Experimental Physics and
Industrial Control System

From: Core-talk <core-talk-bounces at aps.anl.gov> on behalf of Michael Davidsaver via Core-talk <core-talk at aps.anl.gov>
Date: Friday, June 14, 2024 at 8:45 PM
To: EPICS core-talk <core-talk at aps.anl.gov>
Subject: [EXTERNAL] dblsr code question/challenge

A little C code "challenge" which puzzled me today.

Why was the following change a no-op?

As background.  For DB_LINK, the type stored in
'plink->value.pv_link.pvt' changed  from 'DBADDR*'
to 'dbChannel*' back with Base 3.15.0.1.  I was
quite surprised to notice that dblsr() was still
treating it as a 'DBADDR*'.I initially assumed
that no one had tested it since then.  But, after
thinking for awhile, I realized that the
situation was not so simple.

> --- a/modules/database/src/ioc/db/dbLock.c
> +++ b/modules/database/src/ioc/db/dbLock.c
> @@ -918,7 +918,7 @@ long dblsr(char *recordname,int level)
>                  pdbFldDes = pdbRecordType->papFldDes[pdbRecordType->link_ind[link]];
>                  plink = (DBLINK *)((char *)precord + pdbFldDes->offset);
>                  if(plink->type != DB_LINK) continue;
> -                pdbAddr = (DBADDR *)(plink->value.pv_link.pvt);
> +                pdbAddr = &((dbChannel *)(plink->value.pv_link.pvt))->addr;

https://urldefense.us/v3/__https://github.com/epics-base/epics-base/commit/72d50ce2749bbc791907a32c1ed77af1333a21f5__;!!G_uCfscf7eWS!bqiRBiT_Ip0nrcLGxrmm1ZE6yiy1TzCcEIkBjZ30UI_daWaDnRhBenpKVMd1dyXCfbUXcYySljDXqZ16jZH7AIP8Qg$

... my answer below ...
















My key to understanding was noticing that the only usage
of 'pdbAddr' is to print the record name:

> printf(" %s\n",pdbAddr->precord->name);

Now the most obvious answer would be if the struct dbAddr
embedded in the struct dbChannel were the first member.
However, this is not the case.

> typedef struct dbChannel {
>     const char *name;         /**< Channel name */
>     dbAddr addr;              /**< Pointers to record & field */
> ...

The first member of dbAddr is the record pointer:

> typedef struct dbAddr {
>         struct dbCommon *precord;   /* address of record                     */

So

> pdbAddr->precord->name

was actually accessing dbChannel::name with the correct
number of indirections (1) because 'precord->name' is not
actually an indirection since the first member is a 'char[]'.

> typedef struct dbCommon {
>     char                name[61];   /**< @brief Record Name */
> ...

So, I wasn't entirely truthful in calling this a no-op.
If a DB_LINK were targeted at a record alias name, then
between 3.15.0.1 and now, dblsr() would print the alias
instead of the intended canonical record name.

Now for the weekend!