SCons libraries and sub-libraries

Question

I have a hierarchical build system based on SCons. I have a root SConstruct that calls into a SConscript that builds a shared library and then into a different SConscript that builds an executable that depends on the shared library.

So here's my question: my understanding of shared libraries on linux is that when you want to do the final ld link for the executable that will be using the shared lib, the shared lib has to be included on the executable's ld command line as a source to reference it (unless it's in a standard location in which case the -l option works).

So here's something like what my SCons files look like:

=== rootdir/SConstruct

env=DefaultEnvironment()
shared_lib = SConscript('foolib/SConscript')
env.Append( LIBS=[shared_lib] )
executable = SConscript('barexec/SConscript')

=== rootdir/foolib/SConscript

env=DefaultEnvironment()
env.Append(CPPPATH=Glob('inc'))
penv = env.Clone()
penv.Append(CPPPATH=Glob('internal/inc'))
lib = penv.SharedLibrary( 'foo', source=['foo.c', 'morefoo.c']
Return("lib")

=== rootdir/barexec/SConscript

env=DefaultEnvironment()
exe = env.Program( 'bar', source=['main.c', 'bar.c', 'rod.c'] )
Return("exe")

So the hitch here is this line:

env.Append( LIBS=[shared_lib] )

This would be a great way to add generated libraries to the command line for any other libs that need them, EXCEPT that because SCons is doing a two-pass run through the SConscripts (first to generate it's dependency tree, then to do the work), rootdir/foolib/libfoo.so winds up on the command line for ALL products, EVEN libfoo.so itself:

gcc -g -Wall -Werror -o libfoo.so foo.o morefoo.o libfoo.so

So how is this best done with SCons? For now I've resorted to this hack:

=== rootdir/SConstruct

env=DefaultEnvironment()
shared_lib = SConscript('foolib/SConscript')
env['shared_lib'] = shared_lib
executable = SConscript('barexec/SConscript')

...

=== rootdir/barexec/SConscript

env=DefaultEnvironment()
exe = env.Program( 'bar', source=['main.c', 'bar.c', 'rod.c'] + env['shared_lib'] )
Return("exe")

Is there a more SCons-y way of doing this?

Answer 1

You should allow the shared libraries to be found by the build.

Look for the LIBPATH and RPATH variables in the SCons documentation; these are the "Scons-y" way to set up search paths so that any generated -l options find libraries properly.

Having mentioned the above, here's what you should see gcc do based on the setup of SCons (and if it doesn't, you may have to do it manually).

The -l option always finds shared libraries provided that you also give the compiler the location of the library. There are two times this is needed: at compile time (-L option) and at runtime (-rpath generated linker option).

The LIBPATH SCons setup should generate something that looks like -L/some/directory/path for the compile-time search path.

The RPATH SCons setup should generate a linker option to embed a search path; e.g. -Wl,-rpath -Wl,\$ORIGIN/../lib would embed a search path that searches relative to the executable so that executables placed in bin search in the parallel lib directory of the installation.