MALLOC(3) Linux Programmer's Manual MALLOC(3)NAME
malloc, free, calloc, realloc - Allocate and free dynamic memory
SYNOPSIS
#include <stdlib.h>
void *malloc(size_t size);
void free(void *ptr);
void *calloc(size_t nmemb, size_t size);
void *realloc(void *ptr, size_t size);
DESCRIPTION
The malloc() function allocates size bytes and returns a pointer to the
allocated memory. The memory is not initialized. If size is 0, then
malloc() returns either NULL, or a unique pointer value that can later
be successfully passed to free().
The free() function frees the memory space pointed to by ptr, which
must have been returned by a previous call to malloc(), calloc() or
realloc(). Otherwise, or if free(ptr) has already been called before,
undefined behavior occurs. If ptr is NULL, no operation is performed.
The calloc() function allocates memory for an array of nmemb elements
of size bytes each and returns a pointer to the allocated memory. The
memory is set to zero. If nmemb or size is 0, then calloc() returns
either NULL, or a unique pointer value that can later be successfully
passed to free().
The realloc() function changes the size of the memory block pointed to
by ptr to size bytes. The contents will be unchanged in the range from
the start of the region up to the minimum of the old and new sizes. If
the new size is larger than the old size, the added memory will not be
initialized. If ptr is NULL, then the call is equivalent to mal‐
loc(size), for all values of size; if size is equal to zero, and ptr is
not NULL, then the call is equivalent to free(ptr). Unless ptr is
NULL, it must have been returned by an earlier call to malloc(), cal‐
loc() or realloc(). If the area pointed to was moved, a free(ptr) is
done.
RETURN VALUE
The malloc() and calloc() functions return a pointer to the allocated
memory that is suitably aligned for any kind of variable. On error,
these functions return NULL. NULL may also be returned by a successful
call to malloc() with a size of zero, or by a successful call to cal‐
loc() with nmemb or size equal to zero.
The free() function returns no value.
The realloc() function returns a pointer to the newly allocated memory,
which is suitably aligned for any kind of variable and may be different
from ptr, or NULL if the request fails. If size was equal to 0, either
NULL or a pointer suitable to be passed to free() is returned. If
realloc() fails the original block is left untouched; it is not freed
or moved.
CONFORMING TO
C89, C99.
NOTES
By default, Linux follows an optimistic memory allocation strategy.
This means that when malloc() returns non-NULL there is no guarantee
that the memory really is available. In case it turns out that the
system is out of memory, one or more processes will be killed by the
OOM killer. For more information, see the description of
/proc/sys/vm/overcommit_memory and /proc/sys/vm/oom_adj in proc(5), and
the kernel source file Documentation/vm/overcommit-accounting.
Normally, malloc() allocates memory from the heap, and adjusts the size
of the heap as required, using sbrk(2). When allocating blocks of mem‐
ory larger than MMAP_THRESHOLD bytes, the glibc malloc() implementation
allocates the memory as a private anonymous mapping using mmap(2).
MMAP_THRESHOLD is 128 kB by default, but is adjustable using mal‐
lopt(3). Allocations performed using mmap(2) are unaffected by the
RLIMIT_DATA resource limit (see getrlimit(2)).
The UNIX 98 standard requires malloc(), calloc(), and realloc() to set
errno to ENOMEM upon failure. Glibc assumes that this is done (and the
glibc versions of these routines do this); if you use a private malloc
implementation that does not set errno, then certain library routines
may fail without having a reason in errno.
Crashes in malloc(), calloc(), realloc(), or free() are almost always
related to heap corruption, such as overflowing an allocated chunk or
freeing the same pointer twice.
Recent versions of Linux libc (later than 5.4.23) and glibc (2.x)
include a malloc() implementation which is tunable via environment
variables. When MALLOC_CHECK_ is set, a special (less efficient)
implementation is used which is designed to be tolerant against simple
errors, such as double calls of free() with the same argument, or over‐
runs of a single byte (off-by-one bugs). Not all such errors can be
protected against, however, and memory leaks can result. If MAL‐
LOC_CHECK_ is set to 0, any detected heap corruption is silently
ignored; if set to 1, a diagnostic message is printed on stderr; if set
to 2, abort(3) is called immediately; if set to 3, a diagnostic message
is printed on stderr and the program is aborted. Using a nonzero MAL‐
LOC_CHECK_ value can be useful because otherwise a crash may happen
much later, and the true cause for the problem is then very hard to
track down.
SEE ALSObrk(2), mmap(2), alloca(3), posix_memalign(3)COLOPHON
This page is part of release 3.35 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://man7.org/linux/man-pages/.
GNU 2011-09-08 MALLOC(3)
