Experimental Physics and
Industrial Control System

Michael Davidsaver <[email protected]> · Mon, 6 Aug 2018 09:15:59 -0700

On 08/04/2018 08:43 AM, Johnson, Andrew N. wrote:
> My assumption is that they’re wanting direct access to a large block of data without copying it. This should safe for DB links because of the IOC locking, as long as they aren’t copying pointers out of the dbAddr or using it outside of the locked record that is requesting it, so I could see us adding another API to the LSET that allows that for link types that can provide it.
> 
> I would be wary of just handing out the dbAddr* pointer though, since the link could get changed without any notice, so I would like to add a little more enforcement. Some link types could even use this to provide access to their raw data buffers, so I’m wondering about an API like doLocked() where the caller gives us a function pointer which we call back with a dbAddr*.

My thinking has been something like:

> int dbGetPdbAddrFromLink(DBLINK *plink, DBADDR *pout);

which would be used like

> DBLINK *plink = ...;
> DBADDR addr;
> dbScanLock(plink->precord);
> if(dbGetPdbAddrFromLink(plink, &addr)==0) {
>    // do something with addr

Though this would look different in the context of an aSub function, the point
is that this avoids returning a pointer to the internal struct dbAddr by making
a copy.  This copy remains valid in perpetuity, although the assurance that both
records are covered by the same lock expires as soon as the source record is
unlocked.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System