Earlier quoted context omitted.
As you imply, it's not really a standard unless there are multiple implementations. I am in favor of standardizing the behavior of libc and having multiple implementations that are commonly used.
There are many, many implementations of the C standard library. Every OS has one—even Windows. There are also many other POSIX ( UNIX-like ) operating systems such as FreeBSD and OpenBSD which have their own. Redox has a Rust-based C library called Relibc that even works on Linux. There are also other C libraries that work on Linux such as dietLibc and of course Bionic, which is the standard C library on Android. The…
That's incorrect Glibc does follow the relevant standards, you can simply restrict yourself to only use POSIX and C11, it's just that program authors find the glibc-extensions convenient enough to use them over just the bare standard.
That said, some of the other libraries don't even claim to support the relevant standards, e.g. Bionic is not even fully POSIX compliant. So an application that assumes MUSL (which btw, also implements glibc, BSD and Linux extensions), can't use Bionic as a drop in replacement (not sure about Relibc).