readahead(2) System Calls Manual readahead(2)
NAME
readahead - initiate file readahead into page cache
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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);
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 ac-
tivity 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 be-
yond the end of the file. The file offset of the open file description
referred to by the file descriptor fd is left unchanged.
RETURN VALUE
On success, readahead() returns 0; on failure, -1 is returned, with er-
rno set to indicate 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 ap-
plied.
VERSIONS
On some 32-bit architectures, the calling signature for this system
call differs, for the reasons described in syscall(2).
STANDARDS
Linux.
HISTORY
Linux 2.4.13, glibc 2.3.
NOTES
_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 tradi-
tional 32-bit x86 and ARM platforms where off_t's width defaults to 32
bits.
BUGS
readahead() attempts to schedule the reads in the background and return
immediately. However, it may block while it reads the filesystem meta-
data 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)
Linux man-pages 6.7 2023-10-31 readahead(2)
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