Experimental Physics and
Industrial Control System

"Kasemir, Kay" <[email protected]> · Wed, 07 Jul 2010 11:10:36 -0400

Hello:

I ran into the following issue upgrading from EPICS R3.14.8.2 to R3.14.11.
In case others see this, here's a description and workaround.

It starts with device support that detects if the underlying I/O hardware is
not present.

For the AO record, this device support write_ao() routine used to
1) Put the record into alarm
   recGblSetSevr(rec, WRITE_ALARM, INVALID_ALARM);
2) return some error code like 1 or 2.

The return code is critical for the following.

According to the record reference manual, it's OK to return 0 or anything
else for errors:
"When finished, write_ao should ... return one the following values:
 0: Success.
 other: Error.
"

So far, that had worked OK. If there's a write error, the record goes into
WRITE/INVALID. In addition, a Channel Access client sees an error like this:

caput MyRecord 1
CA.Client.Exception...............................................
    Warning: "Channel write request failed"
    Context: "op=1, channel= MyRecord , type=DBR_DOUBLE, count=1, ctx="
MyRecord""
    Source File: ../oldChannelNotify.cpp line 159
    Current Time: Wed Jul 07 2010 08:53:52.816914000

After switching from R3.14.8.2 to R3.14.11, however, sequencer code that
writes to such channels would crash:

0x6474e0 (name_of_my_sequence): Unhandled C++ exception resulted in call to
terminate
Thread name_of_my_sequence (0x6474e0) suspended

This is on vxWorks 6.7 on an MVME5500. Stack trace:
>tt 0x6474e0
0x00137398 vxTaskEntry  +0x48 : 0x014ef464 ()
0x014ef4d0 epicsThreadCreate+0x1e4: sequencer ()
0x014284f4 sequencer    +0x19c: 0x0142817c ()
0x0142817c ss_entry     +0x498: 0x013ff814 ()
0x013ff8d4 name_of_my_sequenceRegistrar+0x138: seq_pvPut ()
0x01425900 seq_pvPut    +0xb0 : 0x0142b54c ()
0x0142b598 pvVarPutNoBlock+0x50 : caVariable::putNoBlock(pvType, int,
pvValue *) ()
0x0142e100 caVariable::putNoBlock(pvType, int, pvValue *)+0x104:
ca_array_put (0)
0x014a5cb8 ca_array_put +0xa4 : dbChannelIO::write(epicsGuard<epicsMutex> &,
unsigned int, unsigned long, const void  ()
0x014851c8 dbChannelIO::write(epicsGuard<epicsMutex> &, unsigned int,
unsigned long, const void +0x1cc: __cxa_throw ()
0x00126ac0 __cxa_throw  +0x70 : std::terminate() ()
0x0012699c __cxxabiv1::__unexpected(void (*)())+0x0  :
__cxxabiv1::__terminate(void (*)()) ()
0x00126968 __cxxabiv1::__terminate(void (*)())+0x18 : cplusTerminate() ()
0x00145e70 cplusTerminate()+0x50 : taskSuspend ()

As far as I could track this down in the source code, the sequencer's
caVariable::putNoBlock calls ca_array_put (pv/pvCa.cc line 292).

ca_array_put calls pChan->io.write (ca/oldChannelNotify.cpp line 487).

That in turn calls the database and finally my write_ao() routine.

Something must throw an exception in response to the non-zero return code of
my write_ao. I don't understand why that causes a problem, because
ca_array_put catches all sorts of exceptions, including a "catch ( ... )" in
ca/oldChannelNotify.cpp line 522.

Still, the sequencer crashes with an unhandled C++ exception as long as the
write_ao of the record's device support returns a non-zero status code.

I stopped trying to debug it, and instead changed my device support to
always return 0 for OK, because the WRITE/INVALID status/severity already
reports the underlying I/O hardware problem to some extend.

Thanks,
Kay

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System