How to get access to current context from XS?

When a user calls XS from main:: package we can not use

caller_cx(0, NULL);

because there is no frames for main:: and for XSUB DOC

Note that XSUBs don't get a stack frame, so C will return information for the immediately-surrounding Perl code

How to get the file:line info the XSUB is called from, hints for main:: scope etc. info?

Solution

Copied from mess_sv (called by Perl API functions warn and croak, which append line information like the Perl functions warn and die):

use strict;
use warnings;
use feature qw( say );

use Inline C => <<'__EOS__';

void testing() {
    dXSARGS;

    /*
     * Try and find the file and line for PL_op.  This will usually be
     * PL_curcop, but it might be a cop that has been optimised away.  We
     * can try to find such a cop by searching through the optree star ting
     * from the sibling of PL_curcop.
     */
    if (PL_curcop) {
        const COP *cop =
            Perl_closest_cop(aTHX_ PL_curcop, OpSIBLING(PL_curcop), PL_op, FALSE);
        if (!cop)
            cop = PL_curcop;

        if (CopLINE(cop)) {
            EXTEND(SP, 2);
            mPUSHs(newSVpv(OutCopFILE(cop), 0));
            mPUSHs(newSViv((IV)CopLINE(cop)));
            XSRETURN(2);
        }
    }

    XSRETURN(0);
}

__EOS__

say join ":", testing();

A little bit about PL_curcop here.