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 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 4.15 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/.