The experience of configuring any software build tool to build a large code base usually, at some point, involves trying to figure out why the tool is behaving a certain way, and how to get it to behave the way you want. SCons is no different. This appendix contains a number of different ways in which you can get some additional insight into SCons' behavior.
Note that we're always interested in trying to improve how you can troubleshoot configuration problems. If you run into a problem that has you scratching your head, and which there just doesn't seem to be a good way to debug, odds are pretty good that someone else will run into the same problem, too. If so, please let the SCons development team know (preferably by filing a bug report or feature request at our project pages at tigris.org) so that we can use your feedback to try to come up with a better way to help you, and others, get the necessary insight into SCons behavior to help identify and fix configuration issues.
Let's look at a simple example of a misconfigured build that causes a target to be rebuilt every time SCons is run:
# Intentionally misspell the output file name in the # command used to create the file: Command('file.out', 'file.in', 'cp $SOURCE file.oout')
(Note to Windows users: The POSIX cp command
copies the first file named on the command line
to the second file.
In our example, it copies the
Now if we run SCons multiple times on this example, we see that it re-runs the cp command every time:
scons -Qcp file.in file.oout %
scons -Qcp file.in file.oout %
scons -Qcp file.in file.oout
In this example,
the underlying cause is obvious:
we've intentionally misspelled the output file name
in the cp command,
so the command doesn't actually
file.out file that we've told SCons to expect.
But if the problem weren't obvious,
it would be helpful
to specify the
on the command line
to have SCons tell us very specifically
why it's decided to rebuild the target:
scons -Q --debug=explainscons: building `file.out' because it doesn't exist cp file.in file.oout
If this had been a more complicated example involving a lot of build output, having SCons tell us that it's trying to rebuild the target file because it doesn't exist would be an important clue that something was wrong with the command that we invoked to build it.
Note that you can also use --warn=target-not-built which checks whether or not expected targets exist after a build rule is executed.
scons -Q --warn=target-not-builtcp file.in file.oout scons: warning: Cannot find target file.out after building File "/scons/as_scons/bootstrap/src/script/scons.py", line 199, in <module>
--debug=explain option also comes in handy
to help figure out what input file changed.
Given a simple configuration that builds
a program from three source files,
changing one of the source files
and rebuilding with the
option shows very specifically
why SCons rebuilds the files that it does:
scons -Qcc -o file1.o -c file1.c cc -o file2.o -c file2.c cc -o file3.o -c file3.c cc -o prog file1.o file2.o file3.o % [CHANGE THE CONTENTS OF file2.c] %
scons -Q --debug=explainscons: rebuilding `file2.o' because `file2.c' changed cc -o file2.o -c file2.c scons: rebuilding `prog' because `file2.o' changed cc -o prog file1.o file2.o file3.o
This becomes even more helpful
in identifying when a file is rebuilt
due to a change in an implicit dependency,
such as an incuded
in our example
both included a
then changing that included file
and re-running SCons with the
will pinpoint that it's the change to the included file
that starts the chain of rebuilds:
scons -Qcc -o file1.o -c -I. file1.c cc -o file2.o -c -I. file2.c cc -o file3.o -c -I. file3.c cc -o prog file1.o file2.o file3.o % [CHANGE THE CONTENTS OF hello.h] %
scons -Q --debug=explainscons: rebuilding `file1.o' because `hello.h' changed cc -o file1.o -c -I. file1.c scons: rebuilding `file3.o' because `hello.h' changed cc -o file3.o -c -I. file3.c scons: rebuilding `prog' because: `file1.o' changed `file3.o' changed cc -o prog file1.o file2.o file3.o
(Note that the
--debug=explain option will only tell you
why SCons decided to rebuild necessary targets.
It does not tell you what files it examined
when deciding not
to rebuild a target file,
which is often a more valuable question to answer.)