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NAME

       posix_memalign, aligned_alloc, memalign, valloc, pvalloc - allocate aligned memory

SYNOPSIS

       #include <stdlib.h>

       int posix_memalign(void **memptr, size_t alignment, size_t size);
       void *aligned_alloc(size_t alignment, size_t size);
       void *valloc(size_t size);

       #include <malloc.h>

       void *memalign(size_t alignment, size_t size);
       void *pvalloc(size_t size);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       posix_memalign(): _POSIX_C_SOURCE >= 200112L

       aligned_alloc(): _ISOC11_SOURCE

       valloc():
           Since glibc 2.12:
               (_XOPEN_SOURCE >= 500) && !(_POSIX_C_SOURCE >= 200112L)
                   || /* Glibc since 2.19: */ _DEFAULT_SOURCE
                   || /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
           Before glibc 2.12:
               _BSD_SOURCE || _XOPEN_SOURCE >= 500
               (The  (nonstandard)  header  file  <malloc.h>  also  exposes  the  declaration  of
               valloc(); no feature test macros are required.)

DESCRIPTION

       The function posix_memalign() allocates size bytes and places the address of the allocated
       memory  in  *memptr.  The address of the allocated memory will be a multiple of alignment,
       which must be a power of two and a multiple of sizeof(void *).  If size  is  0,  then  the
       value  placed  in  *memptr  is  either  NULL,  or a unique pointer value that can later be
       successfully passed to free(3).

       The obsolete function memalign() allocates  size  bytes  and  returns  a  pointer  to  the
       allocated  memory.   The  memory  address will be a multiple of alignment, which must be a
       power of two.

       The function aligned_alloc() is the same as memalign(), except for the  added  restriction
       that size should be a multiple of alignment.

       The obsolete function valloc() allocates size bytes and returns a pointer to the allocated
       memory.  The memory address will be a multiple of the page  size.   It  is  equivalent  to
       memalign(sysconf(_SC_PAGESIZE),size).

       The  obsolete  function  pvalloc()  is  similar  to  valloc(),  but rounds the size of the
       allocation up to the next multiple of the system page size.

       For all of these functions, the memory is not zeroed.

RETURN VALUE

       aligned_alloc(), memalign(), valloc(), and pvalloc() return a  pointer  to  the  allocated
       memory, or NULL if the request fails.

       posix_memalign()  returns  zero  on success, or one of the error values listed in the next
       section on failure.  The value of errno  is  not  set.   On  Linux  (and  other  systems),
       posix_memalign()  does  not  modify  memptr  on failure.  A requirement standardizing this
       behavior was added in POSIX.1-2016.

ERRORS

       EINVAL The alignment argument  was  not  a  power  of  two,  or  was  not  a  multiple  of
              sizeof(void *).

       ENOMEM There was insufficient memory to fulfill the allocation request.

VERSIONS

       The  functions  memalign(),  valloc(), and pvalloc() have been available in all Linux libc
       libraries.

       The function aligned_alloc() was added to glibc in version 2.16.

       The function posix_memalign() is available since glibc 2.1.91.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌─────────────────┬───────────────┬────────────────┐
       │InterfaceAttributeValue          │
       ├─────────────────┼───────────────┼────────────────┤
       │aligned_alloc(), │ Thread safety │ MT-Safe        │
       │memalign(),      │               │                │
       │posix_memalign() │               │                │
       ├─────────────────┼───────────────┼────────────────┤
       │valloc(),        │ Thread safety │ MT-Unsafe init │
       │pvalloc()        │               │                │
       └─────────────────┴───────────────┴────────────────┘

CONFORMING TO

       The function valloc() appeared in 3.0BSD.  It is documented as being obsolete  in  4.3BSD,
       and as legacy in SUSv2.  It does not appear in POSIX.1.

       The function pvalloc() is a GNU extension.

       The function memalign() appears in SunOS 4.1.3 but not in 4.4BSD.

       The  function  posix_memalign()  comes  from POSIX.1d and is specified in POSIX.1-2001 and
       POSIX.1-2008.

       The function aligned_alloc() is specified in the C11 standard.

   Headers
       Everybody agrees that posix_memalign() is declared in <stdlib.h>.

       On some systems memalign() is declared in <stdlib.h> instead of <malloc.h>.

       According to SUSv2, valloc() is declared in <stdlib.h>.  Libc4,5 and glibc declare  it  in
       <malloc.h>,  and  also  in  <stdlib.h>  if  suitable  feature test macros are defined (see
       above).

NOTES

       On many systems there are alignment restrictions, for example, on buffers used for  direct
       block  device  I/O.  POSIX specifies the pathconf(path,_PC_REC_XFER_ALIGN) call that tells
       what alignment is needed.  Now one can use posix_memalign() to satisfy this requirement.

       posix_memalign()  verifies  that  alignment  matches  the  requirements  detailed   above.
       memalign() may not check that the alignment argument is correct.

       POSIX  requires  that  memory  obtained  from posix_memalign() can be freed using free(3).
       Some systems provide no way to  reclaim  memory  allocated  with  memalign()  or  valloc()
       (because  one  can  pass  to  free(3)  only  a pointer obtained from malloc(3), while, for
       example, memalign() would call malloc(3) and then align the obtained  value).   The  glibc
       implementation  allows  memory  obtained  from any of these functions to be reclaimed with
       free(3).

       The glibc malloc(3) always returns 8-byte aligned memory addresses, so these functions are
       needed only if you require larger alignment values.

SEE ALSO

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

COLOPHON

       This  page  is  part of release 4.16 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at https://www.kernel.org/doc/man-pages/.