Experimental Physics and
Industrial Control System

Matthieu Bec <[email protected]> · Fri, 28 Jan 2011 20:38:04 -0300

Hi Patrick,

I would ca_clear_channel first, check what that call returns, and only

then decide to clean pUserPrivate or not.

in ca_disable_preemptive_callback mode, I think it would be safe to free

pUserPrivate, immediately followed by set pointer to NULL. In your

callbacks, checking pUserPrivate != NULL before you decide to use it

won't hurt.

Matthieu

On 01/28/11 20:09, [email protected] wrote:

So in the best case scenario, I could just call ca_clear_channel after I
free pUserPrivate? Is there a way to determine if the worst case scenario
exists? If it does, is there anyway to fix or avoid it?

Yes.  I'm worried about monitor (and other) callbacks that are
in transit while the channel is being cleared.

However, in the cases in which I needed to worry about this there was
a software layer between the decision to clear the channel and the
actual call to ca_clear_channel().  It may be that channel access
somehow claws back sent monitors when a channel is cleared, but that
has never been my understanding of how it works.  If it does work that
way, all that I would worry about is that you're freeing pUserPrivate
before calling ca_clear_channel(), thus opening a tiny time window in
which a race condition is possible.

Tim

----- Original Message -----
From: [email protected]
To: "Tim Mooney"<[email protected]>
Sent: Friday, January 28, 2011 3:04:41 PM
Subject: Re: ca_create_channel memory management

Do you mean calls to the monitor callback function that have occurred
prior to calling ca_clear_channel, but that somehow have not been
processed?

I thought that ca_clear_channel took care of that? From the ca reference
manual:

Clearing a channel does not cause its disconnect handler to be called,
but
clearing a channel does shutdown and reclaim any channel state change
event subscriptions (monitors) registered with the channel.

Thank you,
Patrick

If your channel is doing any callbacks (e.g., you're monitoring it),
you might have outstanding monitors at the time you decide to clear
the channel.  You need to tell your callback routine to ignore any
calls that come after you've called cleanup(), so it doesn't reference
pUserPrivate after it's been freed.
Tim

----- Original Message -----

From: [email protected]
To: "Andrew Johnson"<[email protected]>
Cc: [email protected]
Sent: Friday, January 28, 2011 11:56:37 AM
Subject: Re: ca_create_channel memory management
So:

void cleanup(chid ChanID) {
void* pUserPrivate;

pUserPrivate = ca_puser(ChanID);
free(pUserPrivate);
ca_clear_channel(ChanID);
}
?

Hi Patrick,

On Friday 28 January 2011 11:10:29 [email protected]
wrote:

If you allocate memory and send a pointer to it in the puser field
of
ca_create_channel, does it get freed when ca_clear_channel is
called, or
should I do that explicitly?

There's no way the CA API could know whether it should do that or
not, so
it
doesn't and it falls on you to do it yourself.

- Andrew
--
If a man is offered a fact which goes against his instincts, he will
scrutinize it closely, and unless the evidence is overwhelming, he
will
refuse to believe it. If, on the other hand, he is offered something
which affords a reason for acting in accordance to his instincts, he
will accept it even on the slightest evidence. -- Bertrand Russell

--
Tim Mooney ([email protected]) (630)252-5417
Software Services Group (www.aps.anl.gov)
Advanced Photon Source, Argonne National Lab

--
Tim Mooney ([email protected]) (630)252-5417
Software Services Group (www.aps.anl.gov)
Advanced Photon Source, Argonne National Lab

--
Matthieu Bec              Gemini Observatory
Tel: +56 51 205785        c/o AURA, Casilla 603
Fax: +56 51 205650        La Serena, Chile

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System