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.\" 2004-05-40 Created by Michael Kerrisk <mtk.manpages@gmail.com>
.\" 2004-10-05 aeb, minor correction
.\"
.TH READAHEAD 2 2010-09-10 "Linux" "Linux Programmer's Manual"
.SH NAME
readahead \- perform file readahead into page cache
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
.B #include <fcntl.h>
.sp
.BI "ssize_t readahead(int " fd ", off64_t " offset ", size_t " count );
.fi
.SH DESCRIPTION
.BR readahead ()
populates the page cache with data from a file so that subsequent
reads from that file will not block on disk I/O.
The
.I fd
argument is a file descriptor identifying the file which is
to be read.
The
.I offset
argument specifies the starting point from which data is to be read
and
.I count
specifies the number of bytes to be read.
I/O is performed in whole pages, so that
.I offset
is effectively rounded down to a page boundary
and bytes are read up to the next page boundary greater than or
equal to
.IR "(offset+count)" .
.BR readahead ()
does not read beyond the end of the file.
.BR readahead ()
blocks until the specified data has been read.
The current file offset of the open file referred to by
.I fd
is left unchanged.
.SH "RETURN VALUE"
On success,
.BR readahead ()
returns 0; on failure, \-1 is returned, with
.I errno
set to indicate the cause of the error.
.SH ERRORS
.TP
.B EBADF
.I fd
is not a valid file descriptor or is not open for reading.
.TP
.B EINVAL
.I fd
does not refer to a file type to which
.BR readahead ()
can be applied.
.SH VERSIONS
The
.BR readahead ()
system call appeared in Linux 2.4.13;
glibc support has been provided since version 2.3.
.SH "CONFORMING TO"
The
.BR readahead ()
system call is Linux-specific, and its use should be avoided
in portable applications.
.SH "SEE ALSO"
.BR lseek (2),
.BR madvise (2),
.BR mmap (2),
.BR posix_fadvise (2),
.BR read (2)
.SH COLOPHON
This page is part of release 3.44 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.