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malloc(3)
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);
void *reallocarray(void *ptr, size_t nmemb, size_t size);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
reallocarray(): _GNU_SOURCE
Since glibc 2.29:
_DEFAULT_SOURCE
Glibc 2.28 and earlier:
_GNU_SOURCE
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(). If the multiplication of nmemb and size would result
in integer overflow, then calloc() returns an error. By contrast, an
integer overflow would not be detected in the following call to mal‐
loc(), with the result that an incorrectly sized block of memory would
be allocated:
malloc(nmemb * size);
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.
The reallocarray() function changes the size of the memory block
pointed to by ptr to be large enough for an array of nmemb elements,
each of which is size bytes. It is equivalent to the call
realloc(ptr, nmemb * size);
However, unlike that realloc() call, reallocarray() fails safely in the
case where the multiplication would overflow. If such an overflow
occurs, reallocarray() returns NULL, sets errno to ENOMEM, and leaves
the original block of memory unchanged.
RETURN VALUE
The malloc() and calloc() functions return a pointer to the allocated
memory, which is suitably aligned for any built-in type. 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 built-in type 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.
On success, the reallocarray() function returns a pointer to the newly
allocated memory. On failure, it returns NULL and the original block
of memory is left untouched.
ERRORS
calloc(), malloc(), realloc(), and reallocarray() can fail with the
following error:
ENOMEM Out of memory. Possibly, the application hit the RLIMIT_AS or
RLIMIT_DATA limit described in getrlimit(2).
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
allbox; lbw20 lb lb l l l. Interface Attribute Value T{ malloc(),
free(),
calloc(), realloc() T} Thread safety MT-Safe
CONFORMING TO
malloc(), free(), calloc(), realloc(): POSIX.1-2001, POSIX.1-2008, C89,
C99.
reallocarray() is a nonstandard extension that first appeared in Open‐
BSD 5.6 and FreeBSD 11.0.
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 Linux kernel source file Documentation/vm/overcommit-account‐
ing.rst.
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). Prior to Linux 4.7 allocations performed using mmap(2) were
unaffected by the RLIMIT_DATA resource limit; since Linux 4.7, this
limit is also enforced for allocations performed using mmap(2).
To avoid corruption in multithreaded applications, mutexes are used
internally to protect the memory-management data structures employed by
these functions. In a multithreaded application in which threads
simultaneously allocate and free memory, there could be contention for
these mutexes. To scalably handle memory allocation in multithreaded
applications, glibc creates additional memory allocation arenas if
mutex contention is detected. Each arena is a large region of memory
that is internally allocated by the system (using brk(2) or mmap(2)),
and managed with its own mutexes.
SUSv2 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.
The malloc() implementation is tunable via environment variables; see
mallopt(3) for details.
SEE ALSO
valgrind(1), brk(2), mmap(2), alloca(3), malloc_get_state(3),
malloc_info(3), malloc_trim(3), malloc_usable_size(3), mallopt(3),
mcheck(3), mtrace(3), posix_memalign(3)
For details of the GNU C library implementation, see
⟨https://sourceware.org/glibc/wiki/MallocInternals⟩.
COLOPHON
This page is part of release 5.02 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2019-03-06 MALLOC(3)