Would int* also work?

No, but int would: 
int
foo(ap,bp)
    int ap
    int bp
    PREINIT:
      int a, b;
    CODE:
      a = ap ? ap : -1;
      b = bp ? bp : -1;
      RETVAL = a * b;
    OUTPUT:
      RETVAL

[download]
Though, if warnings are enabled, that produces a "Use of uninitialized value in subroutine entry..." for each undef that's passed to it - no such warning with educated_foo's rendition.

Cheers,
Rob

In reply to Re^3: Passing undef to XSUB by syphilis
in thread Passing undef to XSUB by pileofrogs

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