Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Fri, 12 Apr 2013 00:37:01 +0200

Am Donnerstag, 11. April 2013, 22:37:35 schrieb Andrew Johnson:
> On 2013-04-10 Dirk Zimoch wrote:
> > Today I found a strange problem. I have a device that has 32 bit
> > unsigned int output registers. I want to scale an analog value (e.g.
> > -10.0 .. +10.0) to the raw range 0x00000000 ... 0xffffffff. The
> > special_linconv() function of the ao record sets ESLO and EOFF correctly
> > (ESLO=20/4294967295=4.65661287416E-09, EOFF=-10.0). All negative values
> > work but any positive value gives wrong results. Even worse, the results
> > are different on different machines: 0x80000000 on linux-x86, 0x7fffffff
> > on vxworks-ppc604 for all positive values.
> >
> > What happens? In the convert() function the analog (double) value is
> > converted to epicsInt32 (type of the RVAL field). However if the double
> > value is too large for the integer, strange and obviously implementation
> > dependent things happen. I cannot find anything about this case in K&R.
>
> Benjamin quoted the wrong part of K&R §A6.3 because we're not dealing with
> unsigned types here at all.

Right, I was a bit too quick here. Sorry if this caused confusion.

> ...
> > There's a guy called John Reghr,

John Regehr.

> > a professor at the University of Utah, who
> writes about undefined behaviors in C and C++ code quite a lot; I recommend
> reading his stuff.

Many thanks for the pointer. Very interesting reading indeed.

> He ran an interesting competition recently to write a
> simple ascii-to-long numeric converter that must not exhibit UB, so no
> overflows allowed but it must detect numbers that won't fit into the long.

Some time ago I had to deal with a similar problem in the snc code. Even if
you can use standard libraries and must parse only unsigned values the
solution is less trivial than what one might think at first.

Cheers
--
Ben Franksen
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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System