fileevent(3tcl)
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
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; 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 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.
proc GetData {chan} {
if {![eof $chan]} {
puts [gets $chan]
}
}
fileevent $chan readable [list GetData $chan]
CREDITS
fileevent is based on the addinput command created by Mark Diekhans.
SEE ALSO
fconfigure(3tcl), gets(3tcl), interp(3tcl), puts(3tcl), read(3tcl),
Tcl_StandardChannels(3tcl)
KEYWORDS
asynchronous I/O, blocking, channel, event handler, nonblocking,
readable, script, writable.
Free and Open Source Software