mount_setattr.2: SEE ALSO: place entries in correct order

[man-pages.git] / man2 / close.2

.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
.\" and Copyright (C) 2016 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Modified Wed Jul 21 22:40:25 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Sat Feb 18 15:27:48 1995 by Michael Haardt
.\" Modified Sun Apr 14 11:40:50 1996 by Andries Brouwer <aeb@cwi.nl>:
.\"   corrected description of effect on locks (thanks to
.\"   Tigran Aivazian <tigran@sco.com>).
.\" Modified Fri Jan 31 16:21:46 1997 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2000-07-22 by Nicolás Lichtmaier <nick@debian.org>
.\"   added note about close(2) not guaranteeing that data is safe on close.
.\"
.TH CLOSE 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
close \- close a file descriptor
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int close(int " fd );
.fi
.SH DESCRIPTION
.BR close ()
closes a file descriptor, so that it no longer refers to any file and
may be reused.
Any record locks (see
.BR fcntl (2))
held on the file it was associated with,
and owned by the process, are removed (regardless of the file
descriptor that was used to obtain the lock).
.PP
If
.I fd
is the last file descriptor referring to the underlying
open file description (see
.BR open (2)),
the resources associated with the open file description are freed;
if the file descriptor was the last reference to a file which has been
removed using
.BR unlink (2),
the file is deleted.
.SH RETURN VALUE
.BR close ()
returns zero on success.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I fd
isn't a valid open file descriptor.
.TP
.B EINTR
.\" Though, it's in doubt whether this error can ever occur; see
.\" https://lwn.net/Articles/576478/ "Returning EINTR from close()"
The
.BR close ()
call was interrupted by a signal; see
.BR signal (7).
.TP
.B EIO
An I/O error occurred.
.TP
.BR ENOSPC ", " EDQUOT
On NFS, these errors are not normally reported against the first write
which exceeds the available storage space, but instead against a
subsequent
.BR write (2),
.BR fsync (2),
or
.BR close ().
.PP
See NOTES for a discussion of why
.BR close ()
should not be retried after an error.
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
.\" SVr4 documents an additional ENOLINK error condition.
.SH NOTES
A successful close does not guarantee that the data has been successfully
saved to disk, as the kernel uses the buffer cache to defer writes.
Typically, filesystems do not flush buffers when a file is closed.
If you need to be sure that
the data is physically stored on the underlying disk, use
.BR fsync (2).
(It will depend on the disk hardware at this point.)
.PP
The close-on-exec file descriptor flag can be used to ensure
that a file descriptor is automatically closed upon a successful
.BR execve (2);
see
.BR fcntl (2)
for details.
.\"
.SS Multithreaded processes and close()
It is probably unwise to close file descriptors while
they may be in use by system calls in
other threads in the same process.
Since a file descriptor may be reused,
there are some obscure race conditions
that may cause unintended side effects.
.\" Date: Tue, 4 Sep 2007 13:57:35 +0200
.\" From: Fredrik Noring <noring@nocrew.org>
.\" One such race involves signals and ERESTARTSYS. If a file descriptor
.\" in use by a system call is closed and then reused by e.g. an
.\" independent open() in some unrelated thread, before the original system
.\" call has restarted after ERESTARTSYS, the original system call will
.\" later restart with the reused file descriptor. This is most likely a
.\" serious programming error.
.PP
Furthermore, consider the following scenario where two threads are
performing operations on the same file descriptor:
.IP 1. 3
One thread is blocked in an I/O system call on the file descriptor.
For example, it is trying to
.BR write (2)
to a pipe that is already full, or trying to
.BR read (2)
from a stream socket which currently has no available data.
.IP 2.
Another thread closes the file descriptor.
.PP
The behavior in this situation varies across systems.
On some systems, when the file descriptor is closed,
the blocking system call returns immediately with an error.
.PP
On Linux (and possibly some other systems), the behavior is different:
the blocking I/O system call holds a reference to the underlying
open file description, and this reference keeps the description open
until the I/O system call completes.
.\" 'struct file' in kernel-speak
(See
.BR open (2)
for a discussion of open file descriptions.)
Thus, the blocking system call in the first thread may successfully
complete after the
.BR close ()
in the second thread.
.\"
.SS Dealing with error returns from close()
A careful programmer will check the return value of
.BR close (),
since it is quite possible that errors on a previous
.BR write (2)
operation are reported only on the final
.BR close ()
that releases the open file description.
Failing to check the return value when closing a file may lead to
.I silent
loss of data.
This can especially be observed with NFS and with disk quota.
.PP
Note, however, that a failure return should be used only for
diagnostic purposes (i.e., a warning to the application that there
may still be I/O pending or there may have been failed I/O)
or remedial purposes
(e.g., writing the file once more or creating a backup).
.PP
Retrying the
.BR close ()
after a failure return is the wrong thing to do,
.\" The file descriptor is released early in close();
.\" close() ==> __close_fd():
.\"                     __put_unused_fd() ==> __clear_open_fd()
.\"                     return filp_close(file, files);
.\"
.\" The errors are returned by filp_close() after the FD has been
.\" cleared for re-use.
since this may cause a reused file descriptor
from another thread to be closed.
This can occur because the Linux kernel
.I always
releases the file descriptor early in the close
operation, freeing it for reuse;
the steps that may return an error,
.\" filp_close()
such as flushing data to the filesystem or device,
occur only later in the close operation.
.PP
Many other implementations similarly always close the file descriptor
.\" FreeBSD documents this explicitly. From the look of the source code
.\" SVR4, ancient SunOS, later Solaris, and AIX all do this.
(except in the case of
.BR EBADF ,
meaning that the file descriptor was invalid)
even if they subsequently report an error on return from
.BR close ().
POSIX.1 is currently silent on this point,
but there are plans to mandate this behavior in the next major release
.\" Issue 8
of the standard.
.PP
A careful programmer who wants to know about I/O errors may precede
.BR close ()
with a call to
.BR fsync (2).
.PP
The
.B EINTR
error is a somewhat special case.
Regarding the
.B EINTR
error, POSIX.1-2008 says:
.PP
.RS
If
.BR close ()
is interrupted by a signal that is to be caught, it shall return \-1 with
.I errno
set to
.B EINTR
and the state of
.I fildes
is unspecified.
.RE
.PP
This permits the behavior that occurs on Linux and
many other implementations, where,
as with other errors that may be reported by
.BR close (),
the file descriptor is guaranteed to be closed.
However, it also permits another possibility:
that the implementation returns an
.B EINTR
error and keeps the file descriptor open.
(According to its documentation, HP-UX's
.BR close ()
does this.)
The caller must then once more use
.BR close ()
to close the file descriptor, to avoid file descriptor leaks.
This divergence in implementation behaviors provides
a difficult hurdle for portable applications, since on many implementations,
.BR close ()
must not be called again after an
.B EINTR
error, and on at least one,
.BR close ()
must be called again.
There are plans to address this conundrum for
the next major release of the POSIX.1 standard.
.\" FIXME . for later review when Issue 8 is one day released...
.\" POSIX proposes further changes for EINTR
.\" http://austingroupbugs.net/tag_view_page.php?tag_id=8
.\" http://austingroupbugs.net/view.php?id=529
.\"
.\" FIXME .
.\" Review the following glibc bug later
.\" https://sourceware.org/bugzilla/show_bug.cgi?id=14627
.SH SEE ALSO
.BR close_range (2),
.BR fcntl (2),
.BR fsync (2),
.BR open (2),
.BR shutdown (2),
.BR unlink (2),
.BR fclose (3)