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Re^4: SvPV Segmentation Fault

by adler187 (Initiate)
on Apr 25, 2012 at 22:39 UTC ( #967197=note: print w/replies, xml ) Need Help??

in reply to Re^3: SvPV Segmentation Fault
in thread SvPV Segmentation Fault

Hmm, was wondering about that. Initially I did not have the declaration, but then I got an error about 'my_perl' undeclared. I found some code online (that I didn't read thoroughly, I see) and declared the variable.

Apparently, that is not enough. Any good examples/tutorials on how to do this?

Or perhaps I'm taking the wrong approach here and there is a better way. Here's what I'm trying to do. I'm trying to write a function in C that can be called from perl that takes as input, basically argc and argv. It seems you can't actually pass an array of char * from perl, you need to handle an AV *. I'd like to keep all this code in the external library, instead of writing it in the XS file, but maybe I'm just making it harder for myself that way.

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Re^5: SvPV Segmentation Fault
by dave_the_m (Prior) on Apr 25, 2012 at 23:14 UTC
    See perlguts.pod for more details about the [adp]THX_? macros, but basically, you're using a perl compiled for multiplicility/threading, and in those perls, most perl functions (including SvPV()) expect to take an extra first argument, called my_perl, which is a pointer to the current perl interpreter.

    The idea is that on threaded perls, these macros typically expand as follows:

    aTHX my_perl aTHX_ my_perl, pTHX PerlInterpreter* my_perl pTHX_ PerlInterpreter* my_perl, dTHX PerlInterpreter* my_perl = something_that_retrieves_the_current_perl();
    while on unthreaded perls they expand to null strings.

    You typically declare any function as expecting an interpreter as the first arg:

    void foo(pTHX_ SV * sv) { ... }
    and invoke it as
    foo(aTHX_ some_sv);

    Normally your XS sub will have been passed a my_perl arg, and you can pass this on down the chain of called subs using aTHX/pTHX. In those cases where this isn't possible, dTHX can be used within the declaration section of a function to recreate my_perl 'on the fly', but it's expensive:

    void foo(SV * sv) { dTHX; ... }

    All the above assumes that PERL_NO_GET_CONTEXT is defined in your source. If it isn't then aTHX is redefined to directly compute the interpreter address, like dTHX. This means everything 'just works' without messing with aTHX/pTHX, but is slow. Since you were getting compilation errors about missing my_perl, I'm assuming you must have had PERL_NO_GET_CONTEXT defined.


      Thanks, I also found this page when looking up dTHX on google. Looks like I should be able to figure it out now.

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