Re: [test failure] Re: t4114 binary file becomes symlink

From: Johannes Sixt <j6t@kdbg.org>
Date: Jul 19, 2009, 11:01 UTC
Message-ID: <200907191301.15533.j6t@kdbg.org>
In-Reply-To: <20090718222947.GA31147@coredump.intra.peff.net>

On Sonntag, 19. Juli 2009, Jeff King wrote:

Show 11 quoted lines

> Look at compat/snprintf.c. In git_vsnprintf, we are passed a "va_list
> ap", which we then repeatedly call vsnprintf on, checking the return to
> make sure we have enough space. But using a va_list repeatedly without a
> va_end and va_start in the middle invokes undefined behavior. So we need
> to va_copy it and use the copy.
>
> A patch is below, which fixes the problem for me. However, va_copy is
> C99, so we would generally try to avoid it. But I don't think there is a
> portable way of writing this function without it. And most systems
> shouldn't need to use our snprintf at all, so maybe it is portable
> enough. I dunno.

Problem is, snprintf was made for very old systems, which typically do not have va_copy. (E.g. Windows, but there the situation *might* have changed with the switch to gcc 4.)

The rationale not to use va_copy is that this function is to be used *only* if necessary, i.e. portability is already lacking, and if it can be verified that this function works as is. Portability and correct-by-the-law C code are *not* a goal here. If the function does not work as is, don't use it.

-- Hannes