How can I prevent a CMake external project from updating & reinstalling whenever I make?

Question

After much agony, I've successfully arranged for my CMake project to use some library via ExternalProject. I cmake, I make, it works - life is good.

However, if I make again, it performs the Update and Install steps for the external project. I definitely do not want this to happen. How do I prevent this?

Answer 1

While the way you proposed in your own answer has the same effect, the actual option you might want to use for that is UPDATE_DISCONNECTED.

From CMake's documentation, under the section Update/patch options:
UPDATE_DISCONNECTED <bool>

When enabled, this option causes the update step to be skipped. It does not, however, prevent the download step. The update step can still be added as a step target (see ExternalProject_Add_StepTargets()) and called manually.

So your final thing could look like the following:

ExternalProject_Add( external_lib
    # ...
    UPDATE_DISCONNECTED True
)

If you go a bit further down the excerpt I quoted though, the documentation explicitly advises against doing this and recommends controlling the update behaviour through a directory variable EP_UPDATE_DISCONNECTED, whose value is used as the default value for UPDATE_DISCONNECTED.
This means you can keep your ExternalProject_Add call free of the UPDATE_DISCONNECTED option, and instead run your CMake command like so:

$ cmake .. -DEP_UPDATE_DISCONNECTED:BOOL=True
$ cmake --build . [your_options]

Answer 2

One way to do it is to set UPDATE_COMMAND to "" for your library's external project, that is, have:

ExternalProject_Add(the_external_library_proj_name
    # all sorts of settings here
    # etc. etc.
    # ...
    UPDATE_COMMAND ""
)

in your CMakeLists.txt. That means you might not be able to update the external project via CMake and will have to resort to doing it manually, or cleanly rebuilding everything etc.