pileofrogs has asked for the wisdom of the Perl Monks concerning the following question:

Hello Monks

I'm working on making the unix saved id c functions available in perl. (See Unix::SavedIDs.) The c interface when setting id values is:

setresuid(ruid,euid,suid);

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I would like to accommodate the situation where a user only wants to set the suid, IE I want this to work:

setresuid(undef,undef,$suid);

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But XS seems to take a bleak view of sending undef as a parameter. I can set default values, so:

setresuid($ruid,$euid)

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Works, but that doesn't help the instance where someone specifically passes undef as an arg.

The C functions accept '-1' to leave a value unchanged, so what I need to do is change undef to -1. I'd rather not require my users to say '-1' in a perl script where the perlish thing to do would be to say undef.

Thanks!
--Pileofrogs

Replies are listed 'Best First'.
Re: Passing undef to XSUB
by kyle (Abbot) on May 07, 2008 at 20:35 UTC

    It's a cheap solution, but I would probably write a wrapper. 

    sub setresuid {
    # force @_ to have three elements
    $#_ = 2;

    # replace undefs in @_ with -1
    for ( @_ ) {
        $_ = -1 if ! defined $_;
    }

    # magic goto the real setresuid
    goto &real_setresuid;
}

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[d/l]
Re: Passing undef to XSUB
by educated_foo (Vicar) on May 07, 2008 at 21:21 UTC
    The wrapper's easier and probably better (e.g. it's easy to support keyword parameters), but you can also take raw SVs in your XS code, like so: 
    int
foo(a,b)
    SV * a
    SV * b
    PREINIT:
      int aa, bb;
    CODE:
      aa = SvIOK(a) ? SvIV(a) : -1;
      bb = SvIOK(b) ? SvIV(b) : -1;
      RETVAL = aa * bb;
    OUTPUT:
      RETVAL

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[d/l]
      Would int* also work? Like in 
      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

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[d/l]
[select]
        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

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        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
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[d/l]
[select]
        Not for me -- int* doesn't appear to have a typemap for me (perl 5.8.8).
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