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1 | .\" (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de) |
2 | .\" | |
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. | |
6 | .\" | |
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. | |
11 | .\" | |
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13 | .\" manual page may be incorrect or out-of-date.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 | |
18 | .\" professionally. | |
19 | .\" | |
20 | .\" Formatted or processed versions of this manual, if unaccompanied by | |
21 | .\" the source, must acknowledge the copyright and authors of this work. | |
22 | .\" License. | |
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 | .\" | |
27 | .TH MALLOC 3 1993-04-04 "GNU" "Linux Programmer's Manual" | |
28 | .SH NAME | |
29 | calloc, malloc, free, realloc \- Allocate and free dynamic memory | |
30 | .SH SYNOPSIS | |
31 | .nf | |
32 | .B #include <stdlib.h> | |
33 | .sp | |
34 | .BI "void *calloc(size_t " "nmemb" ", size_t " "size" ); | |
35 | .nl | |
36 | .BI "void *malloc(size_t " "size" ); | |
37 | .nl | |
38 | .BI "void free(void " "*ptr" ); | |
39 | .nl | |
40 | .BI "void *realloc(void " "*ptr" ", size_t " "size" ); | |
41 | .fi | |
42 | .SH DESCRIPTION | |
63aa9df0 | 43 | .BR calloc () |
fea681da MK |
44 | allocates memory for an array of |
45 | .I nmemb | |
46 | elements of | |
47 | .I size | |
48 | bytes each and returns a pointer to the allocated memory. | |
49 | The memory is set to zero. | |
50 | .PP | |
63aa9df0 | 51 | .BR malloc () |
fea681da MK |
52 | allocates |
53 | .I size | |
54 | bytes and returns a pointer to the allocated memory. | |
55 | The memory is not cleared. | |
56 | .PP | |
63aa9df0 | 57 | .BR free () |
fea681da MK |
58 | frees the memory space pointed to by |
59 | .IR ptr , | |
60 | which must have been returned by a previous call to | |
63aa9df0 MK |
61 | .BR malloc (), |
62 | .BR calloc () | |
fea681da | 63 | or |
63aa9df0 | 64 | .BR realloc (). |
fea681da MK |
65 | Otherwise, or if |
66 | .BI "free(" "ptr" ) | |
67 | has already been called before, undefined behaviour occurs. | |
68 | If | |
69 | .I ptr | |
70 | is | |
71 | .BR NULL , | |
72 | no operation is performed. | |
73 | .PP | |
63aa9df0 | 74 | .BR realloc () |
fea681da MK |
75 | changes the size of the memory block pointed to by |
76 | .I ptr | |
77 | to | |
78 | .I size | |
79 | bytes. | |
80 | The contents will be unchanged to the minimum of the old and new sizes; | |
81 | newly allocated memory will be uninitialized. | |
82 | If | |
83 | .I ptr | |
84 | is | |
85 | .BR NULL , | |
86 | the call is equivalent to | |
87 | .BR malloc(size) ; | |
88 | if size is equal to zero, | |
89 | the call is equivalent to | |
90 | .BI "free(" "ptr" ) . | |
91 | Unless | |
92 | .I ptr | |
93 | is | |
94 | .BR NULL , | |
95 | it must have been returned by an earlier call to | |
63aa9df0 MK |
96 | .BR malloc (), |
97 | .BR calloc () | |
fea681da | 98 | or |
63aa9df0 | 99 | .BR realloc (). |
fea681da MK |
100 | If the area pointed to was moved, a |
101 | .BI "free(" "ptr" ) | |
102 | is done. | |
103 | .SH "RETURN VALUE" | |
104 | For | |
e1d6264d MK |
105 | .BR calloc () |
106 | and | |
107 | .BR malloc (), | |
fea681da MK |
108 | the value returned is a pointer to the allocated memory, which is suitably |
109 | aligned for any kind of variable, or | |
110 | .B NULL | |
111 | if the request fails. | |
112 | .PP | |
63aa9df0 | 113 | .BR free () |
fea681da MK |
114 | returns no value. |
115 | .PP | |
