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CLOSE(2)                 DragonFly System Calls Manual                CLOSE(2)


close -- delete a descriptor


Standard C Library (libc, -lc)


#include <unistd.h> int close(int d);


The close() call deletes a descriptor from the per-process object refer- ence table. If this is the last reference to the underlying object, the object will be deactivated. For example, on the last close of a file the current seek pointer associated with the file is lost; on the last close of a socket(2) associated naming information and queued data are dis- carded; on the last close of a file holding an advisory lock the lock is released (see further flock(2)). However, the semantics of System V and IEEE Std 1003.1-1988 (``POSIX.1'') dictate that all fcntl(2) advisory record locks associated with a file for a given process are removed when any file descriptor for that file is closed by that process. When a process exits, all associated file descriptors are freed, but since there is a limit on active descriptors per processes, the close() function call is useful when a large quantity of file descriptors are being handled. When a process forks (see fork(2)), all descriptors for the new child process reference the same objects as they did in the parent before the fork. If a new process is then to be run using execve(2), the process would normally inherit these descriptors. Most of the descriptors can be rearranged with dup2(2) or deleted with close() before the execve(2) is attempted, but if some of these descriptors will still be needed if the execve fails, it is necessary to arrange for them to be closed if the execve succeeds. For this reason, the call ``fcntl(d, F_SETFD, 1)'' is provided, which arranges that a descriptor will be closed after a suc- cessful execve; the call ``fcntl(d, F_SETFD, 0)'' restores the default, which is to not close the descriptor.


The close() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.


Close() will fail if: [EBADF] D is not an active descriptor. [EINTR] An interrupt was received.


accept(2), execve(2), fcntl(2), flock(2), open(2), pipe(2), socket(2), socketpair(2)


The close() function call is expected to conform to ISO/IEC 9945-1:1990 (``POSIX.1'').


A close() function call appeared in Version 7 AT&T UNIX. DragonFly 3.5 April 19, 1994 DragonFly 3.5 close(n) Tcl Built-In Commands close(n) ______________________________________________________________________________


close - Close an open channel


close channelId ?r(ead)|w(rite)? ______________________________________________________________________________


Closes or half-closes the channel given by channelId. ChannelId must be an identifier for an open channel such as a Tcl standard channel (stdin, stdout, or stderr), the return value from an invocation of open or socket, or the result of a channel creation command provided by a Tcl extension. The single-argument form is a simple "full-close": all buffered output is flushed to the channel's output device, any buffered input is discarded, the underlying file or device is closed, and channelId becomes unavailable for use. If the channel is blocking, the command does not return until all output is flushed. If the channel is nonblocking and there is unflushed output, the channel remains open and the command returns immediately; output will be flushed in the background and the channel will be closed when all the flushing is complete. If channelId is a blocking channel for a command pipeline then close waits for the child processes to complete. If the channel is shared between interpreters, then close makes channelId unavailable in the invoking interpreter but has no other effect until all of the sharing interpreters have closed the channel. When the last interpreter in which the channel is registered invokes close, the cleanup actions described above occur. See the interp command for a description of channel sharing. Channels are automatically closed when an interpreter is destroyed and when the process exits. From 8.6 on (TIP#398), nonblocking channels | are no longer switched to blocking mode when exiting; this guarantees a | timely exit even when the peer or a communication channel is stalled. | To ensure proper flushing of stalled nonblocking channels on exit, one | must now either (a) actively switch them back to blocking or (b) use | the environment variable TCL_FLUSH_NONBLOCKING_ON_EXIT, which when set | and not equal to "0" restores the previous behavior. The command returns an empty string, and may generate an error if an error occurs while flushing output. If a command in a command pipeline created with open returns an error, close generates an error (similar to the exec command.) The two-argument form is a "half-close": given a bidirectional channel | like a socket or command pipeline and a (possibly abbreviated) | direction, it closes only the sub-stream going in that direction. This | means a shutdown() on a socket, and a close() of one end of a pipe for | a command pipeline. Then, the Tcl-level channel data structure is | either kept or freed depending on whether the other direction is still | open. | A single-argument close on an already half-closed bidirectional channel | is defined to just "finish the job". A half-close on an already closed | half, or on a wrong-sided unidirectional channel, raises an error. | In the case of a command pipeline, the child-reaping duty falls upon | the shoulders of the last close or half-close, which is thus allowed to | report an abnormal exit error. | Currently only sockets and command pipelines support half-close. A | future extension will allow reflected and stacked channels to do so.


This illustrates how you can use Tcl to ensure that files get closed even when errors happen by combining catch, close and return: proc withOpenFile {filename channelVar script} { upvar 1 $channelVar chan set chan [open $filename] catch { uplevel 1 $script } result options close $chan return -options $options $result }


file(n), open(n), socket(n), eof(n), Tcl_StandardChannels(3)


blocking, channel, close, nonblocking, half-close Tcl 7.5 close(n)

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