348 lines
9.0 KiB
HTML
348 lines
9.0 KiB
HTML
|
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
|
|
<HTML><HEAD><TITLE>Man page of POSIX_FADVISE</TITLE>
|
|
</HEAD><BODY>
|
|
<H1>POSIX_FADVISE</H1>
|
|
Section: Linux Programmer's Manual (2)<BR>Updated: 2019-03-06<BR><A HREF="#index">Index</A>
|
|
<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
|
|
|
|
<A NAME="lbAB"> </A>
|
|
<H2>NAME</H2>
|
|
|
|
posix_fadvise - predeclare an access pattern for file data
|
|
<A NAME="lbAC"> </A>
|
|
<H2>SYNOPSIS</H2>
|
|
|
|
<PRE>
|
|
<B>#include <<A HREF="file:///usr/include/fcntl.h">fcntl.h</A>></B>
|
|
|
|
<B>int posix_fadvise(int </B><I>fd</I><B>, off_t </B><I>offset</I><B>, off_t </B><I>len</I><B>, int </B><I>advice</I><B>);</B>
|
|
</PRE>
|
|
|
|
<P>
|
|
|
|
|
|
|
|
Feature Test Macro Requirements for glibc (see
|
|
<B><A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A></B>(7)):
|
|
|
|
|
|
<P>
|
|
|
|
<B>posix_fadvise</B>():
|
|
|
|
<DL COMPACT><DT id="1"><DD>
|
|
_POSIX_C_SOURCE >= 200112L
|
|
</DL>
|
|
|
|
|
|
<A NAME="lbAD"> </A>
|
|
<H2>DESCRIPTION</H2>
|
|
|
|
Programs can use
|
|
<B>posix_fadvise</B>()
|
|
|
|
to announce an intention to access
|
|
file data in a specific pattern in the future, thus allowing the kernel
|
|
to perform appropriate optimizations.
|
|
<P>
|
|
|
|
The <I>advice</I> applies to a (not necessarily existent) region starting
|
|
at <I>offset</I> and extending for <I>len</I> bytes (or until the end of
|
|
the file if <I>len</I> is 0) within the file referred to by <I>fd</I>.
|
|
The <I>advice</I> is not binding;
|
|
it merely constitutes an expectation on behalf of
|
|
the application.
|
|
<P>
|
|
|
|
Permissible values for <I>advice</I> include:
|
|
<DL COMPACT>
|
|
<DT id="2"><B>POSIX_FADV_NORMAL</B>
|
|
|
|
<DD>
|
|
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.
|
|
<DT id="3"><B>POSIX_FADV_SEQUENTIAL</B>
|
|
|
|
<DD>
|
|
The application expects to access the specified data sequentially (with
|
|
lower offsets read before higher ones).
|
|
<DT id="4"><B>POSIX_FADV_RANDOM</B>
|
|
|
|
<DD>
|
|
The specified data will be accessed in random order.
|
|
<DT id="5"><B>POSIX_FADV_NOREUSE</B>
|
|
|
|
<DD>
|
|
The specified data will be accessed only once.
|
|
<DT id="6"><DD>
|
|
In kernels before 2.6.18, <B>POSIX_FADV_NOREUSE</B> had the
|
|
same semantics as <B>POSIX_FADV_WILLNEED</B>.
|
|
This was probably a bug; since kernel 2.6.18, this flag is a no-op.
|
|
<DT id="7"><B>POSIX_FADV_WILLNEED</B>
|
|
|
|
<DD>
|
|
The specified data will be accessed in the near future.
|
|
<DT id="8"><DD>
|
|
<B>POSIX_FADV_WILLNEED</B> 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.)
|
|
<DT id="9"><B>POSIX_FADV_DONTNEED</B>
|
|
|
|
<DD>
|
|
The specified data will not be accessed in the near future.
|
|
<DT id="10"><DD>
|
|
<B>POSIX_FADV_DONTNEED</B> 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.
|
|
<DT id="11"><DD>
|
|
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
|
|
<I>offset</I>
|
|
|
|
and
|
|
<I>len</I>
|
|
|
|
must be page-aligned.
