Here's the famous "Hello, World!" program in C:
int
main()
{
printf("Hello, world!\n");
}
And here's how to build it using &SCons;.
Enter the following into a file named &SConstruct;:
Program('hello.c')
int main() { printf("Hello, world!\n"); }
That's it. Now run the &scons; command to build the program.
On a POSIX-compliant system like Linux or UNIX,
you'll see something like:
scons
On a Windows system with the Microsoft Visual C++ compiler,
you'll see something like:
scons
First, notice that you only need
to specify the name of the source file,
and that &SCons; deduces the names of
the object and executable files
correctly from the base of the source file name.
Second, notice that the same input &SConstruct; file,
without any changes,
generates the correct output file names on both systems:
hello.o and hello
on POSIX systems,
hello.obj and hello.exe
on Windows systems.
This is a simple example of how &SCons;
makes it extremely easy to
write portable software builds.
(Note that we won't provide duplicate side-by-side
POSIX and Windows output for all of the examples in this guide;
just keep in mind that, unless otherwise specified,
any of the examples should work equally well on both types of systems.)
The &SConstruct; File
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.
Making the Output Less Verbose
You've already seen how &SCons; prints
some messages about what it's doing,
surrounding the actual commands used to build the software:
scons
These messages emphasize the
order in which &SCons; does its work:
the configuration files
(generically referred to as &SConscript; files)
are read and executed first,
and only then are the target files built.
Among other benefits, these messages help to distinguish between
errors that occur while the configuration files are read,
and errors that occur while targets are being built.
The drawback, of course, is that these messages clutter the output.
Fortunately, they're easily disabled by using
the &Q; option when invoking &SCons;:
scons -Q
Because we want this User's Guide to focus
on what &SCons; is actually doing,
we're going use the &Q; option
to remove these messages from the
output of all the remaining examples in this Guide.
Compiling Multiple Source Files
You've just seen how to configure &SCons;
to compile a program from a single source file.
It's more common, of course,
that you'll need to build a program from
many input source files, not just one.
To do this, you need to put the
source files in a Python list
(enclosed in square brackets),
like so:
Program(['prog.c', 'file1.c', 'file2.c'])
int main() { printf("prog.c\n"); }
void file1() { printf("file1.c\n"); }
void file2() { printf("file2.c\n"); }
A build of the above example would look like:
scons -Q
Notice that &SCons;
deduces the output program name
from the first source file specified
in the list--that is,
because the first source file was &prog_c;,
&SCons; will name the resulting program &prog;
(or &prog_exe; on a Windows system).
If you want to specify a different program name,
then you slide the list of source files
over to the right
to make room for the output program file name.
(&SCons; puts the output file name to the left
of the source file names
so that the order mimics that of an
assignment statement: "program = source files".)
This makes our example:
Program('program', ['main.c', 'file1.c', 'file2.c'])
int main() { printf("prog.c\n"); }
void file1() { printf("file1.c\n"); }
void file2() { printf("file2.c\n"); }
On Linux, a build of this example would look like:
scons -Q
Or on Windows:
scons -Q
Keeping &SConstruct; Files Easy to Read
One drawback to the use of a Python list
for source files is that
each file name must be enclosed in quotes
(either single quotes or double quotes).
This can get cumbersome and difficult to read
when the list of file names is long.
Fortunately, &SCons; and Python provide a number of ways
to make sure that
the &SConstruct; file stays easy to read.
To make long lists of file names
easier to deal with, &SCons; provides a
&Split; function
that takes a quoted list of file names,
with the names separated by spaces or other white-space characters,
and turns it into a list of separate file names.
Using the &Split; function turns the
previous example into:
Program('program', Split('main.c file1.c file2.'))
(If you're already familiar with Python,
you'll have realized that this is similar to the
split() method
in the Python standard string module.
Unlike the string.split() method,
however, the &Split; function
does not require a string as input
and will wrap up a single non-string object in a list,
or return its argument untouched if it's already a list.
This comes in handy as a way to make sure
arbitrary values can be passed to &SCons; functions
without having to check the type of the variable by hand.)
Putting the call to the &Split; function
inside the Program call
can also be a little unwieldy.
A more readable alternative is to
assign the output from the &Split; call
to a variable name,
and then use the variable when calling the
Program function:
list = Split('main.c file1.c file2.')
Program('program', list)
Lastly, the &Split; function
doesn't care how much white space separates
the file names in the quoted string.
This allows you to create lists of file
names that span multiple lines,
which often makes for easier editing:
list = Split('main.c
file1.c
file2.c')
Program('program', list)
Keyword Arguments
&SCons; also allows you to identify
the output file and input source files
using Python keyword arguments.
The output file is known as the
target,
and the source file(s) are known (logically enough) as the
source.
The Python syntax for this is:
list = Split('main.c file1.c file2.')
Program(target = 'program', source = list)
Because the keywords explicitly identify
what each argument is,
you can actually reverse the order if you prefer:
list = Split('main.c file1.c file2.')
Program(source = list, target = 'program')
Whether or not you choose to use keyword arguments
to identify the target and source files,
and the order in which you specify them
when using keywords,
are purely personal choices;
&SCons; functions the same regardless.
Compiling Multiple Programs
In order to compile multiple programs
within the same &SConstruct; file,
simply call the Program method
multiple times,
once for each program you need to build:
Program('foo.c')
Program('bar', ['bar1.c', 'bar2.c'])
int main() { printf("foo.c\n"); }
int main() { printf("bar1.c\n"); }
void bar2() { printf("bar2.c\n"); }
&SCons; would then build the programs as follows:
scons -Q
Notice that &SCons; does not necessarily build the
programs in the same order in which you specify
them in the &SConstruct; file.
&SCons; does, however, recognize that
the individual object files must be built
before the resulting program can be built.
We'll discuss this in greater detail in
the "Dependencies" section, below.
Sharing Source Files Between Multiple Programs
It's common to re-use code by sharing source files
between multiple programs.
One way to do this is to create a library
from the common source files,
which can then be linked into resulting programs.
(Creating libraries is discussed in
section XXX, below.)
A more straightforward, but perhaps less convenient,
way to share source files between multiple programs
is simply to include the common files
in the lists of source files for each program:
Program(Split('foo.c common1.c common2.c'))
Program('bar', Split('bar1.c bar2.c common1.c common2.c'))
int main() { printf("foo.c\n"); }
int main() { printf("bar1.c\n"); }
int bar2() { printf("bar2.c\n"); }
void common1() { printf("common1.c\n"); }
void common22() { printf("common2.c\n"); }
&SCons; recognizes that the object files for
the &common1_c; and &common2_c; source files
each only need to be built once,
even though the resulting object files are
each linked in to both of the resulting executable programs:
scons -Q
If two or more programs
share a lot of common source files,
repeating the common files in the list for each program
can be a maintenance problem when you need to change the
list of common files.
You can simplify this by creating a separate Python list
to hold the common file names,
and concatenating it with other lists
using the Python + operator:
common = ['common1.c', 'common2.c']
foo_files = ['foo.c'] + common
bar_files = ['bar1.c', 'bar2.c'] + common
Program('foo', foo_files)
Program('bar', bar_files)
This is functionally equivalent to the previous example.