libmalloc(3LIB) and libbsdmalloc(3LIB) are available for special needs.

Comparative features of the various allocation libraries can be found in the umem_alloc(3MALLOC) manual page.

Attributes

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

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
Safe
Standard
See below.

For malloc(), calloc(), free(), realloc(), and valloc(), see standards(5) .

See Also

brk(2), getrlimit(2), libbsdmalloc(3LIB), libmalloc(3LIB) , libmapmalloc(3LIB), libmtmalloc (3LIB), libumem(3LIB), umem_alloc(3MALLOC), watchmalloc(3MALLOC), attributes(5), standards(5)

Warnings

Undefined results will occur if the size requested for a block of memory exceeds the maximum size of a process's heap, which can be obtained with getrlimit(2)