man2/close.2

   1 .\" This manpage is Copyright (C) 1992 Drew Eckhardt;
   2 .\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
   3 .\" and Copyright (C) 2016 Michael Kerrisk <mtk.manpages@gmail.com>
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  26 .\"
  27 .\" Modified Wed Jul 21 22:40:25 1993 by Rik Faith <faith@cs.unc.edu>
  28 .\" Modified Sat Feb 18 15:27:48 1995 by Michael Haardt
  29 .\" Modified Sun Apr 14 11:40:50 1996 by Andries Brouwer <aeb@cwi.nl>:
  30 .\"   corrected description of effect on locks (thanks to
  31 .\"   Tigran Aivazian <tigran@sco.com>).
  32 .\" Modified Fri Jan 31 16:21:46 1997 by Eric S. Raymond <esr@thyrsus.com>
  33 .\" Modified 2000-07-22 by Nicolás Lichtmaier <nick@debian.org>
  34 .\"   added note about close(2) not guaranteeing that data is safe on close.
  35 .\"
  36 .TH CLOSE 2 2016-10-08 "Linux" "Linux Programmer's Manual"
  37 .SH NAME
  38 close \- close a file descriptor
  39 .SH SYNOPSIS
  40 .nf
  41 .B #include <unistd.h>
  42 .sp
  43 .BI "int close(int " fd );
  44 .fi
  45 .SH DESCRIPTION
  46 .BR close ()
  47 closes a file descriptor, so that it no longer refers to any file and
  48 may be reused.
  49 Any record locks (see
  50 .BR fcntl (2))
  51 held on the file it was associated with,
  52 and owned by the process, are removed (regardless of the file
  53 descriptor that was used to obtain the lock).
  54 .PP
  55 If
  56 .I fd
  57 is the last file descriptor referring to the underlying
  58 open file description (see
  59 .BR open (2)),
  60 the resources associated with the open file description are freed;
  61 if the file descriptor was the last reference to a file which has been
  62 removed using
  63 .BR unlink (2),
  64 the file is deleted.
  65 .SH RETURN VALUE
  66 .BR close ()
  67 returns zero on success.
  68 On error, \-1 is returned, and
  69 .I errno
  70 is set appropriately.
  71 .SH ERRORS
  72 .TP
  73 .B EBADF
  74 .I fd
  75 isn't a valid open file descriptor.
  76 .TP
  77 .B EINTR
  78 The
  79 .BR close ()
  80 call was interrupted by a signal; see
  81 .BR signal (7).
  82 .TP
  83 .B EIO
  84 An I/O error occurred.
  85 .PP
  86 See NOTES for a discussion of why
  87 .BR close ()
  88 should not be retried after an error.
  89 .SH CONFORMING TO
  90 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
  91 .\" SVr4 documents an additional ENOLINK error condition.
  92 .SH NOTES
  93 A careful programmer will check the return value of
  94 .BR close (),
  95 since it is quite possible that errors on a previous
  96 .BR write (2)
  97 operation are reported only on the final
  98 .BR close ()
  99 that releases the open file description.
 100 Failing to check the return value when closing a file may lead to
 101 .I silent
 102 loss of data.
 103 This can especially be observed with NFS and with disk quota.
 104
 105 Note, however, that a failure return should be used only for
 106 diagnostic purposes (i.e., a warning to the application that there
 107 may still be I/O pending or there may have been failed I/O)
 108 or remedial purposes
 109 (e.g., writing the file once more or creating a backup).
 110
 111 Retrying the
 112 .BR close ()
 113 after a failure return is the wrong thing to do,
 114 .\" The file descriptor is released early in close();
 115 .\" close() ==> __close_fd():
 116 .\"                     __put_unused_fd() ==> __clear_open_fd()
 117 .\"                     return filp_close(file, files);
 118 .\"
 119 .\" The errors are returned by filp_close() after the FD has been
 120 .\" cleared for re-use.
 121 since this may cause a reused file descriptor
 122 from another thread to be closed.
 123 This can occur because the Linux kernel
 124 .I always
 125 releases the file descriptor early in the close
 126 operation, freeing it for reuse;
 127 the steps that may return an error,
 128 .\" filp_close()
 129 such as flushing data to the filesystem or device,
 130 occur only later in the close operation.
