man3/malloc.3

   1 .\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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  24 .\"
  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().
  29 .\"
  30 .\" FIXME . Review http://austingroupbugs.net/view.php?id=374
  31 .\" to see what changes are required on this page.
  32 .\"
  33 .TH MALLOC 3  2017-07-13 "GNU" "Linux Programmer's Manual"
  34 .SH NAME
  35 malloc, free, calloc, realloc \- allocate and free dynamic memory
  36 .SH SYNOPSIS
  37 .nf
  38 .B #include <stdlib.h>
  39 .PP
  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" );
  45 .fi
  46 .PP
  47 .in -4n
  48 Feature Test Macro Requirements for glibc (see
  49 .BR feature_test_macros (7)):
  50 .in
  51 .PP
  52 .BR reallocarray ():
  53 .br
  54 .RS 4
  55 .ad l
  56 _GNU_SOURCE
  57 .RE
  58 .ad
  59 .SH DESCRIPTION
  60 .PP
  61 The
  62 .BR malloc ()
  63 function allocates
  64 .I size
  65 bytes and returns a pointer to the allocated memory.
  66 .IR "The memory is not initialized" .
  67 If
  68 .I size
  69 is 0, then
  70 .BR malloc ()
  71 returns either NULL,
  72 .\" glibc does this:
  73 or a unique pointer value that can later be successfully passed to
  74 .BR free ().
  75 .PP
  76 The
  77 .BR free ()
  78 function frees the memory space pointed to by
  79 .IR ptr ,
  80 which must have been returned by a previous call to
  81 .BR malloc (),
  82 .BR calloc (),
  83 or
  84 .BR realloc ().
  85 Otherwise, or if
  86 .I free(ptr)
  87 has already been called before, undefined behavior occurs.
  88 If
  89 .I ptr
  90 is NULL, no operation is performed.
  91 .PP
  92 The
  93 .BR calloc ()
  94 function allocates memory for an array of
  95 .I nmemb
  96 elements of
  97 .I size
  98 bytes each and returns a pointer to the allocated memory.
  99 The memory is set to zero.
 100 If
 101 .I nmemb
 102 or
 103 .I size
 104 is 0, then
 105 .BR calloc ()
 106 returns either NULL,
 107 .\" glibc does this:
 108 or a unique pointer value that can later be successfully passed to
 109 .BR free ().
 110 .PP
 111 The
 112 .BR realloc ()
 113 function changes the size of the memory block pointed to by
 114 .I ptr
 115 to
 116 .I size
 117 bytes.
 118 The contents will be unchanged in the range from the start of the region
 119 up to the minimum of the old and new sizes.
 120 If the new size is larger than the old size, the added memory will
 121 .I not
 122 be initialized.
 123 If
 124 .I ptr
 125 is NULL, then the call is equivalent to
 126 .IR malloc(size) ,
 127 for all values of
 128 .IR size ;
 129 if
 130 .I size
 131 is equal to zero,
 132 and
 133 .I ptr
 134 is not NULL, then the call is equivalent to
 135 .IR free(ptr) .
 136 Unless
 137 .I ptr
 138 is NULL, it must have been returned by an earlier call to
 139 .BR malloc (),
 140 .BR calloc (),
 141 or
 142 .BR realloc ().
 143 If the area pointed to was moved, a
 144 .I free(ptr)
 145 is done.
 146 .PP
 147 The
 148 .BR reallocarray ()
 149 function changes the size of the memory block pointed to by
 150 .I ptr
 151 to be large enough for an array of
 152 .I nmemb
 153 elements, each of which is
 154 .I size
 155 bytes.
 156 It is equivalent to the call
 157 .PP
 158 .in +4n
 159     realloc(ptr, nmemb * size);
 160 .in
 161 .PP
 162 However, unlike that
 163 .BR realloc ()
 164 call,
 165 .BR reallocarray ()
 166 fails safely in the case where the multiplication would overflow.
 167 If such an overflow occurs,
 168 .BR reallocarray ()
 169 returns NULL, sets
 170 .I errno
 171 to
 172 .BR ENOMEM ,
 173 and leaves the original block of memory unchanged.
 174 .SH RETURN VALUE
 175 The
 176 .BR malloc ()
 177 and
 178 .BR calloc ()
 179 functions return a pointer to the allocated memory,
 180 which is suitably aligned for any built-in type.
 181 On error, these functions return NULL.
 182 NULL may also be returned by a successful call to
 183 .BR malloc ()
 184 with a
 185 .I size
 186 of zero,
 187 or by a successful call to
 188 .BR calloc ()
 189 with
 190 .I nmemb
 191 or
 192 .I size
 193 equal to zero.
 194 .PP
 195 The
 196 .BR free ()
 197 function returns no value.
 198 .PP
 199 The
 200 .BR realloc ()
 201 function returns a pointer to the newly allocated memory, which is suitably
 202 aligned for any built-in type and may be different from
 203 .IR ptr ,
 204 or NULL if the request fails.
 205 If
 206 .I size
 207 was equal to 0, either NULL or a pointer suitable to be passed to
 208 .BR free ()
 209 is returned.
 210 If
 211 .BR realloc ()
 212 fails, the original block is left untouched; it is not freed or moved.
 213 .PP
 214 On success, the
 215 .BR reallocarray ()
 216 function returns a pointer to the newly allocated memory.
