On Tue, 23 May 2006, Kasper Daniel Hansen wrote: > Thank you very much. I think I do have a clearer understanding, but I have a > few questions > > On May 23, 2006, at 12:55 AM, Prof Brian Ripley wrote: > >> On Tue, 23 May 2006, Prof Brian Ripley wrote: >> >>> On Mon, 22 May 2006, Kasper Daniel Hansen wrote: >>> >>>> I have a few simple questions about the usage of PROTECT, more >>>> specifically how careful one needs to be. Simple yes/no answers are >>>> fine. >>> >>> (Except that in the last case they would be misleading.) >>> >>>> Most of the uses I have seen do protection when memory is allocated. >>>> But what if one just want to assign a value of another function to a >>>> variable. Say eg. that foo is a function that returns a SEXP. Would >>>> the following code be fine? >>>> >>>> SEXP bar; >>>> PROTECT(bar = foo()); >>> >>> It would be fine but may be unnecessary. It is objects and not pointers >>> which are protected, and a SEXP is a pointer. So protection is needed >>> only if foo() might return a pointer to an unprotected object. > > Ok. I have been coding foo in such a way that I unprotect everything in foo > just before returning its value. I thought that was the "standard" way to do > - is that true? Or should I leave the return value protected and then > unprotect in the function calling foo?
That is indeed standard. The issue is rather that if say foo() extracts an element of a list which has an R-level name, you know that it is already protected. >>>> Also, basically in one use case I would want to return the value of >>>> foo immediately, but I need to do some cleaning up first, which has >>>> nothing to do with R (more specifically, I need to close various >>>> files). Would I then need to protect foo, as in >>>> >>>> SEXP bar; >>>> bar = foo(); >>>> "close the file in C++" >>>> return bar; >>> >>> Fine, as PROTECT protects against R garbage collection, and that can only >>> happen if R's functions are called. >>> >>>> Finally, I am also assigning values to the components of a list. >>>> Would the following be ok >>>> >>>> SEXP bar; >>>> PROTECT(bar = NEW_LIST(2)); >>>> SET_VECTOR_ELT(bar, 0, ScalarInteger(test()); >>>> >>>> (where test is a function returning int, which again has nothing to >>>> do with R - it interfaces to an extern library), or do I need to >>>> hedge myself against garbage collection in the SET_VECTOR_ELT macro? >>> >>> You do need to protect but elsewhere in this call, as ScalarInteger does >>> memory allocation: >>> >>> INLINE_FUN SEXP ScalarInteger(int x) >>> { >>> SEXP ans = allocVector(INTSXP, 1); >>> INTEGER(ans)[0] = x; >>> return ans; >>> } >>> >>> but SET_VECTOR_ELT does not. So you need >>> >>> SEXP bar, tmp; >>> PROTECT(bar = NEW_LIST(2)); >>> PROTECT(tmp = test()); >>> SET_VECTOR_ELT(bar, 0, ScalarInteger(tmp)); >>> UNPROTECT(1); >> >> Or a design that uses fewer PROTECTs >> >> SEXP bar, tmp; >> PROTECT(bar = allocVector(VECSXP, 2)); >> tmp = allocVector(INTSXP, 1); >> SET_VECTOR_ELT(bar, 0, tmp); >> INTEGER(tmp)[0] = test(); > > I thought I got this. Then I grepped the sources and found this in > main/platform.c: > > PROTECT(ans = allocVector(VECSXP, 18)); > PROTECT(nms = allocVector(STRSXP, 18)); > SET_STRING_ELT(nms, 0, mkChar("double.eps")); > SET_VECTOR_ELT(ans, 0, ScalarReal(R_AccuracyInfo.eps)); > > This looks very similar to what I did above. In my case "test" was a C++ > function coming from outside of R returning an int. That was perhaps not > clear from my original mail, since the first suggested correction had > PROTECT(tmp = test()); > indicating that the return value for test is a SEXP. Or am I completely of? If test() iself does not use anthing from R (that it is C++ is enough of the story), then you do not need to protect it. Or as in the platform.c example, if it is a constant. Sorry, the caveats were not clear to me, and I tend not to rely on them as people do sometimes change functions. > I have tried running my original suggestion with gctorture(TRUE) and it did > not give any errors. But neither did the second suggested correction. -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel