What file?

IIRC, I don't think it likes // comments , I think it likes /* */ comments, at least that is my recollection of my toying with this

http://perldoc.perl.org/perlxs.html#The-NO_INIT-Keyword

I'm just finished compiling PDL-2.4.9_005 with ExtUtils::ParseXS: 3.03 as I type this

No problems so far -- I guess I remembered correctly, must be the comments, use /**/

What file?

Basic/Core/Core.xs.PL

I'm just finished compiling PDL-2.4.9_005 with ExtUtils::ParseXS: 3.03

Yes - it should build fine, but only because its Basic/Core/typemap has a pointless typemap entry for 'PDL_Long *'.
Well ... of course, it's *not* pointless .... it prevents xsubpp from complaining about the lack of a typemap entry. But the typemap has no INPUT or OUTPUT entry for 'PDL_Long *' - it therefore does nothing other than trick xsubpp into believing that the typemapping has been done, thereby silencing the complaint.
Of course, no such typemapping has been done and I don't think this is the right solution.

I guess I remembered correctly, must be the comments, use /**/

If you say so ....

Cheers,
Rob

UPDATE: I suppose I should take the time to point out that the function I gave by way of example doesn't contain literally // code that does stuff
That just stands for a block of code whose actual content is irrelevant to the question I've asked.
Anonymous Monk, if you want to see the failure, try building 2.4.9_004 with ExtUtils-ParseXS-3.03. The error should occur about one minute (if that) into the 'make' stage.

...has a pointless typemap entry for 'PDL_Long *'... has no INPUT or OUTPUT ...

I don't think there is any tricking going on

The default typemap has

T_PTR
    $var = INT2PTR($type,SvIV($arg))
[download]

I don't think it needs INPUT/OUTPUT

Since PDL_Long * pos is the RETVAL, and the return type is SV*, you need a typemap entry

You might not need it if you specified T_PTR as return type

T_PTR
at_c(x,position)
   pdl*    x
   PDL_Long * pos = NO_INIT
   CODE:
    // code that does stuff
    OUTPUT:
     RETVAL
[download]

I would test it, but, PDL takes a while to compile :)

Well, you certainly should not put non-arguments there. The line after "pdl* x" should declare the type of the "position" argument. It should not be some random bit of code about "pos", which is not one of the arguments.

The fact that it used to work is certainly an accident.

But the correct thing to do is not what you proposed but more like:

SV *
at_c(x,position)
        pdl*    x
        SV*     position
    PREINIT:
        PDL_Long * pos;
    CODE:
        ...
    OUTPUT:
        RETVAL
[download]

Then you can also replace "ST(1)" with "position" in the code.

"The PREINIT: keyword allows extra variables to be declared immediately before or after the declarations of the parameters from the INPUT: section are emitted."

The fact that it used to work is certainly an accident

Ok - that makes good sense to me, and is one of the things I was hoping to establish.

Then you can also replace "ST(1)" with "position" in the code.

Some of us were wondering what this unused "position" argument was doing. Obviously it wasn't doing anything, but I think I start to understand a purpose for it. (Update: Duh !! It's just an arg that can now be accessed by its name instead of its position on the stack.)

As I now understand it, it's *all* of the other variables (not just 'pos') that should be declared in PREINIT instead of CODE.

I'll rewrite the file in accordance with tye's advice and give it a run.
Many thanks to all respondents.

Cheers,
Rob

Being the guy who did the changes to ExtUtils::ParseXS, let me add my support for what tye said: The code in question worked purely by accident. The proper way to declare or initialize variables in an XSUB is in a PRE_INIT block! ExtUtils::ParseXS 3.XX just became strict in telling you about the problem in your code. It could have instead broken your code to segfault because of the old behaviour being an accidental misfeature and side-effect of the horrors that lurk in EU::ParseXS' code generation.

As I now understand it, it's *all* of the other variables (not just 'pos') that should be declared in PREINIT instead of CODE.

C doesn't allow intermixing of vars declarations and code like Perl and C++. Var declarations can just be placed globally or at the start of the block. PREINIT will be placed before any code. CODE can be preceded by other code. Therefore, declarations must be placed in PREINIT or you must add curlies to CODE.

The PREINIT section of perlxs explains this in more detail and gives example.