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 &file_in; file to the &file_out; file.) Now if we run &SCons; multiple times on this example, we see that it re-runs the &cp; command every time: % scons -Q cp file.in file.oout % scons -Q cp file.in file.oout % scons -Q cp 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 build the &file_out; file that we've told &SCons; to expect. But if the problem weren't obvious, it would be helpful to specify the &debug-explain; option on the command line to have &SCons; tell us very specifically why it's decided to rebuild the target: % scons -Q --debug=explain scons: 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. The &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 &debug-explain; option shows very specifically why &SCons; rebuilds the files that it does: % scons -Q cc -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 % edit file2.c [CHANGE THE CONTENTS OF file2.c] % scons -Q --debug=explain scons: 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 .h file. If the file1.c and file3.c files in our example both included a &hello_h; file, then changing that included file and re-running &SCons; with the &debug-explain; option will pinpoint that it's the change to the included file that starts the chain of rebuilds: % scons -Q cc -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 % edit hello.h [CHANGE THE CONTENTS OF hello.h] % scons -Q --debug=explain scons: 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.)

When you create a construction environment, &SCons; populates it with construction variables that are set up for various compilers, linkers and utilities that it finds on your system. Although this is usually helpful and what you want, it might be frustrating if &SCons; doesn't set certain variables that you expect to be set. In situations like this, it's sometimes helpful to use the construction environment &Dump; method to print all or some of the construction variables. Note that the &Dump; method returns the representation of the variables in the environment for you to print (or otherwise manipulate): env = Environment() print env.Dump() On a POSIX system with gcc installed, this might generate: % scons scons: Reading SConscript files ... { 'BUILDERS': {'_InternalInstall': <function InstallBuilderWrapper at 0x700000>, '_InternalInstallAs': <function InstallAsBuilderWrapper at 0x700000>}, 'CONFIGUREDIR': '#/.sconf_temp', 'CONFIGURELOG': '#/config.log', 'CPPSUFFIXES': [ '.c', '.C', '.cxx', '.cpp', '.c++', '.cc', '.h', '.H', '.hxx', '.hpp', '.hh', '.F', '.fpp', '.FPP', '.m', '.mm', '.S', '.spp', '.SPP'], 'DSUFFIXES': ['.d'], 'Dir': <SCons.Defaults.Variable_Method_Caller object at 0x700000>, 'Dirs': <SCons.Defaults.Variable_Method_Caller object at 0x700000>, 'ENV': { 'PATH': '/usr/local/bin:/opt/bin:/bin:/usr/bin'}, 'ESCAPE': <function escape at 0x700000>, 'File': <SCons.Defaults.Variable_Method_Caller object at 0x700000>, 'HOST_ARCH': None, 'HOST_OS': None, 'IDLSUFFIXES': ['.idl', '.IDL'], 'INSTALL': <function copyFunc at 0x700000>, 'LIBPREFIX': 'lib', 'LIBPREFIXES': ['$LIBPREFIX'], 'LIBSUFFIX': '.a', 'LIBSUFFIXES': ['$LIBSUFFIX', '$SHLIBSUFFIX'], 'MAXLINELENGTH': 128072, 'OBJPREFIX': '', 'OBJSUFFIX': '.o', 