lua/trivial: add some more comments

And remove the unnecessary nothing() function, it was there to test a
crash issue.

1 files changed, 23 insertions, 7 deletions

diff --git a/lua/trivial.lua b/lua/trivial.lua
index abcb5e6..3c775c0 100644
--- a/lua/trivial.lua
+++ b/lua/trivial.lua
@@ -1,23 +1,39 @@
 -- trivial protocol example
 -- declare our protocol
-trivial_proto = Proto("trivial", "Trivial Protocol")
+local trivial_proto = Proto("trivial", "Trivial Protocol")
 
-function dissect_foo(tvb, pinfo, tree)
-    nothing();
-    local subtree = tree:add(trivial_proto, tvb(),"Trivial Protocol Data")
+-- Will be called below in trivial_proto.dissector
+local function dissect_foo(tvb, pinfo, tree)
+    -- Add an additional "layer" (think of IP, TCP, etc.)
+    local subtree = tree:add(trivial_proto, tvb(), "Trivial Protocol Data")
+
+    -- To that layer, add a field that highlights the last two bytes of the
+    -- buffer ("tvb") and add the textual label "Len: " followed by the length
+    -- extracted from the tvb.
     subtree:add(tvb(3,2), "Len: " .. tvb(3,2):uint())
 end
 
-function get_pdu_len(tvb, pinfo, tree)
+-- Will be used in trivial_proto.dissector
+local function get_pdu_len(tvb, pinfo, tree)
+    -- Extract 2 bytes from offset 3 (so the last two bytes of a five-byte
+    -- buffer). This will be the length of the full PDU.
     return tvb(3, 2):uint()
 end
 
 function trivial_proto.dissector(tvb, pinfo, tree)
+    -- Change the "Protocol" column
     pinfo.cols.protocol = "TRIVIAL"
+
+    -- Try to call the "dissect_foo" dissector for each PDU ("message"). The
+    -- PDU is expected to have a header of five bytes and the actual length is
+    -- returned by "get_pdu_len".
     dissect_tcp_pdus(tvb, tree, 5, get_pdu_len, dissect_foo)
 end
 
-
-tcp_table = DissectorTable.get("tcp.port")
+-- Ensure that the dissector is called for TCP port numbers 7777 and 443.
+local tcp_table = DissectorTable.get("tcp.port")
 tcp_table:add(7777, trivial_proto)
 tcp_table:add(443, trivial_proto)
+
+-- For another example, see
+-- https://www.wireshark.org/docs/wsdg_html_chunked/wslua_dissector_example.html