PRB: Using References with va_* Macros from stdarg.h

• The C Run-time (CRT) included with: - 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, 5.0

SYMPTOMS

In Microsoft C++, if you use functions that accept a variable number of arguments, you may encounter problems when trying to use the va_* family of functions to access the parameters if the second parameter used for the va_start macro is a reference type.

CAUSE

This problem is caused by the way that the va_start macro is defined and the way that the C++ language handles taking the address of a reference. Applying the "address of" operator to a reference type results in a pointer to the object that is being referred to. The va_start macro takes the address of the last named parameter to locate subsequent parameters. When the last named parameter is a reference, this causes problems because the macro is no longer referring to the current call stack but whatever follows the object being referred to, which could be a previous call stack or a global memory object.

RESOLUTION

The workaround is to redefine the va_start macro to use inline assembly to subvert the C++ language.

NOTE: This solution is not portable and will require changing if you intend your source code to be used on non-Intel platforms.

MORE INFORMATION

The va_start macro is used in conjunction with the va_arg macro to "walk" the stack to get the parameters passed to the variable argument list. The va_start macro is defined as follows:

   #define va_start(ap,v)  ( ap = (va_list)&v + _INTSIZEOF(v) )

NOTE: The only way to get an l-value from a cast is to cast the value to a reference type, which results in the same problem.

Sample Code

   /* Compile options needed:  none
   */

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

   // Uncomment the following lines to work-around the problem:
   //
   // #ifdef va_start
   // #undef va_start
   //
   // #ifdef _WIN32
   // #define va_start(ap,v) {int var= _INTSIZEOF(v); \
   //                __asm lea eax,v __asm add eax,var __asm mov ap,eax \
   //                }
   // #else
   // #define va_start(ap,v) { int var=_INTSIZEOF(v);\
   //                __asm lea ax,v __asm add ax,var __asm mov ap,ax\
   //                }
   // #endif
   // #endif

   void numprint( int &first ... )
   {
     va_list ap;

     va_start( ap, first );
     printf("%d\n", first );
     int ival = va_arg( ap, int );
     printf("%d\n", ival );
     double dval = va_arg( ap, double );
     printf( "%.2f\n", dval );
     va_end(ap);
   }

   void main()
   {
     int i=100,j=1000;
     float f=999.99;

     numprint( i,j,f );
   }