Beginning C question.

[Richard A. O'Keefe]

In article <4561 at cvl.umd.edu>, arensb at cvl.umd.edu (Andrew Arensburger) writes:
> In article <25440 at nigel.udel.EDU> gdtltr at freezer.it.udel.edu (Gary Duzan) writes:
> >In article <7703 at uudell.dell.com> jrh at mustang.dell.com (James Howard) writes:
> >=>        short a=0x20df;
> >=>        short b=0x3244;
> >=>        int   c;
> >=>
> >=>	c = (a<<16) + b;
> >=>
> >=>I used "short" because they're 16 bits on this machine, and ints are 32.

> 	James's example works correctly because the following assumptions
> are true:

Bad news, friends, the example DOESN'T work correctly, even if the
half-word sex of the machine is right.  Suppose the high bit of b is 1.
That's the _sign_ bit, remember.  The effect in that case is as if 1
were subtracted from a.  (I tried it, and that _does_ happen.)

Instead, use
	c = (a << 16) | (b & 0xFFFF);
or make a and b "unsigned short" and do
	c = (int)((a << 16) + b);

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