1POSIX_MEMALIGN(3)          Linux Programmer's Manual         POSIX_MEMALIGN(3)
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NAME

6       posix_memalign, memalign, valloc - Allocate aligned memory
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SYNOPSIS

9       #include <stdlib.h>
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11       int posix_memalign(void **memptr, size_t alignment, size_t size);
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13       #include <malloc.h>
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15       void *valloc(size_t size);
16       void *memalign(size_t boundary, size_t size);
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18   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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20       posix_memalign(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600
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DESCRIPTION

23       The  function  posix_memalign()  allocates  size  bytes  and places the
24       address of the allocated memory in *memptr.  The address of  the  allo‐
25       cated  memory will be a multiple of alignment, which must be a power of
26       two and a multiple of sizeof(void *).  If size is 0,  then  posix_mema‐
27       lign() returns either NULL, or a unique pointer value that can later be
28       successfully passed to free().
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30       The obsolete function memalign() allocates size  bytes  and  returns  a
31       pointer to the allocated memory.  The memory address will be a multiple
32       of boundary, which must be a power of two.
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34       The obsolete function valloc()  allocates  size  bytes  and  returns  a
35       pointer to the allocated memory.  The memory address will be a multiple
36       of the page  size.   It  is  equivalent  to  memalign(sysconf(_SC_PAGE‐
37       SIZE),size).
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39       For all three routines, the memory is not zeroed.
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RETURN VALUE

42       memalign()  and valloc() return the pointer to the allocated memory, or
43       NULL if the request fails.
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45       posix_memalign() returns zero on success, or one of  the  error  values
46       listed in the next section on failure.  Note that errno is not set.
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ERRORS

49       EINVAL The alignment argument was not a power of two, or was not a mul‐
50              tiple of sizeof(void *).
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52       ENOMEM There was insufficient memory to fulfill the allocation request.
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VERSIONS

55       The functions memalign() and valloc() have been available in all  Linux
56       libc libraries.  The function posix_memalign() is available since glibc
57       2.1.91.
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CONFORMING TO

60       The function valloc() appeared in 3.0BSD.  It is  documented  as  being
61       obsolete  in  4.3BSD,  and  as  legacy in SUSv2.  It does not appear in
62       POSIX.1-2001.  The function memalign() appears in SunOS 4.1.3  but  not
63       in 4.4BSD.  The function posix_memalign() comes from POSIX.1d.
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65   Headers
66       Everybody agrees that posix_memalign() is declared in <stdlib.h>.
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68       On  some  systems memalign() is declared in <stdlib.h> instead of <mal‐
69       loc.h>.
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71       According to SUSv2, valloc() is declared in  <stdlib.h>.   Libc4,5  and
72       glibc  declare it in <malloc.h> and perhaps also in <stdlib.h> (namely,
73       if _GNU_SOURCE is defined, or _BSD_SOURCE is defined, or, for glibc, if
74       _XOPEN_SOURCE_EXTENDED  is  defined, or, equivalently, _XOPEN_SOURCE is
75       defined to a value not less than 500).
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NOTES

78       On many systems there are alignment restrictions, for example, on  buf‐
79       fers  used  for  direct  block  device  I/O.  POSIX specifies the path‐
80       conf(path,_PC_REC_XFER_ALIGN) call that tells what alignment is needed.
81       Now one can use posix_memalign() to satisfy this requirement.
82
83       posix_memalign()  verifies  that  alignment  matches  the  requirements
84       detailed above.  memalign() may not check that the boundary argument is
85       correct.
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87       POSIX  requires that memory obtained from posix_memalign() can be freed
88       using free(3).  Some systems provide no way to reclaim memory allocated
89       with  memalign()  or  valloc()  (because one can only pass to free(3) a
90       pointer gotten from malloc(3), while,  for  example,  memalign()  would
91       call malloc(3) and then align the obtained value).  The glibc implemen‐
92       tation allows memory obtained from any of these three  routines  to  be
93       reclaimed with free(3).
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95       The  glibc malloc(3) always returns 8-byte aligned memory addresses, so
96       these routines are only needed if you require larger alignment values.
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SEE ALSO

99       brk(2), getpagesize(2), free(3), malloc(3)
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COLOPHON

102       This page is part of release 3.25 of the Linux  man-pages  project.   A
103       description  of  the project, and information about reporting bugs, can
104       be found at http://www.kernel.org/doc/man-pages/.
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108GNU                               2009-03-30                 POSIX_MEMALIGN(3)
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