[Freeciv-Dev] Re: (PR#6781) Cleanups

["Jason Short" <jdorje@xxxxxxxxxxxxxxxxxxxxx>]

<URL: http://rt.freeciv.org/Ticket/Display.html?id=6781 >

Gregory Berkolaiko wrote:
> <URL: http://rt.freeciv.org/Ticket/Display.html?id=6781 >
> 
> On Thu, 6 Nov 2003 rt@xxxxxxxxxxx wrote:
> 
> 
>>2)
>>ai/aiunit.c:591:134: Expression has undefined behavior (left operand
>>uses errno, modified by right operand): log(most_desirable / ((((100 *
>>5 * 81 + 100 * (9 - 5) * 51) + 1) + 1) + (100 * 5 * 81 + 100 * (9 - 5)
>>* 51))) / log(0.6)
>>Code has unspecified behavior. Order of evaluation of function
>>parameters or subexpressions is not defined, so if a value is used and
>>modi fied in different places not separated by a sequence point
>>constraining evaluation order, then the result of the expression is
>>unspecified.
> 
> 
> I do not understand this error at all.  Mathmatically, the order of 
> evaluation is well defined.

Presumably log is a macro which modifies the value of errno.  It is 
illegal to do this multiple times within the same "sequence".

Seems like a bug in the implementation of log, to me.  The way to work 
around it would be

   a = log (...)
   b = log (...)
   a / b

although complaining to the library writers is probably just as good of 
a solution.

jason