How can you tell whether an fd refers to inside or the outside of a pseudoterminal?
According to ptsname manual:
char *ptsname(int fd);The
ptsname()function returns the name of the slave pseudoterminal device corresponding to the master referred to byfd.
And it will fail with ENOTTY if
fddoes not refer to a pseudoterminal master device.
So you can first call isatty() and then ptsname().
And ptsname() is
CONFORMING TO POSIX.1-2001, POSIX.1-2008.