Re: How to handle EPICS monitors?

[Marty Kraimer <mrk@aps.anl.gov>]

Geoff Savage wrote:

> Jeff,
>
> Where is the address assigned to the monitor subscription data structures?
> I saw what you are disscussing in db_post_events() but I couldn't determine
> where the address is stored when the monitor is registered.
>
> Geoff
>


J. Frederick Bartlett (bartlett@fnal.gov) wrote:

>...

> To my considerable surprise, I found that the CA client received
> monitor events for the STATUS field but not for the VAL field.
> ...
>   So, how should I handle the monitoring of a VAL (or any other) field
> that has the SPC_DBADDR property?
>
> 						Fritz

This info is in the Application Developers Guide.
In the 3.14.0beta1 version it is section 11.5.7 and 11.5.8.

A brief summary is.

When a CA client connects to a field of a record.

1) A DBADDR structure is allocated.
2) dbNameToAddr is called. It assigns values to the members of DBADDR.
3) If the fields has been declared SPC_DBADDR then the record support routine 
cvt_dbaddr is called. It is allowed to change any of the following members of 
DBADDR: no_elements, field_type, field_size, special, pfield, and dbr_type.

For example the waveformRecord cvt_dbaddr is

static long cvt_dbaddr(paddr)
    struct dbAddr *paddr;
{
    struct waveformRecord *pwf=(struct waveformRecord *)paddr->precord;

    paddr->pfield = (void *)(pwf->bptr);
    paddr->no_elements = pwf->nelm;
    paddr->field_type = pwf->ftvl;
    if(pwf->ftvl==0)  paddr->field_size = MAX_STRING_SIZE;
    else paddr->field_size = sizeofTypes[pwf->ftvl];
    paddr->dbr_field_type = pwf->ftvl;
    return(0);
}


Since it set pfield = bptr it calls

db_post_events(pwf,pwf->bptr,monitor_mask);

rather than

db_post_events(pwf,pwf->val,monitor_mask);

Note that section 11.5.8 says that the record support routine "get_array_info" 
is also allowed to change pfield. This allows record support to implement double 
buffering.

I hope this helps.

Marty Kraimer