va_arg, va_end, va_start

Article
04/22/2008

Access variable-argument lists.

type va_arg(
      va_list arg_ptr,
      type 
);
void va_end(
   va_list arg_ptr 
);
void va_start(
   va_list arg_ptr 
);  // (UNIX version)
void va_start(
   va_list arg_ptr,
      prev_param 
); // (ANSI version)

Parameters

type
Type of argument to be retrieved.
arg_ptr
Pointer to list of arguments.
prev_param
Parameter preceding first optional argument (ANSI only).

Return Value

va_arg returns the current argument; va_start and va_end do not return values.

Remarks

The va_arg, va_end, and va_start macros provide a portable way to access the arguments to a function when the function takes a variable number of arguments. Two versions of the macros are available: The macros defined in STDARG.H conform to the ANSI C standard, and the macros defined in VARARGS.H are compatible with the UNIX System V definition. The macros are:

va_alist
Name of parameter to called function (UNIX version only)
va_arg
Macro to retrieve current argument
va_dcl
Declaration of va_alist (UNIX version only)
va_end
Macro to reset arg_ptr
va_list
typedef for pointer to list of arguments defined in STDIO.H
va_start
Macro to set arg_ptr to beginning of list of optional arguments (UNIX version only)

Both versions of the macros assume that the function takes a fixed number of required arguments, followed by a variable number of optional arguments. The required arguments are declared as ordinary parameters to the function and can be accessed through the parameter names. The optional arguments are accessed through the macros in STDARG.H or VARARGS.H, which set a pointer to the first optional argument in the argument list, retrieve arguments from the list, and reset the pointer when argument processing is completed.

The ANSI C standard macros, defined in STDARG.H, are used as follows:

All required arguments to the function are declared as parameters in the usual way. va_dcl is not used with the STDARG.H macros.
va_start sets arg_ptr to the first optional argument in the list of arguments passed to the function. The argument arg_ptr must have va_list type. The argument prev_param is the name of the required parameter immediately preceding the first optional argument in the argument list. If prev_param is declared with the register storage class, the macro's behavior is undefined. va_start must be used before va_arg is used for the first time.
va_arg retrieves a value of type from the location given by arg_ptr and increments arg_ptr to point to the next argument in the list, using the size of type to determine where the next argument starts. va_arg can be used any number of times within the function to retrieve arguments from the list.
After all arguments have been retrieved, va_end resets the pointer to NULL.

The UNIX System V macros, defined in VARARGS.H, operate somewhat differently:

Any required arguments to the function can be declared as parameters in the usual way.
The last (or only) parameter to the function represents the list of optional arguments. This parameter must be named va_alist (not to be confused with va_list, which is defined as the type of va_alist).
va_dcl appears after the function definition and before the opening left brace of the function. This macro is defined as a complete declaration of the va_alist parameter, including the terminating semicolon; therefore, no semicolon should follow va_dcl.
Within the function, va_start sets arg_ptr to the beginning of the list of optional arguments passed to the function. va_start must be used before va_arg is used for the first time. The argument arg_ptr must have va_list type.
va_arg retrieves a value of type from the location given by arg_ptr and increments arg_ptr to point to the next argument in the list, using the size of type to determine where the next argument starts. va_arg can be used any number of times within the function to retrieve the arguments from the list.
After all arguments have been retrieved, va_end resets the pointer to NULL.

When compiled with /clr (Common Language Runtime Compilation), programs using these macros may generate unexpected results because of differences between native and common language runtime type systems. Consider this program:

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

void testit ( int i, ...)
{
   va_list argptr;
   va_start(argptr, i);

   if ( i == 0 ) {
      int n = va_arg( argptr, int );
      printf( "%d\n", n );
   } else {
      char *s = va_arg( argptr, char* );
      printf( "%s\n", s);
   }
}

int main()
{
   testit( 0, 0xFFFFFFFF ); // 1st problem: 0xffffffff is not an int
   testit( 1, NULL );       // 2nd problem: NULL is not a char*
}

Note that testit expects its second parameter to be either an int or a char*. The arguments being passed are 0xffffffff (an unsigned int, not an int) and NULL (actually an int, not a char*). When compiled for native code, the program produces the output

-1
(null)

However, when compiled with /clr:pure, the type mismatches cause the program to generate an exception. The solution is to use explicit casts:

int main()
{
   testit( 0, (int)0xFFFFFFFF ); // cast unsigned to int
   testit( 1, (char*)NULL );     // cast int to char*
}

Requirements

Routine	Required header	Optional headers
va_arg	<stdio.h> and <stdarg.h>	<varargs.h>*
va_end	<stdio.h> and <stdarg.h>	<varargs.h>*
va_start	<stdio.h> and <stdarg.h>	<varargs.h>*

* Required for UNIX V compatibility.

For additional compatibility information, see Compatibility in the Introduction.

Libraries

All versions of the C run-time libraries.

Example

// crt_va.c
/* The program below illustrates passing a variable
 * number of arguments using the following macros:
 *      va_start            va_arg              va_end
 *      va_list             va_dcl (UNIX only)
 */

#include <stdio.h>
#define ANSI            /* Comment out for UNIX version     */
#ifdef ANSI             /* ANSI compatible version          */
#include <stdarg.h>
int average( int first, ... );
#else                   /* UNIX compatible version          */
#include <varargs.h>
int average( va_list );
#endif

int main( void )
{
   /* Call with 3 integers (-1 is used as terminator). */
   printf( "Average is: %d\n", average( 2, 3, 4, -1 ) );

   /* Call with 4 integers. */
   printf( "Average is: %d\n", average( 5, 7, 9, 11, -1 ) );

   /* Call with just -1 terminator. */
   printf( "Average is: %d\n", average( -1 ) );
}

/* Returns the average of a variable list of integers. */
#ifdef ANSI             /* ANSI compatible version    */
int average( int first, ... )
{
   int count = 0, sum = 0, i = first;
   va_list marker;

   va_start( marker, first );     /* Initialize variable arguments. */
   while( i != -1 )
   {
      sum += i;
      count++;
      i = va_arg( marker, int);
   }
   va_end( marker );              /* Reset variable arguments.      */
   return( sum ? (sum / count) : 0 );
}
#else       /* UNIX compatible version must use old-style definition.  */
int average( va_alist )
va_dcl
{
   int i, count, sum;
   va_list marker;

   va_start( marker );            /* Initialize variable arguments. */
   for( sum = count = 0; (i = va_arg( marker, int)) != -1; count++ )
      sum += i;
   va_end( marker );              /* Reset variable arguments.      */
   return( sum ? (sum / count) : 0 );
}
#endif

Output

Average is: 3
Average is: 8
Average is: 0

.NET Framework Equivalent

System::ParamArrayAttribute Class

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