[CMake] Toolchains and CMAKE_FORCE_C_COMPILER in 3.5

[Tristram, Falk]

Hi,

this is a nasty problem, preventing suitable compiler detection.
I can confirm it with older versions of CMake, e.g. 3.2.x. Did not recognize it has been fixed in the meanwhile.
If anyone has a suitable solution or patched CMake version, I could help testing it.

Regards,
Falk

From: CMake [mailto:cmake-bounces at cmake.org] On Behalf Of Thibault Genessay
Sent: Dienstag, 9. Februar 2016 15:10
To: cmake at cmake.org
Subject: [CMake] Toolchains and CMAKE_FORCE_C_COMPILER in 3.5

Hi,

I'm a long time user of CMake (and huge fan of it) but newly registered on this list, so please forgive me if I have missed something regarding the new 3.5 series.
I use CMake with the GCC ARM Embedded compiler (https://launchpad.net/gcc-arm-embedded<https://urldefense.proofpoint.com/v2/url?u=https-3A__launchpad.net_gcc-2Darm-2Dembedded&d=CwMFaQ&c=VCWpAYkS3z1bOCIxc-BPGZarCq9MRCAVxZJE051VqH8&r=Ee2S2RR_IcVA2riuzsDuYo9W0-YuR65ZmnX9oJ70u5c&m=Ly4Lf_QNoPZw7NLyJgPtadlPnK3hYtbHWzoTFvG3t2k&s=YFTH4j_hy43hY-cPXn1-edRIyxes7EcC5rXO6mAT-Eo&e=>) to cross-compile my project (Windows host, STM32 target, MinGW binutils). Like many users (all?) I had to create a toolchain file because CMake doesn't support this compiler/setup out of the box. I have taken inspiration from the many Google hits and had a working setup until 3.5. This setup relied on the CMAKE_FORCE_<lang>_COMPILER macros.
I installed 3.5 and the build broke, CMake complaining that this macro was deprecated and should not be used because of the many improvements on the compiler detection side. OK, so I removed the offending lines, simply setting the CMAKE_C_COMPILER variable to the compiler's executable ... and suddenly I remember why I did use this FORCE hack in the first place: CMake tells me my compiler is broken because it cannot compile a test program (undefined symbol: _exit). This is somehow correct, as I compile for bare metal and I am providing the _exit function in my code. This is a misunderstanding the CMAKE_FORCE_C_COMPILER works around by simply saying "believe me, this compiler works".
Now I am left without many options: I can't use the FORCE hack, and I can't convince CMake my compiler is OK. I am very open to redesign my toolchain file, or use a stock one from CMake, or whatever this takes, but I simply do not know how to proceed. Actually, I feel my current toolchain file, although functional, is not "clean" but never had enough time to do it. Now that it's a blocking issue, I do have time :)
I rolled back to 3.4 and things work so I'm safe for now. But I would be very glad if somebody knew more about this specific problem, or this compiler, or toolchain file writing in general. I would love to write a "stock" toolchain file for this compiler but I simply do not know enough (nor I can find enough up-to-date documentation).
Thanks for reading,
Cheers
Thibault



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