Re: [Boost-build] How best to combine "using xxx", toolset invocation, and conditions?

Subject: Re: [Boost-build] How best to combine "using xxx", toolset invocation, and conditions?
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2013-12-23 11:26:23

AMDG

On 12/23/2013 02:59 AM, Jess Peterson wrote:
> Sometimes, in jamfiles it is necessary to specify flags based on toolset.
>  It would be nice if those toolset conditions matched the various "using"
> initializations.  That is, it would be nice if I could say something like
> "using gcc : w32cross : /dir/xxx"  and on the command line "b2
> toolset=gcc-w32cross" and in a condition "<toolset>gcc-w32cross:<cflags>-w"
> but it does not seem to work that way.  In particular the gcc.jam file runs
> gcc a couple of times to get the machine and version and then creates a
> condition like gcc-4.8.1-mingw sometimes using the undocumented <flavor>.
>  Unless the generated condition is used in b2 invocation and in conditional
> build lines I have problems getting it to do the right thing.  For example,
> if I do not specify a version in the "using" statement, then it will insert
> one into the condition, which might mean that the various conditions in
> Jamfiles need to change to make sure the toolset is matched.  If I specify
> a version number, gcc.jam will check the compiler version and warn if the
> version does not match.  If I specify a tag for the version, gcc.jam will
> generate a toolset-condition such as "gcc-mingw-mingw" for "using gcc :
> mingw : /xxx/g++" which must be specified rather than "gcc-mingw" or it
> combines with the default and tries to compile with two invocations in a
> row ("g++" "/xxx/g++" ...).  All this has led me to some questions:
> 

gcc-mingw-mingw should be a hard error, because
subfeatures are required to be unique.  Try
using something other than "mingw."

> 1) Is this the way it is supposed to work or am I missing something
> fundamental?
> 
> 2) How does the compiler that is actually used get picked out of the
> various ones defined in "using" statements?
> 

1) using adds the specified (or deduced) version to
  the valid values of <toolset-gcc:version>.  The
  first value seen is the default.
2) using also sets up a rule that says, "if <toolset>gcc
  <toolset-gcc:version>xxx is present in the target
  properties then append /path/to/g++xxx to the variable
  CONFIG_COMMAND (which is used by gcc.compile.c++)"
  It should never be possible to have more than one
  match (i.e. if multiple gcc versions appear on the
  command line, then it's a bug in the gcc toolset).
3) b2 toolset=gcc-4.8.2 sets the features <toolset>gcc
  <toolset-gcc:version>4.8.2.
4) b2 toolset=gcc sets <toolset>gcc, then <toolset-gcc:version>
  is set to its default value (the first one initialized--see (1)).
5) b2 sets <toolset> to its default value (we'll assume that
   gcc is the first toolset).  Then the subfeature <toolset-gcc:version>
   is also set to its default value.

> 3) I would think that the best way for it to work would be to say "using
> gcc : mytag : ..." and then say "b2 toolset=gcc-mytag" which some examples
> seem to indicate should work.  If I want it to work this way, should I
> change gcc.jam so that the condition it generates is "<toolset>gcc-mytag?
> 

It's supposed to work, and I don't think I've ever
had a problem with it.  There may be a problem if
you're declaring both mingw and non-mingw.

In Christ,
Steven Watanabe