Experimental Physics and
Industrial Control System

Marty Kraimer <[email protected]> · Wed, 09 Dec 1998 08:08:15 -0600

Till Straumann, who is working on ioc support for a beam line at BESSY, reported
the following problem.

He wrote device support for an ao record. The device support returns an error
status if the write_ao to the device fails. If a CA client calls ca_put_callback
for this record and write_ao returns an error, the CA event task goes into an
infinite loop.

Till not only found the bug he also found the cause and the solution.

The following happens: putNotify, if necessary, calls dbProcess. If dbProcess
returns an error (In Till's case device support returned an error to aoRecord
which returned an error to dbProcess) then putNotify tells CA that the put is
complete. But because the record was processed CA was already notified.

This bug is almost certainly in all releases of EPICS that support
ca_put_callback.

This will be fixed in future releases.

Here is the diff showing the bug fix.

cvs diff -c dbNotify.c
Index: dbNotify.c
===================================================================
RCS file: /net/phoebus/epicsmgr/cvsroot/epics/base/src/db/dbNotify.c,v
retrieving revision 1.7
diff -c -r1.7 dbNotify.c
*** dbNotify.c  1998/04/20 16:00:17     1.7
--- dbNotify.c  1998/12/08 16:16:59
***************
*** 206,212 ****
      status=dbProcess(precord);
      if(status!=0) {
        ppn->status = status;
-       issueCallback(ppn);
      }
  ret_pending:
      return(S_db_Pending);

Marty Kraimer

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System