Even though the C99 standard allows variable length
arrays on the stack, I would avoid using them much
as the stack is a much more limited resource than the
heap (where malloc allocates memory).  A typical stack
has a few megabytes while a typical heap has a few
gigabytes.  Detecting stack overflows can be hard.

Bill Dunlap
TIBCO Spotfire
wdunlap tibco.com



-----Original Message-----
From: r-devel-bounces@r-project.org on behalf of Murray Efford
Sent: Sat 9/26/2009 3:25 AM
To: Prof Brian Ripley
Cc: r-devel@lists.R-project.org
Subject: Re: [Rd] Compatibility of external C code across platforms
 
Thank you. I'm much clearer on this. I now notice that the messages from gcc distinguish breaches of ISO C and ISO C90, and can see the significance.  My gcc versions were the same - the different behaviour is likely to be explained by the --std flag. And it looks like I have to re-write a lot of C to eliminate nesting of functions.  Point taken about HTML.
Murray Efford
________________________________________
From: Prof Brian Ripley [ripley@stats.ox.ac.uk]
Sent: Saturday, 26 September 2009 9:40 p.m.
To: Murray Efford
Cc: r-devel@lists.R-project.org
Subject: Re: [Rd] Compatibility of external C code across platforms

On Sat, 26 Sep 2009, Murray Efford wrote:

> A package I have written makes considerable use of external C code.
> There appear to be no problems building a Windows binary with the
> GNU C compiler, but now I would like to make the package available
> for other platforms.  A stringent check of the code with the gcc
> options '-Wall -pedantic' flags many nested functions and
> variable-length arrays that are not allowed in ISO C.  Is it
> essential that distributed C source code for R packages conforms to
> ISO C in this respect?  I'm working in Windows XP with R2.9.2 and
> Rtools 2.9.

Yes, it is essential.  R does not assume gcc, and it does not even
assume gcc 4.x (and people are still using gcc 3.x on some platforms,
e.g. the sunfreeware R builds).  This is why we publish test result on
CRAN for a completely different compiler (from SunPro).

However, if is not clear which standard you mean by 'ISO C': assuming
C99 is fairly safe, but GNU extensions from C99 are not -- nested
functions are one such.

> [Incidentally, when I use a makevars.win file to include these gcc
> options in Rcmd build etc. the problem with variable-length arrays
> is not flagged; maybe my command-line gcc is a different version
> from that in Rtools]

Well, you (and only you) can easily check that for yourself, but the
other flags also matter, especially --std.  Because it has been needed
for the use of the MinGW headers, R on Windows is built with
--std=gnu99.  (Linux headers have a similar problem: using --std=c99
disables some features we test for in configure and wish to use -- and
there are OS features, not language extensions.)

>
> Murray Efford
> University of Otago
> Dunedin, New Zealand
> murray.efford@otago.ac.nz<mailto:murray.efford@otago.ac.nz>
>
>       [[alternative HTML version deleted]]

Please note what the posting guide has to say about that.

>
> ______________________________________________
> R-devel@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

--
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595
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