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READAHEAD(2) Linux Programmer's Manual READAHEAD(2)
NAME
readahead - initiate file readahead into page cache
SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h>
ssize_t readahead(int fd, off64_t offset, size_t count);
DESCRIPTION
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 effec‐
tively 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 current file offset of the open file referred to by fd is left unchanged.
RETURN VALUE
On success, readahead() returns 0; on failure, -1 is returned, with errno set to indicate
the cause of the error.
ERRORS
EBADF fd is not a valid file descriptor or is not open for reading.
EINVAL fd does not refer to a file type to which readahead() can be applied.
VERSIONS
The readahead() system call appeared in Linux 2.4.13; glibc support has been provided
since version 2.3.
CONFORMING TO
The readahead() system call is Linux-specific, and its use should be avoided in portable
applications.
NOTES
On some 32-bit architectures, the calling signature for this system call differs, for the
reasons described in syscall(2).
BUGS
readahead() attempts to schedule the reads in the background and return immediately. How‐
ever, 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.
SEE ALSO
lseek(2), madvise(2), mmap(2), posix_fadvise(2), read(2)
COLOPHON
This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.
Linux 2014-03-15 READAHEAD(2)
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