How do I pass a char** in IDL

I'm new to IDL and have not had much luck finding a good tutorial online.

If I have a simple C client program:-

int main(int argc, char **argv) {
...
return(0);
}

How do I declare the interface in IDL?

I've tried the following but it gives the following error:
test.idl: 4:syntax error after string
test.idl: 4:expected one of: <, identifier
test.idl: 4:instead of *

module mymodule {
interface myinterface {
long myoperation(in long a, in string **b);
};
};