fileevent(3tcl)               Tcl Built-In Commands              fileevent(3tcl)


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

       fileevent channelId readable ?script?

       fileevent channelId writable ?script?

       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 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 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; 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 transforms
       ). In all such cases, no events will be processed while the commands

       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.

       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 returns an
       error;  this is done in order to prevent infinite loops due to buggy

       In this setup GetData will be called with the channel as an argument
       whenever $chan becomes readable. The read call will read whatever binary
       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

              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(3tcl), gets(3tcl), interp(3tcl), puts(3tcl), read(3tcl),

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

Tcl                                    7.5                       fileevent(3tcl)