[Boost-build] why generator only for non-top-level target? (was Re: main-target-alternative howto

From: Larry Evans (cppljevans_at_[hidden])
Date: 2007-09-07 07:48:55

On 09/06/07 11:34, Larry Evans wrote:
> On 09/05/07 14:19, Vladimir Prus wrote:
> [snip]
>> Just calling a function does not automatically creates a main target.
>> There are just two ways to create a main target:
>>
>>         - Call the targets.main-target-alternative
>>         - Call some existing rule that creates main target. That will
>>         will eventually call targets.main-target-alternative
>>
> I tried the 1st way by modifying:
> 
> http://svn.boost.org/trac/boost/browser/trunk/tools/build/v2/example/customization/Jamfile
> 
> as follows:
> <--- cut here ---
> import verbatim ;
> import targets ;
> 
> exe codegen : codegen.cpp class.verbatim usage.verbatim
>      t1.verbatim ;
> 
> targets.main-target-alternative t1_cpp ;
> 
> verbatim t1_cpp : t1.verbatim ;
>  >--- cut here ---
[snip]
A simplified Jamfile:
<--- cut here ---
obj t1_obj : t1.verbatim ;

verbatim t1_cpp : t1.verbatim ;
 >--- cut here ---
with some debug ECHO's at:

http://svn.boost.org/trac/boost/browser/trunk/tools/build/v2/build/targets.jam#L1532

shows targets.main-target-alternative is called for both t1_obj and
t1_cpp; however, the attempt to construct t1_cpp gives:

  warn: Unable to construct ./t1_cpp

One difference between the processing is at:

http://svn.boost.org/trac/boost/browser/trunk/tools/build/v2/build/targets.jam#L1409

For t1_obj, r is non-null:

   r='object(property-set)@9'   r='object(file-target)@77'

; however, for t1_cpp, r is null at this point.

This seems like a bug because if t1.cpp is generated for
t1_obj, it should be generated for t1_cpp.
Is there some reason why a target (e.g. t1.cpp) is generated
when it's not a top-level target, but is not generated when it
is a top-level target (e.g. t1_cpp)?

( Maybe I'm mixing my terminology a bit in that I call t1.cpp
a target, when actually it's ./bin/gcc-4.1/debug/t1.cpp and
maybe should be called a "concrete target" whereas t1_cpp should be
called a meta-target or something else.)

TIA,
Larry.