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  2020-04-11 "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 on success.
 148 On error, NULL is returned, and \fIerrno\fP is set
 149 to indicate the cause of the error.
 150 .PP
 151 .BR posix_memalign ()
 152 returns zero on success, or one of the error values listed in the
 153 next section on failure.
 154 The value of
 155 .I errno
 156 is not set.
 157 On Linux (and other systems),
 158 .BR posix_memalign ()
 159 does not modify
 160 .I memptr
 161 on failure.
 162 A requirement standardizing this behavior was added in POSIX.1-2016.
 163 .\" http://austingroupbugs.net/view.php?id=520
 164 .SH ERRORS
 165 .TP
 166 .B EINVAL
 167 The
 168 .I alignment
 169 argument was not a power of two, or was not a multiple of
 170 .IR "sizeof(void\ *)" .
 171 .TP
 172 .B ENOMEM
 173 There was insufficient memory to fulfill the allocation request.
 174 .SH VERSIONS
 175 The functions
 176 .BR memalign (),
 177 .BR valloc (),
 178 and
 179 .BR pvalloc ()
 180 have been available since at least glibc 2.0.
 181 .PP
 182 The function
 183 .BR aligned_alloc ()
 184 was added to glibc in version 2.16.
 185 .PP
 186 The function
 187 .BR posix_memalign ()
 188 is available since glibc 2.1.91.
 189 .SH ATTRIBUTES
 190 For an explanation of the terms used in this section, see
 191 .BR attributes (7).
 192 .TS
 193 allbox;
 194 lb lb lb
 195 l l l.
 196 Interface       Attribute       Value
 197 T{
 198 .BR aligned_alloc (),
 199 .br
 200 .BR memalign (),
 201 .br
 202 .BR posix_memalign ()
 203 T}      Thread safety   MT-Safe
 204 T{
 205 .BR valloc (),
 206 .br
 207 .BR pvalloc ()
 208 T}      Thread safety   MT-Unsafe init
 209 .TE
 210 .sp 1
 211 .SH CONFORMING TO
 212 The function
 213 .BR valloc ()
 214 appeared in 3.0BSD.
 215 It is documented as being obsolete in 4.3BSD,
 216 and as legacy in SUSv2.
 217 It does not appear in POSIX.1.
 218 .PP
 219 The function
 220 .BR pvalloc ()
 221 is a GNU extension.
 222 .PP
 223 The function
 224 .BR memalign ()
 225 appears in SunOS 4.1.3 but not in 4.4BSD.
 226 .PP
 227 The function
 228 .BR posix_memalign ()
 229 comes from POSIX.1d and is specified in POSIX.1-2001 and POSIX.1-2008.
 230 .PP
 231 The function
 232 .BR aligned_alloc ()
 233 is specified in the C11 standard.
 234 .\"
 235 .SS Headers
 236 Everybody agrees that
 237 .BR posix_memalign ()
 238 is declared in \fI<stdlib.h>\fP.
 239 .PP
 240 On some systems
 241 .BR memalign ()
 242 is declared in \fI<stdlib.h>\fP instead of \fI<malloc.h>\fP.
 243 .PP
 244 According to SUSv2,
 245 .BR valloc ()
 246 is declared in \fI<stdlib.h>\fP.
 247 Libc4,5 and glibc declare it in \fI<malloc.h>\fP, and also in
 248 \fI<stdlib.h>\fP
 249 if suitable feature test macros are defined (see above).
 250 .SH NOTES
 251 On many systems there are alignment restrictions, for example, on buffers
 252 used for direct block device I/O.
 253 POSIX specifies the
 254 .I "pathconf(path,_PC_REC_XFER_ALIGN)"
 255 call that tells what alignment is needed.
 256 Now one can use
 257 .BR posix_memalign ()
 258 to satisfy this requirement.
 259 .PP
 260 .BR posix_memalign ()
 261 verifies that
 262 .I alignment
 263 matches the requirements detailed above.
 264 .BR memalign ()
 265 may not check that the
 266 .I alignment
 267 argument is correct.
 268 .PP
 269 POSIX requires that memory obtained from
 270 .BR posix_memalign ()
 271 can be freed using
 272 .BR free (3).
 273 Some systems provide no way to reclaim memory allocated with
 274 .BR memalign ()
 275 or
 276 .BR valloc ()
 277 (because one can pass to
 278 .BR free (3)
 279 only a pointer obtained from
 280 .BR malloc (3),
 281 while, for example,
 282 .BR memalign ()
 283 would call
 284 .BR malloc (3)
 285 and then align the obtained value).
 286 .\" Other systems allow passing the result of
 287 .\" .IR valloc ()
 288 .\" to
 289 .\" .IR free (3),
 290 .\" but not to
 291 .\" .IR realloc (3).
 292 The glibc implementation
 293 allows memory obtained from any of these functions to be
 294 reclaimed with
 295 .BR free (3).
 296 .PP
 297 The glibc
 298 .BR malloc (3)
 299 always returns 8-byte aligned memory addresses, so these functions are
 300 needed only if you require larger alignment values.
 301 .SH SEE ALSO
 302 .BR brk (2),
 303 .BR getpagesize (2),
 304 .BR free (3),
 305 .BR malloc (3)