How to make tcl interpreter not to continue after exit command? - c++

Question

static int
MyReplacementExit(ClientData unused, Tcl_Interp *interp, int argc, const char *argv[])
{
// Tcl_DeleteInterp(interp);
// Tcl_Finalize();
return TCL_OK;
}
int main() {
Tcl_Interp *interp = Tcl_CreateInterp();
Tcl_CreateCommand(interp, "exit", MyReplacementExit, NULL, NULL);
Tcl_Eval(interp, "exit ; puts 11111111");
std::cout << "22222222222" << std::endl;
return 0;
}
I need to handle exit command evaluation of tcl interpreter. By default it tries to delete itself and also calls std::exit which closes whole program.It is not what I want so I am trying to replace it by custom proc. I dont need to delete interpreter in exit handler proc(I can do it later), only need it to not continue evaluating commands after exit command.
In this code I need to change MyReplacementExit proc somehow, so 11111111 doesn't be printed but
22222222222 does printed.
It can be achieved by returning TCL_ERROR from MyReplacementExit proc, but then I can't distinguish other error situations from this.

Answer 1

Make your replacement for exit delete the interpreter (which stops further commands from being executed, but doesn't actually immediately delete the data structure as it is still in use) and, important, wrap the call to Tcl_Eval with calls to Tcl_Preserve and Tcl_Release. Don't call Tcl_Finalize if you can possibly avoid it; that is for when you're about to unload the Tcl library from memory and can be quite tricky (it's easier to just quit the process, frankly).
Here's how to do it with your code (adapted):
static int
MyReplacementExit(ClientData unused, Tcl_Interp *interp, int argc, const char *argv[])
{
Tcl_DeleteInterp(interp); // <------------------
return TCL_OK;
}
int main() {
Tcl_Interp *interp = Tcl_CreateInterp();
Tcl_CreateCommand(interp, "exit", MyReplacementExit, NULL, NULL);
Tcl_Preserve(interp); // <------------------
Tcl_Eval(interp, "exit ; puts 11111111");
Tcl_Release(interp); // <------------------
std::cout << "22222222222" << std::endl;
return 0;
}
Be aware that you should not access the interpreter at all after the Tcl_Release as it might've been destroyed (as in, the memory released and scribbled over with random junk) at that point. If you need to retrieve results and use them (e.g., printing them out) do so beforehand.
Note that in this specific case, you don't need the preserve/release pair; your code isn't actually touching the interpreter after the Tcl_Eval (which does its own preserve/release internally).
If you don't want the interpreter to terminate, that's much trickier. The cleanest way in 8.4 is probably to throw a custom exception code (i.e., anything greater than TCL_CONTINUE) but there's no guarantee that it will work for arbitrary code as Tcl's catch command can still trap it. If you're really in that situation, it's actually easier to create an interpreter, run the arbitrary code in the sub-interp, and tear it down at the end of the script; you can then drop that interpreter without losing much context. Indeed, you could do:
Tcl_Preserve(interp);
if (Tcl_Eval(interp, theScriptToEval) != TCL_OK)
// Handle unexpected errors here
if (!Tcl_InterpDeleted(interp))
Tcl_DeleteInterp(interp);
Tcl_Release(interp);
Yes, this will mean you want to keep the amount of work you do to set up the interpreter fairly small; you probably won't want to try to call this every millisecond on an interrupt…