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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  on success.  On error, NULL is returned, and errno is set to indicate the cause of
       the error.

       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

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