[cmake-developers] A CMAKE_EMULATOR variable

[David Cole]

What does CMAKE_EMULATOR emulate?

>From its name, it sounds like it emulates CMake. But from your description,
I'm thinking that doesn't make sense... Because you actually run CMake and
pass it CMAKE_EMULATOR. So it must be emulating something else while
running CMake?

I'm guessing it emulates the target system when cross-compiling, and
executables built for the target system can be run when passed to the
emulator? Is that right?


D



On Wednesday, March 4, 2015, Matt McCormick <matt.mccormick at kitware.com>
wrote:

> Hi,
>
> I have pushed to stage [1] support for a CMAKE_EMULATOR variable to
> help when cross-compiling.  The goal is to improve cross compiling
> with CMake by making it easier to build and run tests.  In principle,
> the commands
>
>   cmake -DCMAKE_TOOLCHAIN_FILE=/path/to/toolchain.cmake
> -DCMAKE_EMULATOR=/path/to/emulator ~/src/project
>   cmake -D Experimental
>
> are all that is needed generate a dashboard report, complete with test
> results.  This should inch C/C++ closer to being the "write once, run
> anywhere" languages :-).
>
> The emulator is used to run try_run results, which avoids manual
> population of TryRunResults.cmake.  This can be a painful,
> time-consuming process otherwise.
>
> It is also used to run tests on executables that are built for the
> target system.  Without this approach, it is difficult to know which
> tests should be executed on the target system.  Tests are often passed
> absolute paths to input on the host system.  The use of an emulator is
> a way to avoid complexities and transfer overhead related to
> reproducing the host filesystem on the target filesystem to run the
> tests.
>
>
> With some fixes to ITK [2], this was used to build and test for four
> cases of interest.
>
> The emulator approach works best with MinGW and WINE to build and test
> on Linux for Windows [3].
>
> The qemu-arm emulator provided by QEMU User Mode can be used with the
> Android NDK toolchain [4]. A gotcha is that Android tries to be fancy
> and uses its own dynamic loader.  To get around this, I tested with
> completely static executables.  Also, QEMU User Mode does not
> currently support multi-threading well, so tests were run
> single-threaded. An alternative approach may be to use an emulator
> script that is a wrapper around adb.
>
> The qemu-arm emulator was used again with the Raspberry Pi toolchain
> [5].  A symbolic link was created in the expected location for
> ld-linux-armhf.so.3, and dynamic loading works.  To run the tests,
> LD_LIBRARY_PATH was populated with the path to libc and libstdcxx.
>
> One of the most interesting combinations is the Emscripten toolchain
> with NodeJS as the emulator [6]. There are some WIP workarounds to get
> Emscripten to configure cleanly for scientific libraries [7], and code
> had to be injected into the test driver to mount local filesystems for
> node, but this works surprisingly well.
>
>
> Testing and feedback are appreciated.
>
> Thanks,
> Matt
>
>
> [1]
> http://www.cmake.org/gitweb?p=stage/cmake.git;a=shortlog;h=refs/heads/emulator
>
> [2] https://github.com/thewtex/ITK/tree/cmake-emulator
>
> [3] https://open.cdash.org/buildSummary.php?buildid=3694578
>
> [4] https://open.cdash.org/buildSummary.php?buildid=3694810
>
> [5] https://open.cdash.org/buildSummary.php?buildid=3694810
>
> [6] https://open.cdash.org/buildSummary.php?buildid=3705525
>
> [7] https://github.com/thewtex/emscripten/tree/test-big-endian
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