MALLOC(3)

NAME
malloc, free, realloc, calloc, alloca - memory allocator

SYNOPSIS
#include <sys/types.h>
#include <stdlib.h>
#include <alloca.h>
void *malloc(size_t size)
void free(void *ptr)
void *realloc(void *ptr, size_t size)
void *calloc(size_t nelem, size_t elsize)
void *alloca(size_t size)

DESCRIPTION
Malloc and free provide a general-purpose memory allocation package. Malloc returns a pointer to a block of at least size bytes beginning on a word boundary.

The argument to free is a pointer to a block previously allocated by malloc; this space is made available for further allocation, but its contents are left undisturbed. A call with a null ptr is legal and does nothing.

Needless to say, grave disorder will result if the space assigned by malloc is overrun or if some random number is handed to free.

Malloc maintains multiple lists of free blocks according to size, allocating space from the appropriate list. It calls sbrk (see brk(2)) to get more memory from the system when there is no suitable space already free.

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. A call with a null ptr is legal and has the same result as malloc(size).

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

Alloca allocates size bytes of space in the stack frame of the caller. This temporary space is automatically freed on return.

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

To debug malloc-related errors, specify the MALLOC_DEBUG variable in the environment of the program you want to debug. This causes an alternate malloc implementation to be used. This version allocates blocks at the end of random pages so that reads and writes past the end of the buffer cause SIGSEGV. On realloc or free calls, the area just before the buffer is verified to also detect writes before the start of the buffer. Buffer overflows in the BSS section are also more likely to be detected because the brk is never moved. Please note that this flags comes with a considerable performance penalty and dramatically increases memory usage.

SEE ALSO
brk(2).

DIAGNOSTICS
Malloc, realloc and calloc return a null pointer if there is no available memory or if the arena has been detectably corrupted by storing outside the bounds of a block.

NOTES
Other implementations of malloc, realloc or calloc may return a null pointer if the size of the requested block is zero. This implementation will always return a zero length block at a unique address, but you should keep in mind that a null return is possible if the program is run to another system and that this should not be mistakenly seen as an error.

BUGS
When realloc returns a null pointer, the block pointed to by ptr may be destroyed.

Alloca is machine dependent; its use is discouraged.