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Re^5: SvPV Segmentation Fault

by dave_the_m (Parson)
on Apr 25, 2012 at 23:14 UTC ( #967202=note: print w/ replies, xml ) Need Help??


in reply to Re^4: SvPV Segmentation Fault
in thread SvPV Segmentation Fault

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.

Dave


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Re^6: SvPV Segmentation Fault
by adler187 (Initiate) on Apr 25, 2012 at 23:20 UTC
    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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