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Using vec() from XS

by BrowserUk (Pope)
on Sep 23, 2008 at 12:27 UTC ( #713205=perlquestion: print w/ replies, xml ) Need Help??
BrowserUk has asked for the wisdom of the Perl Monks concerning the following question:

Is it possible (to use vec from XS code)?

If yes, does anyone have any examples?


Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

Comment on Using vec() from XS
Re: Using vec() from XS
by salva (Monsignor) on Sep 23, 2008 at 13:48 UTC
    Some ways to do it:
    • Write a wrapper function in Perl and call it.
    • There should be a way to create a sequence of OPs (containing the required pp_vec) and run it.
    • Copy & paste the source code for pp_vec and its helpers into your code changing the interface to suit your needs.
        I don't know the details, but it should be something like this:
        1. synthesize an OP o for vec
        2. set up the runtime environment: push the arguments on the stack, save PL_op and make it point to the synthesized OP o, set any other interpreter globals, stacks, etc.
        3. call CALLRUNOPS
        4. retrieve the result from o->op_targ
        5. clean up
        For and example, look at Perl_fold_constants from op.c from the perl source. Try running it inside a C debugger with a simple fold-able expression as print(1+2).

        The C function Perl_op_dump can be used to dump an OP tree once it has been properly build (otherwise it will crash).

        Most likely, you already know that Perl is compiled to a tree of ops. pp_vec is one such op type -- corresponding to the Perl vec() function.

        Now, you could create op structures from XS and inject them into the running program. B::Generate does stuff like that.

        The one example I'm aware of where this actually works well is List::Util::shuffle. Instead of repeating what's happening there, check for yourself: List::Util XS code.

        I tried to do something similar in order to get caller() in XS, but that experiment mostly failed. I would not suggest going down that route.

        Cheers,
        Steffen

Re: Using vec() from XS
by ysth (Canon) on Sep 23, 2008 at 15:37 UTC

      I'm not sure how that helps? vec doesn't do any inserts, just update in-place or grow if the offest calculates beyond the current end of string.

      It isn't too hard to re-code the math and bit-twiddling, the main problem is getting the memory management right. As vec does everything I need (and more), it would have been nice to find a way to re-use that code without having to either c&p or callback to perl in order to do it.


      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.

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