Re: EPICS 7.0.2.1 release preparation

["Johnson, Andrew N. via Core-talk" <core-talk@aps.anl.gov>]

Hi Dirk,

On 4/3/19 8:31 AM, Dirk Zimoch via Core-talk wrote:

I have seen that dbGetFieldType() and dbGetLinkType() are about to be removed. What is the replacement for dbGetLinkType()? I want to find out if a field is a PV link.

Those routines were deleted before the 7.0.2.1 release that was published (although not formally announced) on March 20th. They were the last vestiges of the dbStatic "Forms" support routines.

EPICS 7 now has extensible link types that don't map to the old "if it isn't a constant or a DB link it must be a CA link" rules. We started introducing these changes in 3.15.1 (the dbLink.h header was created in 2012 and first published in the 3.15.0.1 developer release in 2014). You can now develop your own link types, and the IOC code can't know how to look inside the link data structures for those links. We still support the old CA and DB link types though, so code that sticks its fingers into them will generally still work, but we don't let you do that with the new link types.

The following new routines in dbLink.h allow you to examine the link support that is attached to a particular link field:

int dbLinkIsDefined(const struct link *plink);  /* 0 or 1 */
int dbLinkIsConstant(const struct link *plink); /* 0 or 1 */
int dbLinkIsVolatile(const struct link *plink); /* 0 or 1 */
int dbIsLinkConnected(const struct link *plink); /* 0 or 1 */

The first one dbLinkIsDefined() tells you whether the field is attached to a link support at all. The next two give you some characteristics of the link type; a constant link only provides a value at record initialization time, so is comparable to the old DCT_LINK_CONSTANT; a volatile link is one that may disconnect and reconnect at runtime (a DB link is not volatile, while CA and pva links are since the server they talk to can go down).

You can call the routine dbIsLinkConnected() to find out whether a volatile link is currently connected to its target or not, although there is of course no guarantee that the connection state won't change immediately after the routine has returned. The calcout record now uses this routine to populate the fields that allow you to see the connection state of its link fields.

You can find out more about the other routines in dbLink.h by reading the Doxygen comments that appear above the equivalent function pointers in the definition of struct lset. I haven't talked about dbJLink.h which is how new link types get to parse their addresses.

I use it in a utility function "dbll" (database list links) to show which records link to given records or field: dbll recordpattern[.field]

The core of the function is:

    for (status = dbFirstRecordType(&dbEntry); !status; status = dbNextRecordType(&dbEntry))
    for (status = dbFirstRecord(&dbEntry); !status; status = dbNextRecord(&dbEntry))
    {
#if EPICS_VERSION*10000+EPICS_REVISION*100+EPICS_MODIFICATION >= 31411
        if (dbIsAlias(&dbEntry)) continue;
#endif
        for (status = dbFirstField(&dbEntry, 0); !status; status = dbNextField(&dbEntry, 0))
        {
            if (dbGetLinkType(&dbEntry) == DCT_LINK_PV)
            {
                const char* target = ((DBLINK *)dbEntry.pfield)->value.pv_link.pvname;
                if (!match || (
                        (strchr(target, '.') ? 1 : 0) == (d ? 1 : 0) ?
                            epicsStrGlobMatch(target, match) :
                            alt_match ? epicsStrGlobMatch(target, alt_match) : 0))
                {
                    printf("%s.%s ==> %s\n", dbGetRecordName(&dbEntry), dbGetFieldName(&dbEntry),
                        dbGetString(&dbEntry));
                }
            }
        }
    }
    dbFinishEntry(&dbEntry);

You can look at ((DBLINK *)dbEntry.pfield)->type which gives you one of the link type constants defined at the top of link.h. The above dbGetLinkType(&dbEntry) == DCT_LINK_PV condition is equivalent to that type being any of PV_LINK, PN_LINK, DB_LINK or CA_LINK, which are compatible with the old meanings.

There is now a faster way to find all the DB links that point to a particular record though, the dbDbLink.c code now keeps an ELList of those links for the new lock-set code in dbLock.c to use. I would be happy to see a version of your "dbll" utility which uses those back-links added to dbDbLink.c. If you're interested in contributing such a utility to Base please let us know (both Michael and I have been working in different parts of the link code recently).

- Andrew

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