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Subject: Re: pend_io
From: "Ernest L. Williams Jr." <ernesto@slac.stanford.edu>
To: "Hill, Jeff" <johill@lanl.gov>
Cc: EPICS core-talk <core-talk@aps.anl.gov>
Date: Tue, 26 Jun 2012 10:20:28 -0700
Hi Jeff,

This thread helped me as well?

Is there a short tutorial that you can add somewhere?

Your analogy also made it very clear. :)



Thanks,
Ernest


On 06/26/2012 10:12 AM, Hill, Jeff wrote:
Hi Steve,

I changed caget to caput :)

The value was not written to my ioc
int main (int argc, char *argv[])
{
         double  data=86;
         chid        mychid;
         SEVCHK(ca_create_channel(argv[1], NULL, NULL, 10,
                           &mychid),"failure");
         SEVCHK(ca_pend_io(10.0),"failure");
         SEVCHK(ca_array_put(DBR_DOUBLE, 1, mychid,&data),"failure");
         SEVCHK(ca_pend_io(10.0),"failure");
}
It's true that ca_pend_io causes an implicit flush to occur (i.e the
equivalent of a call to ca_flush_io).

A call to ca_flush_io is analogous to placing a letter in the mail box.
We know now that the problem of delivery is out of our hands and has been
entrusted to the postal service. So far so good, but the problem in
the above program is that a bomb has been dropped on our local postal
service before the letter can been delivered.

When your program exits from main without calling ca_context_destroy
then the ca client library's send threads are very ungracefully
terminated before they have been given a chance to deliver any
message that they might have in their work queues to TCP. Furthermore,
it is very OS dependent what might happen to any undelivered TCP messages
lingering in the local IP kernel if you exit from main without given the
CA client library an opportunity to close its TCP sockets; we end up
with undefined behavior.

In contrast, if the program calls ca_context_destroy before exiting
from main then the CA client library will gracefully shut down its
worker threads delivering all outstanding messages to the IP kernel
before they exit, and also gracefully close all of its sockets;
now we expect defined behavior.

There is a (very brief) discussion of this issue in the troubleshooting
section of the CA reference manual - which perhaps could be expanded.

The bottom line is if you shouldn’t terminate the local postal service
unless you provide also some additional funding for a close out :-)

Jeff

-----Original Message-----
From: core-talk-bounces@aps.anl.gov [mailto:core-talk-bounces@aps.anl.gov]
On Behalf Of Steve Hunt
Sent: Tuesday, June 26, 2012 7:49 AM
To: core-talk@aps.anl.gov
Subject: pend_io

I have a problem running a trivial (too trivial it seems) ca client
program.

I fact I just generated the ca example using makeBaseApp.

I changed caget to caput :)

The value was not written to my ioc

I am running 3.14.12.2, on linux_x86_64 but the behavior seems the same
in older versions (at least 3.14.9(

I am sending this to core-talk rather than tech-talk as this seems to be
a real ca problem


##########################################################################
#######3
#include<stddef.h>
#include<stdlib.h>
#include<stdio.h>
#include<string.h>

#include<cadef.h>

int main (int argc, char *argv[])
{
         double  data=86;
         chid        mychid;

         SEVCHK(ca_create_channel(argv[1], NULL, NULL, 10,
&mychid),"failure");
         SEVCHK(ca_pend_io(10.0),"failure");

         SEVCHK(ca_array_put(DBR_DOUBLE, 1, mychid,&data),"failure");
         SEVCHK(ca_pend_io(10.0),"failure");
//      SEVCHK(ca_array_get(DBR_DOUBLE, 1, mychid,&data),"failure");
//      SEVCHK(ca_pend_io(10.0),"failure");
//      SEVCHK(ca_flush_io(),"failure");
//      SEVCHK(ca_poll(),"failure");

//      sleep(10);
//      ca_context_destroy();

//      return result;
}


When I un-comment ca_poll, ca_context_destroy, ca_get (plus its
pend_io), or sleep!!!  it works!!!

so  a ca_poll, a ca_context_destroy, or a ca_pend_io AFTER a ca_get,
seems to flush the send buffer - but just ca_pend_io, nor ca_flush_io
seem not to do so.

At least the client example generated from makebaseApp should have a
ca_put added, and made to work without the ca_get, but perhaps this
could be considered a bug.








Replies:
RE: pend_io Hill, Jeff
References:
pend_io Steve Hunt
RE: pend_io Hill, Jeff

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