LuaJava Setting Error Handler for LuaState.pcall(a,b, error_function_index)?

4
votes

I am trying to call:

LuaState.pcall(num_args,num_returns, error_handler_index).

I need to know how to set the error handler for this function. In fact, I think it would be nice it someone showed how to invoke a Lua function and get a numerical result back using LuaJava. This might save a lot of time and questions. I am looking but not finding the signature for the error function, and how to place it at the right point on the LuaState stack. All the Java->Lua examples are either printing a value with no return or they are setting values on a Java object passed in using Lua. I would like to see how to call a Lua function directly and get the result back.

Update: one solution is to pass no error handler using LuaState.pcall(1,1,0) by passing zero for the error handler:

String errorStr;
L.getGlobal("foo");
L.pushNumber(8.0);
int retCode=L.pcall(1,1,0);
if (retCode!=0){
    errorStr =  L.toString(-1);
}
double finalResult = L.toNumber(-1);

where calc.lua has been loaded:

function foo(n) 
 return n*2 
end

Now is there a way to set an error handler as well? Thanks

2
lualuajava

2 Answers

1
votes

If you also want the stack traceback (I'm sure you do :), you can pass debug.traceback as the error function. Take a peek at how it's implemented in AndroLua.

Basically, you have to make sure your stack is set up as follows:

  • Error handler (debug.traceback)
  • Function you want to call
  • Parameters

You can do it like this with your example:

L.getGlobal("debug");
L.getField(-1, "traceback");      // the handler
L.getGlobal("foo");               // the function
L.pushNumber(42);                 // the parameters
if (L.pcall(1, 1, -3) != 0) { ... // ... you know the drill...
0
votes

Assuming you have a Lua function somewhere to handle the error:

function err_handler(errstr)
  -- exception in progress, stack's unwinding but control 
  -- hasn't returned to caller yet
  -- do whatever you need in here
  return "I caught an error! " .. errstr
end

You can pass that err_handler function into your pcall:

double finalResult;

L.getGlobal("err_handler"); 
L.getGlobal("foo");
L.pushNumber(8.0);

// err_handler, foo, 8.0
if (L.pcall(1, 1, -3) != 0)
{
    // err_handler, error message
    Log.LogError( L.toString(-1) );  // "I caught an error! " .. errstr
}
else 
{
    // err_handler, foo's result
    finalResult = L.toNumber(-1);
}
// After you're done, leave the stack the way you found it
L.pop(2);