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 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 won't 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 han-dlers handlers
dlers 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 automat-ically automatically
ically 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 fcon-figure fconfigure
figure command. In blocking mode, a puts command may block if you give it more data than the under-lying underlying
lying 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.
EXAMPLE
proc GetData {chan} {
if {![eof $chan]} {
puts [gets $chan]
}
}
fileevent $chan readable [list GetData $chan]
In this setup GetData will be called with the channel as an argument whenever $chan becomes readable.
CREDITS
fileevent is based on the addinput command created by Mark Diekhans.
SEE ALSO
bgerror(n), fconfigure(n), gets(n), puts(n), read(n), Tcl_StandardChannels(3)
KEYWORDS
asynchronous I/O, blocking, channel, event handler, nonblocking, readable, script, writable.
Tcl 7.5 fileevent(n)
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