'PLATFORM': 'posix', 'PROGPREFIX': '', 'PROGSUFFIX': '', 'PSPAWN': <function piped_env_spawn at 0x700000>, 'RDirs': <SCons.Defaults.Variable_Method_Caller object at 0x700000>, 'SCANNERS': [], 'SHELL': 'sh', 'SHLIBPREFIX': '$LIBPREFIX', 'SHLIBSUFFIX': '.so', 'SHOBJPREFIX': '$OBJPREFIX', 'SHOBJSUFFIX': '$OBJSUFFIX', 'SPAWN': <function spawnvpe_spawn at 0x700000>, 'TARGET_ARCH': None, 'TARGET_OS': None, 'TEMPFILE': <class 'SCons.Platform.TempFileMunge'>, 'TEMPFILEPREFIX': '@', 'TOOLS': ['install', 'install'], '_CPPDEFFLAGS': '${_defines(CPPDEFPREFIX, CPPDEFINES, CPPDEFSUFFIX, __env__)}', '_CPPINCFLAGS': '$( ${_concat(INCPREFIX, CPPPATH, INCSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)', '_LIBDIRFLAGS': '$( ${_concat(LIBDIRPREFIX, LIBPATH, LIBDIRSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)', '_LIBFLAGS': '${_concat(LIBLINKPREFIX, LIBS, LIBLINKSUFFIX, __env__)}', '__RPATH': '$_RPATH', '_concat': <function _concat at 0x700000>, '_defines': <function _defines at 0x700000>, '_stripixes': <function _stripixes at 0x700000>} scons: done reading SConscript files. scons: Building targets ... scons: `.' is up to date. scons: done building targets. On a Windows system with Visual C++ the output might look like: C:\>scons scons: Reading SConscript files ... { 'BUILDERS': {'_InternalInstall': <function InstallBuilderWrapper at 0x700000>, 'Object': <SCons.Builder.CompositeBuilder object at 0x700000>, 'PCH': <SCons.Builder.BuilderBase object at 0x700000>, 'RES': <SCons.Builder.BuilderBase object at 0x700000>, 'SharedObject': <SCons.Builder.CompositeBuilder object at 0x700000>, 'StaticObject': <SCons.Builder.CompositeBuilder object at 0x700000>, '_InternalInstallAs': <function InstallAsBuilderWrapper at 0x700000>}, 'CC': 'cl', 'CCCOM': <SCons.Action.FunctionAction object at 0x700000>, 'CCFLAGS': ['/nologo'], 'CCPCHFLAGS': ['${(PCH and "/Yu%s /Fp%s"%(PCHSTOP or "",File(PCH))) or ""}'], 'CCPDBFLAGS': ['${(PDB and "/Z7") or ""}'], 'CFILESUFFIX': '.c', 'CFLAGS': [], 'CONFIGUREDIR': '#/.sconf_temp', 'CONFIGURELOG': '#/config.log', 'CPPDEFPREFIX': '/D', 'CPPDEFSUFFIX': '', 'CPPSUFFIXES': [ '.c', '.C', '.cxx', '.cpp', '.c++', '.cc', '.h', '.H', '.hxx', '.hpp', '.hh', '.F', '.fpp', '.FPP', '.m', '.mm', '.S', '.spp', '.SPP'], 'CXX': '$CC', 'CXXCOM': '$CXX $_MSVC_OUTPUT_FLAG /c $CHANGED_SOURCES $CXXFLAGS $CCFLAGS $_CCCOMCOM', 'CXXFILESUFFIX': '.cc', 'CXXFLAGS': ['$(', '/TP', '$)'], 'DSUFFIXES': ['.d'], 'Dir': <SCons.Defaults.Variable_Method_Caller object at 0x700000>, 'Dirs': <SCons.Defaults.Variable_Method_Caller object at 0x700000>, 'ENV': { 'PATH': 'C:\\WINDOWS\\System32', 'PATHEXT': '.COM;.EXE;.BAT;.CMD', 'SystemRoot': 'C:\\WINDOWS'}, 'ESCAPE': <function escape at 0x700000>, 'File': <SCons.Defaults.Variable_Method_Caller object at 0x700000>, 'HOST_ARCH': '', 'HOST_OS': 'win32', 'IDLSUFFIXES': ['.idl', '.IDL'], 'INCPREFIX': '/I', 'INCSUFFIX': '', 'INSTALL': <function copyFunc at 0x700000>, 'LIBPREFIX': '', 'LIBPREFIXES': ['$LIBPREFIX'], 'LIBSUFFIX': '.lib', 'LIBSUFFIXES': ['$LIBSUFFIX'], 'MAXLINELENGTH': 2048, 'MSVC_SETUP_RUN': True, 'OBJPREFIX': '', 'OBJSUFFIX': '.obj', 'PCHCOM': '$CXX /Fo${TARGETS[1]} $CXXFLAGS $CCFLAGS $CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS /c $SOURCES /Yc$PCHSTOP /Fp${TARGETS[0]} $CCPDBFLAGS $PCHPDBFLAGS', 'PCHPDBFLAGS': ['${(PDB and "/Yd") or ""}'], 'PLATFORM': 'win32', 'PROGPREFIX': '', 'PROGSUFFIX': '.exe', 'PSPAWN': <function piped_spawn at 0x700000>, 'RC': 'rc', 'RCCOM': <SCons.Action.FunctionAction object at 0x700000>, 'RCFLAGS': [], 'RCSUFFIXES': ['.rc', '.rc2'], 'RDirs': <SCons.Defaults.Variable_Method_Caller object at 0x700000>, 'SCANNERS': [], 'SHCC': '$CC', 'SHCCCOM': <SCons.Action.FunctionAction object at 0x700000>, 'SHCCFLAGS': ['$CCFLAGS'], 'SHCFLAGS': ['$CFLAGS'], 'SHCXX': '$CXX', 'SHCXXCOM': '$SHCXX $_MSVC_OUTPUT_FLAG /c $CHANGED_SOURCES $SHCXXFLAGS $SHCCFLAGS $_CCCOMCOM', 'SHCXXFLAGS': ['$CXXFLAGS'], 'SHELL': None, 'SHLIBPREFIX': '', 'SHLIBSUFFIX': '.dll', 'SHOBJPREFIX': '$OBJPREFIX', 'SHOBJSUFFIX': '$OBJSUFFIX', 'SPAWN': <function spawn at 0x700000>, 'STATIC_AND_SHARED_OBJECTS_ARE_THE_SAME': 1, 'TARGET_ARCH': '', 'TARGET_OS': 'win32', 'TEMPFILE': <class 'SCons.Platform.TempFileMunge'>, 'TEMPFILEPREFIX': '@', 'TOOLS': ['msvc', 'install', 'install'], '_CCCOMCOM': '$CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS $CCPCHFLAGS $CCPDBFLAGS', '_CPPDEFFLAGS': '${_defines(CPPDEFPREFIX, CPPDEFINES, CPPDEFSUFFIX, __env__)}', '_CPPINCFLAGS': '$( ${_concat(INCPREFIX, CPPPATH, INCSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)', '_LIBDIRFLAGS': '$( ${_concat(LIBDIRPREFIX, LIBPATH, LIBDIRSUFFIX, __env__, RDirs, TARGET, SOURCE)} $)', '_LIBFLAGS': '${_concat(LIBLINKPREFIX, LIBS, LIBLINKSUFFIX, __env__)}', '_MSVC_OUTPUT_FLAG': <function msvc_output_flag at 0x700000>, '_concat': <function _concat at 0x700000>, '_defines': <function _defines at 0x700000>, '_stripixes': <function _stripixes at 0x700000>} scons: done reading SConscript files. scons: Building targets ... scons: `.' is up to date. scons: done building targets. The construction environments in these examples have actually been restricted to just gcc and Visual C++, respectively. In a real-life situation, the construction environments will likely contain a great many more variables. Also note that we've massaged the example output above to make the memory address of all objects a constant 0x700000. In reality, you would see a different hexadecimal number for each object. To make it easier to see just what you're interested in, the &Dump; method allows you to specify a specific constrcution variable that you want to disply. For example, it's not unusual to want to verify the external environment used to execute build commands, to make sure that the PATH and other environment variables are set up the way they should be. You can do this as follows: env = Environment() print env.Dump('ENV') Which might display the following when executed on a POSIX system: % scons scons: Reading SConscript files ... { 'PATH': '/usr/local/bin:/opt/bin:/bin:/usr/bin'} scons: done reading SConscript files. scons: Building targets ... scons: `.' is up to date. scons: done building targets. And the following when executed on a Windows system: C:\>scons scons: Reading SConscript files ... { 'PATH': 'C:\\WINDOWS\\System32', 'PATHEXT': '.COM;.EXE;.BAT;.CMD', 'SystemRoot': 'C:\\WINDOWS'} scons: done reading SConscript files. scons: Building targets ... scons: `.' is up to date. scons: done building targets.

