remarkable behavior of sprintf

I’ve been working on a library for writing sprintf-like routines. This has led me to learn quite a lot about sprintf. If you’re ever looking to be amazed at how complex one routine can be, look at perldoc -f sprintf. It’s not the most complex builtin in Perl 5 (I think), but it’s up there. I think open wins.

Anyway, here is some Perl:

printf "I will charge you %u percent more than Bob.", 10;

This prints:

I will charge you 10 percent more than Bob.

Great, so let’s use a literal percent character:

printf "I will charge you %u%% more than Bob.", 10;

# I will charge you 10% more than Bob.

This should not be surprising. When looking at how Darren Chamberlain’s String::Format library handles this, I saw that he generates a mapping so that ‘%’ is treated as the literal percent sign. This struck me as wrong, because it meant that all the formatting codes would continue to work. In otherwords, you could say:

print stringf "Look! %10%\n";

# Look!          %

I updated my parser to ignore formatting codes that looked like this, but just for safety’s sake double-checked perl’s behavior:

$ sprintf "Look!  %10%\n"
Look!           %

Woah! Wow. You can left align, too, at least. I didn’t get into many other weird possibilities. This just struck me as totally insane, so probably brought in from C. I decided to check:

#include <stdio.h>

int main() {
  printf("Hello, world %10%\n");
  return 0;
}

…which got me…

~$ gcc hello.c
hello.c: In function ‘main’:
hello.c:4: warning: conversion lacks type at end of format
hello.c:4: warning: unknown conversion type character 0xa in format
~$ ./a.out
Hello, world          %

So, I thought maybe we were seeing the same behavior for a different reason: C was seeing two formats: %10 which lacked a conversion type and %\x0A because of the newline after the terminal percent sign. This is what the warnings suggested, but I got confused when I changed the format to %-10% and the output became:

~$ ./a.out | xxd
0000000: 4865 6c6c 6f2c 2077 6f72 6c64 2025 2020  Hello, world %  
0000010: 2020 2020 2020 200a                             .

So… the percent sign is being left aligned, despite warnings (still there) that seem to imply it’s being interpreted differently.

I was probably a fool to even think about writing anything resembling sprintf. I don’t regret it yet, but I’m sure I will.

Written on November 15, 2009