Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!wuarchive!decwrl!ucbvax!ucsfcgl!pixar!rta
From: rta@pixar.UUCP (Rick Ace)
Newsgroups: comp.lang.c
Subject: ANSI C stdarg questions
Keywords: stdarg ANSI C
Message-ID: <8217@pixar.UUCP>
Date: 21 Dec 89 01:09:01 GMT
Organization: Pixar -- Marin County, California
Lines: 71

Consider the following ANSI C program:

----------------------------------------------------------------------
#include <stdarg.h>

/* print each number in the argument list of test(), stop at 0 */
void print1(va_list ap)
{
	int i;

	while (i = va_arg(ap, int)) 
		printf("%d\n", i);
}

/* print the sum of the numbers in the argument list of test(), stop at 0 */
void print2(va_list ap)
{
	int sum = 0, i;

	while (i = va_arg(ap, int)) 
		sum += i;
	printf("sum %d\n", sum);
}

/* argument "x" is not used, but must be present for stdarg.h */
void test(int x, ...)
{
	va_list ap;

	va_start(ap, x);
	print1(ap);
	print2(ap);
	va_end(x);
}

main(int argc, char **argv)
{
	test(0, 1, 5, 7, 0);
}
----------------------------------------------------------------------

This program works on two different ANSI implementations
(the NeXT and the SGI Iris).  That, of course, does not
guarantee conformance to the standard, and K&R 2E doesn't
address all the details, so here are two questions:

1)  Is it legal ANSI C for test() to pass the argument "ap" to
    print1() and print2() as it does in the above program?

2)  Is it legal ANSI C to traverse the argument list multiple times?
    For example,

        int sum;
        va_list ap;

        va_start(ap, x);              /* start first traversal */
        sum = 0;
        while (i = va_arg(ap, int)) 
                printf("%d\n", i);
        va_end(ap);
        va_start(ap, x);              /* start second traversal */
        sum = 0;
        while (i = va_arg(ap, int)) 
                sum += i;
        va_end(ap);
        printf("sum %d\n", sum);

Please answer both questions.  No partial credit will be given :-)

Rick Ace
{sun,ucbvax}!pixar!rta