Sometimes the best way to try to figure out what &SCons; is doing is simply to take a look at the dependency graph that it constructs based on your &SConscript; files. The --tree option will display all or part of the &SCons; dependency graph in an "ASCII art" graphical format that shows the dependency hierarchy. For example, given the following input &SConstruct; file: env = Environment(CPPPATH = ['.']) env.Program('prog', ['f1.c', 'f2.c', 'f3.c']) Running &SCons; with the --tree=all option yields: % scons -Q --tree=all cc -o f1.o -c -I. f1.c cc -o f2.o -c -I. f2.c cc -o f3.o -c -I. f3.c cc -o prog f1.o f2.o f3.o +-. +-SConstruct +-f1.c +-f1.o | +-f1.c | +-inc.h +-f2.c +-f2.o | +-f2.c | +-inc.h +-f3.c +-f3.o | +-f3.c | +-inc.h +-inc.h +-prog +-f1.o | +-f1.c | +-inc.h +-f2.o | +-f2.c | +-inc.h +-f3.o +-f3.c +-inc.h The tree will also be printed when the -n (no execute) option is used, which allows you to examine the dependency graph for a configuration without actually rebuilding anything in the tree. The --tree option only prints the dependency graph for the specified targets (or the default target(s) if none are specified on the command line). So if you specify a target like f2.o on the command line, the --tree option will only print the dependency graph for that file: % scons -Q --tree=all f2.o cc -o f2.o -c -I. f2.c +-f2.o +-f2.c +-inc.h This is, of course, useful for restricting the output from a very large build configuration to just a portion in which you're interested. Multiple targets are fine, in which case a tree will be printed for each specified target: % scons -Q --tree=all f1.o f3.o cc -o f1.o -c -I. f1.c +-f1.o +-f1.c +-inc.h cc -o f3.o -c -I. f3.c +-f3.o +-f3.c +-inc.h The status argument may be used to tell &SCons; to print status information about each file in the dependency graph: % scons -Q --tree=status cc -o f1.o -c -I. f1.c cc -o f2.o -c -I. f2.c cc -o f3.o -c -I. f3.c cc -o prog f1.o f2.o f3.o E = exists R = exists in repository only b = implicit builder B = explicit builder S = side effect P = precious A = always build C = current N = no clean H = no cache [E b ]+-. [E C ] +-SConstruct [E C ] +-f1.c [E B C ] +-f1.o [E C ] | +-f1.c [E C ] | +-inc.h [E C ] +-f2.c [E B C ] +-f2.o [E C ] | +-f2.c [E C ] | +-inc.h [E C ] +-f3.c [E B C ] +-f3.o [E C ] | +-f3.c [E C ] | +-inc.h [E C ] +-inc.h [E B C ] +-prog [E B C ] +-f1.o [E C ] | +-f1.c [E C ] | +-inc.h [E B C ] +-f2.o [E C ] | +-f2.c [E C ] | +-inc.h [E B C ] +-f3.o [E C ] +-f3.c [E C ] +-inc.h Note that --tree=all,status is equivalent; the all is assumed if only status is present. As an alternative to all, you can specify --tree=derived to have &SCons; only print derived targets in the tree output, skipping source files (like .c and .h files): % scons -Q --tree=derived cc -o f1.o -c -I. f1.c cc -o f2.o -c -I. f2.c cc -o f3.o -c -I. f3.c cc -o prog f1.o f2.o f3.o +-. +-f1.o +-f2.o +-f3.o +-prog +-f1.o +-f2.o +-f3.o You can use the status modifier with derived as well: % scons -Q --tree=derived,status cc -o f1.o -c -I. f1.c cc -o f2.o -c -I. f2.c cc -o f3.o -c -I. f3.c cc -o prog f1.o f2.o f3.o E = exists R = exists in repository only b = implicit builder B = explicit builder S = side effect P = precious A = always build C = current N = no clean H = no cache [E b ]+-. [E B C ] +-f1.o [E B C ] +-f2.o [E B C ] +-f3.o [E B C ] +-prog [E B C ] +-f1.o [E B C ] +-f2.o [E B C ] +-f3.o Note that the order of the --tree= arguments doesn't matter; --tree=status,derived is completely equivalent. The default behavior of the --tree option is to repeat all of the dependencies each time the library dependency (or any other dependency file) is encountered in the tree. If certain target files share other target files, such as two programs that use the same library: env = Environment(CPPPATH = ['.'], LIBS = ['foo'], LIBPATH = ['.']) env.Library('foo', ['f1.c', 'f2.c', 'f3.c']) env.Program('prog1.c') env.Program('prog2.c') Then there can be a lot of repetition in the --tree= output: % scons -Q --tree=all cc -o f1.o -c -I. f1.c cc -o f2.o -c -I. f2.c cc -o f3.o -c -I. f3.c ar rc libfoo.a f1.o f2.o f3.o ranlib libfoo.a cc -o prog1.o -c -I. prog1.c cc -o prog1 prog1.o -L. -lfoo cc -o prog2.o -c -I. prog2.c cc -o prog2 prog2.o -L. -lfoo +-. +-SConstruct +-f1.c +-f1.o | +-f1.c | +-inc.h +-f2.c +-f2.o | +-f2.c | +-inc.h +-f3.c +-f3.o | +-f3.c | +-inc.h +-inc.h +-libfoo.a | +-f1.o | | +-f1.c | | +-inc.h | +-f2.o | | +-f2.c | | +-inc.h | +-f3.o | +-f3.c | +-inc.h +-prog1 | +-prog1.o | | +-prog1.c | | +-inc.h | +-libfoo.a | +-f1.o | | +-f1.c | | +-inc.h | +-f2.o | | +-f2.c | | +-inc.h | +-f3.o | +-f3.c | +-inc.h +-prog1.c +-prog1.o | +-prog1.c | +-inc.h +-prog2 | +-prog2.o | | +-prog2.c | | +-inc.h | +-libfoo.a | +-f1.o | | +-f1.c | | +-inc.h | +-f2.o | | +-f2.c | | +-inc.h | +-f3.o | +-f3.c | +-inc.h +-prog2.c +-prog2.o +-prog2.c +-inc.h In a large configuration with many internal libraries and include files, this can very quickly lead to huge output trees. To help make this more manageable, a prune modifier may be added to the option list, in which case &SCons; will print the name of a target that has already been visited during the tree-printing in [square brackets] as an indication that the dependencies of the target file may be found by looking farther up the tree: % scons -Q --tree=prune cc -o f1.o -c -I. f1.c cc -o f2.o -c -I. f2.c cc -o f3.o -c -I. f3.c ar rc libfoo.a f1.o f2.o f3.o ranlib libfoo.a cc -o prog1.o -c -I. prog1.c cc -o prog1 prog1.o -L. -lfoo cc -o prog2.o -c -I. prog2.c cc -o prog2 prog2.o -L. -lfoo +-. +-SConstruct +-f1.c +-f1.o | +-f1.c | +-inc.h +-f2.c +-f2.o | +-f2.c | +-inc.h +-f3.c +-f3.o | +-f3.c | +-inc.h +-inc.h +-libfoo.a | +-[f1.o] | +-[f2.o] | +-[f3.o] +-prog1 | +-prog1.o | | +-prog1.c | | +-inc.h | +-[libfoo.a] +-prog1.c +-[prog1.o] +-prog2 | +-prog2.o | | +-prog2.c | | +-inc.h | +-[libfoo.a] +-prog2.c +-[prog2.o] Like the status keyword, the prune argument by itself is equivalent to --tree=all,prune.

Sometimes it's useful to look at the pre-substitution string that &SCons; uses to generate the command lines it executes. This can be done with the &debug-presub; option: % scons -Q --debug=presub Building prog.o with action: $CC -o $TARGET -c $CFLAGS $CCFLAGS $_CCOMCOM $SOURCES cc -o prog.o -c -I. prog.c Building prog with action: $SMART_LINKCOM cc -o prog prog.o

To get some insight into what library names &SCons; is searching for, and in which directories it is searching, Use the --debug=findlibs option. Given the following input &SConstruct; file: env = Environment(LIBPATH = ['libs1', 'libs2']) env.Program('prog.c', LIBS=['foo', 'bar']) And the libraries libfoo.a and libbar.a in libs1 and libs2, respectively, use of the --debug=findlibs option yields: % scons -Q --debug=findlibs findlibs: looking for 'libfoo.a' in 'libs1' ... findlibs: ... FOUND 'libfoo.a' in 'libs1' findlibs: looking for 'libfoo.so' in 'libs1' ... findlibs: looking for 'libfoo.so' in 'libs2' ... findlibs: looking for 'libbar.a' in 'libs1' ... findlibs: looking for 'libbar.a' in 'libs2' ... findlibs: ... FOUND 'libbar.a' in 'libs2' findlibs: looking for 'libbar.so' in 'libs1' ... findlibs: looking for 'libbar.so' in 'libs2' ... cc -o prog.o -c prog.c cc -o prog prog.o -Llibs1 -Llibs2 -lfoo -lbar

In general, &SCons; tries to keep its error messages short and informative. That means we usually try to avoid showing the stack traces that are familiar to experienced Python programmers, since they usually contain much more information than is useful to most people. For example, the following &SConstruct; file: Program('prog.c') Generates the following error if the prog.c file does not exist: % scons -Q scons: *** [prog.o] Source `prog.c' not found, needed by target `prog.o'. In this case, the error is pretty obvious. But if it weren't, and you wanted to try to get more information about the error, the &debug-stacktrace; option would show you exactly where in the &SCons; source code the problem occurs: % scons -Q --debug=stacktrace scons: *** [prog.o] Source `prog.c' not found, needed by target `prog.o'. scons: internal stack trace: File "bootstrap/src/engine/SCons/Job.py", line 199, in start task.prepare() File "bootstrap/src/engine/SCons/Script/Main.py", line 167, in prepare return SCons.Taskmaster.OutOfDateTask.prepare(self) File "bootstrap/src/engine/SCons/Taskmaster.py", line 187, in prepare executor.prepare() File "bootstrap/src/engine/SCons/Executor.py", line 392, in prepare raise SCons.Errors.StopError(msg % (s, self.batches[0].targets[0])) Of course, if you do need to dive into the &SCons; source code, we'd like to know if, or how, the error messages or troubleshooting options could have been improved to avoid that. Not everyone has the necessary time or Python skill to dive into the source code, and we'd like to improve &SCons; for those people as well...

