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SconsTutorial1
After you've decided what to program, there is a tedious problem common to all software projects: the build system that will track dependencies and compile everything in required order. Build system is essential for software in compiled languages, such as C, C++, Java, D or Fortran, but also useful for building documentation or making distributives. This tutorial describes SCons primarily for C/C++ programming, but things described are rather simple to be interesting for everyone.
There are several tools available to control a project's build process. One of the first of them is 'make' often used in combination with autoconf. Historically, it became the de-facto standard for *nix systems. Sadly, autoconf is extremely difficult to master. (For a lengthy discussion on autoconf visit a Freshmeat editorial here DEAD LINK.) There were many attempts to create alternative build systems, and they even succeeded in some areas (like Ant for Java), but none of them is universally popular.
There is hope at the end of the rainbow, though, and that hope is SCons. SCons is a very well-thought-out Python based replacement for make. It offers similar functionality, but benefits from the power that a scripting language like Python can bring. One of the key benefits of SCons is its cross-platform abilities, so most of what goes on here will work on Windows even if it was tested on a Linux system.
This series of tutorials will cover a wide variety of topics on using SCons. Each tutorial will cover one small area of SCons. I hope to break down the lengthy documentation at scons.org into a readable and informative set of informal tutorials. This tutorial will cover creating a simple C program.
This series is written very informally - I don't intend this to be a reference manual or anything. It is largely from my own notes that I took while learning SCons, and I'm sure it contains spelling and grammatical errors. Please don't bug me about it. If there are glaring factual mistakes, I would like to hear from you, though.
It is assumed that the reader has a grasp of creating programs in the languages used, and a grasp on basic programming jargon and definitions. However, no prior knowledge of Python is required.
First you need Python installed. Then the most recent version of SCons can always be found on the Scons.org website (http://www.scons.org/download.php). The most convenient way to install (and the preferred way) is to use Python's pip installer. The basic syntax for this is: "python -m pip install scons". Since this writes to system locations, you generally want to install as "python -m pip install --user scons". You can also use pip to install in a virtualenv, but that topic is beyond the scope of this introduction.
For Linux users, many distributions provide a packaged SCons, if you prefer to use the package manager to get SCons, that is fine too. For Red Hat users there is RPM package, install via "sudo dnf install scons" (or in some cases, "sudo dnf install scons-3", or for older versions, substitute yum for dnf). In Ubuntu or other Debian based systems it is enough to run "sudo apt-get install scons". If you are running another distribution, simply download the scons-XYZ.tar.gz package. Unpack this package, and change to the directory it creates. At this point, run - "python setup.py install". Your SCons installation should now be ready. You can test it by running "scons --version". If the install has worked, a Copyright notice and version statement should be displayed.
Every project using SCons must have a SConstruct file. This file directs the operation of the build. The format of the SConstruct file should be familar for Python users. Other uses will be interested in knowing that names are case sensitive, comments can be started with a pound (#) and continue until the end of the line. Also, indentation matters in the SConstruct and should only be used under specific circumstances which we will discuss later.
Comments in the SConstruct file begin with a pound (#) and last to the end of the line. Comments may be placed on the same line as SConstruct commands, but still last to the end of the line.
SCons(or Python) follows a syntax similar to C for that of Data. Strings are contained in either double quotation marks("") or single quotation marks(''): 'hello nurse', "Helloooooo Nurse". In strings there are also escape characters which can be used to express other characters. Escape characters, as in C, as preceded by a backslash('\') and followed by the escaped character. Basic characters are the newline('\n'), quotation('\"'), or the tab('\t'). Integers are simply written out as they would appear: 0, 1, 2, 3.
Now that the environment is created, we will take a little side road and talk about lists in Sconstruct files. Many of the operations that are used in SCons require the use of lists. A list is encapsulated with brackets('[]'). Each list item is followed by a comma, inside these brackets. For example, this would be a list of numbers: [4, 2, 1, 4]. Most of the time, though, we will be using lists of strings: ["abraham.c", "compiler.c", "helpme.cpp"]
As Scons uses Python, the full abilities of the Python programming language are at your disposal. This includes variables. To create a variable, simply assign data to it. Variables are assigned using the ('=') operator. For example...
myFirstSconsVariable = "Hello World"...creates the string object "Hello World" and saves a reference to the object named myFirstSconsVariable.
There are other things that Scons files may contain within them, but these will be considered in future tutorials.
In order to interact with SCons, the first thing done in an SConstruct file is to create an Environment. Environments contain various information about the system, including C compiler flags, linker flags, and so on. We initialize the Environment with a single line:
env = Environment() # Initialize the environmentAgain, recall that the SConstruct file is white space sensitive. This should appear starting in the first column of text.
The environment is not a simple variable. It is an object with various methods or subroutines of its own. The methods and subroutines allow the creation of programs whether they are in Java, C++, or C. Today we will create a simple C program. A basic tutorial for compiling FORTRAN programs can be found here. Once the environment is created, the C program creation is simple:
env.Program(target='helloworld', source=["helloworld.c"])Here we introduce several new concepts. First, the target. A target in SCons is similar to targets in make. The target is the logical result of the operation. Here, the target is the program helloworld. Note that it is not necessary to write helloworld.exe. The extension is chosen by SCons for you. Targets are built from a list of sources. Here, the list of sources contains only one item: the C file "helloworld.c". SCons will automatically pick a compiler, and compile time flags to compile the source for you. While this tutorial only covers C, SCons understand and can build a vast array of source types:
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.c- a C source file -
.cpp/.c++/.C++/.cxx/.cc- a C++ source file -
.f/.for/.fpp- Fortran with optional C preprocessor in POSIX -
.s/.SPP- an assembly file with optional C preprocessor in POSIX
A complete list of options in running SCons can be found at the scons.org website: http://www.scons.org/doc/HTML/scons-man.html. Only the basic options will be considered here. Simply executing SCons will cause all targets to be built in or above the current directory. In addition, it's possible to give SCons a target to have it build, for example: "scons helloworld", would cause the hello world program above to be built. On Windows, note that while SCons will automatically supply a needed .exe suffix for the target you give to the Program builder, you do have to supply the suffix when referring to a target on the command line, so there you would type "scons helloworld.exe". Directory names can also be passed to SCons. When a directory is given, all targets in and below that directory are built. Thus: "scons ." would have the same result, in general, as running "scons". To have everything done, including installation of programs, simply run "scons /", or in Windows, "scons C:". (More on installation later.)
It should be clear from this simple introduction that SCons is a powerful tool, capable of making your life a lot easier in building large projects. Future tutorials will cover shared libraries, and simple programming inside the SConstruct files.
Functions Covered in this Tutorial: Environment, env.Program
Code: sconstutorial1.tar.gz