View source for Undefined Value ← Undefined Value

<syntaxhighlight lang="c">

1. include<stdio.h>

int main() {

   int x=10, y;
   y = (x++) + (++x);
   printf("%d %d %d %d ", y, x++, x, ++x);

} </syntaxhighlight>

a) 22,10,11,13

b) 22,11,11,11

c) 12,10,11,13

d) 22 13 13 13

Solution[edit]

None of the choices are correct as per C standard. This is because the statement

y = (x++) + (++x);

can causes undefined behavior, so does the statement

printf("%d %d %d %d ", y, x++, x, ++x);

C standard says that the side effects of an operation (for b = ++a; modification of a is a side-effect) need to be completed only before the next sequence point. This relaxation is given so that the compiler would be able to generate the most optimal code (which run faster). But as a consequence, programmer shouldn't do a read and a write from a memory location within a sequence point or otherwise the result would be undefined.

In the statement,

y = (x++) + (++x);

There are 2 reads to the memory location of x and 2 writes to the same within a sequence point. So, the result of this statement has no guarantee as per C standard.

Similarly, the statement

printf("%d %d %d %d ", y, x++, x, ++x);

is undefined. In a function call in C, the arguments are pushed onto the stack (even if registers are used, they are written and read in the reverse order) from right to left (the left most argument would be on top of the stack, so that variable no. of arguments can work in C). But the order in which the argument expressions are processed is not defined. The only thing defined is that all arguments must be processes before the next sequence point- which here is the execution of the called function.

I have seen many people saying C is so stupid because of these undefined behaviors. But that's the power of C. A programmer in C should know about these behaviors and avoid writing undefined codes.