Defect Report #082

Submission Date: 03 Dec 93
Submittor: WG14
Source: Clive Feather
Question
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);
}
  1. 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).
  2. 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;
  3. 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.''
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.
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