As the C specification document specifies in section 6.5.2.2 point no 10:
The order of evaluation of the function designator, the actual arguments, and
subexpressions within the actual arguments is unspecified, but there
is a sequence point
before the actual call.
Therefore if in any function call if two or more arguments modify same
variable using some expression
then the order in which the expressions will be evaluated are
unspecified, therefore gcc warning that
operation on 'a' may be undefined. Also there is a sequence point
before the actual call of the function
so all the argument expressions must finish evaluation before the
actual function call.
Also C specification document gives the following example explaining
the above behaviour.
12 EXAMPLE In the function call
(*pf[f1()]) (f2(), f3() + f4())
the functions f1, f2, f3, and f4 may be called in any order. All
side effects have to be completed before
the function pointed to by pf[f1()] is called.
-Dharmendra
On Mon, Jul 20, 2009 at 9:00 PM, <i...@adari.net> wrote:
> Hello,
>
> Here is a program with output in gcc (4.3.2) on pre and post increments:
>
> //--------------------code begin--------------------------------------------
> #include <stdio.h>
>
> main () {
> int a;
> a=1; printf ("1. %d %d\n", ++a, a); // 1. 2 2
> a=1; printf ("2. %d %d\n", a, a++); // 2. 2 1
> a=1; printf ("3. %d %d\n", a++, a); // 3. 1 2
> a=1; printf ("4. %d %d\n", a++, ++a); // 4. 2 3
> a=1; printf ("5. %d %d\n", ++a, a++); // 5. 3 1
> a=1; printf ("6. %d %d %d\n", ++a, a, a++); // 6. 3 3 1
> a=1; printf ("7. %d %d %d\n", a++, a, ++a); // 7. 2 3 3
> a=1; printf ("8. %d %d %d %d\n", a, a++, ++a, a); // 8. 3 2 3 3
> a=1; printf ("9. %d %d %d %d\n", a, ++a, a++, a); // 9. 3 3 1 3
> a=1; printf ("10. %d %d %d %d %d\n", a, a++, a, ++a, a);// 10. 3 2 3 3 3
> a=1; printf ("11. %d %d %d %d %d\n", a, ++a, a, a++, a);// 11. 3 3 3 1 3
> }
> //--------------------code end--------------------------------------------
>
> The output from the program is listed next to it in comments. I thought
> I knew something about pre and post increments, but this program busted
> my understanding of the pre and post increments. I would appreciate
> if someone could explain me the output.
>
> Thanks
>
>
>
