URL in C Puzzle

Summary

URL in C 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 () 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.