[arch-dev-public] RFC: Dropping -DCMAKE_BUILD_TYPE from packages using cmake

Eli Schwartz eschwartz at archlinux.org
Sun Mar 11 20:58:16 UTC 2018

On 03/11/2018 05:00 AM, Antonio Rojas via arch-dev-public wrote:
> This is very poorly documented, so you have to dig into the cmake
> code to figure it out. The default build type is None, which means
> CMAKE_C(XX)_FLAGS is used (see
> /usr/share/cmake-3.10/Modules/CMakeCInformation.cmake:117), whose
> value is taken from the environment C(XX)FLAGS (see
> /usr/share/cmake-3.10/Modules/CMakeCXXInformation.cmake:198). So yes,
> not setting CMAKE_BUILD_TYPE will simply use the build flags from
> C(XX)FLAGS. If you want to test yourself, run cmake with the -LAH
> flag and check the output for CMAKE_C(XX)_FLAGS.

Oh, huh, nice. This seems to work. I wonder why whenever I googled for
information about this it seemed to imply the default buildtype was a
debug build... oh wait, this is CMake we are talking about, google may
be the *only* source of information but it still isn't a *good* source. :p

>> For example, I am currently the maintainer of a package that has, 
>> apparently, historically used:
>> \
> The first two lines should not be necessary (see above)

But I still think this is "necessary". That CMake module mentions

> # use _INIT variables so that this only happens the first time

I've double-checked, and if you don't explicitly set the CFLAGS and
CXXFLAGS then CMake unconditionally reuses the cache, which is ironic
because CMake also contains a touted feature where changing the CFLAGS
causes any object files that depend on those flags, to be properly
rebuilt. Except they don't get rebuilt because they don't even get

I know repository PKGBUILDs are typically built in a clean chroot, so
cached builds make no difference there, but then neither do unquoted
srcdir/pkgdir. It is still not helpful to people who e.g. check out a
PKGBUILD from svntogit and rebuild it locally for any of a number of
reasons to have unpredictable behavior due to assumptions about always
building in a clean chroot. So I think maybe we should still (or start?)
specifying this.

Eli Schwartz
Bug Wrangler and Trusted User

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