Running CMake

Before writing CMake, let's make sure you know how to run it to make things. This is true for almost all CMake projects, which is almost everything.

Building a project

Unless otherwise noted, you should always make a build directory and build from there. You can technically do an in-source build, but you'll have to be careful not to overwrite files or add them to git, so just don't.

Here's the CMake Build Procedure (TM):

~/package $ mkdir build
~/package $ cd build
~/package/build $ cmake ..
~/package/build $ make

You can replace the make line with cmake --build . if you'd like, and it will call make or whatever build tool you are using. You can follow it by make install or cmake --build . --target install if you want to install it.

Picking a compiler

Selecting a compiler must be done on the first run in an empty directory. It's not CMake syntax per se, but you might not be familiar with it. To pick Clang:

~/package/build $ CC=clang CXX=clang++ cmake ..

That sets the environment variables in bash for CC and CXX, and CMake will respect those variables. This sets it just for that one line, but that's the only time you'll need those; afterwards CMake continues to use the paths it deduces from those values.

Picking a generator

You can build with a variety of tools; make is usually the default. To see all the tools CMake knows about on your system, run

~/package/build $ cmake --help

And you can pick a tool with -G"My Tool" (quotes only needed if spaces are in the tool name). You should pick a tool on your first CMake call in a directory, just like the compiler. Feel free to have several build directories, like build/ and buildXcode.

Setting options

You set options in CMake with -D. You can see a list of options with -L, or a list with human-readable help with -LH. If you don't list the source/build directory, the listing will not rerun CMake (cmake -L instead of cmake -L .).

Verbose and partial builds

Again, not really CMake, but if you are using a command line build tool like make, you can get verbose builds:

~/package/build $ VERBOSE=1 make

You can actually write make VERBOSE=1, and make will also do the right thing, though that's a feature of make and not the command line in general.

You can also build just a part of a build by specifying a target, such as the name of a library or executable you've defined in CMake, and make will just build that target.

Standard options

These are common CMake options to most packages:

  • -DCMAKE_BUILD_TYPE= Pick from Release, RelWithDebInfo, Debug, or sometimes more.
  • -DCMAKE_INSTALL_PREFIX= The location to install to. System install on UNIX would often be /usr/local (the default), user directories are often ~/.local, or you can pick a folder.
  • -D BUILD_SHARED_LIBS= You can set this ON or OFF to control the default for shared libraries (the author can pick one vs. the other explicitly instead of using the default, though)

Debugging your CMake files

We've already mentioned verbose output for the build, but you can also see verbose CMake configure output too. The --trace option will print every line of CMake that is run. Since this is very verbose, CMake 3.7 added --trace-source="filename", which will print out every executed line of just the file you are interested in when it runs. If you select the name of the file you are interested in debugging (usually with a parent directory if you are debugging a CMakeLists.txt, since all of those have the same name), you can just see the lines that run in that file. Very useful!

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