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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 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.
RETURN VALUE
On success, readahead() returns 0; on failure, -1 is returned, with errno 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 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. However, it may block while it reads the filesystem metadata needed to lo‐
cate 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 2021-03-22 READAHEAD(2)