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

ID: Q119394

The information in this article applies to:

The C Run-Time (CRT), included with:
- Microsoft C/C++ for MS-DOS, version 7.0
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0

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)&amp;v + _INTSIZEOF(v) )

where va_list is defined as a char * on Intel platforms. The macro parameter "ap" is of type va_list. The problem arises from taking the address of the second parameter, "v", if v is a reference type. The net result of this macro being expanded is that ap is supposed to point to the first of the variable parameters. Casting v to a non-reference type intuitively seems like the logical solution, but because the result of a cast is not an l-value, the compiler returns an error message.

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

The sample code below demonstrates a solution for this problem:


/* 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 );
}

Additional query words: ellipsis

Keywords : kbCRT kbVC100 kbVC150 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Version : 7.0 1.0 1.5 2.0 2.1 4.0 5.0
Platform : MS-DOS NT WINDOWS
Issue type : kbprb