[CMake] Help debug crazy Windows MSVC issue

[Paul Smith]

On Mon, 2014-02-17 at 00:20 -0500, Paul Smith wrote:
> On Sun, 2014-02-16 at 22:38 -0500, David Cole wrote:
> > > How can I structure my cmake file to avoid
> > > this double build?
> > 
> > Put the custom command in a custom target, and make the libraries using 
> > the generated file depend on the custom target.
> > 
> > That works, even for parallel builds.
> > 
> > Google around for examples and similar advice. You'll end up using 
> > add_custom_target and add_dependencies in addition to what you already 
> > have.
> 
> I was thinking about that earlier, but I didn't do it because the docs
> for add_custom_target() say:
> 
>   The target has no output file and is ALWAYS CONSIDERED OUT OF DATE
>   even if the commands try to create a file with the name of the target.

Thanks for the reply, but this isn't working.

First, unlike most of the examples I've found I'm generating C++ source
code, and if I leave the output of the custom command as the generated
source then I ALWAYS get the double build.  In fact if I put
add_custom_target it builds THREE times; I tried this:

  cmake_minimum_required(VERSION 2.8.12)

  project(TEST C CXX)

  add_custom_command(OUTPUT bar.cpp
      COMMAND cmake -E copy ${CMAKE_CURRENT_SOURCE_DIR}/foo.cpp bar.cpp
      DEPENDS foo.cpp
      COMMENT "GENERATING bar.cpp")

  set_source_files_properties(bar.cpp PROPERTIES GENERATED TRUE)

  add_custom_target(BuildBar DEPENDS bar.cpp)

  add_library(Bar STATIC base.cpp bar.cpp)
  add_library(BarDLL SHARED base.cpp bar.cpp)

  add_dependencies(Bar BuildBar)
  add_dependencies(BarDLL BuildBar)

and this caused the "GENERATING bar.cpp" line to be run three times
during my build (setting or not setting the GENERATED property doesn't
appear to make any difference here).

So then I tried to use a separate OUTPUT name for the custom command
which was not the source file.  This time I only get one GENERATING line
so it's only run one time, BUT every time I run the build it regenerates
that source file and hence recompiles and relinks both libraries:

  cmake_minimum_required(VERSION 2.8.12)

  project(TEST C CXX)

  add_custom_command(OUTPUT bldbar
      COMMAND cmake -E copy ${CMAKE_CURRENT_SOURCE_DIR}/foo.cpp bar.cpp
      DEPENDS foo.cpp
      COMMENT "GENERATING bar.cpp")

  set_source_files_properties(bar.cpp PROPERTIES GENERATED TRUE)

  add_custom_target(BuildBar DEPENDS bldbar)

  add_library(Bar STATIC base.cpp bar.cpp)
  add_library(BarDLL SHARED base.cpp bar.cpp)

  add_dependencies(Bar BuildBar)
  add_dependencies(BarDLL BuildBar)

Neither of these is what I need.

Is there an option which gives me reliable builds, but doesn't force the
majority of my codebase to be rebuilt every time even if nothing
changes?