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.\" This manpage is Copyright (C) 2004, Michael Kerrisk
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 2004-05-40 Created by Michael Kerrisk <mtk.manpages@gmail.com>
.\" 2004-10-05 aeb, minor correction
.\"
.TH readahead 2 2024-05-02 "Linux man-pages (unreleased)"
.SH NAME
readahead \- initiate file readahead into page cache
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
.B #define _FILE_OFFSET_BITS 64
.B #include <fcntl.h>
.P
.BI "ssize_t readahead(int " fd ", off_t " offset ", size_t " count );
.fi
.SH DESCRIPTION
.BR readahead ()
initiates readahead on a file so that subsequent reads from that file will
be satisfied from the cache, and not block on disk I/O
(assuming the readahead was initiated early enough and that other activity
on the system did not in the meantime flush pages from the cache).
.P
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.
The file offset of the open file description referred to by the file descriptor
.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 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
On some 32-bit architectures,
the calling signature for this system call differs,
for the reasons described in
.BR syscall (2).
.SH STANDARDS
Linux.
.SH HISTORY
Linux 2.4.13,
glibc 2.3.
.SH NOTES
.B _FILE_OFFSET_BITS
should be defined to be 64 in code that uses a pointer to
.BR readahead ,
if the code is intended to be portable
to traditional 32-bit x86 and ARM platforms where
.BR off_t 's
width defaults to 32 bits.
.SH BUGS
.BR readahead ()
attempts to schedule the reads in the background and return immediately.
However, it may block while it reads the filesystem metadata needed
to locate the requested blocks.
This occurs frequently with ext[234] on large files
using indirect blocks instead of extents,
giving the appearance that the call blocks until the requested data has
been read.
.SH SEE ALSO
.BR lseek (2),
.BR madvise (2),
.BR mmap (2),
.BR posix_fadvise (2),
.BR read (2)
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