1POSIX_MEMALIGN(3)          Linux Programmer's Manual         POSIX_MEMALIGN(3)
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3
4

NAME

6       posix_memalign,  aligned_alloc,  memalign,  valloc,  pvalloc - allocate
7       aligned memory
8

SYNOPSIS

10       #include <stdlib.h>
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12       int posix_memalign(void **memptr, size_t alignment, size_t size);
13       void *aligned_alloc(size_t alignment, size_t size);
14       void *valloc(size_t size);
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16       #include <malloc.h>
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18       void *memalign(size_t alignment, size_t size);
19       void *pvalloc(size_t size);
20
21   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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23       posix_memalign(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600
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25       aligned_alloc(): _ISOC11_SOURCE
26
27       valloc():
28           Since glibc 2.12:
29               _BSD_SOURCE ||
30                   (_XOPEN_SOURCE >= 500 ||
31                       _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) &&
32                   !(_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600)
33           Before glibc 2.12:
34               _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
35               _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
36               (The (nonstandard) header file <malloc.h> also exposes the dec‐
37               laration of valloc(); no feature test macros are required.)
38

DESCRIPTION

40       The function posix_memalign()  allocates  size  bytes  and  places  the
41       address  of  the allocated memory in *memptr.  The address of the allo‐
42       cated memory will be a multiple of alignment, which must be a power  of
43       two  and  a multiple of sizeof(void *).  If size is 0, then posix_mema‐
44       lign() returns either NULL, or a unique pointer value that can later be
45       successfully passed to free(3).
46
47       The  obsolete  function  memalign()  allocates size bytes and returns a
48       pointer to the allocated memory.  The memory address will be a multiple
49       of alignment, which must be a power of two.
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51       The  function aligned_alloc() is the same as memalign(), except for the
52       added restriction that size should be a multiple of alignment.
53
54       The obsolete function valloc()  allocates  size  bytes  and  returns  a
55       pointer to the allocated memory.  The memory address will be a multiple
56       of the page  size.   It  is  equivalent  to  memalign(sysconf(_SC_PAGE‐
57       SIZE),size).
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59       The  obsolete function pvalloc() is similar to valloc(), but rounds the
60       size of the allocation up to the next multiple of the system page size.
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62       For all of these functions, the memory is not zeroed.
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RETURN VALUE

65       aligned_alloc(), memalign(), valloc(), and pvalloc() return  a  pointer
66       to the allocated memory, or NULL if the request fails.
67
68       posix_memalign()  returns  zero  on success, or one of the error values
69       listed in the next section on failure.  Note that errno is not set.
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ERRORS

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

78       The  functions  memalign(), valloc(), and pvalloc() have been available
79       in all Linux libc libraries.
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81       The function aligned_alloc() was added to glibc in version 2.16.
82
83       The function posix_memalign() is available since glibc 2.1.91.
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CONFORMING TO

86       The function valloc() appeared in 3.0BSD.  It is  documented  as  being
87       obsolete  in  4.3BSD,  and  as  legacy in SUSv2.  It does not appear in
88       POSIX.1-2001.
89
90       The function pvalloc() is a GNU extension.
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92       The function memalign() appears in SunOS 4.1.3 but not in 4.4BSD.
93
94       The function posix_memalign() comes from POSIX.1d.
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96       The function aligned_alloc() is specified in the C11 standard.
97
98   Headers
99       Everybody agrees that posix_memalign() is declared in <stdlib.h>.
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101       On some systems memalign() is declared in <stdlib.h> instead  of  <mal‐
102       loc.h>.
103
104       According  to  SUSv2,  valloc() is declared in <stdlib.h>.  Libc4,5 and
105       glibc declare it in <malloc.h>, and also in <stdlib.h> if suitable fea‐
106       ture test macros are defined (see above).
107

NOTES

109       On  many systems there are alignment restrictions, for example, on buf‐
110       fers used for direct block  device  I/O.   POSIX  specifies  the  path‐
111       conf(path,_PC_REC_XFER_ALIGN) call that tells what alignment is needed.
112       Now one can use posix_memalign() to satisfy this requirement.
113
114       posix_memalign()  verifies  that  alignment  matches  the  requirements
115       detailed  above.   memalign() may not check that the alignment argument
116       is correct.
117
118       POSIX requires that memory obtained from posix_memalign() can be  freed
119       using free(3).  Some systems provide no way to reclaim memory allocated
120       with memalign() or valloc() (because one can pass  to  free(3)  only  a
121       pointer  obtained  from malloc(3), while, for example, memalign() would
122       call malloc(3) and then align the obtained value).  The glibc implemen‐
123       tation  allows  memory  obtained  from  any  of  these  functions to be
124       reclaimed with free(3).
125
126       The glibc malloc(3) always returns 8-byte aligned memory addresses,  so
127       these functions are needed only if you require larger alignment values.
128

SEE ALSO

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

133       This  page  is  part of release 3.53 of the Linux man-pages project.  A
134       description of the project, and information about reporting  bugs,  can
135       be found at http://www.kernel.org/doc/man-pages/.
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139GNU                               2012-03-23                 POSIX_MEMALIGN(3)