Manpage of CLOSE

CLOSE

Section: Linux Programmer's Manual (2)
Updated: 2016-10-08
Index
 

NAME

close - close a file descriptor  

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 (regardless of the file descriptor that was used to obtain the lock).

If fdis 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 errnois set appropriately.  

ERRORS

EBADF
fdisn'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.  

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.  

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 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 silentloss 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 alwaysreleases 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 EINTRerror is a somewhat special case. Regarding the EINTRerror, POSIX.1-2013 says:

If close() is interrupted by a signal that is to be caught, it shall return -1 with errnoset to EINTRand the state of fildesis 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 EINTRerror 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 EINTRerror, 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

fcntl(2), fsync(2), open(2), shutdown(2), unlink(2), fclose(3)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
Dealing with error returns from close()
SEE ALSO

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