By Susam Pal on 03 Jun 2011

Here is a silly little C puzzle:

#include <stdio.h>

int main(void)
{
    https://susam.net/
    printf("hello, world\n");
    return 0;
}

This code compiles and runs successfully.

$ c99 hello.c && ./a.out
hello, world

However, the C99 standard draft does not mention anywhere that a URL is a valid syntactic element in C. How does this code work then?

Update on 04 Jun 2011: The puzzle has been solved in the comments section. If you want to think about the problem before you see the solutions, this is a good time to pause and think about it. There are spoilers ahead.

The code works fine because https: is a label and // following it begins a comment. In case, you are wondering if // is indeed a valid comment in C, yes, it is, since C99. Download the C99 standard draft, go to section 6.4.9 (Comments) and read the second point which mentions this:

Except within a character constant, a string literal, or a comment, the characters // introduce a comment that includes all multibyte characters up to, but not including, the next new-line character. The contents of such a comment are examined only to identify multibyte characters and to find the terminating new-line character.