Why PG_SETMASK() change errno to 0 when there is an elog() call before or after it? Is it a bug?

In founction ServerLoop() in code postmaster.c, after select(), we called
PG_SETMASK(). If l add an elog() after PG_SETMASK(), I see errno is reset
to 0 when PG_SETMASK() call suceeded. But the immediately code will
actually check errno from the select() call which now has been overwritten
by PG_SETMASK().

Can someone share some light on this?

Thanks
Congnan