NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON
MALLOC(3) Linux Programmer's Manual MALLOC(3)
calloc, malloc, free, realloc - Allocate and free dynamic memory
#include <stdlib.h>
void *calloc(size_t nmemb, size_t size);
void *malloc(size_t size);
void free(void *ptr);
void *realloc(void *ptr, size_t size);
calloc() 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().
malloc() allocates size bytes and returns a pointer to the allocated memory.
The memory is not cleared. If size is 0, then malloc() returns either NULL,
or a unique pointer value that can later be successfully passed to free().
free() 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.
realloc() changes the size of the memory block pointed to by ptr to size
bytes. The contents will be unchanged to the minimum of the old and new
sizes; newly allocated memory will be uninitialized. If ptr is NULL, then the
call is equivalent to malloc(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(),
calloc() or realloc(). If the area pointed to was moved, a free(ptr) is done.
For calloc() and malloc(), return a pointer to the allocated memory, which 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 calloc() with nmemb or size equal to zero.
free() returns no value.
realloc() 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.
C89, C99.
Normally, malloc() allocates memory from the heap, and adjusts the size of the
heap as required, using sbrk(2). When allocating blocks of memory 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 mallopt(3). Allocations performed using
mmap(2) are unaffected by the RLIMIT_DATA resource limit (see getrlimit(2)).
The Unix98 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 overruns of a single byte (off-by-one bugs).
Not all such errors can be protected against, however, and memory leaks can
result. If MALLOC_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 non-zero MALLOC_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.
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. This is a really bad bug. In case it turns out
that the system is out of memory, one or more processes will be killed by the
infamous OOM killer. In case Linux is employed under circumstances where it
would be less desirable to suddenly lose some randomly picked processes, and
moreover the kernel version is sufficiently recent, one can switch off this
overcommitting behavior using a command like:
# echo 2 > /proc/sys/vm/overcommit_memory
See also the kernel Documentation directory, files vm/overcommit-accounting
and sysctl/vm.txt.
brk(2), mmap(2), alloca(3), posix_memalign(3)
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
GNU 2009-01-13 MALLOC(3)