man3/posix_memalign.3

   1 .\" Copyright (c) 2001 by John Levon <moz@compsoc.man.ac.uk>
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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  2015-08-08 "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 .sp
  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 .sp
  40 .B #include <malloc.h>
  41 .sp
  42 .BI "void *memalign(size_t " alignment ", size_t " size );
  43 .BI "void *pvalloc(size_t " size );
  44 .fi
  45 .sp
  46 .in -4n
  47 Feature Test Macro Requirements for glibc (see
  48 .BR feature_test_macros (7)):
  49 .in
  50 .sp
  51 .ad l
  52 .BR posix_memalign ():
  53 _POSIX_C_SOURCE\ >=\ 200112L || _XOPEN_SOURCE\ >=\ 600
  54 .sp
  55 .BR aligned_alloc ():
  56 _ISOC11_SOURCE
  57 .sp
  58 .BR valloc ():
  59 .br
  60 .PD 0
  61 .RS 4
  62 .TP 4
  63 Since glibc 2.12:
  64 .nf
  65 _BSD_SOURCE ||
  66     (_XOPEN_SOURCE\ >=\ 500 ||
  67         _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED) &&
  68     !(_POSIX_C_SOURCE\ >=\ 200112L || _XOPEN_SOURCE\ >=\ 600)
  69 .br
  70 .fi
  71 .TP
  72 Before glibc 2.12:
  73 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
  74 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
  75 .ad b
  76 .br
  77 (The (nonstandard) header file
  78 .I <malloc.h>
  79 also exposes the declaration of
  80 .BR valloc ();
  81 no feature test macros are required.)
  82 .RE
  83 .PD
  84 .SH DESCRIPTION
  85 The function
  86 .BR posix_memalign ()
  87 allocates
  88 .I size
  89 bytes and places the address of the allocated memory in
  90 .IR "*memptr" .
  91 The address of the allocated memory will be a multiple of
  92 .IR "alignment" ,
  93 which must be a power of two and a multiple of
  94 .IR "sizeof(void\ *)" .
  95 If
  96 .I size
  97 is 0, then
  98 the value placed in
  99 .IR "*memptr"
 100 is either NULL,
 101 .\" glibc does this:
 102 or a unique pointer value that can later be successfully passed to
 103 .BR free (3).
 104
 105 The obsolete function
 106 .BR memalign ()
 107 allocates
 108 .I size
 109 bytes and returns a pointer to the allocated memory.
 110 The memory address will be a multiple of
 111 .IR alignment ,
 112 which must be a power of two.
 113 .\" The behavior of memalign() for size==0 is as for posix_memalign()
 114 .\" but no standards govern this.
 115
 116 The function
 117 .BR aligned_alloc ()
 118 is the same as
 119 .BR memalign (),
 120 except for the added restriction that
 121 .I size
 122 should be a multiple of
 123 .IR alignment .
 124
 125 The obsolete function
 126 .BR valloc ()
 127 allocates
 128 .I size
 129 bytes and returns a pointer to the allocated memory.
 130 The memory address will be a multiple of the page size.
 131 It is equivalent to
 132 .IR "memalign(sysconf(_SC_PAGESIZE),size)" .
 133
 134 The obsolete function
 135 .BR pvalloc ()
 136 is similar to
 137 .BR valloc (),
 138 but rounds the size of the allocation up to
 139 the next multiple of the system page size.
 140
 141 For all of these functions, the memory is not zeroed.
 142 .SH RETURN VALUE
 143 .BR aligned_alloc (),
 144 .BR memalign (),
 145 .BR valloc (),
 146 and
 147 .BR pvalloc ()
 148 return a pointer to the allocated memory, or NULL if the request fails.
 149
 150 .BR posix_memalign ()
 151 returns zero on success, or one of the error values listed in the
 152 next section on failure.
 153 The value of
 154 .I errno
 155 is indeterminate after a call to
 156 .BR posix_memalign ().
 157 .SH ERRORS
 158 .TP
 159 .B EINVAL
 160 The
 161 .I alignment
 162 argument was not a power of two, or was not a multiple of
 163 .IR "sizeof(void\ *)" .
