Using subArray

[Michael Abbott <michael.abbott@diamond.ac.uk>]

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?