#define clrscr() 100 main() { clrscr(); printf( "%dn", clrscr() ); }

Preprocessor executes as a seperate pass before the execution of the compiler. So textual replacement of clrscr() to 100 occurs.The input program to compiler looks like this :

main ()

{

100;

printf("%d\n",100);

}

Note: 100; is an executable statement but with no action. So it doesn't give any problem.