Starting the SNMP Agent

A routine to instantiate and start the SNMP agent thread in the default configuration is provided in PACKAGES/net/snmp/agent/VERSION/src/snmptask.c

It starts the snmpd thread at priority CYGPKG_NET_THREAD_PRIORITY+1 by default, ie. one step less important than the TCP/IP stack service thread. It also statically creates and uses a very large stack of around 100 KiloBytes. To use that convenience function, this code fragment may be copied (in plain C).

#ifdef CYGPKG_SNMPAGENT
{
                extern void cyg_net_snmp_init(void);
                cyg_net_snmp_init();
            
}
#endif

In case you need to perform initialization, for example setting up SNMPv3 security features, when the snmp agent starts and every time it restarts, you can register a callback function by simply writing the global variable:

externC void (*snmpd_reinit_function)( void );
with a suitable function pointer.

The entry point to the SNMP agent is

externC void snmpd( void (*initfunc)( void ) );
so you can of course easily start it in a thread of your choice at another priority instead if required, after performing whatever other initialization your SNMP MIBs need. A larger than default stacksize is required. The initfunc parameter is the callback function mentioned above — a NULL parameter there is safe and obviously means no callback is registered.

Note that if you call snmpd(); yourself and do not call cyg_net_snmp_init(); then that routine, global variable, and the default large stack will not be used. This is the recommended way control such features from your application; create and start the thread yourself at the appropriate moment.

Other APIs from the snmpd module are available, specifically:

void SnmpdShutDown(int a);
which causes the snmpd to restart itself — including the callback to your init function — as soon as possible.

The parameter a is ignored. It is there because in snmpd's “natural environment” this routine is a UNIX signal handler.

The helper functions in the network stack for managing DHCP leases will call SnmpdShutDown() when necessary, for example if network interfaces go down and/or come up again.

2017-02-09
Documentation license for this page: Open Publication License