Experimental Physics and
Industrial Control System

Hi

I am unable to determine how to successfully use ca_attach_context, with the creation of multiple threads to read CA channels.

I suspect I have missed a step but do not find any relevant documentation.

This is all pertinent to EPICS R3.14.9

 

Any help would be appreciated.

 

Two scenarios:

1.     Open a connection, via caRepeater, and periodically read the same channel (no problems)

2.     Open a connection, via caRepeater, and periodically launch a posix thread to read the same channel (does not work). Each thread exits after getting a CA channel value.

 

Starting the connection  to caRepeater.

(code snippets are paraphrased somewhat):

 

      static  struct ca_client_context *ca_context = NULL;

 

      stat  = ca_context_create(ca_enable_preemptive_callback);

 

      ca_context = ca_current_context();

 

      stat = ca_add_exception_event( exception_callback, NULL );

 

      stat = ca_add_fd_registration( fdReg, NULL);

 

Now launch threads to obtain channel values, in my failure scenario each thread is accessing the same channel. The following occurs for the first thread.

 

// The following is done for both scenarios.

// Find the info for the specific channel – details not shown, but essentially an element in an array of structures.

 

If ( info->channelId == NULL ) {

   stat = ca_create_channel( ch_name, changeConnection_callback, info, 20, &info->channelId);

  // Do the following as ca_pend_io() returns immediately because a callback was registered with the channel creation.

   while ( ca_stateinfo->channelId) != cs_conn ) {  <a delay with overall timeout> }   // Completes without timeout.

}

 

The following is done for each thread (the non-threaded scenario effectively falls thru).

If ( ca_context == NULL )
  ca_context = ca_current_context();             // This never occurs, as expected, because ca_context was previously setup

else {

  my_context = ca_current_context();

  if ( my_context != ca_context ) {                   // my_context is always nil for each thread and equality for the non=threaded scenario

    stat = ca_attach_context( ca_context );  // This completes without error for each thread

  }

 

The following is done whether or not threads are created (both scenarios):

 

stat = ca_get( info->getType, info->channelId, buff );  // This fails with ECA_DISCONN error status in the threaded scenario only.

 

Even with a 3 second delay after ca_get and then retrying ca_get, it fails with disconnected status, in the threaded scenario.

 

If a thread is not created then the above completes successfully (the ca_attach_context logic does not occur in that scenario).

 

 

Thanks,

Allan