[CMake] Output of ADD_CUSTOM_COMMAND generated multiple times

[Martin Apel]

Brad King wrote:
> We've built huge projects with thousands of custom build rules while
> still only having a few custom targets.  Your original example can be
> written like this:
>
> ADD_CUSTOM_COMMAND(
>   OUTPUT ${CMAKE_BINARY_DIR}/a
>   COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_BINARY_DIR}/a
>   )
> ADD_CUSTOM_COMMAND(
>   OUTPUT ${CMAKE_BINARY_DIR}/b
>   COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_BINARY_DIR}/b
>   DEPENDS ${CMAKE_BINARY_DIR}/a
>   )
> ADD_CUSTOM_COMMAND(
>   OUTPUT ${CMAKE_BINARY_DIR}/c
>   COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_BINARY_DIR}/c
>   DEPENDS ${CMAKE_BINARY_DIR}/a
>   )
> ADD_CUSTOM_TARGET(world
>   DEPENDS ${CMAKE_BINARY_DIR}/b ${CMAKE_BINARY_DIR}/c
>   )
>
> Now the 'world' target contains all the rules and will build 'a' first,
> then 'b' and 'c' in parallel.
>
>   
Thanks a lot, Brad. This approach improves my build output very much!
>> I think CMake should be smart enough to handle this without the extra
>> targets, at least as long as all of them are in
>> the same directory. That is what the "generated" attribute of a source
>> file is about, isn't it.
>> But for now, I will modify my CMakeLists.txt to contain the extra targets.
>>     
>
> There is a feature request here:
>
> http://www.cmake.org/Bug/view.php?id=6285
>
> However, the solution I have in mind for it will still not handle your
> case.  You're basically telling two different targets how to build a
> file, and not adding any other dependencies to control the order.
>
> Each target gets built with its own make process.  It is at this stage
> that file-level dependencies are evaluated by the build system.  A
> higher-level make process considers only inter-target dependencies with
> symbolic (phony) targets.
>   
It would be nice, if CMake could compute these dependencies
automatically, but there have to be
some features left for CMake 3.0 :-)

Martin

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