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fileevent(n)		     Tcl Built-In Commands		  fileevent(n)


       fileevent  -  Execute  a	 script	 when  a  channel  becomes readable or

       fileevent channelId readable ?script?

       fileevent channelId writable ?script?

       This command is used to create file event handlers.  A file event  han-
       dler  is	a binding between a channel and	a script, such that the	script
       is evaluated whenever the channel becomes readable or  writable.	  File
       event handlers are most commonly	used to	allow data to be received from
       another process on an event-driven basis, so that the receiver can con-
       tinue  to  interact with	the user while waiting for the data to arrive.
       If an application invokes gets or read on a blocking channel when there
       is  no  input  data  available, the process will	block; until the input
       data arrives, it	will not be able to service other events, so  it  will
       appear  to  the	user  to "freeze up".  With fileevent, the process can
       tell when data is present and only invoke gets or read when  they  will
       not block.

       The channelId argument to fileevent refers to 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.

       If the script argument is specified, then fileevent creates a new event
       handler:	  script  will be evaluated whenever the channel becomes read-
       able or writable	(depending on the second argument to  fileevent).   In
       this case fileevent returns an empty string.  The readable and writable
       event handlers for a file are  independent,  and	 may  be  created  and
       deleted separately.  However, there may be at most one readable and one
       writable	handler	for a file at a	given time in a	given interpreter.  If
       fileevent  is  called  when the specified handler already exists	in the
       invoking	interpreter, the new script replaces the old one.

       If the script argument is not specified,	fileevent returns the  current
       script  for  channelId,	or  an	empty string if	there is none.	If the
       script argument is specified as an empty	string then the	event  handler
       is deleted, so that no script will be invoked.  A file event handler is
       also deleted automatically whenever its channel is closed or its	inter-
       preter is deleted.

       A  channel  is considered to be readable	if there is unread data	avail-
       able on the underlying device.  A channel  is  also  considered	to  be
       readable	if there is unread data	in an input buffer, except in the spe-
       cial case where the most	recent attempt to read from the	channel	was  a
       gets  call  that	 could	not  find a complete line in the input buffer.
       This feature allows a file to be	read a line at a time  in  nonblocking
       mode  using  events.  A channel is also considered to be	readable if an
       end of file or error condition is present on the	underlying file	or de-
       vice.   It  is  important  for script to	check for these	conditions and
       handle them appropriately;  for example,	if there is no	special	 check
       for end of file,	an infinite loop may occur where script	reads no data,
       returns,	and is immediately invoked again.

       A channel is considered to be writable if at least one byte of data can
       be  written to the underlying file or device without blocking, or if an
       error condition is present on the underlying file or device.

       Event-driven I/O	works best for channels	that  have  been  placed  into
       nonblocking mode	with the fconfigure command.  In blocking mode,	a puts
       command may block if you	give it	more data than the underlying file  or
       device can accept, and a	gets or	read command will block	if you attempt
       to read more data than is ready;	a readable underlying file  or	device
       may  not	even guarantee that a blocking [read 1]	will succeed (counter-
       examples	being multi-byte encodings, compression	or  encryption	trans-
       forms  ). In all	such cases, no events will be processed	while the com-
       mands block.

       In nonblocking mode puts, read, and gets	never block.  See the documen-
       tation  for  the	individual commands for	information on how they	handle
       blocking	and nonblocking	channels.

       Testing for the end of file condition should be done after any attempts
       read  the channel data. The eof flag is set once	an attempt to read the
       end of data has occurred	and testing before this	read will  require  an
       additional event	to be fired.

       The  script  for	 a file	event is executed at global level (outside the
       context of any Tcl procedure) in	the interpreter	in which the fileevent
       command	was  invoked.	If  an error occurs while executing the	script
       then the	command	registered with	interp bgerror is used to  report  the
       error.	In  addition, the file event handler is	deleted	if it ever re-
       turns an	error;	this is	done in	order to prevent infinite loops	due to
       buggy handlers.

       In  this	 setup	GetData	will be	called with the	channel	as an argument
       whenever	$chan becomes readable.	The read call will read	 whatever  bi-
       nary  data  is  currently available without blocking.  Here the channel
       has the fileevent removed when an end of	file  occurs  to  avoid	 being
       continually called (see above). Alternatively the channel may be	closed
       on this condition.

	      proc GetData {chan} {
		  set data [read $chan]
		  puts "[string	length $data] $data"
		  if {[eof $chan]} {
		      fileevent	$chan readable {}

	      fconfigure $chan -blocking 0 -encoding binary
	      fileevent	$chan readable [list GetData $chan]

       The next	example	demonstrates use of gets to read line-oriented data.

	      proc GetData {chan} {
		  if {[gets $chan line]	>= 0} {
		      puts $line
		  if {[eof $chan]} {
		      close $chan

	      fconfigure $chan -blocking 0 -buffering line -translation	crlf
	      fileevent	$chan readable [list GetData $chan]

       fileevent is based on the addinput command created by Mark Diekhans.

       fconfigure(n), gets(n), interp(n), puts(n), read(n),  Tcl_StandardChan-

       asynchronous  I/O, blocking, channel, event handler, nonblocking, read-
       able, script, writable.

Tcl				      7.5			  fileevent(n)


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