Experimental Physics and
Industrial Control System

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;

... 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!

Replies:

Re: dblsr code question/challenge Zimoch Dirk via Core-talk

Re: dblsr code question/challenge Kasemir, Kay via Core-talk

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