posix_fadvise - predeclare an access pattern for file data


   #include <fcntl.h>

   int posix_fadvise(int fd, off_t offset, off_t len, int advice);

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

       _POSIX_C_SOURCE >= 200112L


   Programs  can  use  posix_fadvise()  to announce an intention to access
   file data in a specific pattern in the future, thus allowing the kernel
   to perform appropriate optimizations.

   The  advice  applies to a (not necessarily existent) region starting at
   offset and extending for len bytes (or until the end of the file if len
   is 0) within the file referred to by fd.  The advice is not binding; it
   merely constitutes an expectation on behalf of the application.

   Permissible values for advice include:

          Indicates that the application has no advice to give  about  its
          access  pattern  for  the specified data.  If no advice is given
          for an open file, this is the default assumption.

          The  application  expects   to   access   the   specified   data
          sequentially (with lower offsets read before higher ones).

          The specified data will be accessed in random order.

          The specified data will be accessed only once.

          The specified data will be accessed in the near future.

          The specified data will not be accessed in the near future.


   On success, zero is returned.  On error, an error number is returned.


   EBADF  The fd argument was not a valid file descriptor.

   EINVAL An invalid value was specified for advice.

   ESPIPE The specified file descriptor refers to a pipe or FIFO.  (ESPIPE
          is the error specified  by  POSIX,  but  before  kernel  version
          2.6.16, Linux returned EINVAL in this case.)


   Kernel  support  first  appeared in Linux 2.5.60; the underlying system
   call is called fadvise64().  Library support has  been  provided  since
   glibc version 2.2, via the wrapper function posix_fadvise().

   Since  Linux  3.18, support for the underlying system call is optional,
   depending on the setting of  the  CONFIG_ADVISE_SYSCALLS  configuration


   POSIX.1-2001, POSIX.1-2008.  Note that the type of the len argument was
   changed from size_t to off_t in POSIX.1-2003 TC1.


   Under Linux, POSIX_FADV_NORMAL sets the readahead window to the default
   size  for  the backing device; POSIX_FADV_SEQUENTIAL doubles this size,
   and POSIX_FADV_RANDOM disables file readahead entirely.  These  changes
   affect  the  entire file, not just the specified region (but other open
   file handles to the same file are unaffected).

   POSIX_FADV_WILLNEED initiates  a  nonblocking  read  of  the  specified
   region  into  the page cache.  The amount of data read may be decreased
   by the kernel depending on virtual memory load.  (A few megabytes  will
   usually be fully satisfied, and more is rarely useful.)

   In  kernels before 2.6.18, POSIX_FADV_NOREUSE had the same semantics as
   POSIX_FADV_WILLNEED.  This was probably a  bug;  since  kernel  2.6.18,
   this flag is a no-op.

   POSIX_FADV_DONTNEED  attempts  to free cached pages associated with the
   specified region.  This is useful, for example, while  streaming  large
   files.   A  program  may periodically request the kernel to free cached
   data that has already been used, so that more useful cached  pages  are
   not discarded instead.

   Requests  to  discard  partial  pages are ignored.  It is preferable to
   preserve needed data than discard unneeded data.   If  the  application
   requires  that  data  be considered for discarding, then offset and len
   must be page-aligned.

   Pages that have not yet been written out will be unaffected, so if  the
   application  wishes to guarantee that pages will be released, it should
   call fsync(2) or fdatasync(2) first.

   C library/kernel differences
   The name of the wrapper function in the C library  is  posix_fadvise().
   The   underlying  system  call  is  called  fadvise64()  (or,  on  some
   architectures, fadvise64_64()).

   Architecture-specific variants
   Some architectures require 64-bit arguments to be aligned in a suitable
   pair  of  registers  (see  syscall(2)  for  further  detail).   On such
   architectures, the call  signature  of  posix_fadvise()  shown  in  the
   SYNOPSIS  would force a register to be wasted as padding between the fd
   and offset arguments.  Therefore, these architectures define a  version
   of the system call that orders the arguments suitably, but is otherwise
   exactly the same as posix_fadvise().

   For example, since Linux 2.6.14, ARM has the following system call:

       long arm_fadvise64_64(int fd, int advice,
                             loff_t offset, loff_t len);

   These  architecture-specific  details   are   generally   hidden   from
   applications  by  the  glibc  posix_fadvise()  wrapper  function, which
   invokes the appropriate architecture-specific system call.


   In kernels before 2.6.6, if len was  specified  as  0,  then  this  was
   interpreted  literally  as  "zero  bytes",  rather than as meaning "all
   bytes through to the end of the file".


   readahead(2), sync_file_range(2), posix_fallocate(3), posix_madvise(3)


   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


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