 164 .TP
 165 .B ENOMEM
 166 There was insufficient memory to fulfill the allocation request.
 167 .SH VERSIONS
 168 The functions
 169 .BR memalign (),
 170 .BR valloc (),
 171 and
 172 .BR pvalloc ()
 173 have been available in all Linux libc libraries.
 174
 175 The function
 176 .BR aligned_alloc ()
 177 was added to glibc in version 2.16.
 178
 179 The function
 180 .BR posix_memalign ()
 181 is available since glibc 2.1.91.
 182 .SH ATTRIBUTES
 183 For an explanation of the terms used in this section, see
 184 .BR attributes (7).
 185 .TS
 186 allbox;
 187 lb lb lb
 188 l l l.
 189 Interface       Attribute       Value
 190 T{
 191 .BR aligned_alloc (),
 192 .br
 193 .BR memalign (),
 194 .br
 195 .BR posix_memalign ()
 196 T}      Thread safety   MT-Safe
 197 T{
 198 .BR valloc (),
 199 .br
 200 .BR pvalloc ()
 201 T}      Thread safety   MT-Unsafe init
 202 .TE
 203
 204 .SH CONFORMING TO
 205 The function
 206 .BR valloc ()
 207 appeared in 3.0BSD.
 208 It is documented as being obsolete in 4.3BSD,
 209 and as legacy in SUSv2.
 210 It does not appear in POSIX.1.
 211
 212 The function
 213 .BR pvalloc ()
 214 is a GNU extension.
 215
 216 The function
 217 .BR memalign ()
 218 appears in SunOS 4.1.3 but not in 4.4BSD.
 219
 220 The function
 221 .BR posix_memalign ()
 222 comes from POSIX.1d and is specified in POSIX.1-2001 and POSIX.1-2008.
 223
 224 The function
 225 .BR aligned_alloc ()
 226 is specified in the C11 standard.
 227 .\"
 228 .SS Headers
 229 Everybody agrees that
 230 .BR posix_memalign ()
 231 is declared in \fI<stdlib.h>\fP.
 232
 233 On some systems
 234 .BR memalign ()
 235 is declared in \fI<stdlib.h>\fP instead of \fI<malloc.h>\fP.
 236
 237 According to SUSv2,
 238 .BR valloc ()
 239 is declared in \fI<stdlib.h>\fP.
 240 Libc4,5 and glibc declare it in \fI<malloc.h>\fP, and also in
 241 \fI<stdlib.h>\fP
 242 if suitable feature test macros are defined (see above).
 243 .SH NOTES
 244 On many systems there are alignment restrictions, for example, on buffers
 245 used for direct block device I/O.
 246 POSIX specifies the
 247 .I "pathconf(path,_PC_REC_XFER_ALIGN)"
 248 call that tells what alignment is needed.
 249 Now one can use
 250 .BR posix_memalign ()
 251 to satisfy this requirement.
 252
 253 .BR posix_memalign ()
 254 verifies that
 255 .I alignment
 256 matches the requirements detailed above.
 257 .BR memalign ()
 258 may not check that the
 259 .I alignment
 260 argument is correct.
 261
 262 POSIX requires that memory obtained from
 263 .BR posix_memalign ()
 264 can be freed using
 265 .BR free (3).
 266 Some systems provide no way to reclaim memory allocated with
 267 .BR memalign ()
 268 or
 269 .BR valloc ()
 270 (because one can pass to
 271 .BR free (3)
 272 only a pointer obtained from
 273 .BR malloc (3),
 274 while, for example,
 275 .BR memalign ()
 276 would call
 277 .BR malloc (3)
 278 and then align the obtained value).
 279 .\" Other systems allow passing the result of
 280 .\" .IR valloc ()
 281 .\" to
 282 .\" .IR free (3),
 283 .\" but not to
 284 .\" .IR realloc (3).
 285 The glibc implementation
 286 allows memory obtained from any of these functions to be
 287 reclaimed with
 288 .BR free (3).
 289
 290 The glibc
 291 .BR malloc (3)
 292 always returns 8-byte aligned memory addresses, so these functions are
 293 needed only if you require larger alignment values.
 294 .SH SEE ALSO
 295 .BR brk (2),
 296 .BR getpagesize (2),
 297 .BR free (3),
 298 .BR malloc (3)