How to predict size of outcome of vsprintf?

[Miles Bader]

bzs at bu-cs.BU.EDU (Barry Shein) writes:
> Ok, there are two thoughts here:
> 
>         1. How to find out the number of characters a printf
>         operation will generate.
> 
>         2. How to limit a printf operation to a buffer size.
> 
> I suggested the first might be the result of printing to a null
> pointer (buffer or file.)

What I'd really like would be a new function that could act as a basis
for all the other printf functions, and make it possible to emulate
them in nice ways.

I would make it a varargs function that takes a bounded output buffer
and is restartable at the point where it stops due to running into the
end of the buffer.

E.g. (pardon the name, but I can't think of a better one):

    /* fmtP and valP are modified so that re-calling the function
     * will start formatting where it left off
     */
    int rvsnprintf(buf,maxlen,fmtP,valP)
    char *buf;
    int maxlen;
    char **fmtP;	/* in/out */
    va_list *valP;	/* in/out */

vsprintf, vfprintf, etc, are all trivially derivable from this (as is
the one someone mentioned a while ago that returned a realloced
buffer).

Say I write my own buffered io package; I can just call rvsnprintf like:

    streamvprintf(stream,fmt,val)
    STREAM *stream;
    char *fmt;
    va_list val;
    {
	...
	while(*fmt!='\0){
	    int written=rvsnprintf(stream->pos,stream->left,&fmt,&val);
	    if(*fmt!='\0')  /* ran out of space */
		streamflush(stream);
	    else{
		stream->pos+=written;
		stream->left-=written;
	    }
	}
	...
    }

None of the other proposals I've seen give you this capability (or do
it expensively, like mallocing the buffer), and I can't see how it
would be much more difficult to implement than existing printfs...

-Miles