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.

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();
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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) { ... }
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and invoke it as

foo(aTHX_ some_sv);
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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;
    ...
}
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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.

Dave

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