Ubuntu Manpage: fileevent - Execute a script when a channel becomes readable or writable

Provided by: tcl8.6-doc_8.6.1-4ubuntu1_all

NAME

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

SYNOPSIS

       fileevent channelId readable ?script?

       fileevent channelId writable ?script?
_________________________________________________________________

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 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 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.

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 handlers.

EXAMPLE

       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 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]

CREDITS

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

KEYWORDS