Creating Tcl Extensions in C

• Tcl can be extended by the addition of new commands.
• Each new command has a name and a number of arguments.
• When Tcl executes a command, a C function is called with the following prototype:

  int newCommand (ClientData cd, Tcl_Interp *interp,
                  int argc, char *argv[]);

  interp is a handle on the Tcl interpretor (there can be more than one).

  newCommand also can place a string result in interp->result.

  newCommand returns TCL_OK or TCL_ERROR.

• New commands must be registered before they can be used:

  Tcl_CreateCommand(interp, "new_command", newCommand,
                    (ClientData) NULL, (Tcl_CmdDeleteProc *) NULL);

• Here the Tcl name of the command is ``new_command''; the C function ``newCommand'' is called when ``new_command'' is interpreted.
• New commands can be created (and destroyed) at any time--enabling object-orientation.
  • The ClientData pointer can be used to reference an object's data.
  • The Tcl_CmdDeleteProc pointer can be used to reference an object destructor.
• A C program is linked with the Tcl library to create a ``shell'' that understands the standard and extended Tcl commands.