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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 descrip-
     tion 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 meta-
     data  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.