[Rd] silent extern error (PR#898)

ripley@stats.ox.ac.uk ripley@stats.ox.ac.uk
Sun, 8 Apr 2001 07:54:10 +0200 (MET DST)

On Sat, 7 Apr 2001 bellis@hsph.harvard.edu wrote:

> Full_Name: Byron Ellis
> Version: 1.2.2
> OS: all
> Submission from: (NULL) (
> R_ext/Arith.h
> #ifdef MAIN
> #define extern
> #endif
> #ifdef __cplusplus
> extern "C" {
> #endif
> these two should be reversed. Its never a problem because builds aren't done
> against C++ compilers, but its still an error (just a low priority one).

Sorry, but as __MAIN__ (not MAIN) is only defined in one C module internal
to R, why is it an error?  It would be an error if there ever was an
intention that module was to be in C++, but there is not.  The #ifdef
__cplusplus constructs are only there to allow the header files to be used
in C++ addons, and defining __MAIN__ is *not* part of the API.

In short, those two #ifdef's are mutually exclusive, so their order is
immaterial.  How can one order be an error?

> also, you could change that to
> #ifdef __cplusplus
> extern "C" {
> #endif
> #ifdef __MAIN__
> #define extern
> #elif _MSC_VER
> #define extern __dllspec(dllimport)
> #endif
> with the corresponding change to the #undef region. This would allow people to
> develop for R using MSVC++ more easily.

However, *that* would be an error.  _MSC_VER may well be defined during the
actual compilation of R, and __MAIN__ is only defined in the compilation of
main.c.  So compilation would fail under VC++ (and other compilers which
define _MSC_VER).

(It is also possible for _MSC_VER to be defined but not have a value, so is
the test what you really meant?)

If we intended to make compilation for other C compilers easier we would
need to add lines like that to lots of the header files, and for more than
one compiler system.  However, we don't particularly want to make the use
of Arith.h under VC++ easier, as VC++ does not handle IEEE arithmetic
correctly!   In short, using Arith.h in addons compiled with VC++ is
likely to give incorrect results.

(We know from past experience that R can be compiled with VC++ (6.0sp3, to
be precise) but then fails make check, giving incorrect results in the
arithmetic tests and elsewhere.)

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 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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