The internal &SCons; subsystem that handles walking the dependency graph and controls the decision-making about what to rebuild is the Taskmaster. &SCons; supports a --taskmastertrace option that tells the Taskmaster to print information about the children (dependencies) of the various Nodes on its walk down the graph, which specific dependent Nodes are being evaluated, and in what order. The --taskmastertrace option takes as an argument the name of a file in which to put the trace output, with - (a single hyphen) indicating that the trace messages should be printed to the standard output: env = Environment(CPPPATH = ['.']) env.Program('prog.c') % scons -Q --taskmastertrace=- prog Taskmaster: Looking for a node to evaluate Taskmaster: Considering node <no_state 0 'prog'> and its children: Taskmaster: <no_state 0 'prog.o'> Taskmaster: adjusted ref count: <pending 1 'prog'>, child 'prog.o' Taskmaster: Considering node <no_state 0 'prog.o'> and its children: Taskmaster: <no_state 0 'prog.c'> Taskmaster: <no_state 0 'inc.h'> Taskmaster: adjusted ref count: <pending 1 'prog.o'>, child 'prog.c' Taskmaster: adjusted ref count: <pending 2 'prog.o'>, child 'inc.h' Taskmaster: Considering node <no_state 0 'prog.c'> and its children: Taskmaster: Evaluating <pending 0 'prog.c'> Task.make_ready_current(): node <pending 0 'prog.c'> Task.prepare(): node <up_to_date 0 'prog.c'> Task.executed_with_callbacks(): node <up_to_date 0 'prog.c'> Task.postprocess(): node <up_to_date 0 'prog.c'> Task.postprocess(): removing <up_to_date 0 'prog.c'> Task.postprocess(): adjusted parent ref count <pending 1 'prog.o'> Taskmaster: Looking for a node to evaluate Taskmaster: Considering node <no_state 0 'inc.h'> and its children: Taskmaster: Evaluating <pending 0 'inc.h'> Task.make_ready_current(): node <pending 0 'inc.h'> Task.prepare(): node <up_to_date 0 'inc.h'> Task.executed_with_callbacks(): node <up_to_date 0 'inc.h'> Task.postprocess(): node <up_to_date 0 'inc.h'> Task.postprocess(): removing <up_to_date 0 'inc.h'> Task.postprocess(): adjusted parent ref count <pending 0 'prog.o'> Taskmaster: Looking for a node to evaluate Taskmaster: Considering node <pending 0 'prog.o'> and its children: Taskmaster: <up_to_date 0 'prog.c'> Taskmaster: <up_to_date 0 'inc.h'> Taskmaster: Evaluating <pending 0 'prog.o'> Task.make_ready_current(): node <pending 0 'prog.o'> Task.prepare(): node <executing 0 'prog.o'> Task.execute(): node <executing 0 'prog.o'> cc -o prog.o -c -I. prog.c Task.executed_with_callbacks(): node <executing 0 'prog.o'> Task.postprocess(): node <executed 0 'prog.o'> Task.postprocess(): removing <executed 0 'prog.o'> Task.postprocess(): adjusted parent ref count <pending 0 'prog'> Taskmaster: Looking for a node to evaluate Taskmaster: Considering node <pending 0 'prog'> and its children: Taskmaster: <executed 0 'prog.o'> Taskmaster: Evaluating <pending 0 'prog'> Task.make_ready_current(): node <pending 0 'prog'> Task.prepare(): node <executing 0 'prog'> Task.execute(): node <executing 0 'prog'> cc -o prog prog.o Task.executed_with_callbacks(): node <executing 0 'prog'> Task.postprocess(): node <executed 0 'prog'> Taskmaster: Looking for a node to evaluate Taskmaster: No candidate anymore. The --taskmastertrace option doesn't provide information about the actual calculations involved in deciding if a file is up-to-date, but it does show all of the dependencies it knows about for each Node, and the order in which those dependencies are evaluated. This can be useful as an alternate way to determine whether or not your &SCons; configuration, or the implicit dependency scan, has actually identified all the correct dependencies you want it to.