Re: [PATCH 1/2] Add strbuf_initf()

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Mar 6, 2008, 12:52 UTC
Message-ID: <alpine.LSU.1.00.0803061319590.15786@racer.site>
In-Reply-To: <3f4fd2640803060353w70651522w4b3896b8106b01c0@mail.gmail.com>

Hi,

On Thu, 6 Mar 2008, Reece Dunn wrote:

Show 25 quoted lines

> On 06/03/2008, Johannes Schindelin <Johannes.Schindelin@gmx.de> wrote:
>
> >  On Thu, 6 Mar 2008, Reece Dunn wrote:
> >
> >  > On 06/03/2008, Mike Hommey <mh@glandium.org> wrote:
> >  > > On Thu, Mar 06, 2008 at 02:14:43AM +0100, Johannes Schindelin wrote:
> >  > >  >
> >  > >  > The most common use of addf() was to init a strbuf and addf() 
> >  > >  > right away. Since it is so common, it makes sense to have a 
> >  > >  > function strbuf_initf() to wrap both calls into one.
> >  > >  >
> >  > >  > Unfortunately, C (and cpp) has no way to make this easy 
> >  > >  > without code duplication, as we need to va_init() in 
> >  > >  > strbuf_addf() possibly a few times.  So the code for addf() is 
> >  > >  > copied.  Fortunately, the code is pretty short, so not too 
> >  > >  > much had to be copied as-is.
> >  >
> >  > Why not have a strbuf_vaddf and strbuf_vinitf that take a va_arg as 
> >  > a parameter. This would mean that you don't have code duplication, 
> >  > and it is flexible enough if you want to add more customisations in 
> >  > the future. No macro needed. This is what the printf/scanf family 
> >  > of functions do.
> 
> Is it possible to pass a void * to a strbuf_vaddf function that you can 
> pass to va_start, so you can then restart the va_list?

AFAIU va_list() works on the stack (on less register-challenged systems than x86, the first parameters are possibly passed as registers, too, but it has to resort to the stack at a certain number of parameters).

Since the stack is also used (at least on register-challenged yadda yadda) to store the return address, va_list() would most likely pick that up, too.

Now, that just _might_ still work, since the printf() family determines the number of arguments from the format string.

But then there are machines which are _not_ completely stack-based, for example SPARC. IIRC the first 8 parameters are passed by a so-called register window, which changes with each function call.

So no, I think there is no portable way to pass them around.

Of course, having a simple implementation for addf() _not_ using vsnprintf() could help, too (and make the process more efficient, probably).

I thought.

The formats we'd have to support are:

$ git grep strbuf_addf |
	sed -e 's/^[^"]*"\([^"]*\)".*$/\1/' -e 's/%%//g' -e 's/%/\
&/g' | sed -n -e 's/\(%[^a-zA-Z]*[a-zA-Z]\).*/\1/p' | sort | uniq

%02X %06o %c %d %l %o %s %.*s %u

So it does not look too bad. 'X', 'o', 'd', 'l' and 'u', possibly with a size specifier (left 0-padded), 'c' and 's' (possibly with a length parameter).

The implementation wouldn't be _too_ short, I'd say, but doable.

Ciao, Dscho