-/* Flags for representing mode (see fopen()). */
-#define _PDCLIB_FREAD 1
-#define _PDCLIB_FWRITE 2
-#define _PDCLIB_FAPPEND 4
-#define _PDCLIB_FRW 8
-#define _PDCLIB_FBIN 16
+/* Flags for representing mode (see fopen()). Note these must fit the same
+ status field as the _IO?BF flags in <stdio.h> and the internal flags below.
+*/
+#define _PDCLIB_FREAD 8u
+#define _PDCLIB_FWRITE 16u
+#define _PDCLIB_FAPPEND 32u
+#define _PDCLIB_FRW 64u
+#define _PDCLIB_FBIN 128u
+
+/* Internal flags, made to fit the same status field as the flags above. */
+#define _PDCLIB_WROTELAST 256u
+#define _PDCLIB_LIBBUFFER 512u
+#define _PDCLIB_VIRGINSTR 1024u
+#define _PDCLIB_ERRORFLAG 2048u
+#define _PDCLIB_EOFFLAG 4096u