Correct .Call() function signatures

[joshuaulrich]

All C functions called via .Call() must return SEXP and have all SEXP arguments. This is not true for do_merge_xts() and isXts(). This may change the API, and impact RcppXts. Need to confirm.

Via email from @kalibera:

Could you please check the signatures of foreign functions in xts? It
seems that do_merge_xts is taking a non-SEXP argument, but it is called
via .Call interface (all arguments have to be SEXP). Also isXts() is
registered as a .Call foreign function, but it does not return SEXP.
Both of these issues could probably lead to segfaults, so it would be
great if you could fix them soon.

[joshuaulrich]

@eddelbuettel My preliminary checks suggest this is going to affect RcppXts. My understanding is that I need to do one of two things to do_merge_xts() and isXts():

1. Do not register them as C-callable functions in init.c, or
2. Change the definitions so both functions only take SEXP arguments and return SEXP (isXts() returns int).

What are your thoughts/preferences?

[kalibera]

To clarify, not registering a function will not solve the problem. If a function is not registered, but is called via .Call, it still has to return SEXP and all of its arguments must be SEXP.

[eddelbuettel]

I got a similar email from Tomas and already fixed it (in digest). Based on that experience, there are two orthogonal issus here:

• Tomas rightly point out that one should not register for .Call() when the signature is different (doh)
• One can still register for export and use by another package, that does not require a SEXP signature (I think...) -- see what digest now does in its init.c

Haven't looked at xts yet.

Edited again, adding an important not above

[kalibera]

Yes, functions for use with RegisterCCallable/GetCCallable only (not .Call) do not have such restrictions.

[joshuaulrich]

Thank you both for your input! It looks like I only need to stop registering those functions for .Call(). Neither of them are called via .Call() in xts:

josh@thinkpad: ~/git/xts/R (master)
> grep -E "\.Call\(.(isXts|do_merge_xts)" *

That also means RcppXts should be okay, since the function signatures do not need to change.