[CMake] dependencies of cross compiliations

[Torsten Robitzki]

Hi,
I hope this question was not asked before. I work in the embedded field and there it is usually to have at least two different build platforms. The Host platform, where unit tests are build (and where CMake is running) and an embedded Target platform, where targets are build with a cross compiler. Sometimes such a system comes with self-written tools that are build and run on the Host platform to build a target for the embedded Target platform (adding meta data to a binary to be used by a bootloader for example).

Usually I have two different build folders, one for the Host platform and one for the Target platform, using different calls to cmake to choose from a set of tools and targets. But when using this approach, it is necessary that the Host platform build ran before the Target platform build, so that tools that are required for the Target platform are build during the Host target build.

One solution I’ve came up with, is to build the required tools during the Target platform build, using an add_custom_target() to invoke the Target compiler directly. This works fine, as long as the tools are basically build just out of a couple of files. 

What would be the „CMake-Way“ to add the tools (that have to be build on the Target platform) as dependency to targets that have to be build for the Target (cross compile) platform?

Kind regards and thanks in advance,

Torsten