man3/posix_memalign.3

   1 .\" Copyright (c) 2001 by John Levon <moz@compsoc.man.ac.uk>
   2 .\" Based in part on GNU libc documentation.
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  26 .\" 2001-10-11, 2003-08-22, aeb, added some details
  27 .\" 2012-03-23, Michael Kerrisk <mtk.manpages@mail.com>
  28 .\"     Document pvalloc() and aligned_alloc()
  29 .TH POSIX_MEMALIGN 3  2017-09-15 "GNU" "Linux Programmer's Manual"
  30 .SH NAME
  31 posix_memalign, aligned_alloc, memalign, valloc, pvalloc \- allocate aligned memory
  32 .SH SYNOPSIS
  33 .nf
  34 .B #include <stdlib.h>
  35 .PP
  36 .BI "int posix_memalign(void **" memptr ", size_t " alignment ", size_t " size );
  37 .BI "void *aligned_alloc(size_t " alignment ", size_t " size );
  38 .BI "void *valloc(size_t " size );
  39
  40 .B #include <malloc.h>
  41 .PP
  42 .BI "void *memalign(size_t " alignment ", size_t " size );
  43 .BI "void *pvalloc(size_t " size );
  44 .fi
  45 .PP
  46 .in -4n
  47 Feature Test Macro Requirements for glibc (see
  48 .BR feature_test_macros (7)):
  49 .in
  50 .PP
  51 .ad l
  52 .BR posix_memalign ():
  53 _POSIX_C_SOURCE\ >=\ 200112L
  54 .PP
  55 .BR aligned_alloc ():
  56 _ISOC11_SOURCE
  57 .PP
  58 .BR valloc ():
  59 .br
  60 .PD 0
  61 .RS 4
  62 .TP 4
  63 Since glibc 2.12:
  64 .nf
  65 (_XOPEN_SOURCE\ >=\ 500) && !(_POSIX_C_SOURCE\ >=\ 200112L)
  66     || /* Glibc since 2.19: */ _DEFAULT_SOURCE
  67     || /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
  68 .br
  69 .fi
  70 .TP
  71 Before glibc 2.12:
  72 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500
  73 .\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  74 .ad b
  75 .br
  76 (The (nonstandard) header file
  77 .I <malloc.h>
  78 also exposes the declaration of
  79 .BR valloc ();
  80 no feature test macros are required.)
  81 .RE
  82 .PD
  83 .SH DESCRIPTION
  84 The function
  85 .BR posix_memalign ()
  86 allocates
  87 .I size
  88 bytes and places the address of the allocated memory in
  89 .IR "*memptr" .
  90 The address of the allocated memory will be a multiple of
  91 .IR "alignment" ,
  92 which must be a power of two and a multiple of
  93 .IR "sizeof(void\ *)" .
  94 If
  95 .I size
  96 is 0, then
  97 the value placed in
  98 .IR "*memptr"
  99 is either NULL,
 100 .\" glibc does this:
 101 or a unique pointer value that can later be successfully passed to
 102 .BR free (3).
 103 .PP
 104 The obsolete function
 105 .BR memalign ()
 106 allocates
 107 .I size
 108 bytes and returns a pointer to the allocated memory.
 109 The memory address will be a multiple of
 110 .IR alignment ,
 111 which must be a power of two.
 112 .\" The behavior of memalign() for size==0 is as for posix_memalign()
 113 .\" but no standards govern this.
 114 .PP
 115 The function
 116 .BR aligned_alloc ()
 117 is the same as
 118 .BR memalign (),
 119 except for the added restriction that
 120 .I size
 121 should be a multiple of
 122 .IR alignment .
 123 .PP
 124 The obsolete function
 125 .BR valloc ()
 126 allocates
 127 .I size
 128 bytes and returns a pointer to the allocated memory.
 129 The memory address will be a multiple of the page size.
 130 It is equivalent to
 131 .IR "memalign(sysconf(_SC_PAGESIZE),size)" .
 132 .PP
 133 The obsolete function
 134 .BR pvalloc ()
 135 is similar to
 136 .BR valloc (),
 137 but rounds the size of the allocation up to
 138 the next multiple of the system page size.
 139 .PP
 140 For all of these functions, the memory is not zeroed.
 141 .SH RETURN VALUE
 142 .BR aligned_alloc (),
 143 .BR memalign (),
 144 .BR valloc (),
 145 and
 146 .BR pvalloc ()
 147 return a pointer to the allocated memory, or NULL if the request fails.
 148 .PP
 149 .BR posix_memalign ()
 150 returns zero on success, or one of the error values listed in the
 151 next section on failure.
 152 The value of
 153 .I errno
 154 is not set.
 155 On Linux (and other systems),
 156 .BR posix_memalign ()
 157 does not modify
 158 .I memptr
 159 on failure.
