Re: Adding CMakeLists.txt files

From: Peter Dimov (pdimov_at_[hidden])
Date: 2021-05-31 15:25:57

Alexander Grund wrote:
> CMake has 2 levels of things you specify because CMake is not a build system
> (as B2, make, ninja, ...) but a buildsystem generator.

In CMake parlance, "build system" (or "buildsystem" as they spell it because
we're all German by heart) refers not to make/ninja, which are merely the
engines that execute the build system. It refers to the collection of Makefiles
that in the dark past had to be written by a human in order to implement
the building of a specific project (and contained a bunch of arcane rules and
hacks.)

(That's why when the CMake documentation refers to the build system
author, it has in mind the person writing the CMakeLists.txt file, and not
the maintainer of make or ninja.)

There's an interesting parallel to be made here with jam (the engine),
which sits at the same level as make/ninja, and b2/Boost.Build, which is
the collection of .jam files that implement the high-level building
functionality.

It's interesting how knowing b2 allows one to understand CMake better,
and conversely, familiarity with CMake allows one to understand b2 better.
For example, since CMake separates configure/generate/build phases, one
realizes that b2 has these same phases, except they are executed at once,
rather than in a clearly delineated way, or with separate invocations.

The similarity between CMake and b2 is most visible when using the Ninja
generator. Then

* you describe your targets in a high-level way (in CMakeLists.txt or
  Jamfile)
* the tool generates a low-level action/dependency graph (build.ninja,
  or an in-memory collection of jam targets)
* the build engine (ninja or bjam) builds the project by executing the
  actions in the graph in dependency order.