[sane-devel] C undefined operations in sane-backends-1.0.7-beta1

[Ingo Wilken]

> let's have a look at the warnings from the Plustek backend.
> The lines that produce the problem include the following statement:
> _SWAP(x,y)	(x)^=(y)^=(x)^=(y)
> So what? Where's the problem here?

From the comp.lang.c FAQ:
Status: O

#  3.3b:	Here's a slick expression:
#  
#  		a ^= b ^= a ^= b
#  
#  	It swaps a and b without using a temporary.
#  
#  A:	Not portably, it doesn't.  It attempts to modify the variable a
#  	twice between sequence points, so its behavior is undefined.
#  
#  	For example, it has been reported that when given the code
#  
#  		int a = 123, b = 7654;
#  		a ^= b ^= a ^= b;
#  
#  	the SCO Optimizing C compiler (icc) sets b to 123 and a to 0.
#  
#  	See also questions 3.1, 3.8, 10.3, and 20.15c.


Regards,
Ingo