Tuesday, 7 May 2013

The SVN_DBG story

Whilst talking on IRC about the exotic design motivation for the --invoke-diff-cmd that I offered, it was realised that I was blissfully ignorant as to how (and why) printf() works, how to escape '%' characters, and what a variadic function is.

As is happens, SVN_DBG which is a test suite friendly SVN debugging tool needed documenting, and it happens to be a very good demonstration of how variadic functions work(esp, if you take time to read the GNU C Preprocessor manual) ,  and so, danielsh kindly tasked me with writing a small precis that introduces the SVN_DBG tool to new devs in the HACKING GUIDE.

As a nice bonus, it turns out that some time ago, I read over a small, but vital clue of how the ANSI C OO stuff works.  I tend to read 'over' some stuff I do not understand in order to keep the universe of confusion minimised, and filed 'va_list' as some funky OO type linked list.   So I was quite chuffed at finally figuring out how the 'and here, a miracle occurs' part works -- it uses a variadic function.

This of course calls for an appropriate video: