Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
In UNIX V7 the format was changed: here the second character was the argument to getty(8) indicating the sequence of line speeds to try ('0' was: cycle through 300-1200-150-110 baud; '4' was for the on-line console DECwriter) while the rest of the line contained the name of the tty. Thus a typical line was "14console".
The glibc2 implementation of this function reads the file _PATH_TTYS, defined in <ttyent.h> as "/etc/ttys". It returns 0 on error. Since Linux systems do not usually have "/etc/ttys", it will always return 0.