When I do this my method is as follows:

I create a dummy function in C such as
bit AsmFunc( data hello)
{
sbuf = hello;// make sure hello is not optimized out
return (TRUE);
}

then I compile it with the .SRC directive and save the assembler listing, column edit out the binary and now know exatly where the C compiler put the input variable and expect the return to be. Then I can code the routine to do what I had planned.

Erik