[CMake] Enabling C99 in CMake

[Michael Hertling]

On 06/24/2011 12:32 AM, Todd Gamblin wrote:
> Thanks to Sean and Jed for useful counterexamples.
> 
> A few more points:
> 
> On Jun 23, 2011, at 8:50 AM, Michael Hertling wrote:
> 
>> While I can understand your point, I'm of a different opinion: In the
>> end, it's the responsibility of the user to provide the project with a
>> suitable compiler.
> 
> If it's not autodetected, then yes.  And given this, the user might supply CMake with a perfectly fine compiler, but since they do not know the proper flags for that compiler they won't be able to use it.  So now you're adding to the user's responsibility, when the developer could just as easily specify this.
> 
> Add to this Jed's point that some projects have certain targets/directories that should be built with C99 or not, and I think simply specifying things at the command line is overly cumbersome.

AFAIK, it is usually a single flag which specifies the dialect although
compilers tend to use countless command line options, even the dialect-
related ones. Thus, I'm quite sure that the user is regularly able to
figure out this flag for its inclusion in the CC environment variable.
However, mixed-dialect projects are special in this regard and could
indeed be handled well with dialect switches which are fine-tuned by
the project itself - as long as the compiler supports every required
dialect. For an approach how to accomplish this specifically for all
compilers through a platform/compiler-independent interface, see my
reply to Jed's posting; of course, I'd be particularly interested
in your opinion whether that approach does suit your needs.

> Re:
>> Case (1) is widely accepted
> 
> It's only widely accepted when CMake can't autodetect your compiler, and CMake goes to great lengths to do that for you.  I don't see what the harm is in having it also make a good-faith effort to know dialect switches.

CMake's autodetection of the compiler already fails if the compiler is
just not in the PATH or unusually named, and that's not uncommon, and
if one has multiple compilers installed - which is not uncommon, too -
CMake will detect only one of them anyway, so it might happen quite
quickly that the wrong compiler is chosen. For this reason, I think
it should be absolutely tolerable for the user to denote a suitable
compiler, and an additional specification of a dialect flag should
also be no unsustainable burden. Of course, an effort to provide
dialect flags in a platform/compiler-independent way is no harm
at all, and if the CMake community will settle on a solution, I
would appreciate it.

>> However, you might possibly add a check to your project, e.g. using
>> TRY_COMPILE() or CheckCSourceCompiles(), with C99 source code a C89
>> compiler can't build, and if this check fails, you can notify the
>> user of the need to reconfigure and specify a C99 compiler. If a
>> project must use the GNU compiler, one would do quite the same.
> 
> You could do this, but it seems like a lot of work for the developer AND the user compared to my suggestion of just having a variable for the compiler-specific C99 flags, or some similar mechanism.  We know an awful lot of other things about compiler switches.  Dialect seems like a useful thing to add to the platform file.  Someone does this once for each known compiler ID and now no one has to bother with this stuff anymore.

CMake provides - and projects use - numerous checks for headers,
libraries, functions, struct/class members etc.; why should a C99
project not examine the compiler for its C99 capabilities in a plat-
form/compiler-independent manner? Your point relies on the assumption
that the compiler accepts multiple dialects, but there are compilers
which do not, and if CMake detects - or the user specifies - a C89
compiler for a C99 project, there're mysterious error messages as

error: expected '=', ',', ';', 'asm' or '__attribute__' before 'p'

when declaring a "void *restrict p" pointer, e.g. IMO, understanding
such error messages and drawing the correct conclusions is much more
difficult for the user than being told explicitly and right from the
very first that the projects needs a C99 compiler.

Regards,

Michael