How does the below program output `C89` when compiled in C89 mode and `C99` when compiled in C99 mode?

Question

I\'ve found this C program from the web:

#include 

int main(){

    printf(\"C%d\\n\",(int)(90-(-4.5//**/
    -4.5)));

    return 0;
}
         


        

Answer 1

Because // comments only exist in C99 and later standards, the code is equivalent to the following:

#include 

int main (void)
{
  int vers;

  #if   __STDC_VERSION__ >= 201112L
    vers = 99; // oops
  #elif __STDC_VERSION__ >= 199901L
    vers = 99;
  #else
    vers = 90;
  #endif

  printf("C%d", vers);

  return 0;
}

Correct code would be:

#include 

int main (void)
{
  int vers;

  #if   __STDC_VERSION__ >= 201112L
    vers = 11;
  #elif __STDC_VERSION__ >= 199901L
    vers = 99;
  #else
    vers = 90;
  #endif

  printf("C%d", vers);

  return 0;
}