fileevent(n) Tcl Built-In Commands fileevent(n) _________________________________________________________________ NAME fileevent - Execute a script when a channel becomes readable or writable SYNOPSIS fileevent _c_h_a_n_n_e_l_I_d readable ?_s_c_r_i_p_t? fileevent _c_h_a_n_n_e_l_I_d writable ?_s_c_r_i_p_t? _________________________________________________________________ DESCRIPTION This command is used to create _f_i_l_e _e_v_e_n_t _h_a_n_d_l_e_r_s. 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 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 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 _c_h_a_n_n_e_l_I_d argument to fileevent refers to an open chan- nel, such as the return value from a previous open or socket command. If the _s_c_r_i_p_t argument is specified, then fileevent creates a new event handler: _s_c_r_i_p_t 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 _s_c_r_i_p_t argument is not specified, fileevent returns the current script for _c_h_a_n_n_e_l_I_d, or an empty string if there is none. If the _s_c_r_i_p_t 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. Tcl Last change: 7.5 1 fileevent(n) Tcl Built-In Commands fileevent(n) 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 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 _s_c_r_i_p_t 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 _s_c_r_i_p_t 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. CREDITS fileevent is based on the addinput command created by Mark Diekhans. SEE ALSO bgerror, fconfigure, gets, puts, read KEYWORDS asynchronous I/O, blocking, channel, event handler, Tcl Last change: 7.5 2 fileevent(n) Tcl Built-In Commands fileevent(n) nonblocking, readable, script, writable. Tcl Last change: 7.5 3