What is the CMake install time?

Question

A quote from the official documentation:

"Specify rules to run at install time."

What exactly is install time?

The problem for me: I am on Linux, software is installed from packages that are just dependencies and data. There is no CMake that can do anything here. So installation time of software is out of scope from CMake. So what exactly do they mean?

Answer 1

Building a CMake project can roughly be divided into three phases:

• Configure time. This includes everything that happens while running cmake itself. This phase is concerned with inspecting certain properties of the host system and generating the specific build files for that platform under the selected configuration.
• Build time. This includes everything that happens while actually building your project from the files generated by CMake (like, when running cmake --build or make). This is where all of the actual compilation and linking happens, so at the end of the build phase, you have a usable binary.
• Install time. This includes everything that happens when running the INSTALL target generated by CMake (like, when running cmake --build --target install or make install). This takes care of copying the binaries that were generated into the build tree to a different directory. Note that the build tree contains a lot of stuff that is no longer needed once the build is completed if you are only interested in running the binary. Examples include all intermediate build artifacts, like the build files generated during the configure phase or the intermediate object files created during the build phase. Furthermore, the install phase might include additional steps to ensure that the binaries produced during the build are portable. For instance, on Linux systems you might want to remove the build directory from the shared library search path in the binary and replace it with a portable equivalent. So the install phase might do more than just copy all the important files to a new directory. It could also include additional steps that change the binaries to make them more portable.

Note that the last phase is optional. If you do not want to support calling make install but prefer another deployment mechanism, you simply don't use the install command in your CMake script and no INSTALL target will be generated.