On 04/08/2017 08:16 AM, Aaron Lun wrote:
On 07/04/17 20:46, luke-tier...@uiowa.edu wrote:
On Fri, 7 Apr 2017, Hervé Pagès wrote:
On 04/07/2017 05:37 AM, luke-tier...@uiowa.edu wrote:
On Fri, 7 Apr 2017, Hervé Pagès wrote:
On 04/06/2017 03:29 AM, Michael Lawrence wrote:
On Thu, Apr 6, 2017 at 2:59 AM, Martin Morgan
<martin.mor...@roswellpark.org> wrote:
On 04/06/2017 05:33 AM, Aaron Lun wrote:
The tool is not perfect, so assess each report
carefully.
I get a lot of warnings because the tool seems to consider that
extracting an attribute (with getAttrib(x, ...)) or extracting the
slot of an S4 object (with GET_SLOT(x, ...) or R_do_slot(x, ...))
returns an SEXP that needs protection. I always assumed that it
didn't because my understanding is that the returned SEXP is pointing
to a part of a pre-existing object ('x') and not to a newly created
one. So I decided I could treat it like the SEXP returned by
VECTOR_ELT(), which, AFAIK, doesn't need protection.
So I suspect these warnings are false positives but I'm not 100%
sure.
If you are not 100% sure then you should protect :-)
There are some cases, in particular related to compact row names on
data frames, where getAttrib will allocate.
Seriously? So setAttrib(x, ..., getAttrib) is not going to be a no-op
anymore? Should I worry that VECTOR_ELT() will also expand some sort
of compact list element? Why not keep these things low-level
getters/setters that return whatever the real thing is and use
higher-level accessors for returning the expanded version of the thing?
Seriously: it's been that way since r37807 in 2006.
If you want to read about some related future directions you can look at
https://svn.r-project.org/R/branches/ALTREP/ALTREP.html.
luke
I was curious about this so I checked out what R-exts had to say
involving set/getAttrib. Funnily enough, the example it gives in Section
5.9.4 seems to be incorrect in its UNPROTECTing.
#include <R.h>
#include <Rinternals.h>
SEXP out(SEXP x, SEXP y)
{
int nx = length(x), ny = length(y);
SEXP ans = PROTECT(allocMatrix(REALSXP, nx, ny));
double *rx = REAL(x), *ry = REAL(y), *rans = REAL(ans);
for(int i = 0; i < nx; i++) {
double tmp = rx[i];
for(int j = 0; j < ny; j++)
rans[i + nx*j] = tmp * ry[j];
}
SEXP dimnames = PROTECT(allocVector(VECSXP, 2));
SET_VECTOR_ELT(dimnames, 0, getAttrib(x, R_NamesSymbol));
SET_VECTOR_ELT(dimnames, 1, getAttrib(y, R_NamesSymbol));
setAttrib(ans, R_DimNamesSymbol, dimnames);
UNPROTECT(3);
return ans;
}
There are two PROTECT calls but an UNPROTECT(3), which triggers a stack
imbalance warning upon trying to run .Call("out", ...) in R.
Yes, that should be UNPROTECT(2). svn blame says the error was
introduced when allocMatrix() was introduced; prior to that the code had
allocVector(), then set dim and dimnames.
As for whether to PROTECT or not, my analysis would be...
SET_VECTOR_ELT does not (currently) allocate (except on error) so there
is no opportunity for the garbage collector to run, hence no need to
PROTECT.
Further, getAttrib() (currently) allocates only if (1) the attribute is
R_RowNamesSymbol and the row names are stored in compact format
c(NA_integer_, nrow); or (2) the first argument is a classic pairlist or
language SEXP. None of these conditions apply, so the return value of
getAttrib() is PROTECTed anyway.
Luke's analysis would be more straight-forward: if in doubt, PROTECT.
I think Herve, Gabe, and perhaps Michael would take Luke's advice, and
maybe also note that my advice, in addition to being an analysis of some
surprisingly complicated code by a practitioner of dubious credibility,
involves the current state of affairs, and you never know (and
apparently ALTREP makes it less certain) what the future will hold. So
they'd probably say PROTECT.
