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       posix_fadvise - predeclare an access pattern for file data


       #define _XOPEN_SOURCE 600
       #include <fcntl.h>

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


       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.  (Linux
              actually returns EINVAL in this case.)


       posix_fadvise() appeared in kernel  2.5.60.   Glibc  support  has  been
       provided since version 2.2.


       POSIX.1-2001.   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.

       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.


       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),           posix_fallocate(3),           posix_madvise(3),


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