Unexpected C behaviour

Question

I'm trying to understand why my program won't to compile,

int myfunction(int x)
{
   return x;
} 

int main(){
  int x = 10;
  int result=0;
  result=myfunction(x) * myfunction(++x);
  printf("Result is = %d", result);
}

After execution i get : warnings being treated as errors In function 'int main()': operation on 'x' may be undefined. Someone has an idea ?

Answer 1

myfunction(x) * myfunction(++x) is undefined because the order of evaluation of the two arguments to operator * is unspecified. So either the first or second call can be executed first, meaning that theoretically x or ++x can be evaluated first, which can lead to different results. Theoretically. In practice, the standard just passes responsibility to you not to do this.