man3/malloc.3

   1 .\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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  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 .\" 2007-09-15 mtk: added notes on malloc()'s use of sbrk() and mmap().
  27 .\"
  28 .TH MALLOC 3  2010-10-03 "GNU" "Linux Programmer's Manual"
  29 .SH NAME
  30 malloc, free, calloc, realloc \- Allocate and free dynamic memory
  31 .SH SYNOPSIS
  32 .nf
  33 .B #include <stdlib.h>
  34 .sp
  35 .BI "void *malloc(size_t " "size" );
  36 .BI "void free(void " "*ptr" );
  37 .BI "void *realloc(void " "*ptr" ", size_t "  "size" );
  38 .BI "void *calloc(size_t " "nmemb" ", size_t " "size" );
  39 .fi
  40 .SH DESCRIPTION
  41 .PP
  42 The
  43 .BR malloc ()
  44 function allocates
  45 .I size
  46 bytes and returns a pointer to the allocated memory.
  47 The memory is not cleared.
  48 If
  49 .I size
  50 is 0, then
  51 .BR malloc ()
  52 returns either NULL,
  53 .\" glibc does this:
  54 or a unique pointer value that can later be successfully passed to
  55 .BR free ().
  56 .PP
  57 The
  58 .BR free ()
  59 function frees the memory space pointed to by
  60 .IR ptr ,
  61 which must have been returned by a previous call to
  62 .BR malloc (),
  63 .BR calloc ()
  64 or
  65 .BR realloc ().
  66 Otherwise, or if
  67 .I free(ptr)
  68 has already been called before, undefined behavior occurs.
  69 If
  70 .I ptr
  71 is NULL, no operation is performed.
  72 .PP
  73 The
  74 .BR calloc ()
  75 function allocates memory for an array of
  76 .I nmemb
  77 elements of
  78 .I size
  79 bytes each and returns a pointer to the allocated memory.
  80 The memory is set to zero.
  81 If
  82 .I nmemb
  83 or
  84 .I size
  85 is 0, then
  86 .BR calloc ()
  87 returns either NULL,
  88 .\" glibc does this:
  89 or a unique pointer value that can later be successfully passed to
  90 .BR free ().
  91 .PP
  92 The
  93 .BR realloc ()
  94 function changes the size of the memory block pointed to by
  95 .I ptr
  96 to
  97 .I size
  98 bytes.
  99 The contents will be unchanged n the range from the start of the region
 100 up to the minimum of the old and new sizes;
 101 newly allocated memory will be uninitialized.
 102 If
 103 .I ptr
 104 is NULL, then the call is equivalent to
 105 .IR malloc(size) ,
 106 for all values of
 107 .IR size ;
 108 if
 109 .I size
 110 is equal to zero,
 111 and
 112 .I ptr
 113 is not NULL, then the call is equivalent to
 114 .IR free(ptr) .
 115 Unless
 116 .I ptr
 117 is NULL, it must have been returned by an earlier call to
 118 .BR malloc (),
 119 .BR calloc ()
 120 or
 121 .BR realloc ().
 122 If the area pointed to was moved, a
 123 .I free(ptr)
 124 is done.
 125 .SH "RETURN VALUE"
 126 The
 127 .BR malloc ()
 128 and
 129 .BR calloc ()
 130 functions return a pointer to the allocated memory
 131 that is suitably aligned for any kind of variable.
 132 On error, these functions return NULL.
 133 NULL may also be returned by a successful call to
 134 .BR malloc ()
 135 with a
 136 .I size
 137 of zero,
 138 or by a successful call to
 139 .BR calloc ()
 140 with
 141 .I nmemb
 142 or
 143 .I size
 144 equal to zero.
 145 .PP
 146 The
 147 .BR free ()
 148 function returns no value.
 149 .PP
 150 The
 151 .BR realloc ()
 152 function returns a pointer to the newly allocated memory, which is suitably
 153 aligned for any kind of variable and may be different from
 154 .IR ptr ,
 155 or NULL if the request fails.
