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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
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READAHEAD(2) Linux Programmer's Manual READAHEAD(2)
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 the file descriptor fd is left unchanged.
On success, readahead() returns 0; on failure, -1 is returned,
with errno set to indicate the cause of the error.
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.
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.
lseek(2), madvise(2), mmap(2), posix_fadvise(2), read(2)
This page is part of release 5.10 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/.
Linux 2019-03-06 READAHEAD(2)
Pages that refer to this page: posix_fadvise(2), syscall(2), syscalls(2), feature_test_macros(7)
Copyright and license for this manual page
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