1 .\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
3 .\" Permission is granted to make and distribute verbatim copies of this
4 .\" manual provided the copyright notice and this permission notice are
5 .\" preserved on all copies.
7 .\" Permission is granted to copy and distribute modified versions of this
8 .\" manual under the conditions for verbatim copying, provided that the
9 .\" entire resulting derived work is distributed under the terms of a
10 .\" permission notice identical to this one.
12 .\" Since the Linux kernel and libraries are constantly changing, this
13 .\" manual page may be incorrect or out-of-date. The author(s) assume no
14 .\" responsibility for errors or omissions, or for damages resulting from
15 .\" the use of the information contained herein. The author(s) may not
16 .\" have taken the same level of care in the production of this manual,
17 .\" which is licensed free of charge, as they might when working
20 .\" Formatted or processed versions of this manual, if unaccompanied by
21 .\" the source, must acknowledge the copyright and authors of this work.
23 .\" Modified Sat Jul 24 19:00:59 1993 by Rik Faith (faith@cs.unc.edu)
24 .\" Clarification concerning realloc, iwj10@cus.cam.ac.uk (Ian Jackson), 950701
25 .\" Documented MALLOC_CHECK_, Wolfram Gloger (wmglo@dent.med.uni-muenchen.de)
26 .\" 2007-09-15 mtk: added notes on malloc()'s use of sbrk() and mmap().
28 .TH MALLOC 3 2009-01-13 "GNU" "Linux Programmer's Manual"
30 calloc, malloc, free, realloc \- Allocate and free dynamic memory
33 .B #include <stdlib.h>
35 .BI "void *calloc(size_t " "nmemb" ", size_t " "size" );
37 .BI "void *malloc(size_t " "size" );
39 .BI "void free(void " "*ptr" );
41 .BI "void *realloc(void " "*ptr" ", size_t " "size" );
45 allocates memory for an array of
49 bytes each and returns a pointer to the allocated memory.
50 The memory is set to zero.
59 or a unique pointer value that can later be successfully passed to
65 bytes and returns a pointer to the allocated memory.
66 The memory is not cleared.
73 or a unique pointer value that can later be successfully passed to
77 frees the memory space pointed to by
79 which must have been returned by a previous call to
86 has already been called before, undefined behavior occurs.
89 is NULL, no operation is performed.
92 changes the size of the memory block pointed to by
97 The contents will be unchanged to the minimum of the old and new sizes;
98 newly allocated memory will be uninitialized.
101 is NULL, then the call is equivalent to
110 is not NULL, then the call is equivalent to
114 is NULL, it must have been returned by an earlier call to
119 If the area pointed to was moved, a
127 return a pointer to the allocated memory, which is suitably
128 aligned for any kind of variable.
129 On error, these functions return NULL.
130 NULL may also be returned by a successful call to
135 or by a successful call to
147 returns a pointer to the newly allocated memory, which is suitably
148 aligned for any kind of variable and may be different from
150 or NULL if the request fails.
153 was equal to 0, either NULL or a pointer suitable to be passed to
158 fails the original block is left untouched; it is not freed or moved.
164 allocates memory from the heap, and adjusts the size of the heap
167 When allocating blocks of memory larger than
171 implementation allocates the memory as a private anonymous mapping using
174 is 128 kB by default, but is adjustable using
176 .\" FIXME . there is no mallopt(3) man page yet.
177 Allocations performed using
179 are unaffected by the
184 The Unix98 standard requires
194 Glibc assumes that this is done
195 (and the glibc versions of these routines do this); if you
196 use a private malloc implementation that does not set
198 then certain library routines may fail without having
208 are almost always related to heap corruption, such as overflowing
209 an allocated chunk or freeing the same pointer twice.
211 Recent versions of Linux libc (later than 5.4.23) and glibc (2.x)
214 implementation which is tunable via environment variables.
217 is set, a special (less efficient) implementation is used which
218 is designed to be tolerant against simple errors, such as double
221 with the same argument, or overruns of a single byte (off-by-one
223 Not all such errors can be protected against, however, and
224 memory leaks can result.
227 is set to 0, any detected heap corruption is silently ignored;
228 if set to 1, a diagnostic message is printed on \fIstderr\fP;
231 is called immediately;
232 if set to 3, a diagnostic message is printed on \fIstderr\fP
233 and the program is aborted.
236 value can be useful because otherwise
237 a crash may happen much later, and the true cause for the problem
238 is then very hard to track down.
240 By default, Linux follows an optimistic memory allocation strategy.
243 returns non-NULL there is no guarantee that the memory really
245 This is a really bad bug.
246 In case it turns out that the system is out of memory,
247 one or more processes will be killed by the infamous OOM killer.
248 In case Linux is employed under circumstances where it would be
249 less desirable to suddenly lose some randomly picked processes,
250 and moreover the kernel version is sufficiently recent,
251 one can switch off this overcommitting behavior using a command like:
255 .RB "#" " echo 2 > /proc/sys/vm/overcommit_memory"
259 See also the kernel Documentation directory, files
260 .I vm/overcommit-accounting
268 .BR posix_memalign (3)