July 23, 2023

Things That Surprised Me About CMake


In my current job for the past year I have been using CMake for all my C++ development, working on financial software in a large CMake project with hundreds of executable targets and around 3 million LoC. In the course of getting up to speed on CMake, I ran into some things that surprised me.

Warped CMake Screenshot

Implicit includes

This is a simple one - when you specify a dependency with target_link_libraries, if the CMake rule that builds that target has specified its Public include directories via target_include_directories, then those include files are propogated.

So if you work in a project that follows the convention:

  • Specify public include files for all targets with target_include_directories( ... PUBLIC)

Then any dependency set up with target_link_libraries will set up the include directories properly as well.

So in short “link” implies that the include files are available, too, for all targets built in a CMake project that follow this convention. I find this a bit surprising since I mentally have thought of linking being different from include directories. But now that I’ve worked in this system for a while, I don’t mind it.

CMake project reloading

Any time a CMakeLists.txt file in a CMake project is modified, the entire CMake project needs to be reloaded for the change to take effect. CMake doesn’t support incrementally loading just one part of a project, so even if you just add one file to a target’s sources, the entire project must be reloaded including any computationally expensive rules. Of course, during project reload we can’t compile or use CLion go-to-definition or refactoring tools. This gets painful for larger projects that have long load times.

Globally scoped targets

All target names in a CMake project inhabit the same global scope, and there isn’t any built in namespacing functionality based (for example) on the directory hierarchy, although there is a recommended convention to use double-colon separated names for imported targets. In a larger project with gratuitous use of add_subdirectory, this ultimately leads to longer target names like ACMEmoduleNameComponent where

  • ACME = company target prefix for ACME Company to distinguish between targets of company source code and third party dependencies
  • moduleName = the module name the target inhabits - for example product group like Accounting
  • Component = the actual piece of software being built by this target, for example a Stock Record Service

This gets annoying especially for command line development since these three components of the target name duplicate information that is already in the filesystem:

# in project root
cd cmake/
cmake ..
cd ModuleName/Component/src
make ACMEmoduleNameComponent

ModuleName/Component/src already specifies the 3 components:

  • it identifies the module as first party (built by us) since 3rd party are in system directories or vendored in third_party/
  • It identifies the module name via ModuleName/
  • It identifies the component name via Component/

Therefore, I would prefer to only need to type make ./Component here.

Named return values

CMake functions return values by setting a variable in the PARENT scope.

The function below is the skeleton of a pretend rule to generate C++ files and associated targets from XML specifications and return them in a variable ${src_name}_GENERATED_TARGETS.

function(acme_generate_targets src_name srcs)
    message("generating C++ targets from sources: ${srcs}")
    ... # generation done here
    src_name =  ...
    targets = ....
    set(${src_name}_GENERATED_TARGETS ${targets} PARENT_SCOPE)

after calling the function, the variable ${src_name}_GENERATED_TARGETS can be used in the parent scope:

set(SRCS "user_model.xml organization_model.xml")
acme_generate_targets("core_models" ${SRCS})
message("Successfully generated targets: ${core_models_GENERATED_TARGETS}")
target_link_libraries(MyCoreService PRIVATE ${core_models_GENERATED_TARGETS})

This is a bit of an odd programming language feature, since the function decides the name of the return variable used in the parent scope when normally (in C++ or Python or most languages) the author of the parent scope would decide the name of the variable.

find_package() must be called after project()

I recently had a CMake project I was developing at home where I had just made the project and installed SDL2 with vcpkg on Windows and I did something like this:

cmake_minimum_required (VERSION 3.8)

find_package(SDL2 REQUIRED)


# Include sub-projects.

And I kept getting errors from find_package that SDL2 wasn’t found. I tried reinstalling multiple ways with vcpkg until finally realizing that I simply needed to put project before find_package:

cmake_minimum_required (VERSION 3.8)

find_package(SDL2 REQUIRED)

# Include sub-projects.

I didn’t notice anything in the documentation for find_package() or project() about this behavior, but maybe I just missed something. In any case, I found it surprising :-)

Variable names

If you write a variable like $VARNAME instead of ${VARNAME} it will work with CMake + Make, but will break when you switch to the Ninja generator.

Missing libraries

CMake won’t complain about a missing or misspelled library in target_link_libraries

        hellowhatsup # invalid lib name

will build fine.

Update July 24: LoC was off by two orders of magnitude. Fixed grammar, added note about variable names and missing libraries.

© Gabriel Dougherty 2019-2023

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