A function to sleep a 1000 years: explained

https://bowero.nl/blog/2019/12/14/a-function-to-sleep-a-1000-years-explained/

by jandeboevrie • 7 years ago

27 17 7 years ago

xg152• 7 years ago

Apart from showing off syntax tricks, what exactly was the reason to write f(sleep(7)) instead of e.g. sleep(7);--i&&f()? Just minimizing the character count?

esnard• 7 years ago

The original challenge [0] was submitted as a "code-golf" challenge, which means that the solution needed to solve the problem in the fewest bytes of source code.

There are a lot of tricks to minimize the character count, and replacing a();b() with b(a()) is one of them.

[0] https://codegolf.stackexchange.com/q/196917/38454

xg152• 7 years ago

Ah, that makes sense. Thanks.

iforgotmypass• 7 years ago

I guess I am the only one missing the point (disliking this as a code golf challenge) and cringing that the solution does not sleep for a precise 1000 years and overthinking that various other rules are not being taken into account (e.g. leap years, extra seconds per year, etc.).

mjevans• 7 years ago

Given the context of 'code golf' I don't mind the submitted solution being 'close enough', it even seems like the programmer in question was precise in their wording (if they were quoted properly).

Overall the thought of a pinned register being software updated once every 7 seconds for near a thousand years is the worst part.

I stand corrected; I'm so used to seeing these unrolled (which hopefully the hypothetical compiler's done) that I didn't consider the stack implications; that's the most cringy part.

joveian• 7 years ago

Not thread-safe. Also, from Alex in the comments: "Sounds good! Although probably we will run out of stack waay before that".

sn• 7 years ago

It won't run out of stack space with a high enough optimization level so that it uses tail recursion. https://en.wikipedia.org/wiki/Tail_call

You can try this yourself by compiling:

#include <stdio.h>

int i = 0; int x() {--i && x(printf("x"));} int main() {x();}

using gcc with and without '-02'.

Dylan16807• 7 years ago

With this, we have proven that this beautiful line of code is correct!

I wouldn't say 'correct', exactly. Not just int size, it depends on undefined behavior and a moderate level of compiler optimizations at the same time. Good luck there.

ddevault• 7 years ago

While we're nit-picking, this also doesn't account for a lot of problems which are inevitable on a 1,000 year scale, such as the lifetime of the computer's parts, dealing with power outages, staffing for the long-term upkeep of the system, etc.

perl4ever5• 7 years ago

That line of code is not beautiful, it's obscene.

At the very least, anything that's intended to last a long time should be written in the simplest, clearest, self-explanatory manner so as to at least give reasonable confidence it's correct.

Not to mention, 952 years is a lousy approximation of a millennium.

  const yearSeconds = 31556926

  for i = 1 to 1000
    sleep yearSeconds

Or, use some calendar library to get the exact number needed.

contravariant• 7 years ago

Why wait for a tropical year though? Surely most people will assume it waits for a calendar year?

Of course, even without leap seconds, this severely complicates the issue since you'd first need to figure out the number of leap days in the next 1000 years, which is unfortunately not fixed since 1000 is not a multiple of 400.

stopreformation• 7 years ago

It relies on undefined behavior since signed integer overflow is undefined.

joelkevinjones• 7 years ago

Another problem is that it makes the assumption that an int is 32 bits. The C89 standard certainly doesn't promise that.

perl4ever5• 7 years ago

You want obfuscated, I'll give you obfuscated:

    void
    f (int i, int j)
    {
        int k = j;
        
        k <<= 2;
        k += j;
        k <<= 3;

        if (--i)
            while (--k)
                f(i, j);
        else
            sleep(k);
    }

    int
    main ()
    {
      f (3, 'O'); /* Sleep for 1000 years */

      return 0;
    }

...this should solve your stack overflow problems and be 99.9% accurate.

perl4ever5• 7 years ago

(IMO enterprise grade C code should not have types other than int, use numbers larger than 2^15-1, or constants with more than one digit/character. This is, I'm sure, the best way to avoid undefined behavior and concomitant nasal demons)

perl4ever5• 7 years ago

Here's a more user-friendly version with self-explanatory parameters:

  f(i, j, k, l) { for (j = (j << 2) + j << 3; i + 1 && --j; f(i - 1, k, l, l)); sleep(j); }

  int
  main ()
  {
    f(1, 'O', 'O', 'O'); /* Sleep for 1000 years */
  }

A function to sleep a 1000 years: explained

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