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

The information in this article applies to:

• The C Run-time (CRT) included with: - Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0, 1.5 - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0,

       4.1, 4.2, 5.0
  

SYMPTOMS

When the va_arg() macro is used more than once in a function parameter list, an application produces different results depending on the version of the C compiler used to build the application.

CAUSE

The C standard does not precisely define expression evaluation order. In many cases, the compiler implementation determines the order in which an application evaluates expressions.

RESOLUTION

To preserve the original order of the arguments, use the va_arg() macro only once in a given expression. One method is to evaluate the expression in a loop that processes each variable argument and stops when the last parameter is reached.

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));
   }

   1 4 3 2

   1 4 4 4

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);
      }
   }

    1 2 3 4

Keywords          : CRTIss
Version           : MS-DOS:6.0,6.00a,6.00ax,7.0;WINDOWS:1.0,1.5;WINDOWS  NT:1.0,2.0,2.1,4.0,4.1,4.2,5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbprb