man3/posix_memalign.3

   1 .\" (c) 2001 by John Levon <moz@compsoc.man.ac.uk>
   2 .\" Based in part on GNU libc documentation.
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  25 .\" 2001-10-11, 2003-08-22, aeb, added some details
  26 .TH POSIX_MEMALIGN 3  2003-08-22 "GNU" "Linux Programmer's Manual"
  27 .SH NAME
  28 posix_memalign, memalign, valloc \- Allocate aligned memory
  29 .SH SYNOPSIS
  30 .nf
  31 .B #define _XOPEN_SOURCE 600
  32 .B #include <stdlib.h>
  33 .sp
  34 .BI "int posix_memalign(void **" memptr ", size_t " alignment ", size_t " size );
  35 .sp
  36 .B #include <malloc.h>
  37 .sp
  38 .BI "void *valloc(size_t " size );
  39 .BI "void *memalign(size_t " boundary ", size_t " size );
  40 .nl
  41 .fi
  42 .SH DESCRIPTION
  43 The function
  44 .B posix_memalign()
  45 allocates
  46 .I size
  47 bytes and places the address of the allocated memory in
  48 .IR "*memptr".
  49 The address of the allocated memory will be a multiple of
  50 .IR "alignment",
  51 which must be a power of two and a multiple of
  52 .IR "sizeof(void *)".
  53
  54 The obsolete function
  55 .B memalign()
  56 allocates
  57 .I size
  58 bytes and returns a pointer to the allocated memory.
  59 The memory address will be a multiple of
  60 .IR "boundary",
  61 which must be a power of two.
  62
  63 The obsolete function
  64 .B valloc()
  65 allocates
  66 .I size
  67 bytes and returns a pointer to the allocated memory.
  68 The memory address will be a multiple of the page size.
  69 It is equivalent to
  70 .IR "memalign(sysconf(_SC_PAGESIZE),size)" .
  71
  72 For all three routines, the memory is not zeroed.
  73
  74 .SH "RETURN VALUE"
  75 .BR memalign()
  76 and
  77 .BR valloc()
  78 return the pointer to the allocated memory, or
  79 .B NULL
  80 if the request fails.
  81
  82 .BR posix_memalign()
  83 returns zero on success, or one of the error values listed in the
  84 next section on failure. Note that
  85 .IR errno
  86 is not set.
  87
  88 .SH "ERRORS"
  89 .TP
  90 .B EINVAL
  91 The
  92 .IR alignment
  93 parameter was not a power of two, or was not a multiple of
  94 .IR "sizeof(void *)" .
  95 .TP
  96 .B ENOMEM
  97 There was insufficient memory to fulfill the allocation request.
  98
  99 .SH NOTES
 100 On many systems there are alignment restrictions, e.g. on buffers
 101 used for direct block device I/O. POSIX specifies the
 102 .I "pathconf(path,_PC_REC_XFER_ALIGN)"
 103 call that tells what alignment is needed. Now one can use
 104 .BR posix_memalign ()
 105 to satisfy this requirement.
 106
 107 .BR posix_memalign ()
 108 verifies that
 109 .IR alignment
 110 matches the requirements detailed above.
 111 .BR memalign()
 112 may not check that the
 113 .IR boundary
 114 parameter is correct.
 115
 116 POSIX requires that memory obtained from
 117 .BR posix_memalign()
 118 can be freed using
 119 .IR free ().
 120 Some systems provide no way to reclaim memory allocated with
 121 .IR memalign ()
 122 or
 123 .IR valloc ()
 124 (because one can only pass to
 125 .IR free ()
 126 a pointer gotten from
 127 .IR malloc (),
 128 while e.g.
 129 .IR memalign ()
 130 would call
 131 .IR malloc ()
 132 and then align the obtained value).
 133 .\" Other systems allow passing the result of
 134 .\" .IR valloc ()
 135 .\" to
 136 .\" .IR free (),
 137 .\" but not to
 138 .\" .IR realloc ().
 139 GNU libc allows memory obtained from any of these three routines to be
 140 reclaimed with
 141 .IR free ().
 142
 143 GNU libc
 144 .BR "malloc()"
 145 always returns 8-byte aligned memory addresses, so these routines are only
 146 needed if you require larger alignment values.
 147
 148 .SH AVAILABILITY
 149 The functions
 150 .IR memalign ()
 151 and
 152 .IR valloc ()
 153 have been available in all Linux libc libraries.
 154 The function
 155 .IR posix_memalign ()
 156 is available since glibc 2.1.91.
 157
 158 .SH "CONFORMING TO"
 159 The function
 160 .IR valloc ()
 161 appeared in 3.0 BSD. It is documented as being obsolete in BSD 4.3,
 162 and as legacy in SUSv2. It no longer occurs in SUSv3.
 163 The function
 164 .IR memalign ()
 165 appears in SunOS 4.1.3 but not in BSD 4.4.
 166 The function
 167 .IR posix_memalign ()
 168 comes from POSIX 1003.1d.
 169
 170 .SH HEADERS
 171 Everybody agrees that
 172 .IR posix_memalign ()
 173 is declared in <stdlib.h>. In order to declare it, glibc needs
 174 _GNU_SOURCE defined, or _XOPEN_SOURCE defined to a value not less than 600.
 175
 176 Everybody agrees that
 177 .IR memalign ()
 178 is declared in <malloc.h>.
 179
 180 According to SUSv2,
 181 .IR valloc ()
 182 is declared in <stdlib.h>.
 183 Libc4,5 and glibc declare it in <malloc.h> and perhaps also in <stdlib.h>
 184 (namely, if _GNU_SOURCE is defined, or _BSD_SOURCE is defined, or,
 185 for glibc, if _XOPEN_SOURCE_EXTENDED is defined, or, equivalently,
 186 _XOPEN_SOURCE is defined to a value not less than 500).
 187
 188 .SH "SEE ALSO"
 189 .BR brk (2),
 190 .BR getpagesize (2),
 191 .BR free (3),
 192 .BR malloc (3)