[aaronmdjones]

`errno` is a userland concept; the kernel returns negative error numbers that libc then turns into -1 and sets errno. Thus the correct manpage is errno(3).

[eqvinox]

OpenBSD up to 5.9 had errno(2) symlinked to intro(2), describing error codes:

https://man.freebsd.org/cgi/man.cgi?query=errno&apropos=0&se...

Also, your statements about the kernel and libc are OS specific.

[WhyNotHugo]

Why does libc do this instead of simply returning that same negative number?

[matheusmoreira]

Legacy. It's always been this way and it can't change without breaking everything.

[rcxdude]

POSIX, basically. It was already a convention by the time linux/glibc implemented it.