Re: dblsr code question/challenge

[Zimoch Dirk via Core-talk <core-talk at aps.anl.gov>]

It seems I missed that line in my commit b1f445925d when I changed dbLinks from
DBADDR to dbChannel in order to allow the use of link filters.

Dirk

On Fri, 2024-06-14 at 17:45 -0700, Michael Davidsaver via Core-talk wrote:
> 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 ...
> 
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> 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!
>