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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
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MALLOC(3) Linux Programmer's Manual MALLOC(3)
malloc, free, calloc, realloc, reallocarray - allocate and free
dynamic memory
#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():
Since glibc 2.29:
_DEFAULT_SOURCE
Glibc 2.28 and earlier:
_GNU_SOURCE
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
malloc(), 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 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) (this behavior is nonportable;
see NOTES). 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.
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.
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 calloc() 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, or NULL
if the request failed. The returned pointer may be the same as
ptr if the allocation was not moved (e.g., there was room to
expand the allocation in-place), or different from ptr if the
allocation was moved to a new address. 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.
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).
reallocarray() first appeared in glibc in version 2.26.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│malloc(), free(), calloc(), realloc() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
malloc(), free(), calloc(), realloc(): POSIX.1-2001,
POSIX.1-2008, C89, C99.
reallocarray() is a nonstandard extension that first appeared in
OpenBSD 5.6 and FreeBSD 11.0.
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-accounting.rst.
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). 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.
Nonportable behavior
The behavior of realloc() when size is equal to zero, and ptr is
not NULL, is glibc specific; other implementations may return
NULL, and set errno. Portable POSIX programs should avoid it.
See realloc(3p).
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⟩.
This page is part of release 5.13 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 2021-03-22 MALLOC(3)
Pages that refer to this page: memusage(1), brk(2), clone(2), getrlimit(2), io_uring_register(2), mlock(2), mmap(2), mremap(2), alloca(3), argz_add(3), asprintf(3), ausearch_add_expression(3), avc_init(3), backtrace(3), basename(3), canonicalize_file_name(3), cfree(3), CPU_SET(3), dbopen(3), end(3), exec(3), fopen(3), fseek(3), fts(3), ftw(3), getcwd(3), getgrent(3), getgrnam(3), getifaddrs(3), getline(3), getpwent(3), getpwnam(3), glob(3), hsearch(3), if_nameindex(3), lber-memory(3), ldap_memory(3), mallinfo(3), malloc_get_state(3), malloc_hook(3), malloc_info(3), malloc_stats(3), malloc_trim(3), malloc_usable_size(3), mallopt(3), mcheck(3), mpool(3), mtrace(3), numa(3), open_memstream(3), pam_conv(3), pmaddderived(3), __pmaf(3), pmagetlog(3), pmapi(3), pmarewritedata(3), pmarewritemeta(3), pmdachildren(3), pmdafetch(3), pmdainstance(3), pmdalabel(3), pmdatext(3), pmdatrace(3), pmdiscoverservices(3), pmextractvalue(3), pmfault(3), pmfetch(3), pmfetcharchive(3), pmfetchgroup(3), pmfreelabelsets(3), pmfreeprofile(3), pmfreeresult(3), pmfstring(3), pmgetchildren(3), pmgetchildrenstatus(3), pmgetindom(3), pmgetindomarchive(3), pmlookupindomtext(3), pmlookuptext(3), pmnameall(3), pmnameid(3), pmnameindom(3), pmnameindomarchive(3), pmnewcontextzone(3), pmnewzone(3), pmnomem(3), __pmparsectime(3), pmparsehostattrsspec(3), pmparsehostspec(3), pmparseinterval(3), pmparsemetricspec(3), __pmparsetime(3), pmparsetimewindow(3), pmparseunitsstr(3), pmregisterderived(3), posix_memalign(3), pthread_setcancelstate(3), random_r(3), readdir(3), readline(3), realpath(3), scandir(3), scanf(3), sd_bus_creds_get_pid(3), sd_bus_error(3), sd_bus_path_encode(3), sd_get_seats(3), sd_journal_get_catalog(3), sd_journal_get_cursor(3), sd-login(3), sd_machine_get_class(3), sd_path_lookup(3), sd_pid_get_owner_uid(3), sd_seat_get_active(3), sd_session_is_active(3), sd_uid_get_state(3), seccomp_syscall_resolve_name(3), security_class_to_string(3), selabel_get_digests_all_partial_matches(3), selinux_boolean_sub(3), selinux_getpolicytype(3), selinux_raw_context_to_color(3), setbuf(3), strdup(3), string(3), tempnam(3), wcsdup(3), proc(5), environ(7), signal-safety(7), system_data_types(7)
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