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.
Why is That Target Being Rebuilt? the &debug-explain; Option
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.)
What's in That Construction Environment? the &Dump; Method
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': {'InstallAs': <function InstallAsBuilderWrapper at 0x700000>, 'Install': <function InstallBuilderWrapper 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 instance at 0x700000>,
'Dirs': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
'ENV': {'PATH': '/usr/local/bin:/opt/bin:/bin:/usr/bin'},
'ESCAPE': <function escape at 0x700000>,
'File': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
'IDLSUFFIXES': ['.idl', '.IDL'],
'INSTALL': <function copyFunc at 0x700000>,
'LATEXSUFFIXES': ['.tex', '.ltx', '.latex'],
'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 instance at 0x700000>,
'SCANNERS': [],
'SHELL': 'sh',
'SHLIBPREFIX': '$LIBPREFIX',
'SHLIBSUFFIX': '.so',
'SHOBJPREFIX': '$OBJPREFIX',
'SHOBJSUFFIX': '$OBJSUFFIX',
'SPAWN': <function spawnvpe_spawn at 0x700000>,
'TEMPFILE': <class SCons.Platform.TempFileMunge at 0x700000>,
'TEMPFILEPREFIX': '@',
'TOOLS': ['install', 'install'],
'_CPPDEFFLAGS': '${_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': '$_RPATH',
'_concat': <function _concat at 0x700000>,
'_defines': <function _defines at 0x700000>,
'_stripixes': <function _stripixes at 0x700000>}
scons: done reading SConscript files.
scons: Building targets ...
scons: `.' is up to date.
scons: done building targets.
On a Windows system with Visual C++
the output might look like:
C:\>scons
scons: Reading SConscript files ...
{ 'BUILDERS': {'RES': <SCons.Builder.BuilderBase instance at 0x700000>, 'Object': <SCons.Builder.CompositeBuilder instance at 0x700000>, 'InstallAs': <function InstallAsBuilderWrapper at 0x700000>, 'PCH': <SCons.Builder.BuilderBase instance at 0x700000>, 'Install': <function InstallBuilderWrapper at 0x700000>, 'SharedObject': <SCons.Builder.CompositeBuilder instance at 0x700000>, 'StaticObject': <SCons.Builder.CompositeBuilder instance at 0x700000>},
'CC': 'cl',
'CCCOM': <SCons.Action.FunctionAction instance at 0x700000>,
'CCCOMFLAGS': '$CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS /c $SOURCES /Fo$TARGET $CCPCHFLAGS $CCPDBFLAGS',
'CCFLAGS': ['/nologo'],
'CCPCHFLAGS': ['${(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 $CXXFLAGS $CCCOMFLAGS',
'CXXFILESUFFIX': '.cc',
'CXXFLAGS': ['$CCFLAGS', '$(', '/TP', '$)'],
'DSUFFIXES': ['.d'],
'Dir': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
'Dirs': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
'ENV': { 'INCLUDE': 'C:\\Program Files\\Microsoft Visual Studio/VC98\\include',
'LIB': 'C:\\Program Files\\Microsoft Visual Studio/VC98\\lib',
'PATH': 'C:\\Program Files\\Microsoft Visual Studio\\Common\\tools\\WIN95;C:\\Program Files\\Microsoft Visual Studio\\Common\\MSDev98\\bin;C:\\Program Files\\Microsoft Visual Studio\\Common\\tools;C:\\Program Files\\Microsoft Visual Studio/VC98\\bin',
'PATHEXT': '.COM;.EXE;.BAT;.CMD',
'SystemRoot': 'C:/WINDOWS'},
'ESCAPE': <function escape at 0x700000>,
'File': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
'IDLSUFFIXES': ['.idl', '.IDL'],
'INCPREFIX': '/I',
'INCSUFFIX': '',
'INSTALL': <function copyFunc at 0x700000>,
'LATEXSUFFIXES': ['.tex', '.ltx', '.latex'],
