Scons custom dependency scanner not called for .cpp files - scons

Question

I have a scons project directory containing following files:
dep.txt
main.cpp
SConstruct
content of SConstruct:
env = Environment()
def f_scan(node, env, path):
print("[.] Scanner invoked. -> %s" % str(node))
return env.File(["dep.txt"])
fscan = Scanner(function = f_scan, skeys = ['.o', '.cpp'])
env.Append(SCANNERS = fscan)
env.Program('main', ['main.cpp'])
content of main.cpp:
#include <iostream>
using namespace std;
int main(){
cout << 1 << endl;
return 0;
}
content in dep.txt does not have any specific meaning.
scons -Q output(if dep.txt was changed after last build):
[.] Scanner invoked. -> main.o
g++ -o main main.o
scons -Q output(if dep.txt was not changed after last build):
[.] Scanner invoked. -> main.o
scons: `.' is up to date.
I am trying to implement a custom scanner for .cpp files. I want to add extra dependencies for all .cpp files on top of one that is detected by inbuilt Scanner. But I notice that scanner never called on .cpp files. However, it gets called correctly for .o files.

Answer 1

From the manpage: https://scons.org/doc/production/HTML/scons-man.html#scanner_objects
Note that scons has a global SourceFileScanner object that is used by
the Object, SharedObject and StaticObject builders to decide which
scanner should be used for different file extensions. You can use the
SourceFileScanner.add_scanner() method to add your own Scanner object
to the SCons infrastructure that builds target programs or libraries
from a list of source files of different types:
So if you change to:
env = Environment(CPPPATH=['.'])
def f_scan(node, env, path):
print("[.] Scanner invoked. -> %s" % str(node))
return env.File(["dep.txt"])
fscan = Scanner(f_scan)
SourceFileScanner.add_scanner('.cpp', fscan)
SourceFileScanner.add_scanner('.o', fscan)
env.Program('main', ['main.cpp'])
Note that you are not adding to the scanners applied to .cpp and .o, you are replacing them.
So if I create an x.h and change main.cpp to:
#include <iostream>
#include "x.h"
using namespace std;
int main(){
cout << 1 << endl;
return 0;
}
You will see this:
% python ~/devel/scons/git/as_scons/scripts/scons.py --tree=prune
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
[.] Scanner invoked. -> main.o
[.] Scanner invoked. -> main.cpp
g++ -o main.o -c -I. main.cpp
[.] Scanner invoked. -> main.o
g++ -o main main.o
+-.
+-SConstruct
+-dep.txt
+-main
| +-main.o
| | +-main.cpp
| | +-dep.txt
| | +-/usr/bin/g++
| +-dep.txt
| +-/usr/bin/g++
+-main.cpp
+-[main.o]
scons: done building targets.
Whereas without your custom scanner:
% python ~/devel/scons/git/as_scons/scripts/scons.py --tree=prune
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
g++ -o main main.o
+-.
+-SConstruct
+-main
| +-main.o
| | +-main.cpp
| | +-x.h
| | +-/usr/bin/g++
| +-/usr/bin/g++
+-main.cpp
+-[main.o]
+-x.h
scons: done building targets.
So what you likely want is to still call the existing scanner and add your file to it:
env = Environment(CPPPATH=['.'])
# get existing .cpp scanner
origin_cpp_scanner = SourceFileScanner.function['.cpp']
def f_scan(node, env, path):
filename = str(node)
print("[.] Scanner invoked. -> %s" % filename)
if filename.endswith('.cpp'):
current_files = origin_cpp_scanner(node, env, path)
else:
current_files = []
print("Files:%s"%[str(f) for f in current_files])
return current_files + env.File(["dep.txt"])
fscan = Scanner(f_scan)
SourceFileScanner.add_scanner('.cpp', fscan)
SourceFileScanner.add_scanner('.o', fscan)
env.Program('main', ['main.cpp'])
Which then yields:
% python ~/devel/scons/git/as_scons/scripts/scons.py --tree=prune
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
[.] Scanner invoked. -> main.o
Files:[]
[.] Scanner invoked. -> main.cpp
Files:['x.h']
g++ -o main.o -c -I. main.cpp
[.] Scanner invoked. -> main.o
Files:[]
g++ -o main main.o
+-.
+-SConstruct
+-dep.txt
+-main
| +-main.o
| | +-main.cpp
| | +-x.h
| | +-dep.txt
| | +-/usr/bin/g++
| +-dep.txt
| +-/usr/bin/g++
+-main.cpp
+-[main.o]
+-x.h
scons: done building targets.