Under IRIX 4.0.x, programs which access files served by NIS must be
  linked with -lsun. The functions in libc.a which deal with these files
  are not NIS-aware; the NIS versions are in libsun.a. Using libsun.a
  usually means adding '-lsun' to the variable LIBS or LDFLAGS in one's
  Makefile. If '-lc' or '-lc_s' (libc_s.a is the shared version of
  libc.a) is given explicitly, '-lsun' must precede it.

  Under IRIX 5.x, libc.so is NIS-aware and -lsun is unnecessary.
  However, linking with -lnsl and/or -lsocket will confuse libc.so and
  NIS access won't work. Either remove -lnsl and/or -lsocket or put an
  explicit -lc *before* them in the link line.