[Rd] Patch to fix small bug in do_External and do_dotcall

Olaf Mersmann olafm at kimberly.tako.de
Sun Dec 28 17:36:17 CET 2008

Excerpts from Prof Brian Ripley's message of Sun Dec 28 15:03:28 +0100 2008:
> Thank you.  You do realize that your example is not passing a 
> NativeSymbolInfo object, don't you?  I believe the intention is that you 
> pass 'sym'.

Yes, I had originally used sym and later changed it to sym$address to
verify, that the fix also works for case b) described in the comment
above checkValidSymboldId(), namly passing in a pointer to a
function. I'm not sure if this is currently a 'supported' method of
calling a function, since it is only mentioned in the man page for
getNativeSymbolInfo() and not in the .Call() man page.

> I'll incoporate the patch once I have worked out an accuracy description 
> of what it does ....

The behavior before the patch is to assume that the head of args is a
string. When it is anything else, the call to translateChar() when
deriving the function name for the error message fails.

Instead of dealing with each possible type, the patch reuses the
function name that was returned by resolveNativeRoutine() which in
turn calls checkValidSymbolId() (all defined in dotcode.c). If
CAR(args) is a string, checkValidSymboldId simply returns and
resolveNativeRoutine() copies the name into buf. If CAR(args) is a
NativeSymbolInfo object, checkValidSymbolId() recalls itself with the
second element of the NativeSymbolInfo object (its address
member). Lastly if CAR(args) is a EXTRPTRSXP (the type of the address
member of a NativeSymbolInfo object) checkValidSymboldId() extracts
the symbol name from the EXTPTRSXP if it is a registered symbol. This
is the only loophole. If I where to pass an address to .Call() or
.External() which was a "native symbol" but not a "registered native
symbol", the buffer holding the function name would never be
filled. I'm not sure how to deal with this corner case. One option
would be to copy some (un)descriptive like '<Unknwon>' into the
buffer. If this is acceptable I can add it and post a revised patch.

Greetings from Dortmund
Olaf Mersmann

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