 217 On failure,
 218 it returns NULL and the original block of memory is left untouched.
 219 .SH ERRORS
 220 .BR calloc (),
 221 .BR malloc (),
 222 .BR realloc (),
 223 and
 224 .BR reallocarray ()
 225 can fail with the following error:
 226 .TP
 227 .B ENOMEM
 228 Out of memory.
 229 Possibly, the application hit the
 230 .BR RLIMIT_AS
 231 or
 232 .BR RLIMIT_DATA
 233 limit described in
 234 .BR getrlimit (2).
 235 .SH ATTRIBUTES
 236 For an explanation of the terms used in this section, see
 237 .BR attributes (7).
 238 .TS
 239 allbox;
 240 lbw20 lb lb
 241 l l l.
 242 Interface       Attribute       Value
 243 T{
 244 .BR malloc (),
 245 .BR free (),
 246 .br
 247 .BR calloc (),
 248 .BR realloc ()
 249 T}      Thread safety   MT-Safe
 250 .TE
 251 .SH CONFORMING TO
 252 .BR malloc (),
 253 .BR free (),
 254 .BR calloc (),
 255 .BR realloc ():
 256 POSIX.1-2001, POSIX.1-2008, C89, C99.
 257 .PP
 258 .BR reallocarray ()
 259 is a nonstandard extension that first appeared in OpenBSD 5.6 and FreeBSD 11.0.
 260 .SH NOTES
 261 By default, Linux follows an optimistic memory allocation strategy.
 262 This means that when
 263 .BR malloc ()
 264 returns non-NULL there is no guarantee that the memory really
 265 is available.
 266 In case it turns out that the system is out of memory,
 267 one or more processes will be killed by the OOM killer.
 268 For more information, see the description of
 269 .IR /proc/sys/vm/overcommit_memory
 270 and
 271 .IR /proc/sys/vm/oom_adj
 272 in
 273 .BR proc (5),
 274 and the Linux kernel source file
 275 .IR Documentation/vm/overcommit-accounting .
 276 .PP
 277 Normally,
 278 .BR malloc ()
 279 allocates memory from the heap, and adjusts the size of the heap
 280 as required, using
 281 .BR sbrk (2).
 282 When allocating blocks of memory larger than
 283 .B MMAP_THRESHOLD
 284 bytes, the glibc
 285 .BR malloc ()
 286 implementation allocates the memory as a private anonymous mapping using
 287 .BR mmap (2).
 288 .B MMAP_THRESHOLD
 289 is 128\ kB by default, but is adjustable using
 290 .BR mallopt (3).
 291 Allocations performed using
 292 .BR mmap (2)
 293 are unaffected by the
 294 .B RLIMIT_DATA
 295 resource limit only prior to Linux kernel 4.7 (see
 296 .BR getrlimit (2)).
 297 .PP
 298 To avoid corruption in multithreaded applications,
 299 mutexes are used internally to protect the memory-management
 300 data structures employed by these functions.
 301 In a multithreaded application in which threads simultaneously
 302 allocate and free memory,
 303 there could be contention for these mutexes.
 304 To scalably handle memory allocation in multithreaded applications,
 305 glibc creates additional
 306 .IR "memory allocation arenas"
 307 if mutex contention is detected.
 308 Each arena is a large region of memory that is internally allocated
 309 by the system
 310 (using
 311 .BR brk (2)
 312 or
 313 .BR mmap (2)),
 314 and managed with its own mutexes.
 315 .PP
 316 SUSv2 requires
 317 .BR malloc (),
 318 .BR calloc (),
 319 and
 320 .BR realloc ()
 321 to set
 322 .I errno
 323 to
 324 .B ENOMEM
 325 upon failure.
 326 Glibc assumes that this is done
 327 (and the glibc versions of these routines do this); if you
 328 use a private malloc implementation that does not set
 329 .IR errno ,
 330 then certain library routines may fail without having
 331 a reason in
 332 .IR errno .
 333 .PP
 334 Crashes in
 335 .BR malloc (),
 336 .BR calloc (),
 337 .BR realloc (),
 338 or
 339 .BR free ()
 340 are almost always related to heap corruption, such as overflowing
 341 an allocated chunk or freeing the same pointer twice.
 342 .PP
 343 The
 344 .BR malloc ()
 345 implementation is tunable via environment variables; see
 346 .BR mallopt (3)
 347 for details.
 348 .SH SEE ALSO
 349 .\" http://g.oswego.edu/dl/html/malloc.html
 350 .\" A Memory Allocator - by Doug Lea
 351 .\"
 352 .\" http://www.bozemanpass.com/info/linux/malloc/Linux_Heap_Contention.html
 353 .\" Linux Heap, Contention in free() - David Boreham
 354 .\"
 355 .\" http://www.citi.umich.edu/projects/linux-scalability/reports/malloc.html
 356 .\" malloc() Performance in a Multithreaded Linux Environment -
 357 .\"     Check Lever, David Boreham
 358 .\"
 359 .ad l
 360 .nh
 361 .BR brk (2),
 362 .BR mmap (2),
 363 .BR alloca (3),
 364 .BR malloc_get_state (3),
 365 .BR malloc_info (3),
 366 .BR malloc_trim (3),
 367 .BR malloc_usable_size (3),
 368 .BR mallopt (3),
 369 .BR mcheck (3),
 370 .BR mtrace (3),
 371 .BR posix_memalign (3)