1 .\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
3 .\" %%%LICENSE_START(VERBATIM)
4 .\" Permission is granted to make and distribute verbatim copies of this
5 .\" manual provided the copyright notice and this permission notice are
6 .\" preserved on all copies.
8 .\" Permission is granted to copy and distribute modified versions of this
9 .\" manual under the conditions for verbatim copying, provided that the
10 .\" entire resulting derived work is distributed under the terms of a
11 .\" permission notice identical to this one.
13 .\" Since the Linux kernel and libraries are constantly changing, this
14 .\" manual page may be incorrect or out-of-date. The author(s) assume no
15 .\" responsibility for errors or omissions, or for damages resulting from
16 .\" the use of the information contained herein. The author(s) may not
17 .\" have taken the same level of care in the production of this manual,
18 .\" which is licensed free of charge, as they might when working
21 .\" Formatted or processed versions of this manual, if unaccompanied by
22 .\" the source, must acknowledge the copyright and authors of this work.
25 .\" Modified Sat Jul 24 19:00:59 1993 by Rik Faith (faith@cs.unc.edu)
26 .\" Clarification concerning realloc, iwj10@cus.cam.ac.uk (Ian Jackson), 950701
27 .\" Documented MALLOC_CHECK_, Wolfram Gloger (wmglo@dent.med.uni-muenchen.de)
28 .\" 2007-09-15 mtk: added notes on malloc()'s use of sbrk() and mmap().
30 .\" FIXME . Review http://austingroupbugs.net/view.php?id=374
31 .\" to see what changes are required on this page.
33 .TH MALLOC 3 2019-03-06 "GNU" "Linux Programmer's Manual"
35 malloc, free, calloc, realloc \- allocate and free dynamic memory
38 .B #include <stdlib.h>
40 .BI "void *malloc(size_t " "size" );
41 .BI "void free(void " "*ptr" );
42 .BI "void *calloc(size_t " "nmemb" ", size_t " "size" );
43 .BI "void *realloc(void " "*ptr" ", size_t " "size" );
44 .BI "void *reallocarray(void " "*ptr" ", size_t " nmemb ", size_t " "size" );
48 Feature Test Macro Requirements for glibc (see
49 .BR feature_test_macros (7)):
57 Glibc 2.28 and earlier:
66 bytes and returns a pointer to the allocated memory.
67 .IR "The memory is not initialized" .
74 or a unique pointer value that can later be successfully passed to
79 function frees the memory space pointed to by
81 which must have been returned by a previous call to
88 has already been called before, undefined behavior occurs.
91 is NULL, no operation is performed.
95 function allocates memory for an array of
99 bytes each and returns a pointer to the allocated memory.
100 The memory is set to zero.
109 or a unique pointer value that can later be successfully passed to
111 If the multiplication of
115 would result in integer overflow, then
119 an integer overflow would not be detected in the following call to
121 with the result that an incorrectly sized block of memory would be allocated:
125 malloc(nmemb * size);
131 function changes the size of the memory block pointed to by
136 The contents will be unchanged in the range from the start of the region
137 up to the minimum of the old and new sizes.
138 If the new size is larger than the old size, the added memory will
143 is NULL, then the call is equivalent to
152 is not NULL, then the call is equivalent to
156 is NULL, it must have been returned by an earlier call to
161 If the area pointed to was moved, a
167 function changes the size of the memory block pointed to by
169 to be large enough for an array of
171 elements, each of which is
174 It is equivalent to the call
177 realloc(ptr, nmemb * size);
184 fails safely in the case where the multiplication would overflow.
185 If such an overflow occurs,
191 and leaves the original block of memory unchanged.
197 functions return a pointer to the allocated memory,
198 which is suitably aligned for any built-in type.
199 On error, these functions return NULL.
200 NULL may also be returned by a successful call to
205 or by a successful call to
215 function returns no value.
219 function returns a pointer to the newly allocated memory, which is suitably
220 aligned for any built-in type and may be different from
222 or NULL if the request fails.
225 was equal to 0, either NULL or a pointer suitable to be passed to
230 fails, the original block is left untouched; it is not freed or moved.
234 function returns a pointer to the newly allocated memory.
236 it returns NULL and the original block of memory is left untouched.
243 can fail with the following error:
247 Possibly, the application hit the
254 For an explanation of the terms used in this section, see
260 Interface Attribute Value
267 T} Thread safety MT-Safe
274 POSIX.1-2001, POSIX.1-2008, C89, C99.
277 is a nonstandard extension that first appeared in OpenBSD 5.6 and FreeBSD 11.0.
279 By default, Linux follows an optimistic memory allocation strategy.
282 returns non-NULL there is no guarantee that the memory really
284 In case it turns out that the system is out of memory,
285 one or more processes will be killed by the OOM killer.
286 For more information, see the description of
287 .IR /proc/sys/vm/overcommit_memory
289 .IR /proc/sys/vm/oom_adj
292 and the Linux kernel source file
293 .IR Documentation/vm/overcommit-accounting.rst .
297 allocates memory from the heap, and adjusts the size of the heap
300 When allocating blocks of memory larger than
304 implementation allocates the memory as a private anonymous mapping using
307 is 128\ kB by default, but is adjustable using
310 allocations performed using
312 were unaffected by the
315 since Linux 4.7, this limit is also enforced for allocations performed using
318 To avoid corruption in multithreaded applications,
319 mutexes are used internally to protect the memory-management
320 data structures employed by these functions.
321 In a multithreaded application in which threads simultaneously
322 allocate and free memory,
323 there could be contention for these mutexes.
324 To scalably handle memory allocation in multithreaded applications,
325 glibc creates additional
326 .IR "memory allocation arenas"
327 if mutex contention is detected.
328 Each arena is a large region of memory that is internally allocated
334 and managed with its own mutexes.
346 Glibc assumes that this is done
347 (and the glibc versions of these routines do this); if you
348 use a private malloc implementation that does not set
350 then certain library routines may fail without having
360 are almost always related to heap corruption, such as overflowing
361 an allocated chunk or freeing the same pointer twice.
365 implementation is tunable via environment variables; see
369 .\" http://g.oswego.edu/dl/html/malloc.html
370 .\" A Memory Allocator - by Doug Lea
372 .\" http://www.bozemanpass.com/info/linux/malloc/Linux_Heap_Contention.html
373 .\" Linux Heap, Contention in free() - David Boreham
375 .\" http://www.citi.umich.edu/projects/linux-scalability/reports/malloc.html
376 .\" malloc() Performance in a Multithreaded Linux Environment -
377 .\" Check Lever, David Boreham
385 .BR malloc_get_state (3),
388 .BR malloc_usable_size (3),
392 .BR posix_memalign (3)
394 For details of the GNU C library implementation, see
395 .UR https://sourceware.org/glibc/wiki/MallocInternals