[Rd] C/C++ 'assert' should not be used in R packages

Duncan Temple Lang duncan at wald.ucdavis.edu
Sat Nov 10 19:00:32 CET 2007

Hash: SHA1

Duncan Murdoch wrote:
> Prof Brian Ripley wrote:
>> Please don't use 'assert' in R packages.  If called, this means that an 
>> error in your code aborts the whole R process, including your user's work. 
>> I see several R packages doing this, and one of them called 'assert' on me 
>> earlier in the week.
> I partly disagree about this.  If assert() is triggered, it clearly 
> indicates a bug in the package.  If it just generated an R error, most 
> users would ignore it, and not report it to the package maintainer. 
> It may well be that when an assertion fails, none of the subsequent 
> calculations are reliable, in which case returning control to the user 
> could result in data corruption.  That's worse than losing a session, 
> because at least when you lose a session, you know it.
> Could we write our own implementation of assert() that displays an R 
> error and unloads the package?  I think I could do something like that 
> in Windows by calling FreeLibrary to unload the DLL, but I'd prefer a 
> cross-platform solution.

I am not sure why you think we need to discard the DLL.
What if we want to access variables and routines merely to find out
the resulting state after the assert().
And it is not clear whether you mean any "subsequent calculations"
or any "subsequent calculations using code within this DLL"
might be unreliable.

But regardless of that and the assumption that users will ignore this
error, why would you want to call FreeLibrary? Since R has induced the
DLL to be loaded either directly or indirectly, R holds a handle to the
DLL. If you don't use R's cross-platform facilities for releasing the
DLL (either at the R or C-level), you will corrupt the session,
specifically in the list of assumed-live DLLs.


>> We provide 'error': please do use it to return control to the user when 
>> your code misbehaves.
>> Similarly 'exit' and 'abort' should never be used in R packages.
>> Sometimes it is not under your control: I sometimes see an rgl failure at
>> R: indirect_vertex_array.c:659: emit_DrawArrays_old: Assertion 
>> `elements_per_request >= count' failed.
>> that is coming from the Mesa GL libraries.
> I'd say that's a bug, either in Mesa GL or in rgl.  If you can make it 
> reproducible, I'll try to track it down.
> Duncan Murdoch
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