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

       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  sequen-
	      tially (with lower offsets read before higher ones).

	      The specified data will be accessed in random order.

	      The specified data will be accessed only once.

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

	      The specified data will be accessed in the near future.

	      POSIX_FADV_WILLNEED  initiates  a nonblocking read of the speci-
	      fied 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.)

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

	      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.

	      The implementation may attempt to write back dirty pages in  the
	      specified	 region,  but  this  is not guaranteed.	 Any unwritten
	      dirty pages will not be freed.  If  the  application  wishes  to
	      ensure  that  dirty  pages  will	be  released,  it  should call
	      fsync(2) or fdatasync(2) first.

       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).

       The  contents  of  the  kernel  buffer  cache  can  be  cleared via the
       /proc/sys/vm/drop_caches interface described in proc(5).

       One can obtain a snapshot of which pages of a file are resident in  the
       buffer  cache  by  opening  a  file,  mapping it with mmap(2), and then
       applying mincore(2) to the mapping.

   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 architec-
       tures, 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 archi-
       tectures, the call signature of posix_fadvise() shown in	 the  SYNOPSIS
       would  force a register to be wasted as padding between the fd and off-
       set 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  applica-
       tions  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".

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

       This  page  is  part of release 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
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Linux				  2017-03-13		      POSIX_FADVISE(2)