One might be tempted to write
SET_VECTOR_ELT(dimnames, 0, PROTECT(getAttrib(x, R_NamesSymbol)));
but I'm not sure whether C guarantees that function arguments are fully
evaluated, maybe it's legal for a compiler to evaluate getAttrib(), then
do some more work with other arguments, then evaluate PROTECT(), so long
as the overall logic is not disrupted. So the 'if in doubt' argument
would make me write
SEXP nms = PROTECT(getAttrib(x, R_NamesSymbol));
SET_VECTOR_ELT(dimnames, 0, nms);
I think , in C is called a 'sequence point'. Google takes me to
https://msdn.microsoft.com/en-us/library/azk8zbxd.aspx
where it seems like the left operand of ',' are fully evaluated before
proceeding, and furthermore that arguments to functions are evaluated
before entering the function, implying that it is safe to use the one-liner
SET_VECTOR_ELT(dimnames, 0, PROTECT(getAttrib(x, R_NamesSymbol)));
At any rate I changed the example in R-exts to UNPROTECT(2), leaving the
nuances for further discussion.
Martin
Anyway, getting back to the topic of this thread; if we were to pretend
that getAttrib() allocates in the above example, would that mean that
both getAttrib() calls now need to be PROTECTed by the developer? Or is
this handled somewhere internally?
Thanks,
H.
Best,
luke
I also get a warning on almost every C++
function I've written,
because
I use the following code to handle exceptions:
SEXP output=PROTECT(allocVector(...));
try {
// do something that might raise an exception
} catch (std::exception& e) {
UNPROTECT(1);
throw; // break out of this part of the function
}
UNPROTECT(1);
return output;
Presumably the check doesn't account for transfer of
control to > > > > the
catch block. I find that R itself is pretty good at
complaining > > > > about
stack imbalances during execution of tests, examples, etc.
'My' packages
(Rsamtools, DirichletMultinomial) had several false
positives > > > > > (all
associated with use of an attribute of a protected SEXP),
one > > > > > subtle
problem (a symbol from a PROTECT'ed package name space;
the > > > > > symbol
could
in theory be an active binding and the value obtained not
PROTECTed by
the name space), and a genuine bug
tag = NEW_CHARACTER(n);
for (int j = 0; j < n; ++j)
SET_STRING_ELT(tag, j, NA_STRING);
if ('A' == aux[0]) {
buf_A = R_alloc(2, sizeof(char)); #
<<- bug
buf_A[1] = '\0';
}
...
SET_VECTOR_ELT(tags, i, tag); # PROTECT
tag, too
late!
I assume the bug refers to the un-PROTECT'd
NEW_CHARACTER here - > > > > the
R_alloc call looks okay to me...
yes, tag needs protection.
I attributed the bug to R_alloc because I failed to
reason that > > > R_alloc
(obviously) allocates and hence can trigger a garbage collection.
Somehow it reflects my approach to PROTECTion, probably
not shared > > > by
everyone. I like to PROTECT only when necessary, rather than
indiscriminately. Probably this has no practical consequence in
terms of
performance, making the code a little easier to read at the
expense > > > of
exposing me to bugs like this.
I guess it's a tradeoff between syntactic complexity
and logical
complexity. You have to think pretty hard to minimize use of the
protect stack.
I prefer to call it logical obscurity ;-)
The hard thinking consists in assessing whether or not the code
between
the line where a new SEXP is allocated (line X) and the line where
it's put in a safe place (line Y) can trigger garbage collection.
Hard to figure out in general indeed, but not impossible! Problem
is that the result of this assessment is valid at a certain point
in time but might change in the future, even if your code has not
changed.
So a dangerous game for virtually zero benefits.
One recommendation might be to UNPROTECT() as soon as the
pointer on
the top is unneeded, rather than trying to figure out the number to
pop just before returning to R.
If you PROTECT() in a loop, you definitely want to do that.
Otherwise,
does it make a big difference?
One thing that got me is that the order in which C
evaluates function
call arguments is undefined. I did a lot of R_setAttrib(x,
install("foo"), allocBar()), thinking that the symbol would be
automatically protected, and allocBar() would not need protection,
since it happened last. Unfortunately, it might be evaluated first.
I got hit by this too long time ago but with defineVar() instead of
R_setAttrib():
https://urldefense.proofpoint.com/v2/url?u=https-3A__stat.ethz.ch_pipermail_r-2Ddevel_2008-2DJanuary_048040.html&d=DwID-g&c=eRAMFD45gAfqt84VtBcfhQ&r=BK7q3XeAvimeWdGbWY_wJYbW0WYiZvSXAJJKaaPhzWA&m=FscW1HcPCwUqtMwKVFDfd1NyW0oHh0tJOPdFb3C1IWk&s=O3CcB-Z_OkVKaC1aV0aIc5SCDNqGQrkvGSmPf0TRAsw&e=
H.
Btw, I think my package RGtk2 got the record: 1952 errors.
Luckily
almost all of them happened inside a few macros and autogenerated
code.
Martin
Cheers,
Aaron
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