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From: NYMAN@intellicorp.com (Lars Nyman)
Newsgroups: comp.unix.questions
Subject: variable number of arguments in ANSI C
Message-ID: <20070@adm.BRL.MIL>
Date: 21 Jun 89 19:48:04 GMT
Sender: news@adm.BRL.MIL
Lines: 58

What assumptions is OK to make about variable number of arguments in ANSI C ?

Example:

#include <stdarg.h>

void foo(int i, ...)
{
  va_list args;

  va_start(args, i);         /* initialize args pointer */
  va_arg(args, char *);      /* pop the first argument */
  bar(i - 1, args);          /* let bar() see the rest, Q1 */

  va_start(args, i);         /* initialize args pointer again, Q2 */
  baz(i, args);              /* let baz() see them all, Q1 */

  va_end(args);              /* done */
}

void bar(int i, va_list args)
{
  while (i > 0) {
    printf("%s\n", va_arg(args, char *)); /* pop arguments, Q1 */
    i--;
  }
}

void baz(int i, va_list args)
{
  while (i > 0) {
    printf("%s\n", va_arg(args, char *)); /* pop arguments, Q1 */
    i--;
  }
}

main()
{
  foo(3, "Monday", "Tuesday", "Wednesday");
}



I know this will work in atleast some implementations of ANSI C, but is it
portable, legal ANSI C code ?

Q1: Is it OK to pass the va_list to another function and let that function pop
    all or some of the arguments with va_arg() ?

    I believe this is OK according to ANSI, since vprintf() etc. in the 
    standard library takes a va_list argument.  Am I correct ?

Q2: Is it OK to "restart" by calling va_start() more than once ?

Lars Nyman

nyman@intellicorp.com
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