[klodolph]

Well…

I do not know details here. But there is, in fact, a separate “proprietary” API for interacting with files on macOS, provided for backwards compatibility. This API exists because the POSIX file functions do not provide similar enough functionality to the earlier API. This API is called the File Manager. Documentation is hard to find on the Apple website, but you can find some fairly extensive documentation for it in the old Inside Macintosh series (and sometimes you can dig HTML or PDF versions of this on the Apple website, but whether this is possible, and the exact location, seems to change every couple years).

The way you would specify a file on old versions of macOS used a length-delimited string for the filename, and referred to directories by 32-bit IDs. This was packed into an FSSpec structure which was passed to most functions which operated on files. Being length-delimited, it is possible to pass a NUL byte.

From what I understand, the macOS kernel exposed some additional entry points for these functions to operate correctly.

[opless]

Pre-posix macOS (previously known as ‘system N’ where N was the version number) did indeed use pascal calling conventions, so the posters assertions aren’t unreasonable.