diff --git a/man/libgps.xml b/man/libgps.xml
index 7cb30dee..4dd68e4c 100644
--- a/man/libgps.xml
+++ b/man/libgps.xml
@@ -156,7 +156,15 @@ either a nonblocking read for data from the daemon or a fetch from
 shared memory; it returns a count of bytes read for success, -1 with
 errno set on a Unix-level read error, -1 with errno not set if the
 socket to the daemon has closed or if the shared-memory segment was
-unavailable, and 0 if no data is available.</para>
+unavailable, and 0 if no data is available. This function can deliver
+the messages from gpsd to your application to a buffer you provide. If
+you do not want the messages, provide NULL and 0 for
+<parameter>message</parameter> and <parameter>message_size</parameter>
+respectively. If you do want them, make <parameter>message</parameter>
+at least 4096 bytes, and provide that size as
+<parameter>message_size</parameter>. If
+<parameter>message_size</parameter> is too small, libgps will truncate
+the data.</para>
 
 <para><function>gps_waiting()</function> can be used to check whether
 there is new data from the daemon. The second argument is the maximum