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<== Date ==> <== Thread ==>
Subject: Re: How to handle EPICS monitors?
From: Marty Kraimer <[email protected]>
Cc: [email protected]
Date: Mon, 22 Apr 2002 07:56:12 -0500
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 ([email protected]) 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

References:
How to handle EPICS monitors? J. Frederick Bartlett ([email protected])
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