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

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NAME

       fileevent - Execute a script when a channel becomes readable or writable

SYNOPSIS

       fileevent channelId readable ?script?

       fileevent channelId writable ?script?
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DESCRIPTION

       This command is used to create file event handlers.  A file event handler 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 continue 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 won't block.

       The channelId argument to fileevent refers to an open channel, such as the return value from a previous open or	socket	command.   If  the
       script  argument  is specified, then fileevent creates a new event handler:  script will be evaluated whenever the channel becomes readable
       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 interpreter is deleted.

       A  channel  is  considered  to  be readable if there is unread data available on the underlying device.	A channel is also considered to be
       readable if there is unread data in an input buffer, except in the special 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
       device.	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;  no events will be processed while the commands block.  In nonblocking mode puts, read, and gets
       never block.  See the documentation for the individual commands for information on how they handle blocking and nonblocking channels.

       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 bgerror mechanism is used to report the error.	In
       addition, the file event handler is deleted if it ever returns an error;  this is done in order to prevent infinite loops due to buggy han-
       dlers.

CREDITS

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

SEE ALSO

       bgerror(n), fconfigure(n), gets(n), puts(n), read(n)

KEYWORDS

       asynchronous I/O, blocking, channel, event handler, nonblocking, readable, script, writable.

Tcl									7.5							      fileevent(n)