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 suc‐
cessfully 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(). Other‐
wise, or if free(ptr) has already been called before, undefined behav‐
ior 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 realloc()
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 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 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 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 non-zero 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.
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 pro‐
cesses will be killed by the infamous OOM killer. In case Linux is
employed under circumstances where it would be less desirable to sud‐
denly lose some randomly picked processes, and moreover the kernel ver‐
sion 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 3.05 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 2008-04-05 MALLOC(3)