[Rd] survival bug? - solved

[brodie gaslam]

I _think_ the relevant section of the C standard is 6.5.6 Additive Operators Par 8, excerpted here:

> If both the pointer operand and the result point to elements 
> of the same array object, or one past the last element of the 
> array object, the evaluation shall not produce an overflow; 
> otherwise, **the behavior is undefined**.

This is from the [C11 draft][1], though I imagine has been part of the standard for a while.  So by doing id[-1], in this case the pointer operand is id, and the result is one element _before_ the array object, thus undefined behavior which is bad news.

I'm not an expert in these matters though.

[1]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf


On Thursday, March 5, 2020, 11:39:38 AM EST, Therneau, Terry M., Ph.D. via R-devel <r-devel using r-project.org> wrote: 





 I ended up finding the issue by a focused code review.

Once in the past, I had a version that would fail under one architecture but not another, 
in that case some help from Brian Ripley pointed me to the offending line of C code.   
That line read, but did not write, at an invalid memory location.   Starting with the 
question of "what C routines have I added or modified most recently" along with where the 
fault appeared to occur in my test suite, I started reading C code and found one.   
Revised code passes tests on the winbuilder site.

For the curious, I had a line asking "is this patient id different than the last patient 
id" in the C routine underneath survcheck(); I'm making sure that patients don't go 
backwards in time. Essentially
 for (i=0; i< n; i) {
     if (id[i] != id[i-1] )  { ...}

It is still a surprise to me that just LOOKING at this out of range element would cause a 
failure,  [i-1] never appears on the left hand side of any expressions in the ... chunk 
above. Nevertheless, it was an error.   Que sera sera

A strong thanks to those who gave solid suggestions for bringing up a local copy of Windows.

Terry T

>>> My latest submission of survival3.1-10 to CRAN fails  a check, but only on
>>> windows, which
>>> I don't use.
>>> How do I track this down?
>>> The test in question works fine on my Linux box.
>>>
>>> Terry
>>>
>>>
>>>
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