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SCO Visual Tcl commands are synchronous. The command runs, it performs an action, and returns both a Tcl return result (TCL_OK or TCL_ERROR) and often a return string (some returned information or an error message).
The return code and string must be received before the interpreter can execute the next command.
The application interaction model is event driven, where most of the application is concerned with building an application widget tree and a number of procedures to run when ``interesting'' events happen. The only asynchronous event the interpreter may receive is an indication to execute a callback procedure set up earlier, all other interaction is synchronous with respect to the interpreter.