[etherlab-users] how to get slave count from userspace ?

[Gavin Lambert]

On 26 July 2014, quoth Sebastien Blanchet:
> I would like to know which is the right way to get the number of slaves
> on the EtherCAT bus in a usespace program.
> 
> I have tried 3 ways:
> 
> // way #1:
>      ec_master_info_t masterInfo;
>      ec_master_t* master = ecrt_request_master(0);
>      ec_master( master, &masterInfo );
> 
>      // the answer should be in masterInfo.slave_count
>      // but it does not work because the compiler says
>      //"error: invalid use of incomplete type 'struct ec_master'"

That's because the API call is "ecrt_master", not "ec_master".  If you
correct that, then this should work, but it's not the most efficient method.

> // way #2
>      ec_master_state_t state;
>      ec_master_t* master = ecrt_request_master(0);
>      ecrt_master_state( master, &state );
> 
>      // it seems to work,
>      // the answer is in state.slaves_responding

This is the correct method.

> // way #3  (very ugly)
>      ec_slave_info_t dummySlaveInfo;
>      int slave_count = 0;
>      ec_master_t* master = ecrt_request_master(0);
> 
>      while (!ecrt_master_get_slave( master, slave_count,
> &dummySlaveInfo)) {
> 	slave_count++;
>      }

Don't do this (unless you actually want to keep each slave's info).
Remember that if you're using the usermode lib, each time you call a master
API you are making a user->kernel->user transition, which will cause a
performance hit.  While it doesn't really matter at this point (since you're
still pre-realtime) making excessive API calls is not a good habit to get
into.