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       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)):

           _XOPEN_SOURCE >= 600 || _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

       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

       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

              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 2.16, Linux returned EINVAL  in  this


       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 option.


       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

       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)


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