How to target older SDK than host macOS

[jeroen]

I would like to build some bottles for arm64_big_sur on a M1 machine running arm64_ventura with the big-sur SDK installed.

When I manually compile things I set the following variables to target big-sur:

export SDKROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX11.sdk
export MACOSX_DEPLOYMENT_TARGET=11.0
export OS_VER=20

Is there a way to trick homebrew to target an other SDK version than the host, such that it will download arm64_big_sur bottles for the dependencies, and target this SDK when building bottles?

[Bo98]

Not officially, but the idea of how to do this would be to trick MacOS.full_version (or sw_vers) and then pretty much everything else will work from there. Internally we have a HOMEBREW_FAKE_EL_CAPITAN, but that doesn't extend to arbitrary versions.

When doing anything like this though, I recommend enabling cross-compiling in any configure scripts (e.g. --build=aarch64-apple-darwin22 --host=aarch64-apple-darwin20). That is because some may do runtime tests when cross-compiling is disabled, which will misconfigure your build for your target.

[jeroen]

OK for future reference in case somebody needs this:

I had difficulty getting this to work on ventura, homebrew kept complaining about missing big-sur command line utilities, but it is not possible to install big-sur command line utilities on ventura. Also it is tricky to make homebrew target the correct SDK.

However I found a neat solution:

In ventura, we cannot virtualize big sur, but we can easily virtualize macos 12 (monterey). So we start with a basic monterey vm:

tart clone ghcr.io/cirruslabs/macos-monterey-base:latest monterey
tart run monterey

Now we download and install Command_Line_Tools_for_Xcode_13.dmg from: https://developer.apple.com/xcode/resources/
Note that it has to be exactly v13.0 and not newer. This version can be installed on MacOS 12 (monterey), and if we look in the xcode support table version 13 of the command line utilities target the macOS 11.3 SDK.

So we now have a virtual machine that has (only) the big-sur SDK and command line utilities, but still can be virtualized. So the only thing left to do is what Bo98 says above, either shim sw_vers to say 11.7.10 or manually hack HOMEBREW_MACOS_VERSION in brew.sh and voila, we have a fully working big-sur build environment.