Fixing C++ compilation bugs for the MacOS Catalina upgrade

I recently updated to macOS Catalina and encountered several issues with programs that use a C/C++ compiler.
If you see any of these errors this post might be helpful for you:

/usr/local/Cellar/llvm/7.0.1/include/c++/v1/wchar.h:119:15: fatal error: 'wchar.h' file not found #include_next ^~~~~~~~~ 1 error generated.

Or any error indicating that a standard library couldn't be found:

ld: library not found for -ldl clang-10: error: linker command failed with exit code 1 (use -v to see invocation)

What's the issue?

The issue is that macOS Catalina doesn't use the /usr/include and /usr/lib/ directories anymore for all C++ headers and libraries.
A lot of projects rely on these directories to exist, can't find the files and then break.
Instead they are under the xcode path and you need to tell the C/C++ compiler where to find these libraries now.

How to fix it?

Optionally, install a C++ compiler like clang / gcc through brew if one is missing.

Install Xcode through the App store to get access to the developer command-line tools.
This will install the C++ header files and libraries, albeit, in a different directory. They will be installed in /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk or a similar directory. To get the correct path for your system you can run the xcrun --show-sdk-path command.

The C++ compiler needs to be told to look at this root path instead of /usr/include. This can be done by setting the CPLUS_INCLUDE_PATH env variable. It's important to also include LLVM's normal include path because this is where it should search first.

# adjust your llvm and CLT include paths to match your setup
export CPLUS_INCLUDE_PATH=/usr/local/opt/llvm/include/c++/v1:/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include

This should fix any fatal error: 'xxx.h' file not found errors.

You might still run into linker errors like ld: library not found for -lxxx. The linker also needs to be told to look for libraries in the CommandLineTools/Xcode paths by setting the LIBRARY_PATH env variable.

export LIBRARY_PATH=$LIBRARY_PATH:/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/lib

Make sure to add these export statements to your .bash_rc, .zshrc, .bash_profile, or whatever shell you use, to make these adjusted environment variables available in all terminals.

Resources

Thanks a lot to this GitHub post for helping me figure out what's wrong.