Issue with linker for Mac OS

I have recently started using ada for a lab in my systems programming class and I can say it is really interesting.

I tried most of it at the computers labs for testing and I decided to install the compiler and its linking tools in a MacBook.

Altough the installation is done and there seems to be no problem, when I tried to print the classic hello world program I am getting this message:

gprbuild greet.adb                                                                                                                                                                                                            
using project file /Users/xyderos/opt/GNAT/2020/share/gpr/_default.gpr
Bind
[gprbind]      greet.bexch
[Ada]          greet.ali
Link
[link]         greet.adb
ld: library not found for -lSystem
collect2: error: ld returned 1 exit status
gprbuild: link of greet.adb failed
gprbuild: failed command was: /users/name/opt/gnat/2020/bin/gcc greet.o b__greet.o - L/Users/name/ada/ -L/Users/name/ada// -L/users/name/opt/gnat/2020/lib/gcc/x86_64-apple-darwin17.7.0/8.4.1/adalib/ /users/name/opt/gnat/2020/lib/gcc/x86_64-apple-darwin17.7.0/8.4.1/adalib/libgnat.a -Wl,-rpath,@executable_path/ -Wl,-rpath,@executable_path/..//opt/gnat/2020/lib/gcc/x86_64-apple-darwin17.7.0/8.4.1/adalib -o greet

I am getting really confused about the lSystem library.

It seems that the library doesn't exist on Mac(maybe I need to install Xcode?).

Does anyone have a clue about that issue or is there a probably better way to install it via brew?

Thanks a lot in advance!

Solution

On my MacOS 10.14, the following worked for me, without having to download the massive blob that is Xcode, which I had neither the bandwidth nor the diskspace for. I don't know if this still works on MacOS 10.15 (not planning to upgrade).

Most of this info was from a blog post by Simon Wright: https://forward-in-code.blogspot.com/2019/06/macos-software-development-kit-changes.html

You only need the Xcode command line tools only (not Xcode): xcode-select --install
Now unpack the included headers: sudo installer -pkg /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg -target /
And tell GNAT where to find them: mv ce2109-specs /usr/local/opt/GNAT/2019/lib/gcc/x86_64-apple-darwin17.7.0/8.3.1/specs

The ce2109-specs file is from the blog post above. This did the job for me and GNAT can now link working binaries on MacOS 10.14.

If you're on 10.15 and it doesn't work (because there's no more Xcode command line tools or they're missing the headers package), you will unfortunately have to install Xcode as per the GNAT Readme.