I don't really know C++, that is C++ code. Can you post the function from the .c file that xsubpp generates from your XS syntax func?

I suggest you use Devel::Peek's Dump function. You can also call it from XS/C as dump.c#l2139 in perl.git as "void sv_dump(SV *sv)" It will need a SV *, I think that ST(1) is the passfail param as a SV *, since ST(0) will be your THIS pointer in a SV * I think from Script land. Using sv_dump with your code unmodified is impossible as written since the setting of SV * that matches passfail is after your CODE: section (again look at the .c file produced), so you would need to

int 
MyLib::strToInt(passfailSV)
    SV* passfailSV
PREINIT:
    bool passfail;
CODE:
    foo(&passfail);
    SvSetMagicSV(passfailSV, passfail ? &PL_sv_yes : &PL_sv_no);
OUTPUT:
    RETVAL
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I think there is a bug in the typemap,

T_BOOL
    $arg = boolSV($var);
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That is from the default typemap. But look at boolSV.

/*
=for apidoc Am|SV *|boolSV|bool b

Returns a true SV if C<b> is a true value, or a false SV if C<b> is 0.

See also C<PL_sv_yes> and C<PL_sv_no>.

=cut
*/

#define boolSV(b) ((b) ? &PL_sv_yes : &PL_sv_no)
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We assigned a SV * to a SV *, but if this is a in parameter and not an out parameter, you can't just change the SV * to another SV *, you have to change the SV * you got from the caller (return/result's of XSUBs are *usually* brand new SV *s you created and mortalized, then boolSV would work, it won't work to change the inside value of an existing scalar). BTW, you never need to mortal in SV *s you get through the param list (AKA ST(123456) macros), your caller owns the param list SV *s. Also never mortal &PL_sv_yes, &PL_sv_no, &PL_sv_undef, they are process globals/statics.

I probably should go file a bug report with ExtUtils::ParseXS but I dont have the time.

update: bug filed