Calling a C function that alters the passed argument?

[Bob Greschke]

Here's the C function I want to call from Python:

char *boo(ANumber)
int *ANumber;
{
    *ANumber = 42;

    if (something went wrong)
        return "An error message";

    return (char *)NULL;
}


How would I write the wrapper function for this?  All I can find are
examples where the wrapper functions deal with the returned value from
C functions.

????

Thanks!

Bob