[Rd] Question about copying arguments in C.

[Simon Urbanek]

On Sep 15, 2012, at 2:57 PM, Simon Urbanek wrote:

> 
> On Sep 15, 2012, at 11:28 AM, Simon Knapp wrote:
> 
>> Hi Simon,
>> 
>> Thanks for your advice, but I'm still not clear. In my case I don't
>> want to modify the result - the integer acts as a handle for indexing
>> an array in later calls back into my library.
>> 
>> As I understand it, returning result like
>> 
>> SEXP func(SEXP arg) {return arg;}
>> 
>> would not copy arg and hence I would have two pointers to the same
>> object immediately after the call... and (if this is the case) I'm not
>> sure whether this is OK.
>> 
> 
> My answer is was, yes, it's ok.
> 
> 
>> Just to be clear, are you saying that the proper way to do things is
>> (at the end of my function in the original post):
>> 
>> SET_SLOT(ans, Rf_install("myInteger"), duplicate(thingysInteger));
>> return ans;
>> 
>> rather than
>> 
>> SET_SLOT(ans, Rf_install("myInteger"), thingysInteger);
>> return ans;
>> 
>> ?
>> 
> 
> No, because you're not modifying anything in that case.
> 
> 
>> The last thing I'm not clear on is if it is OK to create a new SEXP
>> (with a call like duplicate) in a call to another function (as in the
>> first case above) or does this leave it unprotected?
>> 
> 
> Most functions (but not all - a notable exception is eval) protect their arguments, so it's ok in most cases. But this is no different than any other SEXP result - not specific to duplicate() in particular.
> 

Actually, I should explain a bit, because the above case is not about SET_SLOT protecting or not protecting arguments. In fact it does protect its arguments, but even when you are calling a function that protects its arguments, you can get into trouble. But let's take a more common example (since you really don't want to call duplicate() in your example) - let's say you want to do something like

setAttrib(foo, install("bar"), mkString("bar"));

Although setAttrib() is friendly an protects its arguments, the above is bad, because the mkString() is unprotected while install() is called. Now, symbols don't need protection, but it is possible that install() will trigger allocation, so the mkString() result is in danger. The best way around is something like

SEXP bar = install("bar")
setAttrib(foo, bar, mkString("bar"));

Here mkString() is fine, because there cannot be an allocation before setAttrib() protects its arguments. Also bar is a symbol so it doesn't need to be protected, so the above is ok.

Cheers,
Simon



> Cheers,
> Simon
> 
> 
> 
>> Thanx again for your help,
>> Simon
>> 
>> On Sat, Sep 15, 2012 at 10:40 PM, Simon Urbanek
>> <simon.urbanek at r-project.org> wrote:
>>> 
>>> On Sep 14, 2012, at 11:10 PM, Simon Knapp wrote:
>>> 
>>>> Hi List,
>>>> 
>>>> I'd imagine this is a question that has been answered before, but I
>>>> can't seem to track it down, sorry for the duplication if it has.
>>>> 
>>>> I am writing an interface for a C library and want to return an S4
>>>> class from the 'constructing' method. One of the slots of the argument
>>>> to be returned will be filled with one of the arguments passed to the
>>>> function. My question is about whether I can directly pass arguments
>>>> to the function directly to slots of the returned object (or to a
>>>> return value more generally for that matter), or whether I have to
>>>> copy them. If it is the latter, then how may I do this. The question
>>>> is phrased in the following (simplified) code.
>>>> 
>>>> int constructThingy(int thingysInteger);
>>>> 
>>>> SEXP constructThingy(SEXP thingysInteger) {
>>>>  SEXP ans, TClass, ti;
>>>>  if(!isInteger(thingysInteger)) error("thingysIntegermust be an integer.");
>>>>  if(constructThingy(INTEGER(thingysInteger)[0])) error("error in
>>>> getting a thingy");
>>>>  TClass = MAKE_CLASS("thingy");
>>>>  PROTECT(ans = NEW_OBJECT(TClass));
>>>> 
>>>> 
>>>>  // *****QUESTION STARTS HERE*****
>>>>  // CAN I SAY:
>>>>  SET_SLOT(ans, Rf_install("myInteger"), thingysInteger);
>>>> 
>>>>  // IF NOT, CAN I SAY
>>>>  SET_SLOT(ans, Rf_install("myInteger"), AS_INTEGER(thingysInteger));
>>>> 
>>>>  // OR DO I NEED TO SAY
>>>>  PROTECT(ti = allocVector(INTSXP, 1)); INTEGER(pns)[0] =
>>>> INTEGER(thingysInteger)[0];
>>>>  SET_SLOT(ans, Rf_install("myInteger"), ti);
>>>>  // *****END OF QUESTION*****
>>>> 
>>>>  UNPROTECT(1); // or UNPROTECT(2) in latter case.
>>>>  return ans;
>>>> }
>>>> 
>>>> 
>>>> 
>>>> I think this is the same as asking whether the following is OK:
>>>> 
>>>> SEXP func(SEXP arg) {
>>>>  return arg;
>>>> }
>>>> 
>>> 
>>> Yes.
>>> 
>>> In fact if you wanted to duplicate (e.g, if you want to modify an incoming argument and return the modified result), the proper way would be to use duplicate() and not the contortions your were trying.
>>> 
>>> Cheers,
>>> Simon
>>> 
>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> Thanks in advance,
>>>> Simon
>>>> 
>>>> ______________________________________________
>>>> R-devel at r-project.org mailing list
>>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>> 
>>>> 
>>> 
>> 
>> 
>