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Root compilation with G++ on MacOS


I need to compile ROOT with gcc/g++. I am having difficulty with the objective C libraries.

If I link a true g++ (installed with brew) to the command gg++ and the same for gcc -> ggcc - I can start to compile with

cmake ../ -DCMAKE_CXX_COMPILER=gg++ -DCMAKE_C_COMPILER=ggcc <the rest of my options>

The compilation gets about 70% done before it gives me an error that “Option ‘-O’ takes an integer argument”. I tracked this down. In ‘root/core/macosx/CMakeLists.txt’, there is a line

set_source_files_properties(${sources} COMPILE_FLAGS "-ObjC++ -std=c++11")

So g++ is not recognizing the Objective C flag. I’ve changed this line to

set_source_files_properties(${sources} COMPILE_FLAGS "-x objective-c++-header -lobjc -std=c++11")

Which is the g++ equivalent. I’ve also tried ‘-x objective-c++’. This attempt fails at around 70% with hundreds of errors. It basically looks like the compiler does not understand the Objective C++.

Has anyone had success in compiling ROOT from source with g++ on OSX (not clang)?

PS - As a side note, I believe ‘g++’ in /usr/bin on OSX is actually a link to clang - verified on my machine by typing ‘g++ --version’. This is why I installed g++ from brew and linked it to gg++.

ROOT Version: Github Master branch, commit # 2762a32
Platform: OSX
Compiler: GCC 8.1.0

Please note that building ROOT on OS X with GCC is not officially supported. There are many problems with this combination. That said, it may be the case that your GCC was not compiled with support for Objective C++ (as support for it is optional in GCC). If your GCC does have support for Objective C++, however, you may be able to compile ROOT. You may want to make sure that you are not using the wrong compiler by setting -DCMAKE_C_COMPILER=... -DCMAKE_CXX_COMPILER=... with full paths to the compilers you want to use.

1 Like

Thanks for the info. I think my best bet is to use ROOT compiled with clang and fix my downstream problems another way. Cheers!

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