fileevent(n) Tcl Built-In Commands fileevent(n) NAME fileevent - Execute a script when a channel becomes readable or writable SYNOPSIS fileevent channelId readable ?script? fileevent channelId writable ?script? DESCRIPTION This command is used to create file(1,n) event handlers. A file(1,n) 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(3,n) or read(2,n,1 builtins) 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(3,n) or read(2,n,1 builtins) when they won't block. The channelId argument to fileevent refers to an open(2,3,n) channel such as a Tcl standard channel (stdin, stdout, or stderr), the return value from an invocation of open(2,3,n) or socket(2,7,n), 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 read- able 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(1,n) are independent, and may be created and deleted separately. However, there may be at most one readable and one writable handler for a file(1,n) at a given time(1,2,n) in(1,8) a given interpreter. If fileevent is called when the specified handler already exists in(1,8) 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(3,n) if(3,n) there is none. If the script argument is specified as an empty string(3,n) then the event handler is deleted, so that no script will be invoked. A file(1,n) 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(3,n) there is unread data avail- able on the underlying device. A channel is also considered to be readable if(3,n) there is unread data in(1,8) an input buffer, except in(1,8) the spe- cial case where the most recent attempt to read(2,n,1 builtins) from the channel was a gets(3,n) call that could not find a complete line in(1,8) the input buffer. This feature allows a file(1,n) to be read(2,n,1 builtins) a line at a time(1,2,n) in(1,8) nonblocking mode using events. A channel is also considered to be readable if(3,n) an end of file(1,n) or error(8,n) condition is present on the underlying file(1,n) or device. It is important for script to check for these conditions and handle them appropriately; for example, if(3,n) there is no special check for end of file(1,n), an infinite loop may occur where script reads no data, returns, and is immediately invoked again. A channel is considered to be writable if(3,n) at least one byte of data can be written to the underlying file(1,n) or device without blocking, or if(3,n) an error(8,n) condition is present on the underlying file(1,n) 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(3,n) command may block if(3,n) you give it more data than the underlying file(1,n) or device can accept(2,8), and a gets(3,n) or read(2,n,1 builtins) command will block if(3,n) you attempt to read(2,n,1 builtins) more data than is ready; no events will be processed while the commands block. In nonblocking mode puts(3,n), read(2,n,1 builtins), and gets(3,n) never block. See the documentation for the individual commands for information on how they handle blocking and nonblocking channels. The script for a file(1,n) event is executed at global level (outside the context of any Tcl procedure) in(1,8) the interpreter in(1,8) which the fileevent command was invoked. If an error(8,n) occurs while executing the script then the bgerror mechanism is used to report the error. In addition, the file(1,n) event handler is deleted if(3,n) it ever returns an error(8,n); this is done in(1,8) order to prevent infinite loops due to buggy handlers. EXAMPLE proc(5,n) GetData {chan} { if(3,n) {![eof $chan]} { puts(3,n) [gets(3,n) $chan] } } fileevent $chan readable [list GetData $chan] In this setup(2,8) 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(3,n)(n), puts(3,n)(n), read(2,n,1 builtins)(n), Tcl_StandardChan- nels(3) KEYWORDS asynchronous I/O, blocking, channel, event handler, nonblocking, read- able, script, writable. Tcl 7.5 fileevent(n)