man2/readahead.2

   1 .\" This manpage is Copyright (C) 2004, Michael Kerrisk
   2 .\"
   3 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   4 .\"
   5 .\" 2004-05-40 Created by Michael Kerrisk <mtk.manpages@gmail.com>
   6 .\" 2004-10-05 aeb, minor correction
   7 .\"
   8 .TH READAHEAD 2 2022-09-09 "Linux man-pages (unreleased)"
   9 .SH NAME
  10 readahead \- initiate file readahead into page cache
  11 .SH LIBRARY
  12 Standard C library
  13 .RI ( libc ", " \-lc )
  14 .SH SYNOPSIS
  15 .nf
  16 .BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
  17 .B #include <fcntl.h>
  18 .PP
  19 .BI "ssize_t readahead(int " fd ", off64_t " offset ", size_t " count );
  20 .fi
  21 .SH DESCRIPTION
  22 .BR readahead ()
  23 initiates readahead on a file so that subsequent reads from that file will
  24 be satisfied from the cache, and not block on disk I/O
  25 (assuming the readahead was initiated early enough and that other activity
  26 on the system did not in the meantime flush pages from the cache).
  27 .PP
  28 The
  29 .I fd
  30 argument is a file descriptor identifying the file which is
  31 to be read.
  32 The
  33 .I offset
  34 argument specifies the starting point from which data is to be read
  35 and
  36 .I count
  37 specifies the number of bytes to be read.
  38 I/O is performed in whole pages, so that
  39 .I offset
  40 is effectively rounded down to a page boundary
  41 and bytes are read up to the next page boundary greater than or
  42 equal to
  43 .IR "(offset+count)" .
  44 .BR readahead ()
  45 does not read beyond the end of the file.
  46 The file offset of the open file description referred to by the file descriptor
  47 .I fd
  48 is left unchanged.
  49 .SH RETURN VALUE
  50 On success,
  51 .BR readahead ()
  52 returns 0; on failure, \-1 is returned, with
  53 .I errno
  54 set to indicate the error.
  55 .SH ERRORS
  56 .TP
  57 .B EBADF
  58 .I fd
  59 is not a valid file descriptor or is not open for reading.
  60 .TP
  61 .B EINVAL
  62 .I fd
  63 does not refer to a file type to which
  64 .BR readahead ()
  65 can be applied.
  66 .SH VERSIONS
  67 The
  68 .BR readahead ()
  69 system call appeared in Linux 2.4.13;
  70 glibc support has been provided since version 2.3.
  71 .SH STANDARDS
  72 The
  73 .BR readahead ()
  74 system call is Linux-specific, and its use should be avoided
  75 in portable applications.
  76 .SH NOTES
  77 On some 32-bit architectures,
  78 the calling signature for this system call differs,
  79 for the reasons described in
  80 .BR syscall (2).
  81 .SH BUGS
  82 .BR readahead ()
  83 attempts to schedule the reads in the background and return immediately.
  84 However, it may block while it reads the filesystem metadata needed
  85 to locate the requested blocks.
  86 This occurs frequently with ext[234] on large files
  87 using indirect blocks instead of extents,
  88 giving the appearance that the call blocks until the requested data has
  89 been read.
  90 .SH SEE ALSO
  91 .BR lseek (2),
  92 .BR madvise (2),
  93 .BR mmap (2),
  94 .BR posix_fadvise (2),
  95 .BR read (2)