Why don't we get a compile time error even if we don't include stdio.h in a C program?

/********************************/
int main()
{
 short int i;
 for(i = 0; i<5; i++)
 {
    printf("%d",i);`print("code sample");`
    sleep(1);
 }
 return 0;
}

int sleep();

int sleep(t)
int t;
{
   /* do something with t */
}

int sleep(int);