[CMake] Targets where the command decides if the target needs rebuilding

[David Gobbi]

Never mind, I found an easy solution.

I just added my fake ".target" file as a second output of the command,
and use "cmake -E touch" to update the .target timestamp whenever my
custom command executes.  Even though the timestamp of my "true"
output isn't being updated, CMake doesn't re-execute the command.

   David


On Mon, May 31, 2010 at 10:13 AM, David Gobbi <david.gobbi at gmail.com> wrote:
> Hi all,
>
> There are times when a CMake custom command will generate exactly the
> same output file even if its inputs have changed.  In these cases, I
> would like the timestamp on the output file to remain unchanged, but
> to generate a fake ".target" file with the current timestamp so the
> custom command will not be re-execute the next time I run CMake.
>
> Here is an example:
>
> I've written a tool called vtkBuildHeirarchy that goes through the VTK
> header files and writes a text file that describes the class
> hierarchy.  Then, I have some special wrappers that use this text file
> to do compile-time type introspection.
>
> A naive approach would force a complete re-build of the wrappers if
> even a single header file has changed.  However, the only important
> changes to the wrapper are those that change the actual hierarchy.
>
> So, I have made my vtkBuildHeirarchy executable so that it pre-reads
> the output and then only overwrites it if the hierarchy has changed.
> This keeps my wrappers from re-compiling when they don't have to.
> However, now the vtkBuildHeirarchy executable runs every time
> (actually several times) when I run cmake because its output file's
> timestamp is not updated.
>
> This is why I want to create a second, "fake" output file with a
> .target extension, so that vtkBuildHeirarchy won't execute if this
> "target" file is more recent than its inputs.  However, commands that
> depend on vtkBuildHeirachy's output will have to continue to check the
> timestamp of the "real" output file.
>
> Is there a way of splitting a target into a real target/fake target in cmake?
>
>   David
>