Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Sun, 14 Mar 2004 01:32:18 +0100

Hello Jeff & other CA experts,

The CA client reference manual says: "The user's connection state change 
function will be run immediately from within ca_create_channel if the CA 
client and the server are both hosted within the same address space (within 
the same process)."

(1) From the above follows that if (for whatever reason) the connection 
handler waits for an event that will happen only after ca_create_channel 
completes, the application will be deadlocked. Is this also the case if CA is 
used multi-threaded, i.e. initialized with the ca_enable_preemptive_callback 
flag?

(2) Does a similar rule apply to ca_change_connection_event if previously 
ca_create_channel was called with a NULL connection handler?

Btw, ca_change_connection_event is nowhere mentioned in the new CA client 
reference. I hope it was just forgotten and is not deprecated.

(3) The manual further says: "If a connection state change call back function 
is not installed when ca_create_channel is called (if a nil function pointer 
is supplied) then the application program must wait for successful status 
from ca_pend_io prior to using the channel for the first time."

I have some difficulty infering what "must" and "using" means in this case.

At first, i thought it means that the behavior of CA routines that take a chid 
as argument is undefined if called with the chid returned from 
ca_create_channel (with null connection handler) before ca_pend_io returns 
success. However, this seems to contradict what is said in the description of 
ca_create_channel: "The function ca_state(CHID) can be used to test the 
connection state of a channel. Valid connections may be isolated from invalid 
ones with this function if ca_pend_io() times out." This statement suggests 
that at least ca_state seems to have a defined behavior.

So, what about calls to ca_change_connection_event? (I.e is it possible to 
circumvent the above mentioned deadlock situation by calling 
ca_create_channel without connection handler and afterwards call 
ca_change_connection_event?)

Or am I completely wrong and all that "must" means is that the user should be 
aware that certain CA routines will return an error code, saying that the 
channel is disconnected??? I'm at a loss...

Ben

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System