The fileHandle argument to fileevent refers to an open filehandle, such as the return value from a previous open or socket command. If the callback argument is specified, then fileevent creates a new event handler: callback will be evaluated whenever the filehandle becomes readable or writable (depending on the 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 callback replaces the old one.
If the callback argument is not specified, fileevent returns the current callback for fileHandle, or an empty string if there is none. If the callback argument is specified as an empty string then the event handler is deleted, so that no callback will be invoked. A file event handler is also deleted automatically whenever its filehandle is closed or its interpreter is deleted.
A filehandle is considered to be readable if there is unread data available on the underlying device. A filehandle 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 callback 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 callback reads no data, returns, and is immediately invoked again.
A filehandle 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 filehandles that have been placed into nonblocking mode. In blocking mode, a "print" command may block if you give it more data than the underlying file or device can accept, and a "<>", "sysread" 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 "print", "<>", "sysread" and "read" never block. See the documentation for the individual commands for information on how they handle blocking and nonblocking filehandles.
The callback for a file event is executed in the context of $widget with which fileevent was invoked. If an error occurs while executing the callback then the Tk::Error 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.