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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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readahead(2) System Calls Manual readahead(2)
readahead - initiate file readahead into page cache
Standard C library (libc, -lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#define _FILE_OFFSET_BITS 64
#include <fcntl.h>
ssize_t readahead(int fd, off_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 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.
On some 32-bit architectures, the calling signature for this
system call differs, for the reasons described in syscall(2).
Linux.
Linux 2.4.13, glibc 2.3.
_FILE_OFFSET_BITS should be defined to be 64 in code that uses a
pointer to readahead, if the code is intended to be portable to
traditional 32-bit x86 and ARM platforms where off_t's width
defaults to 32 bits.
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 readahead(2)
Pages that refer to this page: posix_fadvise(2), syscall(2), syscalls(2), off_t(3type), feature_test_macros(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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