undefined tests

rubadub · 01-27-2008, 04:54 PM

Hi,
from stdarg.h i'm looking at va_list and i'm wondering if it's possible to test for 'undefined', in the sense of:

Quote:

va_arg
Expands to the next argument in the paramater list of the function with type type. Note that ap must be initialized with va_start. If there is no next argument, then the result is undefined.

These are two way's that I can already do:

Code:

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

void version_a(int arg1, ...)
{
	va_list ap;
	int n;
	
	va_start(ap, arg1); 
	for (n=arg1; n!=-1; n=va_arg(ap, int))
	{
		printf("%d ", n);
	}
	va_end(ap);
	printf("\n");
}

void version_b(int n, ...)
{
	va_list ap;
	int i;
	
	va_start(ap,n);
	for(i=0; i<n; i++)
	{
		printf("%d ", va_arg(ap,int));
	}
	va_end(ap);
	printf("\n");
}

int main()
{
	version_a(1, 2, 3, 4, -1);
	version_b(3, 5, 6, 7);
	
	return 0;
}

I was trying along these types of lines, but i'm assuming i'm banging my noggin against an undefined wall!

Code:

while( (i=va_arg(ap,int)) != NULL)
while(i=va_arg(ap,int))
while( ... )	// This was used in an example from old version?
for (p=fmt;*p;p++)
etc...

oh, the idea is to not have to pass a terminating value as last arg!

osor · 01-28-2008, 06:57 PM

Quote:

Originally Posted by rubadub

from stdarg.h i'm looking at va_list and i'm wondering if it's possible to test for 'undefined', in the sense of:

When the C standard says “the result is undefined” it is not talking about a particular value “undefined” which you get when you test the result. It means that doing such results in “undefined behavior” of the implementation. This pretty much means anything can happen and whatever does happen is nonportable. A prime example of undefined behavior (besides va_arg), is dereferencing a pointer to an object whose lifetime has ended.

This is different from unspecified behavior in which there are two or more choices, and the standard does not favor one choice over another (an example of unspecified behavior is the order of evaluation of function arguments).

Quote:

Originally Posted by rubadub

oh, the idea is to not have to pass a terminating value as last arg!

Unfortunately, the way C was set up (and the way it is often implemented—with function calling on a stack), it is impossible to have runtime detection of the “last” argument of a function call.

osor · 01-28-2008, 07:00 PM

Quote:

Originally Posted by osor

Unfortunately, the way C was set up (and the way it is often implemented—with function calling on a stack), it is impossible to have runtime detection of the “last” argument of a function call.

Btw, you don’t have to use a terminating value to figure out the number of arguments you have. For example, the printf() function will deduce the number of arguments it has by reading the format string and counting the number of format specifiers.

rubadub · 01-29-2008, 11:47 PM

Cheers,
This is what I thought... but I like to check all avenues...

P.S. nice tip about printf though (does printf itself use va_list?)