Skip folders to build using scons after full build - c

Question

I have large number of source files ~10,000 and they are scattered across several folders.
I wanted to know if there is a way to skip certain folders, I know that havent changed.
For ex, consider the following folder structure
A (Sconstruct is here)
|
->B (unchanged 1000 files)
->C (unchanged 1000 files)
->D (changed 1 file)
Once I do a complete build for the first time, I want it to compile everything (B, C, D) but when I modify a file in D (I know that), I would like to build folder D only, skip B and C and finally link them all together to form the final binary (B, C and new D).
I have been looking for quite some time now but not able to figure it out. Is it even possible? Can I specify only to look into a particular folder for changes?

Answer 1

First, I'd investigate using Decider('timestamp-match') or even building a custom Decider function. That should speed up your dependency-checking time.
But to answer your specific question, yes it is possible to not build the targets in B and C. If you don't invoke a builder for the targets in those subdirectories, you just won't build them. Just have an if that selectively chooses which env.Object() (or similar) functions to invoke.
When I fleshed out your example, I chose to have each subdirectory create a library that would be linked into the main executable, and to only invoke env.SConscript() for the directories that the user chooses. Here is one way to implement that:
A/SConstruct:
subdirs = ['B','C','D']
AddOption('--exclude', default=[], action='append', choices=subdirs)
env = Environment(EXCLUDES = GetOption('exclude'))
env.SConscript(
dirs=[subdir for subdir in subdirs
if subdir not in env['EXCLUDES']],
exports='env')
env2 = env.Clone()
env2.PrependUnique(LIBPATH=subdirs,
LIBS=subdirs)
env2.Program('main.c')
B/SConscript:
Import('env')
env.Library('B', env.Glob('*.c'))
C/SConscript:
Import('env')
env.Library('C', env.Glob('*.c'))
D/SConscript:
Import('env')
env.Library('D', env.Glob('*.c'))
To do a global build: scons
To do a build after modifying a single file in D: scons --exclude=B --exclude=C
EDIT
Similarly, you can add a whitelist option to your SConstruct. The idea is the same: only invoke builders for certain objects.
Here is a SConstruct similar to above, but with a whitelist option:
subdirs = ['B','C','D']
AddOption('--only', default=[], action='append', choices=subdirs)
env = Environment(ONLY = GetOption('only') or subdirs)
env.SConscript(
dirs=env['ONLY'],
exports='env')
env2 = env.Clone()
env2.PrependUnique(LIBPATH=subdirs,
LIBS=subdirs)
env2.Program('main.c')
To build everything: scons
To rebuild D and relink main program: scons --only=D

Answer 2

If D is independent of B and C just specify your target in D (program/library), or the whole directory, as target explicitly on the command line like scons D/myprog.exe. SCons will expand the required dependencies automatically, and such doesn't traverse the unrelated folders B and C.
Note how you can specify an arbitrary number of targets, so
scons D/myprog.exe B
is allowed too.