'LIBPREFIX': '',
'LIBPREFIXES': ['$LIBPREFIX'],
'LIBSUFFIX': '.lib',
'LIBSUFFIXES': ['$LIBSUFFIX'],
'MAXLINELENGTH': 2048,
'MSVS': {'VERSION': '6.0', 'VERSIONS': ['6.0']},
'MSVS_VERSION': '6.0',
'OBJPREFIX': '',
'OBJSUFFIX': '.obj',
'PCHCOM': '$CXX $CXXFLAGS $CPPFLAGS $_CPPDEFFLAGS $_CPPINCFLAGS /c $SOURCES /Fo${TARGETS[1]} /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': '$RC $_CPPDEFFLAGS $_CPPINCFLAGS $RCFLAGS /fo$TARGET $SOURCES',
'RCFLAGS': [],
'RDirs': <SCons.Defaults.Variable_Method_Caller instance at 0x700000>,
'SCANNERS': [],
'SHCC': '$CC',
'SHCCCOM': <SCons.Action.FunctionAction instance at 0x700000>,
'SHCCFLAGS': ['$CCFLAGS'],
'SHCFLAGS': ['$CFLAGS'],
'SHCXX': '$CXX',
'SHCXXCOM': '$SHCXX $SHCXXFLAGS $CCCOMFLAGS',
'SHCXXFLAGS': ['$CXXFLAGS'],
'SHELL': None,
'SHLIBPREFIX': '',
'SHLIBSUFFIX': '.dll',
'SHOBJPREFIX': '$OBJPREFIX',
'SHOBJSUFFIX': '$OBJSUFFIX',
'SPAWN': <function spawn at 0x700000>,
'STATIC_AND_SHARED_OBJECTS_ARE_THE_SAME': 1,
'TEMPFILE': <class SCons.Platform.TempFileMunge at 0x700000>,
'TEMPFILEPREFIX': '@',
'TOOLS': ['msvc', '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__)}',
'_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 ...
{ 'INCLUDE': 'C:\\Program Files\\Microsoft Visual Studio/VC98\\include',
'LIB': 'C:\\Program Files\\Microsoft Visual Studio/VC98\\lib',
'PATH': 'C:\\Program Files\\Microsoft Visual Studio\\Common\\tools\\WIN95;C:\\Program Files\\Microsoft Visual Studio\\Common\\MSDev98\\bin;C:\\Program Files\\Microsoft Visual Studio\\Common\\tools;C:\\Program Files\\Microsoft Visual Studio/VC98\\bin',
'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.
What Dependencies Does &SCons; Know About? the &tree; Option
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.
How is &SCons; Constructing the Command Lines It Executes? the &debug-presub; Option
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
Where is &SCons; Searching for Libraries? the &debug-findlibs; Option
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
Where is &SCons; Blowing Up? the &debug-stacktrace; Option
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: *** Source `prog.c' not found, needed by target `prog.o'. Stop.
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: *** Source `prog.c' not found, needed by target `prog.o'. Stop.
scons: internal stack trace:
File "bootstrap/src/engine/SCons/Job.py", line 114, in start
File "bootstrap/src/engine/SCons/Script/Main.py", line 157, in prepare
File "bootstrap/src/engine/SCons/Taskmaster.py", line 169, in prepare
File "bootstrap/src/engine/SCons/Node/FS.py", line 2568, in prepare
File "bootstrap/src/engine/SCons/Node/__init__.py", line 349, in prepare
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...
How is &SCons; Making Its Decisions? the &taskmastertrace; Option
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: 'prog': children:
['prog.o']
waiting on unfinished children:
['prog.o']
Taskmaster: 'prog.o': children:
['inc.h', 'prog.c']
waiting on unfinished children:
['inc.h', 'prog.c']
Taskmaster: 'prog.c': evaluating prog.c
Taskmaster: 'inc.h': evaluating inc.h
Taskmaster: 'prog.o': children:
['inc.h', 'prog.c']
evaluating prog.o
cc -o prog.o -c -I. prog.c
Taskmaster: 'prog': children:
['prog.o']
evaluating prog
cc -o prog prog.o
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.