Q119394: PRB: Using References with va_* Macros from stdarg.h

Article: Q119394
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,2.1,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbCRT kbVC100 kbVC150 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Last Modified: 11-FEB-2002

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The information in this article applies to:

- The C Run-Time (CRT), included with:
   - Microsoft C/C++ for MS-DOS 
   - 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 
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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) )

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 
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : :1.0,1.5,2.0,2.1,4.0,5.0,6.0
Issue type        : kbprb

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