🚀 go-pugleaf

Jeff.Dunlop@dbsoft.gigo.com (Jeff Dunlop) wrote:

>Is it an allowable side effect for vprintf or any other vararg
>function to modify the value of the passed arglist? The following
>code fails under Watcom 10.0 because p_arg is modified by the first
>call to vprintf. If I reinitialize p_arg with va_end and va_start,
>the second vprintf works.
>
>#include <stdarg.h>
>#include <stdio.h>
>
>void test(char *fmt, ...)
>{
>    va_list p_arg;
>    va_start(p_arg, fmt);
>    vprintf(fmt, p_arg);
>    vprintf(fmt, p_arg);
>    va_end(p_arg);
>}

Yes.  From ISO 7.8:

	If access to the varying arguments is desired, the called function 
	shall declare an object (refered to as ap in this subclause) having 
	type va_list.  The object ap may be passed as an argument to another 
	function; if that function invokes the va_arg macro with parameter 
	ap, the value of ap in the calling function is indeterminate and 
	shall be passed to the va_end macro prior to any further reference to 
	ap.

To get it to work right, just change your code to:

    	va_list p_arg;

    	va_start(p_arg, fmt);
    	vprintf(fmt, p_arg);
    	va_end(p_arg);

    	va_start(p_arg, fmt);
	vprintf(fmt, p_arg);
    	va_end(p_arg);

Michael M Rubenstein