Manpages - close.2

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) to a pipe that is already full, or trying to *read*(2) 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) 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.

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.1-2008 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 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.