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NAME

       posix_fadvise - predeclare an access pattern for file data

SYNOPSIS

       #include <fcntl.h>

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

DESCRIPTION

       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 optimisations.

       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:

       POSIX_FADV_NORMAL
              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.

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

       POSIX_FADV_RANDOM
              The specified data will be accessed in random order.

       POSIX_FADV_NOREUSE
              The specified data will be accessed only once.

       POSIX_FADV_WILLNEED
              The specified data will be accessed in the near future.

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

RETURN VALUE

       On  success,  zero  is returned. On error, -1 is returned, and errno is
       set appropriately.

ERRORS

       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.  (Linux
              actually returns EINVAL in this case.)

NOTES

       posix_fadvise() appeared in kernel 2.5.60.

       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 and POSIX_FADV_NOREUSE both initiate a non-blocking
       read of the specified region into the page cache. The  amount  of  data
       read  may  be  decreased  by  the  kernel  depending on VM load. (A few
       megabytes will usually be fully satisfied, and more is rarely  useful.)

       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.

       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() or fdatasync() first.

CONFORMING TO

       SUSv3  (Advanced  Realtime  Option),  POSIX 1003.1-2003.  Note that the
       type of the len parameter was changed from size_t  to  off_t  in  POSIX
       1003.1-2003 TC5.

BUGS

       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".

SEE ALSO

       posix_fallocate(2), posix_madvise(2).