close(2) - Linux manual page (original) (raw)
close(2) System Calls Manual close(2)
NAME top
close - close a file descriptorLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <unistd.h>**
**int close(int** _fd_**);**DESCRIPTION top
**close**() closes a file descriptor, so that it no longer refers to
any file and may be reused. Any record locks (see [fcntl(2)](../man2/fcntl.2.html)) 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. This has some unfortunate consequences and one should
be extra careful when using advisory record locking. See [fcntl(2)](../man2/fcntl.2.html)
for discussion of the risks and consequences as well as for the
(probably preferred) open file description locks.
If _fd_ is the last file descriptor referring to the underlying open
file description (see [open(2)](../man2/open.2.html)), 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 [unlink(2)](../man2/unlink.2.html),
the file is deleted.RETURN VALUE top
**close**() returns zero on success. On error, -1 is returned, and
_[errno](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**EBADF** _fd_ isn't a valid open file descriptor.
**EINTR** The **close**() call was interrupted by a signal; see
[signal(7)](../man7/signal.7.html).
**EIO** An I/O error occurred.
**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 [write(2)](../man2/write.2.html), [fsync(2)](../man2/fsync.2.html), or
**close**().
See NOTES for a discussion of why **close**() should not be retried
after an error.STANDARDS top
POSIX.1-2008.HISTORY top
POSIX.1-2001, SVr4, 4.3BSD.NOTES top
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 [fsync(2)](../man2/fsync.2.html). (It will
depend on the disk hardware at this point.)
The close-on-exec file descriptor flag can be used to ensure that
a file descriptor is automatically closed upon a successful
[execve(2)](../man2/execve.2.html); see [fcntl(2)](../man2/fcntl.2.html) for details.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.
Furthermore, consider the following scenario where two threads are
performing operations on the same file descriptor:
(1) One thread is blocked in an I/O system call on the file
descriptor. For example, it is trying to [write(2)](../man2/write.2.html) to a pipe
that is already full, or trying to [read(2)](../man2/read.2.html) from a stream
socket which currently has no available data.
(2) Another thread closes the file descriptor.
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.
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. (See
[open(2)](../man2/open.2.html) for a discussion of open file descriptions.) Thus, the
blocking system call in the first thread may successfully complete
after the **close**() in the second thread.Dealing with error returns from close() A careful programmer will check the return value of close(), since it is quite possible that errors on a previous write(2) operation are reported only on the final close() that releases the open file description. Failing to check the return value when closing a file may lead to silent loss of data. This can especially be observed with NFS and with disk quota.
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).
Retrying the **close**() after a failure return is the wrong thing to
do, since this may cause a reused file descriptor from another
thread to be closed. This can occur because the Linux kernel
_always_ releases the file descriptor early in the close operation,
freeing it for reuse; the steps that may return an error, such as
flushing data to the filesystem or device, occur only later in the
close operation.
Many other implementations similarly always close the file
descriptor (except in the case of **EBADF**, meaning that the file
descriptor was invalid) even if they subsequently report an error
on return from **close**(). POSIX.1 is currently silent on this
point, but there are plans to mandate this behavior in the next
major release of the standard.
A careful programmer who wants to know about I/O errors may
precede **close**() with a call to [fsync(2)](../man2/fsync.2.html).
The **EINTR** error is a somewhat special case. Regarding the **EINTR**
error, POSIX.1-2008 says:
If **close**() is interrupted by a signal that is to be caught,
it shall return -1 with _[errno](../man3/errno.3.html)_ set to **EINTR** and the state of
_fildes_ is unspecified.
This permits the behavior that occurs on Linux and many other
implementations, where, as with other errors that may be reported
by **close**(), the file descriptor is guaranteed to be closed.
However, it also permits another possibility: that the
implementation returns an **EINTR** error and keeps the file
descriptor open. (According to its documentation, HP-UX's **close**()
does this.) The caller must then once more use **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, **close**()
must not be called again after an **EINTR** error, and on at least
one, **close**() must be called again. There are plans to address
this conundrum for the next major release of the POSIX.1 standard.SEE ALSO top
[close_range(2)](../man2/close%5Frange.2.html), [fcntl(2)](../man2/fcntl.2.html), [fsync(2)](../man2/fsync.2.html), [open(2)](../man2/open.2.html), [shutdown(2)](../man2/shutdown.2.html),
[unlink(2)](../man2/unlink.2.html), [fclose(3)](../man3/fclose.3.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 close(2)
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