Provided by: tcl8.5-doc_8.5.19-4_all 
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.