NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <stdlib.h>

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

       #include <malloc.h>

       [[deprecated]] void *memalign(size_t alignment, size_t size);
       [[deprecated]] 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 >= 2.19: */ _DEFAULT_SOURCE
                   || /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
           Before glibc 2.12:
               _BSD_SOURCE || _XOPEN_SOURCE >= 500

DESCRIPTION

       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 *).   This  address can later be successfully passed to free(3).  If size is 0,
       then the value placed in *memptr is either NULL or a unique pointer value.

       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.

       aligned_alloc()  is  the  same  as  memalign(), except for the added restriction that alignment must be a
       power of two.

       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 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.1-2008 TC2.

ERRORS

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

       ENOMEM Out of memory.

ATTRIBUTES

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

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

STANDARDS

       aligned_alloc()
              C11.

       posix_memalign()
              POSIX.1-2008.

       memalign()
       valloc()
              None.

       pvalloc()
              GNU.

HISTORY

       aligned_alloc()
              glibc 2.16.  C11.

       posix_memalign()
              glibc 2.1.91.  POSIX.1d, POSIX.1-2001.

       memalign()
              glibc 2.0.  SunOS 4.1.3.

       valloc()
              glibc 2.0.  3.0BSD.  Documented as obsolete in 4.3BSD, and as legacy in SUSv2.

       pvalloc()
              glibc 2.0.

   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>.  glibc declares 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)