fileevent
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 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 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 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
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 handlers.
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, read-
able, script, writable.
Tcl 7.5 fileevent(n)