 156 If
 157 .I size
 158 was equal to 0, either NULL or a pointer suitable to be passed to
 159 .BR free ()
 160 is returned.
 161 If
 162 .BR realloc ()
 163 fails the original block is left untouched; it is not freed or moved.
 164 .SH "CONFORMING TO"
 165 C89, C99.
 166 .SH NOTES
 167 By default, Linux follows an optimistic memory allocation strategy.
 168 This means that when
 169 .BR malloc ()
 170 returns non-NULL there is no guarantee that the memory really
 171 is available.
 172 In case it turns out that the system is out of memory,
 173 one or more processes will be killed by the OOM killer.
 174 For more information, see the description of
 175 .IR /proc/sys/vm/overcommit_memory
 176 and
 177 .IR /proc/sys/vm/oom_adj
 178 in
 179 .BR proc (5),
 180 and the kernel source file
 181 .IR Documentation/vm/overcommit-accounting .
 182
 183 Normally,
 184 .BR malloc ()
 185 allocates memory from the heap, and adjusts the size of the heap
 186 as required, using
 187 .BR sbrk (2).
 188 When allocating blocks of memory larger than
 189 .B MMAP_THRESHOLD
 190 bytes, the glibc
 191 .BR malloc ()
 192 implementation allocates the memory as a private anonymous mapping using
 193 .BR mmap (2).
 194 .B MMAP_THRESHOLD
 195 is 128 kB by default, but is adjustable using
 196 .BR mallopt (3).
 197 .\" FIXME . there is no mallopt(3) man page yet.
 198 Allocations performed using
 199 .BR mmap (2)
 200 are unaffected by the
 201 .B RLIMIT_DATA
 202 resource limit (see
 203 .BR getrlimit (2)).
 204
 205 The UNIX 98 standard requires
 206 .BR malloc (),
 207 .BR calloc (),
 208 and
 209 .BR realloc ()
 210 to set
 211 .I errno
 212 to
 213 .B ENOMEM
 214 upon failure.
 215 Glibc assumes that this is done
 216 (and the glibc versions of these routines do this); if you
 217 use a private malloc implementation that does not set
 218 .IR errno ,
 219 then certain library routines may fail without having
 220 a reason in
 221 .IR errno .
 222 .LP
 223 Crashes in
 224 .BR malloc (),
 225 .BR calloc (),
 226 .BR realloc (),
 227 or
 228 .BR free ()
 229 are almost always related to heap corruption, such as overflowing
 230 an allocated chunk or freeing the same pointer twice.
 231 .PP
 232 Recent versions of Linux libc (later than 5.4.23) and glibc (2.x)
 233 include a
 234 .BR malloc ()
 235 implementation which is tunable via environment variables.
 236 When
 237 .B MALLOC_CHECK_
 238 is set, a special (less efficient) implementation is used which
 239 is designed to be tolerant against simple errors, such as double
 240 calls of
 241 .BR free ()
 242 with the same argument, or overruns of a single byte (off-by-one
 243 bugs).
 244 Not all such errors can be protected against, however, and
 245 memory leaks can result.
 246 If
 247 .B MALLOC_CHECK_
 248 is set to 0, any detected heap corruption is silently ignored;
 249 if set to 1, a diagnostic message is printed on \fIstderr\fP;
 250 if set to 2,
 251 .BR abort (3)
 252 is called immediately;
 253 if set to 3, a diagnostic message is printed on \fIstderr\fP
 254 and the program is aborted.
 255 Using a nonzero
 256 .B MALLOC_CHECK_
 257 value can be useful because otherwise
 258 a crash may happen much later, and the true cause for the problem
 259 is then very hard to track down.
 260 .SH "SEE ALSO"
 261 .BR brk (2),
 262 .\" .BR mallopt (3),
 263 .BR mmap (2),
 264 .BR alloca (3),
 265 .BR posix_memalign (3)