close - close a file descriptor
int close(int fd);
() 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).
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.
() returns zero on success. On error, -1 is returned, and
is set appropriately.
- fd isn't a valid open file descriptor.
- The close() call was interrupted by a signal; see
- 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
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
(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) 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
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
loss of data. This can especially be observed with NFS and with
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
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
() with a call to fsync
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
() 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.