On Wed, 23 Jun 2004 19:05:09 +1000 Gary wrote:
GP> As an example, I want to periodically update a variable from my
GP> application, and when there is a GET request for it, supply a scalar
GP> that is the change since the last request. This requires that when a
GP> request is made I remember the value at that time, for the subsequent
GP> call. Maybe there is a role for a cache handler here. I am not sure.
Actually, simply adding a static variable to you handler should work ok.
GP> The lack of questions suggests that there is in fact some clear
GP> documentation somewhere, and I have missed it.
No, it just means that for most people, the simple case works for them.
GP> I will eventually absorb the source code and understand how to use this
GP> facility, but in the meantime I would appreciate any roadmap-like
GP> information that people can supply. The kind of thing I am interested in
GP> is
GP>
GP> - what do all the terms used mean? Examples are 'helper handler', 'mib
GP> handler', 'handler chain', 'access method', 'inject', 'serialize
GP> handler'. This terminology seems well established - is there a
GP> reference?
No, no reference. The closest thing would be the hander overview.
http://www.net-snmp.org/tutorial-5/agent/group__handler.html
Handlers and helpers are really the same thing. And instead of a single hander,
there is a chain of handlers, each of which does some work to make things
easier. A table creates the chain when it registers, either explicity, or by
calling a function which does some of the work. For example, the tcpTable init
looks something like this:
reginfo = netsnmp_create_handler_registration("tcp", tcp_handler,
tcp_oid, OID_LENGTH(tcp_oid), HANDLER_CAN_RONLY);
netsnmp_inject_handler( reginfo,
netsnmp_get_cache_handler(TCP_STATS_CACHE_TIMEOUT,
tcp_load, tcp_free,
tcp_oid, OID_LENGTH(tcp_oid)));
This registers the tcp_handler function, which will create a default chain, and
then injects a cache handler into that chain. These two calls result in a
chain that looks like this:
cache_handler -> bulk_to_next -> table -> table_iterator -> tcpTable
GP> - what is the sequence of processing steps for a GET request and where
GP> do 'handlers' fit into that.
The agent looks up the registration for each OID in an incoming request, then
calls the handler for that registration. That handler looks at the request,
possibly does some work, and decides whether or not to pass the requests down
to lower handlers in it's chain. The last handler in the chain is usually the
user's handler, and it does the work of actually finding the data.
GP> - for each function, what do all its parameters mean, what is its return
GP> value and how does it relate to other function.
I was suprised to find that there isn't a list of functions available. It
appears you have to browse by module. Or check out the file list, where you can
view a file with hyperlinks for functions/structure.
GP> - for each struct, what do all its data members mean?
http://www.net-snmp.org/tutorial-5/agent/annotated.html
--
Robert Story; NET-SNMP Junkie <http://www.net-snmp.org/>
<irc://irc.freenode.net/#net-snmp>
Archive: <http://sourceforge.net/mailarchive/forum.php?forum=net-snmp-coders>
You are lost in a twisty maze of little standards, all different.
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