Re: [GD-General] variadic functions

[Brett B. <res...@ga...>]

Andras,
Thanks for the post.  Pretty much my conclusion too.  But I'm not using
this for printf so I need a generalized solution, although I could
theoretically create bindings for every possible case (which is normal
for scripting engine binding anyway).  Since the callee doesn't know how
many parameters there are, it seems I just need to push the values to
the stack in assembly code and then call the variadic function.  Does
that sound right?
Brett

----- Original Message ----- 
From: "Andras Balogh" <bn...@ma...>
To: <gam...@li...>
Sent: Thursday, November 27, 2003 4:28 PM
Subject: Re: [GD-General] variadic functions


> Hi Brett,
>
> > The compiler strips the "..." off the back and makes a normal
function
> > out of the variadic one, and the preceeding four functions are then
used
> > to access these unknown parameters.  The start function takes the
last
> > known parameter as a value so at first glance it seems the function
> > knows how many paramters there are because because it can compare
the
> > last known argument with the value on the stack.  The problem is
that
> > since the parameters were pushed in reverse order how would it know
how
> > many parameters there are?  If it were in direct order, you could
take
> > the stack pointer minus the stack pointer at the last parameter
position
> > (which is provided) and the diffference would be the number of
> > arguments.  But since they are reversed how does the va_arg function
> > know when to stop?
>
> The answer is simple: It does not know! :(
>
> That's exactly why vararg functions always use the cdecl calling
> convention. This requires that the caller cleans the stack, because
> the caller always knows the number of params on the stack, while the
> calle doesn't (in the case of vararg).
>
> Think of printf! The first param is supposed to be a pointer to a
> format string. Reading that string printf determines how many (and of
> what type) values it has to read from the stack. I'm pretty sure that
> every one of us crashes a program every once in a while because of
> unmatching params and format string. The reason for this is that there
> is just no way printf can verify the stuff you gave is valid.
>
> > Has anybody else run into this before?  How did you solve it?
>
> Well, you could use a format string like printf, or something similar.
>
> If you do this for scripting, then your script binding engine knows
> all the params, so it might be a good idea to generate the format
> description string automatically. This way the script writers could
> call the function with any number of params without explicitly
> describing their type and number in a format string.
>
> The ultimate solution would be of course the introduction of a new
> calling convention, but I don't really know who's responsibility is
> this... anyone? :)
>
>
> Andras
>
>
>
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