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)