diff --git a/cf/sunos.m4 b/cf/sunos.m4
index d81be70d4..1e2bac6a0 100644
--- a/cf/sunos.m4
+++ b/cf/sunos.m4
@@ -8,7 +8,10 @@ case "$host" in
 *-*-solaris2.7)
 	sunos=57
 	;;
-*-*-solaris2.[[89]] | *-*-solaris2.1[[0-9]])
+*-*-solaris2.1[[0-9]])
+	sunos=511
+	;;
+*-*-solaris2.[[89]])
 	sunos=58
 	;;
 *-*-solaris2*)
@@ -19,4 +22,8 @@ if test "$sunos" != no; then
 	AC_DEFINE_UNQUOTED(SunOS, $sunos, 
 		[Define to what version of SunOS you are running.])
 fi
-])
\ No newline at end of file
+if test "$sunos" = 511; then
+	AC_DEFINE_UNQUOTED(_POSIX_PTHREAD_SEMANTICS, 1,
+		[Define to get the standards-compliant, 5-argument getpwnam_r.])
+fi
+])