man2/fsync.2

   1 .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) and
   2 .\" and Copyright 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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  25 .\"
  26 .\" Modified 21 Aug 1994 by Michael Chastain <mec@shell.portal.com>:
  27 .\"   Removed note about old libc (pre-4.5.26) translating to 'sync'.
  28 .\" Modified 15 Apr 1995 by Michael Chastain <mec@shell.portal.com>:
  29 .\"   Added `see also' section.
  30 .\" Modified 13 Apr 1996 by Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
  31 .\"   Added remarks about fdatasync.
  32 .\" Modified 31 Jan 1997 by Eric S. Raymond <esr@thyrsus.com>
  33 .\" Modified 18 Apr 2001 by Andi Kleen
  34 .\"   Fix description to describe what it really does; add a few caveats.
  35 .\" 2006-04-28, mtk, substantial rewrite of various parts.
  36 .\" 2012-02-27 Various changes by Christoph Hellwig <hch@lst.de>
  37 .\"
  38 .TH FSYNC 2 2019-03-06 "Linux" "Linux Programmer's Manual"
  39 .SH NAME
  40 fsync, fdatasync \- synchronize a file's in-core state with storage device
  41 .SH SYNOPSIS
  42 .B #include <unistd.h>
  43 .PP
  44 .BI "int fsync(int " fd );
  45 .PP
  46 .BI "int fdatasync(int " fd );
  47 .PP
  48 .in -4n
  49 Feature Test Macro Requirements for glibc (see
  50 .BR feature_test_macros (7)):
  51 .in
  52 .PP
  53 .BR fsync ():
  54     Glibc 2.16 and later:
  55         No feature test macros need be defined
  56     Glibc up to and including 2.15:
  57         _BSD_SOURCE || _XOPEN_SOURCE
  58             || /* since glibc 2.8: */ _POSIX_C_SOURCE\ >=\ 200112L
  59 .br
  60 .BR fdatasync ():
  61     _POSIX_C_SOURCE\ >=\ 199309L || _XOPEN_SOURCE\ >=\ 500
  62 .SH DESCRIPTION
  63 .BR fsync ()
  64 transfers ("flushes") all modified in-core data of
  65 (i.e., modified buffer cache pages for) the
  66 file referred to by the file descriptor
  67 .I fd
  68 to the disk device (or other permanent storage device) so that all
  69 changed information can be retrieved even if the system crashes or
  70 is rebooted.
  71 This includes writing through or flushing a disk cache if present.
  72 The call blocks until the device reports that the transfer has completed.
  73 .PP
  74 As well as flushing the file data,
  75 .BR fsync ()
  76 also flushes the metadata information associated with the file (see
  77 .BR inode (7)).
  78 .PP
  79 Calling
  80 .BR fsync ()
  81 does not necessarily ensure
  82 that the entry in the directory containing the file has also reached disk.
  83 For that an explicit
  84 .BR fsync ()
  85 on a file descriptor for the directory is also needed.
  86 .PP
  87 .BR fdatasync ()
  88 is similar to
  89 .BR fsync (),
  90 but does not flush modified metadata unless that metadata
  91 is needed in order to allow a subsequent data retrieval to be
  92 correctly handled.
  93 For example, changes to
  94 .I st_atime
  95 or
  96 .I st_mtime
  97 (respectively, time of last access and
  98 time of last modification; see
  99 .BR inode (7))
 100 do not require flushing because they are not necessary for
 101 a subsequent data read to be handled correctly.
 102 On the other hand, a change to the file size
 103 .RI ( st_size ,
 104 as made by say
 105 .BR ftruncate (2)),
 106 would require a metadata flush.
 107 .PP
 108 The aim of
 109 .BR fdatasync ()
 110 is to reduce disk activity for applications that do not
 111 require all metadata to be synchronized with the disk.
 112 .SH RETURN VALUE
 113 On success, these system calls return zero.
 114 On error, \-1 is returned, and
 115 .I errno
 116 is set appropriately.
 117 .SH ERRORS
 118 .TP
 119 .B EBADF
 120 .I fd
 121 is not a valid open file descriptor.
 122 .TP
 123 .B EIO
 124 An error occurred during synchronization.
 125 This error may relate to data written to some other file descriptor
 126 on the same file.
 127 Since Linux 4.13,
 128 .\" commit 088737f44bbf6378745f5b57b035e57ee3dc4750
 129 errors from write-back will be reported to
 130 all file descriptors that might have written the data which triggered
 131 the error.
 132 Some filesystems (e.g., NFS) keep close track of which data
 133 came through which file descriptor, and give more precise reporting.
 134 Other filesystems (e.g., most local filesystems) will report errors to
 135 all file descriptors that were open on the file when the error was recorded.
 136 .TP
 137 .B ENOSPC
 138 Disk space was exhausted while synchronizing.
 139 .TP
 140 .BR EROFS ", " EINVAL
 141 .I fd
 142 is bound to a special file (e.g., a pipe, FIFO, or socket)
 143 which does not support synchronization.
 144 .TP
 145 .BR ENOSPC ", " EDQUOT
 146 .I fd
 147 is bound to a file on NFS or another filesystem which does not allocate
 148 space at the time of a
 149 .BR write (2)
 150 system call, and some previous write failed due to insufficient
 151 storage space.
 152 .SH CONFORMING TO
 153 POSIX.1-2001, POSIX.1-2008, 4.3BSD.
 154 .PP
 155 On POSIX systems on which
 156 .BR fdatasync ()
 157 is available,
 158 .B _POSIX_SYNCHRONIZED_IO
 159 is defined in
 160 .I <unistd.h>
 161 to a value greater than 0.
 162 (See also
 163 .BR sysconf (3).)
 164 .\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.
 165 .\" -1: unavailable, 0: ask using sysconf().
 166 .\" glibc defines them to 1.
 167 .SH NOTES
 168 On some UNIX systems (but not Linux),
 169 .I fd
 170 must be a
 171 .I writable
 172 file descriptor.
 173 .PP
 174 In Linux 2.2 and earlier,
 175 .BR fdatasync ()
 176 is equivalent to
 177 .BR fsync (),
 178 and so has no performance advantage.
 179 .PP
 180 The
 181 .BR fsync ()
 182 implementations in older kernels and lesser used filesystems
 183 do not know how to flush disk caches.
 184 In these cases disk caches need to be disabled using
 185 .BR hdparm (8)
 186 or
 187 .BR sdparm (8)
 188 to guarantee safe operation.
 189 .SH SEE ALSO
 190 .BR sync (1),
 191 .BR bdflush (2),
 192 .BR open (2),
 193 .BR posix_fadvise (2),
 194 .BR pwritev (2),
 195 .BR sync (2),
 196 .BR sync_file_range (2),
 197 .BR fflush (3),
 198 .BR fileno (3),
 199 .BR hdparm (8),
 200 .BR mount (8)