memalign(3)

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       #define _XOPEN_SOURCE 600
10       #include <stdlib.h>
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12       int posix_memalign(void **memptr, size_t alignment, size_t size);
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14       #include <malloc.h>
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16       void *valloc(size_t size);
17       void *memalign(size_t boundary, size_t size);
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DESCRIPTION

20       The  function  posix_memalign()  allocates  size  bytes  and places the
21       address of the allocated memory in *memptr.  The address of  the  allo‐
22       cated  memory will be a multiple of alignment, which must be a power of
23       two and a multiple of sizeof(void *).
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25       The obsolete function memalign() allocates size  bytes  and  returns  a
26       pointer to the allocated memory.  The memory address will be a multiple
27       of boundary, which must be a power of two.
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29       The obsolete function valloc()  allocates  size  bytes  and  returns  a
30       pointer to the allocated memory.  The memory address will be a multiple
31       of the page  size.   It  is  equivalent  to  memalign(sysconf(_SC_PAGE‐
32       SIZE),size).
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34       For all three routines, the memory is not zeroed.
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RETURN VALUE

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

46       EINVAL The  alignment  parameter  was  not a power of two, or was not a
47              multiple of sizeof(void *).
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49       ENOMEM There was insufficient memory to fulfill the allocation request.
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NOTES

53       On many systems there are alignment restrictions, e.g. on buffers  used
54       for    direct   block   device   I/O.   POSIX   specifies   the   path‐
55       conf(path,_PC_REC_XFER_ALIGN) call that tells what alignment is needed.
56       Now one can use posix_memalign() to satisfy this requirement.
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58       posix_memalign()  verifies  that  alignment  matches  the  requirements
59       detailed above.  memalign() may not check that the  boundary  parameter
60       is correct.
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62       POSIX  requires that memory obtained from posix_memalign() can be freed
63       using free().  Some systems provide no way to reclaim memory  allocated
64       with  memalign()  or  valloc()  (because  one can only pass to free() a
65       pointer gotten from malloc(), while e.g.  memalign()  would  call  mal‐
66       loc()  and  then  align  the  obtained  value).  GNU libc allows memory
67       obtained from any of these three routines to be reclaimed with free().
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69       GNU libc malloc() always returns 8-byte aligned  memory  addresses,  so
70       these routines are only needed if you require larger alignment values.
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AVAILABILITY

74       The  functions memalign() and valloc() have been available in all Linux
75       libc libraries.  The function posix_memalign() is available since glibc
76       2.1.91.
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CONFORMING TO

80       The  function  valloc()  appeared in 3.0BSD.  It is documented as being
81       obsolete in 4.3BSD, and as legacy in SUSv2.   It  does  not  appear  in
82       POSIX.1-2001.   The  function memalign() appears in SunOS 4.1.3 but not
83       in 4.4BSD.  The function posix_memalign() comes from POSIX.1d.
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HEADERS

87       Everybody agrees that posix_memalign() is declared  in  <stdlib.h>.  In
88       order  to declare it, glibc needs _GNU_SOURCE defined, or _XOPEN_SOURCE
89       defined to a value not less than 600.
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91       On some systems memalign() is declared in <stdlib.h> instead  of  <mal‐
92       loc.h>.
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94       According  to  SUSv2,  valloc() is declared in <stdlib.h>.  Libc4,5 and
95       glibc declare it in <malloc.h> and perhaps also in <stdlib.h>  (namely,
96       if _GNU_SOURCE is defined, or _BSD_SOURCE is defined, or, for glibc, if
97       _XOPEN_SOURCE_EXTENDED is defined, or, equivalently,  _XOPEN_SOURCE  is
98       defined to a value not less than 500).
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