Introduction to Software Development Tooling

Homework 6: BLD, Constructive

Build me up, buttercup

In this assignment, you’ll be adding a build system to an existing software project written in C. The project is based on a real open-source project, but we’ve modified it to be more conducive to the task at hand.

The project in question, called jvm, has gotten to the point where manually rebuilding every file by hand is frustrating and error-prone. To make its developers’ lives easier, you’ll write a Makefile to build it automatically.

Start by accepting the assignment on GitHub Classroom. Then clone your own personal copy of the project, which we’ve pre-populated for you using GitHub Classroom, from git@github.com:cs50isdt/hw6-{yourgithubusername}. (The URL should closely resemble that of your VCS Constructive repository.) This is also where you’ll push your solution after completing the assignment.

Step 1: determine the build graph

Before you write a Makefile to automate the build process, however, you need to figure out what that build process is. The following questions will help you discern the proper way to build the code you’ve been given. We designed the questions to help you, but you do not have to provide us the answers. Skip them at your own peril.

  1. How many executable files is this project designed to have? What are they called? How do you know? (Hint: there are multiple ways to determine this, but it may help to think about what piece of code every C or C++ program must have.)
  2. Starting from the main .c file for each executable, list all the other .c files needed for that executable to compile and run. (Hint: follow the #includes.)
  3. Assume that, during the build, every .c file will be compiled into a matching .o file then those .o files will be linked together as appropriate to create the final executable(s). Draw a graph showing the compile-time1 dependencies between the executable(s), the .o files, and the source (.c and .h) files. Your answer to the previous question will be useful here. Do not use wildcards or globs–each file should be its own node in the graph.
    1. You may hand draw this on paper or use a free online service like Excalidraw to draw it digitally.
    2. We recommend that you organize the graph with the executable(s) horizontally across the top, the object files below them, and the source files below those. For each .o file an executable depends on, draw an arrow pointing from the executable to the .o file. Likewise, for each source file a .o file depends on, draw an arrow pointing from the .o file to the source file.
    3. If you’re unsure of your drawing, feel free to make a private post on Piazza and we can let you know if you’re headed in the right direction!

Step 2: express the graph in a Makefile

Now that you’ve drawn out the different files involved in the build and the dependencies between them, it’s time to start on a Makefile! Please create a file called Makefile at the root of the repository that builds the executable(s) as described above.

When writing your Makefile, you must do the following:

  1. If an implicit rule exists to do what you want, use it instead of writing your own recipe. (Hint: remember that most implicit rules are customizable by setting specially-named variables.)
  2. Ensure that both make and make all cause all executables to be built.
  3. Ensure that any individual .o or executable file can be built by passing its filename to Make as a target.
  4. Specify the minimal correct set of prerequisites for each target (i.e. rebuild each target only when strictly needed).
  5. Use a C compiler such as gcc or clang for compilation and linking. The choice of which is up to you, except the default of cc, which you must not use.
  6. Include the following flags in the compiler invocation: -Wall -Wextra -std=gnu99.
  7. Include the following flags at the very end of the linker invocation: -lm. (Without this, the linker will fail. Hint: take a look at Make’s documentation for the “Linking a single object file” implicit rule.)
  8. Ensure that running make clean removes all generated files.
  9. Ensure that the clean and all rules do their jobs even if files named clean or all are present on disk.

Apart from these requirements, you may use your discretion about how to implement things. Please try to follow the general practices set out in lecture and the GNU Make manual, though.

Once you’ve fulfilled these requirements to the best of your ability, test out your Makefile. Try building various targets. Does Make do what you expect? Next, try updating various source files (you can use touch to indicate to Make that file has changed without actually altering it), rerunning make along the way. Do the right targets get rebuilt? Does anything get rebuilt that shouldn’t?

Step 3: teach an old DAG new tricks

C code isn’t the only thing in this repository that can be compiled. As you have hopefully seen by now, one of the executables you’ve built is named java and implements a small Java Virtual Machine (JVM) capable of running a subset of valid Java programs. Inside the tests/ directory are a number of Java programs, intended to test out this JVM.

Before you can run the Java programs, however, you have to compile the .java source you’ve been given into Java bytecode, which is what the JVM knows how to read. Java bytecode is stored in files with the extension .class. You can use the standard Java compiler, javac (which is installed on the homework server), to perform this compilation.

Add a pattern rule to your Makefile that can compile any given .java file inside tests/ into a matching .class file. (The .class files must also reside in tests/ and have an identical name save for the changed extension). In you recipe for this rule, please use automatic variables wherever possible. The rule should build only the given .class file, and it should rebuild that file if the source changes. Update your clean rule as appropriate.

Take a look inside the tests/ directory for a suite of sample Java programs. Try running the HelloWorld program using your interpreter: first build HelloWorld and the interpreter with make java tests/HelloWorld.class, then run HelloWorld with ./java -cp tests HelloWorld! (Note how you must provide the directory and class name separately instead of as a single path.)

Step 4: classy tests

Makefiles can do more than just compile code; since recipes can consist of arbitrary commands, developers often create special rules to perform common tasks, even if those tasks don’t produce files on disk. Like running unit tests, for example.

Add a pattern rule to your Makefile that can run any given test. The rule must have targets of the form run-ClassName. To run a test, use ./java -cp tests ClassName. You may find the automatic variable $* helpful here. When you invoke such a target, it should run successfully even if you haven’t explicitly built anything else yet. There is no need to make this rule phony, but you get an imaginary gold star if you make it work.2

Add a rule, test, that runs all the tests. This rule should rely on the run-* rules you just created to actually perform its work. This rule should be phony, as it needs to run even if a file called test exists on disk.

Submitting your work

To submit, commit your changes to the main branch of your repository and push that branch back to GitHub (see the cloning instructions). If you do this correctly, you’ll see your changes in the GitHub UI. This is a good opportunity to exercise your Git skills from last module–you’re free to make your changes using as many commits as you want!

  1. Technically, there is compile-time, link-time, load-time, and run-time. In C, the compile-time dependencies for a .o file almost always consist of a single C file and one or more header files. Header files contain enough information for the resulting .o file to know how to call the functions they declare (the interfaces), but they don’t contain the actual implementations of those functions. Those implementations live in their own .o files, and they’re only needed while linking the final executable, which has link-time dependencies on all of the implementations. This particular kind of interface is known as an application binary interface, or ABI, and the compile-time dependency is why interfaces are specified in header files. 

  2. It’s not as simple as you might think. Test out your hypothesis by running, for example, touch run-HelloWorld and seeing if Make will still run your Java program with make run-HelloWorld