Tcl Built-In Commands fileevent(1T)
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NAME
fileevent - Execute a script when a channel becomes readable
or writableSYNOPSIS
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 mostcommonly used to allow data to be received from another pro-
cess 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 tothe 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 chan- |
nel 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 thesecond 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 anddeleted separately. However, there may be at most one read-
able and one writable handler for a file at a given time ina given interpreter. If fileevent is called when the speci-
fied 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 Tcl Last change: 7.5 1Tcl Built-In Commands fileevent(1T)
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. Thisfeature allows a file to be read a line at a time in non-
blocking 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 inter-
preter 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
In this setup GetData will be called with the channel as anargument whenever $chan becomes readable.
proc GetData {chan} {if {![eof $chan]} {
puts [gets $chan]
} } Tcl Last change: 7.5 2Tcl Built-In Commands fileevent(1T)
fileevent $chan readable [list GetData $chan]
CREDITSfileevent is based on the addinput command created by Mark
Diekhans.SEE ALSO
bgerror(1T), fconfigure(1T), gets(1T), puts(1T), read(1T),Tcl_StandardChannels(3TCL)
KEYWORDSasynchronous I/O, blocking, channel, event handler, non-
blocking, readable, script, writable.ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:_______________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE|
|____________________|__________________|_
| Availability | runtime/tcl-8 |
|____________________|__________________|_
| Interface Stability| Uncommitted ||____________________|_________________|
NOTES Source for Tcl is available on http://opensolaris.org. Tcl Last change: 7.5 3