rta@pixar.UUCP (Rick Ace) (12/21/89)
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);
}
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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!rtachris@mimsy.umd.edu (Chris Torek) (12/21/89)
In article <8217@pixar.UUCP> rta@pixar.UUCP (Rick Ace) writes: >1) Is it legal ANSI C for test() to pass the argument "ap" to > print1() and print2() as it does in the above program [deleted]? Yes. Unfortunately, it is not legal afterwards to do more with `ap' in test(). >2) Is it legal ANSI C to traverse the argument list multiple times? >[using va_start + series of va_arg + va_end + va_start + va_arg + va_end] Yes, this is explicitly allowed. As you noted, the only real problem is the obnoxious (and completely unnecessary, since the `...' syntax must be recognised by the compiler and therefore compilers could recognise some other hidden syntax for va_start's expansion) requirement for an `anchor' (the `x' in your `test'). -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163) Domain: chris@cs.umd.edu Path: uunet!mimsy!chris