readahead — initiate file readahead into page cache
#define _GNU_SOURCE /* See feature_test_macros(7) */ #include <fcntl.h>
ssize_t
readahead( |
int fd, |
| off64_t offset, | |
size_t count); |
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).
The fd argument is
a file descriptor identifying the file which is to be read.
The offset argument
specifies the starting point from which data is to be read
and count specifies
the number of bytes to be read. I/O is performed in whole
pages, so that offset
is effectively rounded down to a page boundary and bytes are
read up to the next page boundary greater than or equal to
(offset+count).
readahead() does not read
beyond the end of the file. The file offset of the open file
description referred to by fd is left unchanged.
On success, readahead()
returns 0; on failure, −1 is returned, with
errno set to indicate the cause
of the error.
fd is not a
valid file descriptor or is not open for reading.
fd does not
refer to a file type to which readahead() can be applied.
The readahead() system call
appeared in Linux 2.4.13; glibc support has been provided
since version 2.3.
The readahead() system call
is Linux-specific, and its use should be avoided in portable
applications.
On some 32-bit architectures, the calling signature for this system call differs, for the reasons described in syscall(2).
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.
This page is part of release 4.07 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
|
This manpage is Copyright (C) 2004, Michael Kerrisk %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END 2004-05-40 Created by Michael Kerrisk <mtk.manpagesgmail.com> 2004-10-05 aeb, minor correction |