R devel - Apr 2023 - Warnings created during R_eval or R_tryEval not shown before R ending or R error.

Hi,

I have an embedded R, with the evaluation of expressions happening over
time during the lifespan of the process. I tried either `R_eval()`  and
`R_tryEval()` for the evaluation. The issue I have is that the processing
of warnings does not happen until the process exits and/or R is shut down.
Evaluating `warnings()` returns NULL (NILSXP).

It seems like the warnings are stuck in a list out of `warnings()`'s reach,
waiting to be processed. I cannot seem to find why or how to force that to
happen. The embedded R is interactive (R_Interactive is 1).

Moreover, when the evaluation of another R expression after the warnings
results in an R error I get the error message shown followed by pending
warnings.  When tracing what happens during an error I found that
verrorcall_dflt() in src/main/errors.c calls PrintWarnings(). That function
is not part of R's C-API though.

Would anyone have a suggestion about what I am missing?

Best,


Laurent

	[[alternative HTML version deleted]]

? Sun, 23 Apr 2023 13:33:16 -0400
Laurent Gautier <lgautier at gmail.com> ?????:
> When tracing what happens during an error I found that
> verrorcall_dflt() in src/main/errors.c calls PrintWarnings(). That
> function is not part of R's C-API though.
I've tried reading the source code and came to a similar conclusion.

PrintWarnings() is required for warnings() to work because it creates
the last.warning variable for warnings() to access. When driving an
embedded R, R_ReplDLLdo1() will call it for you between expressions it
processes, but there doesn't seem to be a way to call it yourself.

Interestingly, there is .Internal(printDeferredWarnings()) which
eventually calls PrintWarnings(), but it's not exported as an API, only
used in a few places like try().

Would it help to run with options(warn = 1)? I think that a different
code path is taken in this case, which should emit warnings even
without a working REPL. warnings() would still be empty, unfortunately.

-- 
Best regards,
Ivan