Why we can't extend the DSET or DRVET

[Andrew Johnson <anj@aps.anl.gov>]

Jeff Hill wrote:
> I assume that the device and driver support would be handled the same way -
> by adding a new routine at the end of the device and driver support entry
> tables.
That's not possible because in reality the structure definition for the 
DSET is not defined centrally - it can be (and usually is) different for 
each record type, and is usually defined by the device support layer 
itself.  For example, in R3.13 src/dev/atDev/devAt5Vxi.c contains two 
typedefs like this:
typedef struct {
        long            number;
        DEVSUPFUN       report;
        DEVSUPFUN       init;
        DEVSUPFUN       init_record;
        DEVSUPFUN       get_ioint_info;
        DEVSUPFUN       read_write;
        DEVSUPFUN       special_linconv;} AT5VXIDSET;

There are also several very commonly used device supports that have 
already extended the entry tables to add routines for their own purposes. 
 One of the worst is devCommonGpib.h which contains this:
typedef struct {
  long          number;
  DEVSUPFUN     funPtr[20];
} gDset;

It would be impossible to insert a new routine into the struct dset 
defined in devSup.h without causing major headaches for years to come.
Similarly the driver support entry table gets defined and extended by 
individual drivers.  This is the definition in drvBitBusInterface.h:
struct drvBitBusEt {
        long            number;
        DRVSUPFUN       report; /* Report status of Bit Bus links */
        DRVSUPFUN       init;   /* Init the xvme card */
        DRVSUPFUN       qReq;   /* Queue a transaction request */
};

I don't like the way these interfaces were designed, but there's not much 
we can do about them now if we're going to keep to the principle that any 
change we make in Base which breaks backwards comptibility must cause a 
compiler error rather than a runtime fault, which might be very subtle and 
hard to trace.
- Andrew
--
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