Question on parsing R code from C

First of all thanks a lot to you both for all the replies, they have been of 
great help to me!

I got the basic embedding running, however I still have some issues to solve in 
order to complete the interface to R I am working on:

1. I initialize with Rf_initEmbeddR().
- Is there a way to have more than one R "state" (per single thread)? Or is 
everything necessarily global?
There are no threads in R. You have only one, global instance of R.
- I also read somewhere that the fpu settings may be touched. Is there a way to 
make sure that this is not the case? Is this related to fpu_setup()?

All following answers assume that you're not running REPL. If you did, you would get all the errors on the console callback, so I'm assuming that's not what you want.
2. I create a string and parse it via Rf_mkString() and R_parseVector() (yes I 
protect/un-protect the SEXPs).
- Is it possible to obtain the precise error message, like "unexpected symbol 
in..." (as would be reported by R.exe) in case of error as a const char* string?
AFAIR you have to evaluate parse(text=...) for that, there is no C-level access to parser errors.
- If I pass a wrongly escaped string (for instance 'ggsave("C:\gtest.png")', 
please notice the missing \) I get on stderr: Error '\g' is an unrecognized 
escape and I get a crash. This does not happen if for instance i try to parse 
'rnorm(10a10)' in which case I get the error flag and the NULL ptr return. I 
suspect I need to initialize something / set a callback to avoid this but I am 
not sure...

If you get a crash, you're not setting up you R correctly. If your R quits then you are in non-interactive mode and you didn't setup an error handler.
3. I eval with R_tryEvalSilent().
- Again, is it possible to obtain the precise error message in case of 
evaluation error as would be reported from R.exe?

I prefer using Rf_eval() and try(..., silent=TRUE) - you can check on the class of the result to see if there was an error.
4. Regarding the returned result of evaluation.
- What is the correct way to obtain a const char* string representing the result 
as would be "printed" in the R shell by executing a command (for instance 
"summary(c(1,2,3))") ?
See ?capture.output
- Why is the return type (as reported from typeof in R and TYPEOF in C) of 
"summary(c(1,2,3))" a double?

Because the constants 1, 2 and 3 are all doubles. For example 1L is an integer and "1" is a string.

BTW: you are asking a lot of questions the are answered in the Rserve FAQ (since what you do is exactly what Rserve provides) so you may want to have a quick look:
http://rforge.net/Rserve/faq.html

Cheers,
Simon
Any help / feedback on these issues would be greatly appreciated.

Thanks again.

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Question on parsing R code from C

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