Merged own patch for the bug #391 (debugging and comments mostly).
diff --git a/lib/zebra.h b/lib/zebra.h
index 2c7edd9..150aa2c 100644
--- a/lib/zebra.h
+++ b/lib/zebra.h
@@ -454,10 +454,12 @@
 #define ZEBRA_FAMILY_MAX                 3
 
 /* Error codes of zebra. */
+#define ZEBRA_ERR_NOERROR                0
 #define ZEBRA_ERR_RTEXIST               -1
 #define ZEBRA_ERR_RTUNREACH             -2
 #define ZEBRA_ERR_EPERM                 -3
 #define ZEBRA_ERR_RTNOEXIST             -4
+#define ZEBRA_ERR_KERNEL                -5
 
 /* Zebra message flags */
 #define ZEBRA_FLAG_INTERNAL           0x01