Experimental Physics and
Industrial Control System

Michael Abbott <[email protected]> · Sun, 27 Nov 2005 12:24:20 +0000 (GMT)

I am writing device support for a subArray record so that I can read small
slices from a very large array provided by a device. The device in
question has limited memory, and so I cannot afford more than one copy of
this array in memory, namely the copy in the device driver itself. My
plan is to read the data like this:
	record(subArray, "SA")
	{
	    field(DTYP, "My driver")
	    field(INP,  "my driver's arguments")
	    field(MALM, "1024")
	    field(FTVL, "LONG")
	}
	$ caput SA.NELM 1024
	$ caput SA.INDX 1024000
	$ caput SA.PROC 0
	$ caget SA
Unfortunately there is one very interesting and annoying fly in the
ointment. In subArrayRecord.c:readValue we see the following two lines of
code:
	if (psa->indx >= psa->malm)
	    psa->indx = psa->malm - 1;
Ooops. Most unhelpful!
This stupid code means that MALM, which is used to allocate the storage
used by the subArray record, also determines the maximum index into the
device array. I think this is missing the point of a subArray; it
certainly gets in my way.
Now, it turns out I am lucky: malm is used to allocate storage *before* it
calls my init_record, so I can fake malm back to the value I want to to
be. However, this is hardly very satisfactory, and is going to have a
mildly embarassing side effect: get_{graphic,control}_double will return
bogus values for the upper limit on nelm.
Any thoughts on this?

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System