 131
 132 Many other implementations similarly always close the file descriptor
 133 .\" FreeBSD documents this explicitly. From the look of the source code
 134 .\" SVR4, ancient SunOS, later Solaris, and AIX all do this.
 135 (except in the case of
 136 .BR EBADF ,
 137 meaning that the file descriptor was invalid)
 138 even if they subsequently report an error on return from
 139 .BR close ().
 140 POSIX.1 is currently silent on this point,
 141 but there are plans to mandate this behavior in the next major release
 142 .\" Issue 8
 143 of the standard
 144
 145 A careful programmer who wants to know about I/O errors may precede
 146 .BR close ()
 147 with a call to
 148 .BR fsync (2).
 149
 150 The
 151 .B EINTR
 152 error is a somewhat special case.
 153 Regarding the
 154 .B EINTR
 155 error, POSIX.1-2013 says:
 156
 157 .RS
 158 If
 159 .BR close ()
 160 is interrupted by a signal that is to be caught, it shall return \-1 with
 161 .I errno
 162 set to
 163 .B EINTR
 164 and the state of
 165 .I fildes
 166 is unspecified.
 167 .RE
 168
 169 This permits the behavior that occurs on Linux and
 170 many other implementations, where,
 171 as with other errors that may be reported by
 172 .BR close (),
 173 the file descriptor is guaranteed to be closed.
 174 However, it also permits another possibility:
 175 that the implementation returns an
 176 .B EINTR
 177 error and keeps the file descriptor open.
 178 (According to its documentation, HP-UX's
 179 .BR close ()
 180 does this.)
 181 The caller must then once more use
 182 .BR close ()
 183 to close the file descriptor, to avoid file descriptor leaks.
 184 This divergence in implementation behaviors provides
 185 a difficult hurdle for portable applications, since on many implementations,
 186 .BR close ()
 187 must not be called again after an
 188 .B EINTR
 189 error, and on at least one,
 190 .BR close ()
 191 must be called again.
 192 There are plans to address this conundrum for
 193 the next major release of the POSIX.1 standard.
 194 .\" FIXME . for later review when Issue 8 is one day released...
 195 .\" POSIX proposes further changes for EINTR
 196 .\" http://austingroupbugs.net/tag_view_page.php?tag_id=8
 197 .\" http://austingroupbugs.net/view.php?id=529
 198 .\"
 199 .\" FIXME .
 200 .\" Review the following glibc bug later
 201 .\" https://sourceware.org/bugzilla/show_bug.cgi?id=14627
 202
 203 A successful close does not guarantee that the data has been successfully
 204 saved to disk, as the kernel uses the buffer cache to defer writes.
 205 Typically, filesystems do not flush buffers when a file is closed.
 206 If you need to be sure that
 207 the data is physically stored on the underlying disk, use
 208 .BR fsync (2).
 209 (It will depend on the disk hardware at this point.)
 210 .PP
 211 The close-on-exec file descriptor flag can be used to ensure
 212 that a file descriptor is automatically closed upon a successful
 213 .BR execve (2);
 214 see
 215 .BR fcntl (2)
 216 for details.
 217 .PP
 218 It is probably unwise to close file descriptors while
 219 they may be in use by system calls in
 220 other threads in the same process.
 221 Since a file descriptor may be reused,
 222 there are some obscure race conditions
 223 that may cause unintended side effects.
 224 .\" Date: Tue, 4 Sep 2007 13:57:35 +0200
 225 .\" From: Fredrik Noring <noring@nocrew.org>
 226 .\" One such race involves signals and ERESTARTSYS. If a file descriptor
 227 .\" in use by a system call is closed and then reused by e.g. an
 228 .\" independent open() in some unrelated thread, before the original system
 229 .\" call has restarted after ERESTARTSYS, the original system call will
 230 .\" later restart with the reused file descriptor. This is most likely a
 231 .\" serious programming error.
 232 .SH SEE ALSO
 233 .BR fcntl (2),
 234 .BR fsync (2),
 235 .BR open (2),
 236 .BR shutdown (2),
 237 .BR unlink (2),
 238 .BR fclose (3)