Issue 0082: Many varargs questions

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Clive Feather, WG14
Date: 1993-12-03
Submitted against: C90
Status: Fixed
Fixed in: C90 TC2
Converted from: dr.htm, dr_082.html

Item 19 - multiple varargs

Consider the following translation unit:

#include <stdarg.h>
 #include <stdio.h>

 extern int is_final_arg (int);

 void f1 (int n, ...)
        {
        va_list ap1, ap2;

        va_start (ap1, n);
        va_start (ap2, n);
        while (va_arg (ap1, int) != 0)
                printf ("Value is %d\n", va_arg (ap2, int));
        va_end (ap1);
        va_end (ap2);
        }

 void f2 (int n, ...)
        {
        va_list ap;

        va_start (ap, n);
        for (;;)
                {
                n = va_arg (ap, int);
                if (is_final_arg (n))
                        {
                        va_end (ap);
                        return;
                        }
                printf ("Value is %d\n", n);
                }
        }

 void f3 (int n, ...)
        {
        va_list ap;

        va_start (ap, n);
        while (n = va_arg (ap, int), n != 0)
                printf ("Value is %d\n", n);
        va_start (ap, n);
        while (n = va_arg (ap, int), n != 0)
                printf ("Value is still %d\n", n);
        va_end (ap);
        }

 void f4a (va_list *pap)
        {
        int n;

        while (n = va_arg (*pap, int), n != 0)
                printf ("Value is %d\n", n);
        }

 void f4 (int n, ...)
        {
        va_list ap;

        va_start (ap, n);
        f4a (&ap);
        va_end (ap);
        }

 void f5a (va_list apc)
        {
        int n;

        while (n = va_arg (apc, int), n != 0)
                printf ("Value is %d\n", n);
        }

 void f5 (int n, ...)
        {
        va_list ap;

        va_start (ap, n);
        f5a (ap);
        va_end (ap);
        }

a. Is each function in this translation unit strictly conforming? Note in particular:

in f1, the use of simultaneous va_lists in f1;

in f2, va_start and va_end are in different scopes;

in f3, there are two va_starts and one va_end;

in f4, the address of an object of type va_list is taken;

in f4a and f5a, va_arg is called with a first parameter which is not “the same as the va_list ap initialized by va_start” (subclause 7.8.1.2).

b. Is the following implementation conforming?

va_start allocates a block of memory with malloc;

a va_list is a pointer to the block;

va_end frees the same block;

c. Is there any portable method to copy the current state of a va_list, for example in order that the remaining arguments can be scanned twice without knowledge of the va_arg calls made previous to that point? If the answer to (b) is “yes,” I believe the answer to (c) must be “no.”

Comment from WG14 on 1997-09-23:

Response

a) All functions listed except for f3 contain strictly conforming code. The function f3 violates the intended requirement for va_start and va_end to be invoked in matching pairs, as reflected in the following Correction.

b) There is nothing described in this section that would make such an implementation non-conforming.

c) No.

Correction

In subclause 7.8.1, page 122, change the last sentence from:

The va_start and va_end macros shall be invoked in the function accepting a varying number of arguments, if access to the varying arguments is desired.

to:

The va_start and va_end macros shall be invoked in corresponding pairs in the function accepting a varying number of arguments, if access to the varying arguments is desired.

In subclause 7.8.1.1, page 122, add at the end of the second paragraph of the Description:

va_start shall not be invoked again for the same ap without an intervening invocation of va_end for the same ap.