select.2: Show correspondence between select() and poll() readiness notifications

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/select.2

@@ -469,6 +469,30 @@ when
 .B _GNU_SOURCE
 is defined.
 Since glibc 2.2.2, the requirements are as shown in the SYNOPSIS.
+.\"
+.SS Correspondence between select() and poll() notifications
+Within the Linux kernel source,
+.\" fs/select.c
+we find the following definitions which show the correspondence
+between the readable, writable, and exceptional condition notifications of
+.BR select ()
+and the event notifications provided by
+.BR poll (2)
+(and
+.BR epoll (7)):
+
+.nf
+.in +4n
+#define POLLIN_SET (POLLRDNORM | POLLRDBAND | POLLIN | POLLHUP |
+                    POLLERR)
+                   /* Ready for reading */
+#define POLLOUT_SET (POLLWRBAND | POLLWRNORM | POLLOUT | POLLERR)
+                   /* Ready for writing */
+#define POLLEX_SET (POLLPRI)
+                   /* Exceptional condition */
+.in
+.fi
+.\"
 .SS Multithreaded applications
 If a file descriptor being monitored by
 .BR select ()