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POSIX_FADVISE(2)           Linux Programmer's Manual          POSIX_FADVISE(2)

       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

              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  unaf-

       POSIX_FADV_WILLNEED  initiates  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 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), feature_test_macros(7)

       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at

Linux                             2003-02-14                  POSIX_FADVISE(2)