But do real apps actually use them directly? Or only through the userland libraries?
Just FYI, the https://www.darlinghq.org project is doing MacOS app compat on Linux - i.e. building the userland libraries.
Yes, real apps use Mach messaging directly. i.e.: https://source.chromium.org/chromium/chromium/src/+/main:ipc...
Yes, real apps use Mach messaging directly. i.e.: https://source.chromium.org/chromium/chromium/src/+/main:ipc...