C implementation of Tic-Tac-Toe in a single call to printf

yokohummer7 wrote: post #3

> %n takes a pointer and writes (!!) the number of bytes printed so far. > Okay, everyone probably knows this. Let's get a bit more advanced. Ok, but I didn't know about that. What's the use?

I seem to recall using it to auto-adjust column widths.

And we did something with it involving string translations. Since we didn't control the translated format string we couldn't just count the characters in the source code.

But it's been a really long time and I don't recall the details.

Re: C implementation of Tic-Tac-Toe in a single call to printf

#5

Post by mariomatos » Sun, Jun 07, 2020, 8:39 AM UTC

Some advanced clickbait we have here.

Re: C implementation of Tic-Tac-Toe in a single call to printf

#6

Post by tikej » Sun, Jun 07, 2020, 8:39 AM UTC

Looks a bit like scientific codes from 3 decades ago :)

Re: C implementation of Tic-Tac-Toe in a single call to printf

#7

Post by encom » Sun, Jun 07, 2020, 8:42 AM UTC

mariomatos wrote: post #5

Some advanced clickbait we have here.

Nah.

Dennis Ritchie HATES him. Program like a pro with this one weird trick! You won't believe what happens!

That's proper bait.

Re: C implementation of Tic-Tac-Toe in a single call to printf

#8

Post by [deleted] » Sun, Jun 07, 2020, 8:43 AM UTC

yokohummer7 wrote: post #3

> %n takes a pointer and writes (!!) the number of bytes printed so far. > Okay, everyone probably knows this. Let's get a bit more advanced. Ok, but I didn't know about that. What's the use?

[deleted]

Re: C implementation of Tic-Tac-Toe in a single call to printf

#9

Post by Someone » Sun, Jun 07, 2020, 8:48 AM UTC

yokohummer7 wrote: post #3

> %n takes a pointer and writes (!!) the number of bytes printed so far. > Okay, everyone probably knows this. Let's get a bit more advanced. Ok, but I didn't know about that. What's the use?

One use is aligning outputs:

  char *prefix = "example";
  char *line1 = "line 1";
  char *line2 = "line 2";
  printf("%s: %n%s\n", prefix, &n, line1);
  printf("%*s%s\n", n, "", line2);

will output

  example: line 1
           line 2

That’s a bit more robust than using strlen(s)+2, where you have to keep that magic constant 2 in sync with ": ". Moving ": " to a variable and using strlen(s)+strlen(separator) would fix that, though (at the price of speed, unless you’ve a compiler that optimizes that away)

Re: C implementation of Tic-Tac-Toe in a single call to printf

#10

Post by MertsA » Sun, Jun 07, 2020, 9:04 AM UTC

yokohummer7 wrote: post #3

> %n takes a pointer and writes (!!) the number of bytes printed so far. > Okay, everyone probably knows this. Let's get a bit more advanced. Ok, but I didn't know about that. What's the use?

Well the fun use is using it to exploit printf format string vulnerabilities. Microcorruption has a fun level involving that IIRC.