That isn't a valid reason, given the existence of Solaris, HP-UX, DG/UX, Tru64, NeXTSTEP, and so many other UNIXes that grew beyond AT&T UNIX System V.

All of them provide C APIs to their additional features not covered by POSIX.

What Linux has is that due to the way syscalls are exposed there is a certain laziness to cover everything on glibc, or its replacements like musl.

I think rather than "laziness" I would say it's an instance of the widespread phenomenon of "shipping the orgchart". Because for Linux the kernel developers and the libc developers are separate communities, the boundary between those components becomes more meaningful, more visible to the end-user, and more likely to have lag where one side supports something and the other doesn't yet. (That goes both ways, incidentally -- the handling of POSIX threads was driven more from the libc side of the fence and it took a while before the kernel provided enough facilities to make it cleanly doable, and there are still corners like setuid() where there is a mismatch between what the standard wants and the primitives the kernel provides). Where an OS has a more tightly integrated development team the kernel/libc boundary is more likely to stay an internal one.

This description matches my own experience. E.g., I recall having to use my own macro-based syscall() things when the inotify system was first introduced because glibc did not have support for years and then it was years more for slow moving Linux distros to pick up the new glibc version.

Unsaid was that much of this project separation comes from glibc being born as (and probably still being) a "portable libc with extra GNU-ish features", not a Linux-specific thing.

Honestly, some of this pain might have been avoided had the Bell Labs guys made two libraries - the syscall interface part of `libc`, called say `libos`, and the more articulated language run-time (string/buffered IO/etc./etc) the actual `libc`. Then the kernel could "easily" ship with libos and libc's could vary. To even realize this might be helpful someday likely required foresight beyond reason in the mid-1970s. Then, afterwards, Makefile's and other build system stuff probably wanted to stay with "-lc" in various places and then glibc/others wanted to support that and so it goes. Integration can be hard to un-do.

IIRC it’s not just laziness, there are things glibc explicitly doesn’t want to expose for various reasons, and since the two projects are essentially unrelated you get the intersection of what both sides are happy with.

Traditional unices develop the kernel and the libc together, as a system, so any kernel feature they want to expose they can just do so.

You are right, but I did not comment on what might be desirable, but on what is the current status.

Because I do not like certain decisions in the design of glibc, I am skeptical about their ability do define good standard APIs for the more recent syscalls, so perhaps it is better that they did not attempt to do this.