PRB: Unpredictable Results When >1 va_arg() in Parameter List (66738)

MORE INFORMATION

The va_arg() macro provides a method to access the arguments of a function when the function can accept a variable number of arguments. The macro returns a variable parameter by incrementing the value of the arg_ptr that points to the list of arguments. The values returned depend on the order in which the application evaluates the va_arg() macros.

However, because the C language does not guarantee the order in which most expressions are evaluated, application code should not depend on a particular evaluation order. The sample code below demonstrates this behavior and the unwanted side effects that can occur when code that depends on evaluation order is compiled with different versions of the same compiler.

Sample Code #1

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

void myfunc(int, ...);

void main(void)
{
   myfunc(1, 2, 3, 4);
}

void myfunc(int first, ...)
{
   va_list arg_ptr;
   va_start(arg_ptr, first);

   printf("%d %d %d %d\n", first, va_arg(arg_ptr, int),
                                  va_arg(arg_ptr, int),
                                  va_arg(arg_ptr, int));
}
				

When this sample is compiled with C version 5.1 or C/C++ version 8.0 for MS- DOS, the sample produces the following output:

   1 4 3 2
				

However, when this sample is compiled with Microsoft C versions 6.0, 6.0a, 6.0ax, Microsoft C/C++ version 7.0, or Microsoft Visual C++ 32-bit Edition, versions 1.0 through 4.2, the sample produces the following output:

   1 4 4 4
				

In Microsoft Visual C++ versions 5.0 and 6.0, the output is the same as C 5.1 and C/C++ 8.0.

In the following sample code, the expressions are evaluated correctly without regard to the expression evaluation order.

Sample Code #2

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

void myfunc(int, ...);

void main(void)
{
   myfunc(1, 2, 3, 4, -1);     /* -1 is a flag */ 
}

void myfunc(int first, ...)
{
   int temp;
   va_list arg_ptr;
   va_start(arg_ptr, first);

   temp = first;
   while(temp != -1)        /* test for flag (-1) */ 
   {
      printf("%d ", temp);
      temp = va_arg(arg_ptr, int);
   }
}
				

The output is as follows:

    1 2 3 4