mirror of https://github.com/mkerrisk/man-pages
126 lines
4.6 KiB
Groff
126 lines
4.6 KiB
Groff
.\" Hey Emacs! This file is -*- nroff -*- source.
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.\"
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.\" Copyright 2003 Abhijit Menon-Sen <ams@wiw.org>
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.\" 2005-04-08 mtk, noted kernel version and added BUGS
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.\"
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.TH POSIX_FADVISE 2 "14 Feb 2003" "Linux 2.5.60" "Linux Programmer's Manual"
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.SH NAME
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posix_fadvise \- predeclare an access pattern for file data
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.SH SYNOPSIS
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.nf
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.B #define _XOPEN_SOURCE 600
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.B #include <fcntl.h>
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.sp
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.BI "int posix_fadvise(int " fd ", off_t " offset ", off_t " len ", int " advice ");"
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.fi
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.SH DESCRIPTION
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Programs can use \fBposix_fadvise\fP() to announce an intention to access
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file data in a specific pattern in the future, thus allowing the kernel
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to perform appropriate optimisations.
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The \fIadvice\fP applies to a (not necessarily existent) region starting
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at \fIoffset\fP and extending for \fIlen\fP bytes (or until the end of
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the file if \fIlen\fP is 0) within the file referred to by \fIfd\fP. The
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advice is not binding; it merely constitutes an expectation on behalf of
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the application.
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Permissible values for \fIadvice\fP include:
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.TP
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.B POSIX_FADV_NORMAL
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Indicates that the application has no advice to give about its access
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pattern for the specified data. If no advice is given for an open file,
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this is the default assumption.
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.TP
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.B POSIX_FADV_SEQUENTIAL
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The application expects to access the specified data sequentially (with
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lower offsets read before higher ones).
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.TP
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.B POSIX_FADV_RANDOM
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The specified data will be accessed in random order.
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.TP
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.B POSIX_FADV_NOREUSE
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The specified data will be accessed only once.
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.TP
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.B POSIX_FADV_WILLNEED
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The specified data will be accessed in the near future.
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.TP
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.B POSIX_FADV_DONTNEED
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The specified data will not be accessed in the near future.
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.SH "RETURN VALUE"
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On success, zero is returned. On error, \-1 is returned, and \fIerrno\fP
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is set appropriately.
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.SH ERRORS
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.TP
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.B EBADF
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The \fIfd\fP argument was not a valid file descriptor.
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.TP
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.B EINVAL
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An invalid value was specified for \fIadvice\fP.
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.TP
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.B ESPIPE
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The specified file descriptor refers to a pipe or FIFO. (Linux actually
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returns EINVAL in this case.)
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.SH NOTES
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.BR posix_fadvise ()
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appeared in kernel 2.5.60.
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.\" Actually as fadvise64() -- MTK
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Under Linux, \fBPOSIX_FADV_NORMAL\fP sets the readahead window to the
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default size for the backing device; \fBPOSIX_FADV_SEQUENTIAL\fP doubles
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this size, and \fBPOSIX_FADV_RANDOM\fP disables file readahead entirely.
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These changes affect the entire file, not just the specified region
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(but other open file handles to the same file are unaffected).
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\fBPOSIX_FADV_WILLNEED\fP and \fBPOSIX_FADV_NOREUSE\fP both initiate a
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non-blocking read of the specified region into the page cache. The amount
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of data read may be decreased by the kernel depending on VM load. (A few
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megabytes will usually be fully satisfied, and more is rarely useful.)
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\fBPOSIX_FADV_DONTNEED\fP attempts to free cached pages associated with
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the specified region. This is useful, for example, while streaming large
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files. A program may periodically request the kernel to free cached data
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that has already been used, so that more useful cached pages are not
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discarded instead.
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Pages that have not yet been written out will be unaffected, so if the
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application wishes to guarantee that pages will be released, it should
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call \fBfsync\fP() or \fBfdatasync\fP() first.
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.SH "CONFORMING TO"
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POSIX.1-2001.
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Note that the type of the
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.I len
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parameter was changed from
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.I size_t
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to
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.I off_t
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in POSIX.1-2003 TC5.
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.SH BUGS
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In kernels before 2.6.6, if
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.I len
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was specified as 0, then this was interpreted literally as "zero bytes",
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rather than as meaning "all bytes through to the end of the file".
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.SH "SEE ALSO"
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.BR posix_madvise (2),
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.BR readahead (2),
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.BR posix_fallocate (3)
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