Hi,

On Mon, Oct 12, 2015 at 10:39 AM, Ren=C3=A9 J. V. <= rjvbertin@gmail.co= m> wrote:

Hello,

I'm using cmake in conjunction with a packaging/distribution system tha= t aims to
control the compiler and linker flags, a priori via the usual environment variables. (We're talking about MacPorts.)

Using one of the CMAKE_BUILD_TYPE presets, the value of those env. variable= s
appears at most to be added in front of the preset options, which means use= r
options can be overridden. That may be intended behaviour, but not ideal fo= r my
use case.

Working with Debian and Ubuntu systems, I deduced that using a non-pre-defi= ned
BUILD_TYPE make cmake use the values of CFLAGS, CXXFLAGS etc, through
CMAKE_C_FLAGS, CMAKE_CXX_FLAGS etc (instead of CMAKE_C*_FLAGS_RELEASE, for<= br> instance).

Experimenting with -DCMAKE_BUILD_TYPE=3DMacPorts in the toplevel control fi= le
(cmake PortGroup), that appeared indeed to work, but I appear to have been<= br> mistaken. Adding -DCMAKE_C*_FLAGS_MACPORTS definitions has no effect, nor h= as
setting CMAKE_C*_FLAGS from the CMake command line.

Which leads me to the following questions:
- Is it indeed possible to get CMake to take all compiler and linker flags = from
the environment, and if so, how?

- If not, what is the best/official way to get exact control over the compi= ler
and linker options used?

No this is no= t possible in general. A CMakeLists.txt file can always just set their own = compiler/linker flags.

However if I understand you= correctly (in what kind of flags you want to change), then this could be d= oable with a toolchain file. These are usually used to teach CMake speciali= ties about compilers/linkers that it does not support itself and behave suf= ficiently different from one it does know. That is for example gcc builds f= or doing cross-compilation to some specialized hardware may not work with c= ertain flags CMake uses by default for gcc builds.

The toolchain files are just cmake scripts, you can see some examples in t= he Modules/Platform directory of your cmake install. They set certain speci= al variables that CMake will read out again when creating compiler/linker c= ommandlines. The variables are explained in the documentation of CMake IIRC= .

=C2=A0

- Out of curiosity, what's special about the CMAKE_BUILD_TYPE=3DDebian = build type?

There's no such build t= ype in CMake, see the Compilers and Tools section here:=C2=A0https://cmake.o= rg/Wiki/CMake_Useful_Variables#Various_Options that details the built-i= n types in CMake.

Andreas