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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 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 4.09 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/.



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