|
|
<DT id="12"><DD>
|
|
The implementation
|
|
<I>may</I>
|
|
|
|
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
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fsync">fsync</A></B>(2)
|
|
|
|
or
|
|
<B><A HREF="/cgi-bin/man/man2html?2+fdatasync">fdatasync</A></B>(2)
|
|
|
|
first.
|
|
</DL>
|
|
<A NAME="lbAE"> </A>
|
|
<H2>RETURN VALUE</H2>
|
|
|
|
On success, zero is returned.
|
|
On error, an error number is returned.
|
|
<A NAME="lbAF"> </A>
|
|
<H2>ERRORS</H2>
|
|
|
|
<DL COMPACT>
|
|
<DT id="13"><B>EBADF</B>
|
|
|
|
<DD>
|
|
The <I>fd</I> argument was not a valid file descriptor.
|
|
<DT id="14"><B>EINVAL</B>
|
|
|
|
<DD>
|
|
An invalid value was specified for <I>advice</I>.
|
|
<DT id="15"><B>ESPIPE</B>
|
|
|
|
<DD>
|
|
The specified file descriptor refers to a pipe or FIFO.
|
|
(<B>ESPIPE</B>
|
|
|
|
is the error specified by POSIX,
|
|
but before kernel version 2.6.16,
|
|
|
|
Linux returned
|
|
<B>EINVAL</B>
|
|
|
|
in this case.)
|
|
</DL>
|
|
<A NAME="lbAG"> </A>
|
|
<H2>VERSIONS</H2>
|
|
|
|
Kernel support first appeared in Linux 2.5.60;
|
|
the underlying system call is called
|
|
<B>fadvise64</B>().
|
|
|
|
|
|
Library support has been provided since glibc version 2.2,
|
|
via the wrapper function
|
|
<B>posix_fadvise</B>().
|
|
|
|
<P>
|
|
|
|
Since Linux 3.18,
|
|
|
|
support for the underlying system call is optional,
|
|
depending on the setting of the
|
|
<B>CONFIG_ADVISE_SYSCALLS</B>
|
|
|
|
configuration option.
|
|
<A NAME="lbAH"> </A>
|
|
<H2>CONFORMING TO</H2>
|
|
|
|
POSIX.1-2001, POSIX.1-2008.
|
|
Note that the type of the
|
|
<I>len</I>
|
|
|
|
argument was changed from
|
|
<I>size_t</I>
|
|
|
|
to
|
|
<I>off_t</I>
|
|
|
|
in POSIX.1-2003 TC1.
|
|
<A NAME="lbAI"> </A>
|
|
<H2>NOTES</H2>
|
|
|
|
Under Linux, <B>POSIX_FADV_NORMAL</B> sets the readahead window to the
|
|
default size for the backing device; <B>POSIX_FADV_SEQUENTIAL</B> doubles
|
|
this size, and <B>POSIX_FADV_RANDOM</B> 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).
|
|
<P>
|
|
|
|
The contents of the kernel buffer cache can be cleared via the
|
|
<I>/proc/sys/vm/drop_caches</I>
|
|
|
|
interface described in
|
|
<B><A HREF="/cgi-bin/man/man2html?5+proc">proc</A></B>(5).
|
|
|
|
<P>
|
|
|
|
One can obtain a snapshot of which pages of a file are resident
|
|
in the buffer cache by opening a file, mapping it with
|
|
<B><A HREF="/cgi-bin/man/man2html?2+mmap">mmap</A></B>(2),
|
|
|
|
and then applying
|
|
<B><A HREF="/cgi-bin/man/man2html?2+mincore">mincore</A></B>(2)
|
|
|
|
to the mapping.
|
|
<A NAME="lbAJ"> </A>
|
|
<H3>C library/kernel differences</H3>
|
|
|
|
The name of the wrapper function in the C library is
|
|
<B>posix_fadvise</B>().
|
|
|
|
The underlying system call is called
|
|
<B>fadvise64</B>()
|
|
|
|
(or, on some architectures,
|
|
<B>fadvise64_64</B>());
|
|
|
|
the difference between the two is that the former system call
|
|
assumes that the type of the <I>len</I> argument is <I>size_t</I>,
|
|
while the latter expects <I>loff_t</I> there.
