close(2) System Calls Manual close(2)
NAME
close - close a file descriptor
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int close(int fd);
DESCRIPTION
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 re-
gardless 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) 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)), 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.
RETURN VALUE
close() returns zero on success. On error, -1 is returned, and errno
is set to indicate the error.
ERRORS
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.
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), fsync(2), or close().
See NOTES for a discussion of why close() should not be retried after
an error.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, SVr4, 4.3BSD.
NOTES
A successful close does not guarantee that the data has been success-
fully 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 hard-
ware 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.
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 per-
forming operations on the same file descriptor:
(1) One thread is blocked in an I/O system call on the file descrip-
tor. For example, it is trying to write(2) to a pipe that is al-
ready full, or trying to read(2) from a stream socket which cur-
rently 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) 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 re-
ported only on the final close() that releases the open file descrip-
tion. 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 in-
valid) 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 stan-
dard.
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.1-2008 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 implemen-
tations, where, as with other errors that may be reported by close(),
the file descriptor is guaranteed to be closed. However, it also per-
mits another possibility: that the implementation returns an EINTR er-
ror and keeps the file descriptor open. (According to its documenta-
tion, 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
close_range(2), fcntl(2), fsync(2), open(2), shutdown(2), unlink(2),
fclose(3)
Linux man-pages 6.7 2023-10-31 close(2)
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