// Define a command function in C style
    
    int aCommand( ClientData data, Tcl_Interp* interp,
  				 int args, char* argv[]) {
  
    some_type* x = (some_type*) data;        // conversion by cast
    
    // some processing
  
    }
    
    // Declare the function aCommand as a Tcl command
    // for example in the  main  function
    
    some_type* user = new some_type(); // to create the client data
    Tcl_CreateCommand( interp, "aco", aCommand, (ClientData) user );