[CMake] Looking for an explanation: What exactly means "install" in cmake language?

[Cornelis Bockemühl]

Constantly I am stumbling over the word "install" in the context of cmake scripts - while it is pretty clear that the word cannot mean what nowadays people would understand by that term! But even reading the docs forwards and backwards, studying examples and some generic cmake tutorials I still do hot have a PRECISE idea about what it is actually doing and what the purpose is. Well, I can see everywhere HOW it is done - but it is sometimes an advantage if you also know WHAT!

I know that this question is probably so stupid that I do not really expect anybody to answer it here in the mailing list, but maybe somebody can point me to a resource that explains it in a language that a "mere mortal programmer" can follow?

So far I have a certain kind of understanding, and sometimes it looks like this is correct, but then again I have my certain doubts. I am using install(TARGETS), install(FILES) and install(EXPORT) so far, and the way I am doing it I mostly picked from some general intros and tutorials. And from what I understand, this is what I expect to happen:

install(FILES): some files are copied from some location in the build tree to some installation location
install(EXPORT): some cmake script is written that a "derived" project can call (through find_package) in order to get knowledge about my project
install(TARGET): not really an idea what should happen: also some copying from build tree to installation tree I suppose

Now I also learned that the install step does not happen during a normal build "all" - like "ninja all", but requires an extra "ninja install" step. And also I saw that you can adapt the target of the installation process like this in your project - if you never ever expect the build to be "installed" anywhere else:

set(CMAKE_INSTALL_PREFIX ${CMAKE_CURRENT_BINARY_DIR}/../install CACHE PATH "" FORCE)

With this I would expect to generate an install folder in parallel to the bin, lib64 and other folders in the build tree, collecting there all the "products" of the build for further processing by a derived project.

But what I see is actually - almost nothing!

Well, the install(FILES) actually copies a file, and the install(EXPORT) indeed generates a cmake script, but from the install(TARGET) I see no effect: the install folder remains completely empty! Still I see in the log many things like "Installing: /.../install/bin/<somefile>" - but I never find then <somefile> in that indicated location. But yes, it said "installing", not "copying" - so I am back to my initial question: what the hell is "installing" in cmake language?

So basically my problem is: I cannot judge whether my script is working because I simply do not know what SHOULD be the effect of install(TARGET)!

Ah yes, my build tree is after a "normal build" ("all", not "install") full of cmake_install.cmake files that would probably have something to do with that "installation" process, but what? Am I supposed to call them on any occasion or are they supposed to be called automatically at some moment?

Finally, I managed to get also some find_package() working for my project. The "heart" of this is a <Project>Config.cmake script that I am generating during the project build, and which I can write in whatever way I want. For example, I am reading everywhere that I am supposed to do an install(EXPORT) in order to make that find_package() happen, but since I do not really understand what it is supposed to do, and I also see that I can simply write that Config file in such a way that it picks things from where the build process puts them, I do not really see the point.

In other words: I tend to simply write config scripts for the import that rely on the build tree and simply skip that install step - because getting this to work is so difficult if you never know what is supposed to happen how and at what stage of the process and under which conditions!

But then I see that others are still using the install() steps, so there is probably still some meaning in it...

So IF such a thing exists, like a comprehensive explanation about what "install" actually means and what it is supposed to do at what stage of the process, I would be very happy and thankful!

Best regards,
Cornelis
 
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