+/* VAL_O_NONBLOCK:
+ * This symbol is to be used during open() or fcntl(F_SETFL) to turn on
+ * non-blocking I/O for the file descriptor. Note that there is no way
+ * back, i.e. you cannot turn it blocking again this way. If you wish to
+ * alternatively switch between blocking and non-blocking, use the
+ * ioctl(FIOSNBIO) call instead, but that is not supported by all devices.
+ */
+/* VAL_EAGAIN:
+ * This symbol holds the errno error code set by read() when no data was
+ * present on the non-blocking file descriptor.
+ */
+/* RD_NODATA:
+ * This symbol holds the return code from read() when no data is present
+ * on the non-blocking file descriptor. Be careful! If EOF_NONBLOCK is
+ * not defined, then you can't distinguish between no data and EOF by
+ * issuing a read(). You'll have to find another way to tell for sure!
+ */
+/* EOF_NONBLOCK:
+ * This symbol, if defined, indicates to the C program that a read() on
+ * a non-blocking file descriptor will return 0 on EOF, and not the value
+ * held in RD_NODATA (-1 usually, in that case!).
+ */
+#define VAL_O_NONBLOCK O_NONBLOCK
+#define VAL_EAGAIN EAGAIN
+#define RD_NODATA -1
+#define EOF_NONBLOCK
+