Hi and welcome to a brand-new blog series! Over the coming weeks, I'll be presenting build systems to you, and today I want to set the stage for this. I'll be looking at the topic through a C++ lens, as my sample project will be written in C++, but that's just because I'm a C++ person and that's what is most interesting for me. That's just a minor detail though -- we'll cover multiple build systems which support many different languages. Before we look into the project I want to build, let's take a look first and what we're going to discuss.
A build system solves a rather simple problem at its core: It transforms data using rules and takes dependencies into account. You can think of any build system as a graph traversal engine. Every node in that graph has one or more inputs, produces one or more outputs, and the node itself encodes the rules how to transform the inputs into the desired outputs. That's it, once everything is laid out in this way, you just need to find out which inputs have changed, and then build the nodes which (transitively) depend on those inputs.
Unfortunately, that's a really hard problem, as the task of finding all dependencies is a tough one -- as the set of dependencies include the compilers, system SDKs, any library which happens to be in your project, the specific settings you pass on to your compiler and any file that's read by one of your transformation steps (and more!). Spelling out all those dependencies is not realistic, and thus most build systems will provide some shortcuts to reasonably approximate it. That's where the language support of a build system comes in handy, as this allows you to express fewer of those dependencies manually and just let the build system figure out the details.
The language support part can also extend to dependency management. Strictly speaking, it's not part of the core business, but a build system has a lot of knowledge about the things it builds, and that makes it convenient to build more than just the shared library file you asked for. For instance, a build system could also deploy all headers you need to include said shared library, the linker options you need to link against it, the version, and so on, up to the point that you can consume it in another project just "as if" it was built locally.
With this background, let's look at the sample project I want to build. It's a small project consisting of three separate parts:
- A static library, which contains a generated source file. That source file is generated from a Python script.
- A dynamic library linking against the static library.
- An executable consuming the dynamic library.
Let's look at the tricky parts for each of them. The static library has two interesting requirements to a build system. It needs to find Python before you can actually generate the source file. If the build system provides some way to find a dependency or binary, that's a big plus because it'll make this part more reliable. The other requirement is a tricky one -- on Linux, a static library which goes into a dynamic library must be compiled with -fPIC, i.e. as position-independent code. This can be only discovered by looking at the consumers of the static library. A C++ aware build system should at least notice this and warn you.
The dynamic library is straightforward, except that it requires a preprocessor definition to build. That definition must be set only for the dynamic library, not for the consumer of it.
The executable itself has no extra requirements. It just includes the dynamic library header and links it. Includes is a good topic though -- there's one external constraint I'm putting on the build systems which is that each part must have its own build file, to show how the build system handles modularity. This also means the build system needs to set up the include directories, as by default, the individual parts just #include the headers of their dependencies. That's it -- a small, but not trivial project, and the "benchmark" I'll be using going forward.
You can find the whole setup -- without build system files -- in my build system repository on Bitbucket. With every new blog post, I'll be adding one new set of build files so you can follow along. As a small teaser, we're going to start next week with Make!