|
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
CLOSE(2) Linux Programmer's Manual CLOSE(2)
close - close a file descriptor
#include <unistd.h>
int close(int fd);
close() closes a file descriptor, so that it no longer refers to any
file and may be reused. Any record locks (see 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).
If fd is the last file descriptor referring to the underlying open
file description (see 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 unlink(2), the file
is deleted.
close() returns zero on success. On error, -1 is returned, and errno
is set appropriately.
EBADF fd isn't a valid open file descriptor.
EINTR The close() call was interrupted by a signal; see signal(7).
EIO An I/O error occurred.
See NOTES for a discussion of why close() should not be retried after
an error.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
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). (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);
see fcntl(2) for details.
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.
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).
The EINTR error is a somewhat special case. Regarding the EINTR
error, POSIX.1-2013 says:
If close() is interrupted by a signal that is to be caught, it
shall return -1 with errno 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.
fcntl(2), fsync(2), open(2), shutdown(2), unlink(2), fclose(3)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-10-08 CLOSE(2)
Pages that refer to this page: bpf(2), dup(2), epoll_create(2), eventfd(2), flock(2), open(2), perfmonctl(2), read(2), signalfd(2), socket(2), spu_create(2), spu_run(2), syscalls(2), timerfd_create(2), write(2), closedir(3), dbopen(3), fclose(3), fcloseall(3), fts(3), getdtablesize(3), mkfifo(3), pmconnectlogger(3), posix_spawn(3), shm_open(3), stdio(3), nfs(5), systemd.socket(5), cpuset(7), epoll(7), fanotify(7), inotify(7), pipe(7), shm_overview(7), signal-safety(7), socket(7), spufs(7)