build Qt project with openmp on macOS

Question

I have a qmake project developed on windows that uses openmp and I want to build it on macOS. I followed this instruction How to compile with OpenMP in Qt on macOS?

adding the following to my .pro file:

win32: {
    QMAKE_CXXFLAGS+= -fopenmp
    QMAKE_LFLAGS += -fopenmp
}

unix:!macx {
    QMAKE_CXXFLAGS+= -fopenmp
    QMAKE_LFLAGS += -fopenmp
}

macx: {
    QMAKE_CXXFLAGS += -Xpreprocessor -fopenmp -lomp -I/usr/local/include
    QMAKE_LFLAGS += -lomp
    LIBS += -L /usr/local/lib /usr/local/lib/libomp.dylib
}

But I'm getting this error.

Compiler feature detection failure!
The command "/usr/bin/clang++ -stdlib=libc++ -fopenmp -std=gnu++11 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -mmacosx-version-min=10.13 -fPIC -x c++ -E -v - -target x86_64-apple-darwin19.4.0" terminated with exit code 1.
Apple clang version 11.0.3 (clang-1103.0.32.62)
Target: x86_64-apple-darwin19.4.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
clang: error: unsupported option '-fopenmp'

As far as I understand this message I have something missing on my system but I can't figure out what

score 2 · Answer 1 · answered Sep 21 '20 at 09:34

I have found the reason.

Qt creator looks for flags in QMAKE_CXXFLAGS and if -fopenmp is found it automatically uses it when performing the compiler feature detection, which fails because it's missing the
-Xpreprocessor part.

The trick is to add the flags as a whole string, like this:

QMAKE_CXXFLAGS *= "-Xpreprocessor -fopenmp"

score 1 · Answer 2 · answered May 27 '20 at 09:51

I don't think there's anything missing. I met this notice after updating Xcode and QtCreator some time ago. Someone reports that simply Xcode no longer supports OpenMP.. which is possible even if I don't know why. Doing a quick test, with a check on basic features like omp_get/set_max_threads and omp parallel, I see that OpenMP is working properly anyway. The only real drawback, however, is that QtCreator can no longer find the standard template library includes. So for example #include is not resolved correctly and autocompletion doesn't work. I don't know why, but I checked it out on two different macs. Currently I'm using Clang 10 (installed by HomeBrew) to bypass this limit.

I'm using Qt Creator on Win7 SP1 x64, and Clang was going clang and crashing on me too. I just installed Qt Creator within the last week, and I got vers. 8. I installed vers. 10 into the same dir, which I had already cleaned out, and clang quit crashing every time I started Qt Creator. — CodeLurker, Jul 08 '20 at 20:25

build Qt project with openmp on macOS

2 Answers2