Re: [Caml-list] Suggested way to determine platform specific capabilities in build system?

[Adrien Nader <adrien@notk.org>]

Hi,

On Tue, Apr 14, 2015, Daniel Bünzli wrote:
> Le mardi, 14 avril 2015 à 12:51, Malcolm Matalka a écrit :
> > What is the current suggested way to determine what, roughly, autoconf
> > would do for you? I have some platform specific functionality to be
> > included (or excluded) depending on the OS.
> 
> I don't know if there's a suggested way but here are various ways to proceed.
> 
> If you are using ocamlbuild, you can use `Sys.win32` since 4.01.0 or `Sys.os_type = "Win32"` to determine if you are on windows and otherwise get the (stripped and lowercased) result of `uname -s`, see e.g (but it's missing the win bit):  
> 
> https://github.com/dbuenzli/tsdl/blob/master/myocamlbuild.ml#L6
> 
> If you are using Makefiles you may want include `$(ocamlc -where)/lib/ocaml/Makefile.config` and use the `$SYSTEM` variable.
> 
> If this is only needed for C stubs you can also try solve this at the CPP level — but I guess this can be quite brittle — see e.g (here again missing the win bit as it's undefined for now):  
> 
> https://github.com/dbuenzli/mtime/blob/master/src-os/mtime_stubs.c#L11-L21
> 
> In any case make sure the value can be overridden from the command line for cross compilation scenarios.  

This is the wrong approach. Please do not test for systems but for
capabilities. Testing for systems hinders progress and is a huge
long-term cost.

Consider the case where you test for windows on the mingw-w64 toolchain.
Something isn't available and you therefore disable it. Then mingw-w64
add it and tt becomes available at the end of the year.
Either you must push a new release (typically a development one that is
completely experimental) or the packager must patch (and that's one of
the most annoying kind of patching to do) or the software never takes
advantage of the new feature.

The proper way to do it is to test for features instead. Want to know if
function foo is available in library bar? Well, just reference it and
see if compilation and/or linking fails. You can also try to run it, if
not cross-compiling; at which point you would have to guess (in practice
it's < 1% of cases).

-- 
Adrien Nader