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
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
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
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
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