 160 A requirement standardizing this behavior was added in POSIX.1-2016.
 161 .\" http://austingroupbugs.net/view.php?id=520
 162 .SH ERRORS
 163 .TP
 164 .B EINVAL
 165 The
 166 .I alignment
 167 argument was not a power of two, or was not a multiple of
 168 .IR "sizeof(void\ *)" .
 169 .TP
 170 .B ENOMEM
 171 There was insufficient memory to fulfill the allocation request.
 172 .SH VERSIONS
 173 The functions
 174 .BR memalign (),
 175 .BR valloc (),
 176 and
 177 .BR pvalloc ()
 178 have been available in all Linux libc libraries.
 179 .PP
 180 The function
 181 .BR aligned_alloc ()
 182 was added to glibc in version 2.16.
 183 .PP
 184 The function
 185 .BR posix_memalign ()
 186 is available since glibc 2.1.91.
 187 .SH ATTRIBUTES
 188 For an explanation of the terms used in this section, see
 189 .BR attributes (7).
 190 .TS
 191 allbox;
 192 lb lb lb
 193 l l l.
 194 Interface       Attribute       Value
 195 T{
 196 .BR aligned_alloc (),
 197 .br
 198 .BR memalign (),
 199 .br
 200 .BR posix_memalign ()
 201 T}      Thread safety   MT-Safe
 202 T{
 203 .BR valloc (),
 204 .br
 205 .BR pvalloc ()
 206 T}      Thread safety   MT-Unsafe init
 207 .TE
 208 .sp 1
 209 .SH CONFORMING TO
 210 The function
 211 .BR valloc ()
 212 appeared in 3.0BSD.
 213 It is documented as being obsolete in 4.3BSD,
 214 and as legacy in SUSv2.
 215 It does not appear in POSIX.1.
 216 .PP
 217 The function
 218 .BR pvalloc ()
 219 is a GNU extension.
 220 .PP
 221 The function
 222 .BR memalign ()
 223 appears in SunOS 4.1.3 but not in 4.4BSD.
 224 .PP
 225 The function
 226 .BR posix_memalign ()
 227 comes from POSIX.1d and is specified in POSIX.1-2001 and POSIX.1-2008.
 228 .PP
 229 The function
 230 .BR aligned_alloc ()
 231 is specified in the C11 standard.
 232 .\"
 233 .SS Headers
 234 Everybody agrees that
 235 .BR posix_memalign ()
 236 is declared in \fI<stdlib.h>\fP.
 237 .PP
 238 On some systems
 239 .BR memalign ()
 240 is declared in \fI<stdlib.h>\fP instead of \fI<malloc.h>\fP.
 241 .PP
 242 According to SUSv2,
 243 .BR valloc ()
 244 is declared in \fI<stdlib.h>\fP.
 245 Libc4,5 and glibc declare it in \fI<malloc.h>\fP, and also in
 246 \fI<stdlib.h>\fP
 247 if suitable feature test macros are defined (see above).
 248 .SH NOTES
 249 On many systems there are alignment restrictions, for example, on buffers
 250 used for direct block device I/O.
 251 POSIX specifies the
 252 .I "pathconf(path,_PC_REC_XFER_ALIGN)"
 253 call that tells what alignment is needed.
 254 Now one can use
 255 .BR posix_memalign ()
 256 to satisfy this requirement.
 257 .PP
 258 .BR posix_memalign ()
 259 verifies that
 260 .I alignment
 261 matches the requirements detailed above.
 262 .BR memalign ()
 263 may not check that the
 264 .I alignment
 265 argument is correct.
 266 .PP
 267 POSIX requires that memory obtained from
 268 .BR posix_memalign ()
 269 can be freed using
 270 .BR free (3).
 271 Some systems provide no way to reclaim memory allocated with
 272 .BR memalign ()
 273 or
 274 .BR valloc ()
 275 (because one can pass to
 276 .BR free (3)
 277 only a pointer obtained from
 278 .BR malloc (3),
 279 while, for example,
 280 .BR memalign ()
 281 would call
 282 .BR malloc (3)
 283 and then align the obtained value).
 284 .\" Other systems allow passing the result of
 285 .\" .IR valloc ()
 286 .\" to
 287 .\" .IR free (3),
 288 .\" but not to
 289 .\" .IR realloc (3).
 290 The glibc implementation
 291 allows memory obtained from any of these functions to be
 292 reclaimed with
 293 .BR free (3).
 294 .PP
 295 The glibc
 296 .BR malloc (3)
 297 always returns 8-byte aligned memory addresses, so these functions are
 298 needed only if you require larger alignment values.
 299 .SH SEE ALSO
 300 .BR brk (2),
 301 .BR getpagesize (2),
 302 .BR free (3),
 303 .BR malloc (3)