The number of arguments to a varags C function is known at compile time. The number of arguments to a Perl function isn't known until runtime. That is, when you code:

XS_func( char *templ, ... ) {
   ...
   vsprintf( templ, x, y, z );
}
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The C compiler knows it is passing 4 parameters to the varargs function printf and stacks them appropriately. But when you call the XS function from Perl, you can pass any number of arguments at runtime, so there is no way to write that call to vsprintf() with an appropriate number of arguments.

About the best you could do is code the call to vsprintf() with an arbitrarily large number of arguments and yell if the Perl code passes you more:

#! perl -slw
use strict;
#use Inline 'FORCE';
use Inline C => 'DATA', NAME => 'varargs', CLEAN_AFTER_BUILD => 0;


myPrintf( '%s ' x @ARGV . "\n", @ARGV );

__DATA__
__C__
#include <windows.h>
#include <stdio.h>
#include <stdarg.h>

void eprintf( const char *template, ... ) {
    va_list ap;

    va_start( ap, template );
    vfprintf( win32_stderr(), template, ap );
    va_end( ap );
}

void myPrintf( char *templ, ... ) {
    inline_stack_vars;
    char* a[20];
    int i;

    if( inline_stack_items > 20 )
        croak( "myPrintf can handle at most 20 arguments." );

    for( i = 0; i < inline_stack_items; i++ ) {
        a[ i ] = SvPVX( inline_stack_item( i ) );
    }

    eprintf(
        a[ 0], a[ 1], a[ 2], a[ 3], a[ 4], a[ 5], a[ 6], a[ 7], a[ 8],
+ a[ 9],
        a[10], a[11], a[12], a[13], a[14], a[15], a[16], a[17], a[18],
+ a[19]
    );
    inline_stack_reset;
}
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That's crude. it should be checking that the SVs have a PV assigned and lots of other stuff, but it will pass whatever args (upto 20) you give on the command line and print them to stderr via C and eprintf().

It could also inspect the contents of the template and attempt to extract the relevant IV/NV/PV, but then you'd have to do something trick with the declaration of a[].

Another way to do this would be for the XS/Inline C function to push the (relevant parts of) the SVs from the Perl stack, onto the C stack. That is, it would have to emulate the C compilers prologue and epilogue code manually, before transferring control (assembler call instruction) to the varargs C function.

(And it would have to be the relevant prologue code depending upon the calling convention (__stdcall, __cdecl or __fastcall) declared for that function.)

This approach would require compiler dependant asm{} hacks.