[CMake] Correct way to specify multiple build configurations for single config generators?

[Zaak Beekman]

Hi Craig, et al,

Craig, once again, thank you for writing such a great reference and
learning book! I have a few responses inline below.

On Mon, Dec 10, 2018 at 6:34 AM Craig Scott <craig.scott at crascit.com> wrote:

> This is only going to be an issue for single-configuration generators. For
> multi-config generators, you don't set CMAKE_BUILD_TYPE and the flags you
> set as cache variables after the first project() call should be honoured.
> For single-config generators, you have a few choices:
>
>    - Run CMake with CMAKE_BUILD_TYPE set to some other config first, then
>    change it to the one you want after the first run. If the custom config is
>    one that is only occasionally used and typically just one you switch to
>    temporarily from time-to-time, then this may be okay for your situation.
>    It's also easy enough to handle in a CI build, albeit slightly less
>    efficient since you have to run CMake twice. It's a bit annoying, but it is
>    at least easy.
>
> Come to think of it, this might be why my example below was working.
During the CI process for that project we test the different
configurations, so if the config is in question is *not* set first, then
it's cache variables have a chance to initialize themselves before the
build is switched to that config.

>
>    - Rather than setting the custom config's flags in the project, do it
>    in a toolchain file instead (with the various ..._<CONFIG>_INIT variables).
>    The flags are going to be toolchain-specific anyway, so you don't lose
>    generality. You also gain the advantage that you can take that same
>    toolchain file and apply it to other projects, so you get better
>    reusability. This would be what I would use personally, but I'm comfortable
>    with toolchain files. The main downside to this approach is that those new
>    to CMake can feel a bit intimidated by toolchain files or see them as an
>    unnecessary complication for a build that should just pick up the default
>    compilers and work out of the box. It's a tradeoff, so I erred on the side
>    of simplicity in the book and didn't make toolchains a recommendation for
>    defining custom configs. If you and your users are happy with toolchain
>    files though, I'd go this route.
>
> Hhhmmm. Looks like I'm off to read about toolchain files. Fingers crossed
its covered in your book!

>
>    - You could set your custom config's cache variables (again I'd
>    recommend setting the ..._<CONFIG>_INIT variables) before the first
>    project() command, but you won't have the compiler ID or any other compiler
>    information at that point. You also won't have any of the default configs'
>    flags available to you, so you can't make your custom config an extension
>    of an existing one (e.g. you can't start with flags from, say,
>    RelWithDebInfo and add a few more to enable profiling like in the book's
>    example). If your project only needs to consider one compiler and the flags
>    are simple and well-defined, this is going to be the easiest for your
>    developers, since it will work out-of-the-box with no extra effort on their
>    part.
>    - Instead of setting cache variables, you could append to regular
>    non-cache ...<CONFIG> variables after the project() command. This has the
>    advantage of being simple, but your config-specific flags won't show in the
>    cache and developers can't override them without editing the project.
>
> I find cache variables to be a large enough source of confusion as it is.
In my experience users rarely use ccmake or cmake-gui. They tend to give up
if they can't pass a few flags on the command line and achieve the desired
results. And if they do, and see one value of the variable, yet the project
seems to use a different value caused by shadowing the cache variable with
a regular variable, this leads to extreme confusion.

I am aware of `CMAKE_USER_MAKE_RULES_OVERRIDE` and all of the
>> `CMAKE_<LANG|TARGET_TYPE>_FLAGS_<CONFIG>_INIT` variable, but as far as I
>> can tell, there is no access to knowing what compiler is being used, since
>> the compilers have yet to be probed. Is it possible to use the
>> `CMAKE_..._INIT` variables to set per-compiler flags some how?
>>
>
> Use a toolchain file where you are in control of which compiler is
> selected. Use different toolchain files for different compilers.
>

I actually just seem to have had some success through the combination of
`CMAKE_..._INIT` variables passed in `CMAKE_USER_MAKE_RULES_OVERRIDE` when
used in combination with setting the corresponding CACHE variables in the
CMakeLists.txt. Now, this may not be a robust solution because it relies on
some interesting behavior of the `if()` statement used from the
`CMAKE_USER_MAKE_RULES_OVERRIDE` file. In the file specified by
`CMAKE_USER_MAKE_RULES_OVERRIDE`  includes `if()` statements of the form
`if("${CMAKE_C_COMPILER_ID}" STREQUAL "GNU")` then the following code block
is never actually executed. However, if one uses `if(CMAKE_C_COMPILER_ID
STREQUAL "GNU")` it seems that the evaluation of the `if()` statement is
deferred until a point in which the value of `CMAKE_C_COMPILER` has already
been set.

It sounds like ToolChain files are probably a better solution here,
however, It's nice that this quick and dirty trick seems to work (at least
during my testing with 3.10 on Linux....). Does anyone have any insight
into if this will be a robust and reliable approach?

This didn't work for me, it showed the same problem you described in your
> original issue. Perhaps you forgot to clear out a previous build first? It
> would only work after an initial run of CMake where you didn't set
> CMAKE_BUILD_TYPE or you set it to something else.
>

Yes,  That is probably true. This config is normally just used under CI
testing in which we loop through the build configs, starting with standard
pre-defined ones. So the cache is probably set by a previous call with
CMAKE_BUILD_TYPE set to Debug or something.

Thanks for the swift response and the great book!

-Zaak
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://cmake.org/pipermail/cmake/attachments/20181210/d18d292e/attachment.html>