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?
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]
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.