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 current file offset of the open file referred to by 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  4.04  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
       http://www.kernel.org/doc/man-pages/.