Question

I got the Box2D project source and want to compile the testbed portion of it.
The project folder contains folders like: freeglu glui testbed(a demo) helloword(a demo)
Box2D Build CMakeFiles

There are many CMakeLists.txt in all the different folders.
I was thinking that I should cmake all those files so that make files are created in all places required.
I read this (as instructions to do do want I want) :

wget http://box2d.googlecode.com/files/Box2D_v2.2.1.zip
unzip Box2D_v2.2.1.zip
cd Box2D_v2.2.1/Build
cmake ..
make

What does the cmake .. do?
There is no CMakeLists.txt in the build folder.

Answer 1

cmake is a Makefile generator.

When you call cmake [path], you ask it to generate a Makefile in the current directory following instructions given in [path]/CMakeLists.txt

Usually cmake output some messages while it is working, and after it is done without errors, you can type “make” to execute your newly created Makefile.

CMakeLists.txt files can reference other CMakeLists.txt file in sub-directories, so you are usually only interested by the CMakeLists.txt of the top directory, not the other ones.

Using an empty “build” directory is a technique called “out-of-source build”, in which all your generated files (.o, executable, Makefile, .anything) are generated in the separate “build” directory and not mixed with source files. If you want to clean all, you can delete all the content of the build directory.

In fact, you can put your “build” directory in any place, as long as you give cmake the correct path of the top CMakeLists.txt. You can even have several build directories. It is very useful if you need several different builds at the same time (with different options, different versions of gcc, etc.)

In old programs, you generate the Makefile too, but using ./configure (this is called auto-tools. You may have encountered that already). cmake is considered a successor of the auto-tools.