[CMake] COMMAND dependencies with add_custom_command()

[Craig Scott]

Great, thanks. That was a similar case to the example I was using to
experiment with this stuff, I just hadn't picked up that if generated.out
already existed, it wouldn't be rebuilt if generate.cpp changed. Cheers.

On Thu, Feb 2, 2017 at 7:12 AM, Matthew Woehlke <matthew.woehlke at kitware.com
> wrote:

> On 2017-01-25 20:25, Craig Scott wrote:
> > The documentation for add_custom_command() contains this little nugget
> when
> > discussing the COMMAND keyword:
> >
> > If COMMAND specifies an executable target name ...<snip>... Additionally
> a
> >> target-level dependency will be added so that the executable target
> will be
> >> built before any target using this custom command. However this does NOT
> >> add a file-level dependency that would cause the custom command to
> re-run
> >> whenever the executable is recompiled.
> >
> > I'm struggling to come up with a scenario where that last part about file
> > level dependencies becomes relevant. Can anyone come up with a simple use
> > case which highlights that particular aspect of dependencies and
> commands?
>
> Let's start with an example, so we can talk about this concretely:
>
>   add_executable(my_generator generate.cpp)
>   add_custom_command(
>     OUTPUT generated.out
>     DEPENDS content.in
>     COMMAND my_generator ...)
>
> I think what this is saying is that the dependencies will be such that
> if `generated.out` needs to be built, the build will ensure that
> `my_generator` is built first (since the command would obviously fail
> otherwise). However, the build will NOT be set up such that a change to
> `generate.cpp` causes `generated.out` to be recreated. IOW, if
> `generated.out` exists, it will only be considered "dirty" if it is
> older than `content.in`, but NOT if it is older than `my_generator`.
>
> Does that help?
>
> --
> Matthew
> --
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-- 
Craig Scott
Melbourne, Australia
https://crascit.com
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