|
|
<A NAME="lbAK"> </A>
|
|
<H3>Architecture-specific variants</H3>
|
|
|
|
Some architectures require
|
|
64-bit arguments to be aligned in a suitable pair of registers (see
|
|
<B><A HREF="/cgi-bin/man/man2html?2+syscall">syscall</A></B>(2)
|
|
|
|
for further detail).
|
|
On such architectures, the call signature of
|
|
<B>posix_fadvise</B>()
|
|
|
|
shown in the SYNOPSIS would force
|
|
a register to be wasted as padding between the
|
|
<I>fd</I>
|
|
|
|
and
|
|
<I>offset</I>
|
|
|
|
arguments.
|
|
Therefore, these architectures define a version of the
|
|
system call that orders the arguments suitably,
|
|
but is otherwise exactly the same as
|
|
<B>posix_fadvise</B>().
|
|
|
|
<P>
|
|
|
|
For example, since Linux 2.6.14, ARM has the following system call:
|
|
<P>
|
|
|
|
|
|
|
|
<B>long arm_fadvise64_64(int </B><I>fd</I><B>, int </B><I>advice</I><B>,</B>
|
|
|
|
<B> loff_t </B><I>offset</I><B>, loff_t </B><I>len</I><B>);</B>
|
|
|
|
|
|
|
|
<P>
|
|
|
|
These architecture-specific details are generally
|
|
hidden from applications by the glibc
|
|
<B>posix_fadvise</B>()
|
|
|
|
wrapper function,
|
|
which invokes the appropriate architecture-specific system call.
|
|
<A NAME="lbAL"> </A>
|
|
<H2>BUGS</H2>
|
|
|
|
In kernels before 2.6.6, if
|
|
<I>len</I>
|
|
|
|
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".
|
|
<A NAME="lbAM"> </A>
|
|
<H2>SEE ALSO</H2>
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?1+fincore">fincore</A></B>(1),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+mincore">mincore</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+readahead">readahead</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+sync_file_range">sync_file_range</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?3+posix_fallocate">posix_fallocate</A></B>(3),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?3+posix_madvise">posix_madvise</A></B>(3)
|
|
|
|
<A NAME="lbAN"> </A>
|
|
<H2>COLOPHON</H2>
|
|
|
|
This page is part of release 5.05 of the Linux
|
|
<I>man-pages</I>
|
|
|
|
project.
|
|
A description of the project,
|
|
information about reporting bugs,
|
|
and the latest version of this page,
|
|
can be found at
|
|
<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
|
|
<P>
|
|
|
|
<HR>
|
|
<A NAME="index"> </A><H2>Index</H2>
|
|
<DL>
|
|
<DT id="16"><A HREF="#lbAB">NAME</A><DD>
|
|
<DT id="17"><A HREF="#lbAC">SYNOPSIS</A><DD>
|
|
<DT id="18"><A HREF="#lbAD">DESCRIPTION</A><DD>
|
|
<DT id="19"><A HREF="#lbAE">RETURN VALUE</A><DD>
|
|
<DT id="20"><A HREF="#lbAF">ERRORS</A><DD>
|
|
<DT id="21"><A HREF="#lbAG">VERSIONS</A><DD>
|
|
<DT id="22"><A HREF="#lbAH">CONFORMING TO</A><DD>
|
|
<DT id="23"><A HREF="#lbAI">NOTES</A><DD>
|
|
<DL>
|
|
<DT id="24"><A HREF="#lbAJ">C library/kernel differences</A><DD>
|
|
<DT id="25"><A HREF="#lbAK">Architecture-specific variants</A><DD>
|
|
</DL>
|
|
<DT id="26"><A HREF="#lbAL">BUGS</A><DD>
|
|
<DT id="27"><A HREF="#lbAM">SEE ALSO</A><DD>
|
|
<DT id="28"><A HREF="#lbAN">COLOPHON</A><DD>
|
|
</DL>
|
|
<HR>
|
|
This document was created by
|
|
<A HREF="/cgi-bin/man/man2html">man2html</A>,
|
|
using the manual pages.<BR>
|
|
Time: 00:05:33 GMT, March 31, 2021
|
|
</BODY>
|
|
</HTML>
|