If you're used to build systems like Make you've already figured out that the SConstruct file is the SCons equivalent of a Makefile. That is, the SConstruct file is the input file that SCons reads to control the build.
There is, however, an important difference between an SConstruct file and a Makefile: the SConstruct file is actually a Python script. If you're not already familiar with Python, don't worry. This User's Guide will introduce you step-by-step to the relatively small amount of Python you'll need to know to be able to use SCons effectively. And Python is very easy to learn.
One aspect of using Python as the scripting language is that you can put comments in your SConstruct file using Python's commenting convention; that is, everything between a '#' and the end of the line will be ignored:
# Arrange to build the "hello" program. Program('hello.c') # "hello.c" is the source file.
You'll see throughout the remainder of this Guide that being able to use the power of a real scripting language can greatly simplify the solutions to complex requirements of real-world builds.
One important way in which the SConstruct
file is not exactly like a normal Python script,
and is more like a Makefile,
is that the order in which
the SCons functions are called in
the SConstruct file
affect the order in which SCons
actually builds the programs and object files
you want it to build.
In other words, when you call the
(or any other builder method),
you're not telling SCons to build
the program at the instant the builder method is called.
Instead, you're telling SCons to build the program
that you want, for example,
a program built from a file named hello.c,
and it's up to SCons to build that program
(and any other files) whenever it's necessary.
(We'll learn more about how
SCons decides when building or rebuilding a file
is necessary in Chapter 6, below.)
SCons reflects this distinction between
calling a builder method like
and actually building the program
by printing the status messages that indicate
when it's "just reading" the SConstruct file,
and when it's actually building the target files.
This is to make it clear when SCons is
executing the Python statements that make up the SConstruct file,
and when SCons is actually executing the
commands or other actions to
build the necessary files.
Let's clarify this with an example.
Python has a print statement that
prints a string of characters to the screen.
If we put print statements around
our calls to the
Program builder method:
print "Calling Program('hello.c')" Program('hello.c') print "Calling Program('goodbye.c')" Program('goodbye.c') print "Finished calling Program()"
Then when we execute SCons, we see the output from the print statements in between the messages about reading the SConscript files, indicating that that is when the Python statements are being executed:
% scons scons: Reading SConscript files ... Calling Program('hello.c') Calling Program('goodbye.c') Finished calling Program() scons: done reading SConscript files. scons: Building targets ... cc -o goodbye.o -c goodbye.c cc -o goodbye goodbye.o cc -o hello.o -c hello.c cc -o hello hello.o scons: done building targets.
Notice also that SCons built the goodbye program first, even though the "reading SConscript" output shows that we called Program('hello.c') first in the SConstruct file.
In programming parlance, the SConstruct file is declarative, meaning you tell SCons what you want done and let it figure out the order in which to do it, rather than strictly imperative, where you specify explicitly the order in which to do things.