Subtle difference between FreeBSD, MacOSX and Linux printf

I’m working on polishing my mini_printf implementation and making a final, verified and documented code release, and the thing I made work in the past and something that stopped working when I moved to MacOSX was a randomized stress-test.

https://github.com/wkoszek/mini_printf

It’s pretty neat: it uses its own PRNG generator to achieve repeatability and uses its numbers to construct randomized format strings which are then played against OSes printf() and my mini_printf() API. Both versions are later compared. I exposed a lot of bugs in mini_printf that way, of which all have been fixed.

So anyway: I refactored mini_printf to the state where I can show it to other people. I also made the stress-test work on MacOSX, and I could play 1,000,000 random format string patterns with 0 failures in couple of seconds. But the same code failed miserably on Linux.

It looks like my stressing testbench found a usability problem between Linux and MacOSX printf. Problem shows up when you decide to use unsupported format string specifier. While Linux leaves the % signs, MacOSX doesn’t propagate them to the string. I guess I’ll have to find a way to elegantly solve it, and preferably without any #defines.

Program

wk:/w/repos/mini_printf> cat p.c
#include <stdio.h>

int
main()
{
	printf("%k%k%k\n");
}

Linux

vagrant@vagrant-ubuntu-trusty-64:/vagrant/mini_printf$ ./p
%k%k%k

MacOSX and FreeBSD

wk:/w/repos/mini_printf> ./p
kkk