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 cause of
       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  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)

COLOPHON

       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)