pvalloc(3)

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).

   ┌─────────────────┬───────────────┬────────────────┐
   │Interface        │ Attribute     │ Value          │
   ├─────────────────┼───────────────┼────────────────┤
   │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 4.09 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/.

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