DragonFly On-Line Manual Pages
CLOSE(2) DragonFly System Calls Manual CLOSE(2)
NAME
close -- delete a descriptor
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int
close(int d);
DESCRIPTION
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.
RETURN VALUES
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.
ERRORS
Close() will fail if:
[EBADF] D is not an active descriptor.
[EINTR] An interrupt was received.
SEE ALSO
accept(2), execve(2), fcntl(2), flock(2), open(2), pipe(2), socket(2),
socketpair(2)
STANDARDS
The close() function call is expected to conform to ISO/IEC 9945-1:1990
(``POSIX.1'').
HISTORY
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)
______________________________________________________________________________
NAME
close - Close an open channel
SYNOPSIS
close channelId ?r(ead)|w(rite)?
______________________________________________________________________________
DESCRIPTION
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.
EXAMPLE
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
}
SEE ALSO
file(n), open(n), socket(n), eof(n), Tcl_StandardChannels(3)
KEYWORDS
blocking, channel, close, nonblocking, half-close
Tcl 7.5 close(n)