63aa9df0 | 116 | .BR realloc () |
fea681da MK |
117 | returns a pointer to the newly allocated memory, which is suitably |
118 | aligned for any kind of variable and may be different from | |
119 | .IR ptr , | |
120 | or | |
121 | .B NULL | |
122 | if the request fails. If | |
123 | .I size | |
124 | was equal to 0, either NULL or a pointer suitable to be passed to | |
31e9a9ec | 125 | .BR free () |
fea681da | 126 | is returned. If |
63aa9df0 | 127 | .BR realloc () |
c65433e6 | 128 | fails the original block is left untouched; it is not freed or moved. |
fea681da MK |
129 | .SH "CONFORMING TO" |
130 | ANSI-C | |
131 | .SH "SEE ALSO" | |
132 | .BR brk (2), | |
133 | .BR posix_memalign (3) | |
134 | .SH NOTES | |
135 | The Unix98 standard requires | |
63aa9df0 MK |
136 | .BR malloc (), |
137 | .BR calloc (), | |
fea681da MK |
138 | and |
139 | .BR realloc () | |
140 | to set | |
141 | .I errno | |
142 | to ENOMEM upon failure. Glibc assumes that this is done | |
143 | (and the glibc versions of these routines do this); if you | |
144 | use a private malloc implementation that does not set | |
145 | .IR errno , | |
146 | then certain library routines may fail without having | |
147 | a reason in | |
148 | .IR errno . | |
149 | .LP | |
150 | Crashes in | |
63aa9df0 MK |
151 | .BR malloc (), |
152 | .BR free () | |
fea681da | 153 | or |
63aa9df0 | 154 | .BR realloc () |
fea681da MK |
155 | are almost always related to heap corruption, such as overflowing |
156 | an allocated chunk or freeing the same pointer twice. | |
157 | .PP | |
158 | Recent versions of Linux libc (later than 5.4.23) and GNU libc (2.x) | |
159 | include a malloc implementation which is tunable via environment | |
160 | variables. When | |
161 | .BR MALLOC_CHECK_ | |
162 | is set, a special (less efficient) implementation is used which | |
163 | is designed to be tolerant against simple errors, such as double | |
164 | calls of | |
63aa9df0 | 165 | .BR free () |
fea681da MK |
166 | with the same argument, or overruns of a single byte (off-by-one |
167 | bugs). Not all such errors can be protected against, however, and | |
168 | memory leaks can result. | |
169 | If | |
170 | .BR MALLOC_CHECK_ | |
171 | is set to 0, any detected heap corruption is silently ignored; | |
172 | if set to 1, a diagnostic is printed on stderr; | |
173 | if set to 2, | |
63aa9df0 | 174 | .BR abort () |
fea681da MK |
175 | is called immediately. This can be useful because otherwise |
176 | a crash may happen much later, and the true cause for the problem | |
177 | is then very hard to track down. | |
178 | .SH BUGS | |
179 | By default, Linux follows an optimistic memory allocation strategy. | |
180 | This means that when | |
63aa9df0 | 181 | .BR malloc () |
fea681da MK |
182 | returns non-NULL there is no guarantee that the memory really |
183 | is available. This is a really bad bug. | |
184 | In case it turns out that the system is out of memory, | |
185 | one or more processes will be killed by the infamous OOM killer. | |
186 | In case Linux is employed under circumstances where it would be | |
187 | less desirable to suddenly lose some randomly picked processes, | |
188 | and moreover the kernel version is sufficiently recent, | |
189 | one can switch off this overcommitting behavior using a command like | |
190 | .RS | |
191 | # echo 2 > /proc/sys/vm/overcommit_memory | |
192 | .RE | |
193 | See also the kernel Documentation directory, files | |
194 | .I vm/overcommit-accounting | |
195 | and | |
196 | .IR sysctl/vm.txt . |