[cfe-dev] How do I try out C++ modules with clang?

[Stephen Kelly]

Stephen Kelly wrote:

> To "fix" that, I created this module.modulemap in /usr/include :
> 
> module systemheaders {
> header "stdio.h"
> header "stdlib.h"
> header "math.h"
> }
> 
> After I did that, I was able to create a Qt5Core.modulemap on linux. I
> expect that file to grow if I add more modules for other Qt libraries.

The Qt headers include very few external headers. Apart from the QtTest 
module, it's only a few STL headers, and platform-specific opengl headers.

In order to be able to build the Qt5Test.modulemap, I had to extend the file 
I hacked together in /usr/include/module.modulemap like so:

 module system {
   header "stdio.h"
   header "stdlib.h"
   header "math.h"
   module system_c [extern_c] {
     header "inttypes.h"
   }
 }


If I keep going like this, my /usr/include/module.modulemap will refer to 
headers which come from different debian packages. This seems unacceptable 
to me, but you'd have to talk to some packagers to make sure. (for modules 
to be viable on linux, I'm sure you have to somehow solve the system headers 
case or make it solvable).

So, a solution would be to use the new mechanism to specify multiple module 
map files. However, it would be unsatisfying to have to specify many 

 /usr/include/{foo,bar,bat}.modulemap 

files using -fmodule-map-file, because it is a system location. I can easily 
think of some options for improving that situation, and I'm sure there are 
more options:

1) Add option to give the clang driver a module map directory

Then packages would install their modulemap to a well-known location and all 
modulemaps in that location would get loaded

 ${prefix}/modulemaps/{foo,bar,bat}.modulemap

and 
 
 clang -fmodules -fmodule-map-dir=${prefix}

loads all module maps in ${prefix}. 

This solution has the disadvantage that authors would have to write 

 header "../foo.h" 

and similar, unless clang had built-in logic to look up a directory.

System locations should be built-in without being specified, so I never have 
to write 

 -fmodule-map-dir=/usr/include/modulemaps

or anything. Maybe clang should automatically look in ${dir}/modulemaps for 
each -I${dir} passed on the command line (and each built-in dir) and load 
any modulemaps found in those directories.


2) Clang could load *.modulemap, not only module.modulemap from each dir

This seems like a simple solution and doesn't require '..' paths.


3) Files in ${dir}/modulemaps/* are redirects to '..'

Ie, packages would install

 /usr/include/foo.modulemap
 /usr/include/foo.h
 /usr/include/modulemaps/foo.redirect

and foo.redirect would contain some syntax to make clang behave as if 

 -fmodule-map-file=/usr/include/foo.modulemap 

was passed. Eg, foo.redirect might contain only

 ../foo.modulemap

and then foo.modulemap would not need '..' paths.


Thanks,

Steve.