There should be a way to create a sequence of OPs (containing the required pp_vec) and run it.

Could you expand on this a little please?

I don't know the details, but it should be something like this:
1. synthesize an OP o for vec
2. set up the runtime environment: push the arguments on the stack, save PL_op and make it point to the synthesized OP o, set any other interpreter globals, stacks, etc.
3. call CALLRUNOPS
4. retrieve the result from o->op_targ
5. clean up
For and example, look at Perl_fold_constants from op.c from the perl source. Try running it inside a C debugger with a simple fold-able expression as print(1+2).

The C function Perl_op_dump can be used to dump an OP tree once it has been properly build (otherwise it will crash).

Most likely, you already know that Perl is compiled to a tree of ops. pp_vec is one such op type -- corresponding to the Perl vec() function.

Now, you could create op structures from XS and inject them into the running program. B::Generate does stuff like that.

The one example I'm aware of where this actually works well is List::Util::shuffle. Instead of repeating what's happening there, check for yourself: List::Util XS code.

I tried to do something similar in order to get caller() in XS, but that experiment mostly failed. I would not suggest going down that route.

Cheers,
Steffen

I'm not sure how that helps? vec doesn't do any inserts, just update in-place or grow if the offest calculates beyond the current end of string.

It isn't too hard to re-code the math and bit-twiddling, the main problem is getting the memory management right. As vec does everything I need (and more), it would have been nice to find a way to re-use that code without having to either c&p or callback to perl in order to do it.

sv_insert doesn't just insert, it's the guts behind substr and s///. But I see your point.

Non-lvalue use of vec boils down to:

    sv_setuv(TARG, do_vecget(src, offset, size));
[download]

which should give you what you want.
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