pipe.7: Document FIONREAD

Reported-by: Patrick McLean <patrickm@gaikai.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/pipe.7

@@ -143,6 +143,21 @@ operations.
 See
 .BR fcntl (2)
 for more information.
+
+The following
+.BR ioctl (2)
+operation, which can be applied to a file descriptor
+that refers to either end of a pipe,
+places a count of the number of unread bytes in the pipe in the
+.I int
+buffer pointed to by the final argument of the call:
+
+    ioctl(fd, FIONREAD, &nbytes);
+
+The
+.B FIONREAD
+operation is not specified in any standard,
+but is provided on many implementations.
 .\"
 .SS PIPE_BUF
 POSIX.1 says that