The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


close - close a file descriptor


#include <unistd.h>

int close(int fildes);


The close() function will deallocate the file descriptor indicated by fildes. To deallocate means to make the file descriptor available for return by subsequent calls to open() or other functions that allocate file descriptors. All outstanding record locks owned by the process on the file associated with the file descriptor will be removed (that is, unlocked).

If close() is interrupted by a signal that is to be caught, it will return -1 with errno set to [EINTR] and the state of fildes is unspecified.

When all file descriptors associated with a pipe or FIFO special file are closed, any data remaining in the pipe or FIFO will be discarded.

When all file descriptors associated with an open file description have been closed the open file description will be freed.

If the link count of the file is 0, when all file descriptors associated with the file are closed, the space occupied by the file will be freed and the file will no longer be accessible.

If a STREAMS-based fildes is closed and the calling process was previously registered to receive a SIGPOLL signal for events associated with that STREAM, the calling process will be unregistered for events associated with the STREAM. The last close() for a STREAM causes the STREAM associated with fildes to be dismantled. If O_NONBLOCK is not set and there have been no signals posted for the STREAM, and if there is data on the module's write queue, close() waits for an unspecified time (for each module and driver) for any output to drain before dismantling the STREAM. The time delay can be changed via an I_SETCLTIME ioctl() request. If the O_NONBLOCK flag is set, or if there are any pending signals, close() does not wait for output to drain, and dismantles the STREAM immediately.

If the implementation supports STREAMS-based pipes, and fildes is associated with one end of a pipe, the last close() causes a hangup to occur on the other end of the pipe. In addition, if the other end of the pipe has been named by fattach(), then the last close() forces the named end to be detached by fdetach(). If the named end has no open file descriptors associated with it and gets detached, the STREAM associated with that end is also dismantled.

If fildes refers to the master side of a pseudo-terminal, and this is the last close, a SIGHUP signal is sent to the process group, if any, for which the slave side of the pseudo-terminal is the controlling terminal. It is unspecified whether closing the master side of the pseudo-terminal flushes all queued input and output.

If fildes refers to the slave side of a STREAMS-based pseudo-terminal, a zero-length message may be sent to the master.

If the Asynchronous Input and Output option is supported:

When there is an outstanding cancelable asynchronous I/O operation against fildes when close() is called, that I/O operation may be canceled. An I/O operation that is not canceled completes as if the close() operation had not yet occurred. All operations that are not canceled complete as if the close() blocked until the operations completed. The close() operation itself need not block awaiting such I/O completion. Whether any I/O operation is cancelled, and which I/O operation may be cancelled upon close(), is implementation-dependent.

If the Mapped Files or Shared Memory Objects option is supported:

If a memory object remains referenced at the last close (that is, a process has it mapped), then the entire contents of the memory object persist until the memory object becomes unreferenced. If this is the last close of a memory object and the close results in the memory object becoming unreferenced, and the memory object has been unlinked, then the memory object will be removed.


Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.


The close() function will fail if:
The fildes argument is not a valid file descriptor.
The close() function was interrupted by a signal.

The close() function may fail if:

An I/O error occurred while reading from or writing to the file system.




An application that had used the stdio routine fopen() to open a file should use the corresponding fclose() routine rather than close().




fattach(), fclose(), fdetach(), fopen(), ioctl(), open(), <unistd.h>, STREAMS overview.


Derived from Issue 1 of the SVID.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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