va_arg

  #include <cstdarg>
  type va_arg( va_list argptr, type );
  void va_end( va_list argptr );
  void va_start( va_list argptr, last_parm );

The va_arg() macros are used to pass a variable number of arguments to a function.

1. First, you must have a call to va_start() passing a valid va_list and the mandatory first argument of the function. This first argument can be anything; one way to use it is to have it be an integer describing the number of parameters being passed.
2. Next, you call va_arg() passing the va_list and the type of the argument to be returned. The return value of va_arg() is the current parameter.
3. Repeat calls to va_arg() for however many arguments you have.
4. Finally, a call to va_end() passing the va_list is necessary for proper cleanup.

For example:

  int sum( int num, ... ) {
    int answer = 0;
    va_list argptr;            

    va_start( argptr, num );            

    for( ; num > 0; num-- ) {
      answer += va_arg( argptr, int );
    }           

    va_end( argptr );           

    return( answer );
  }             
                

  int main( void ) {            

    int answer = sum( 4, 4, 3, 2, 1 );
    printf( "The answer is %d\n", answer );           

    return( 0 );
  }             

This code displays 10, which is 4+3+2+1.

Here is another example of variable argument function, which is a simple printing function:

 void my_printf( char *format, ... ) {
   va_list argptr;             

   va_start( argptr, format );          

   while( *format != '\0' ) {
     // string
     if( *format == 's' ) {
       char* s = va_arg( argptr, char * );
       printf( "Printing a string: %s\n", s );
     }
     // character
     else if( *format == 'c' ) {
       char c = (char) va_arg( argptr, int );
       printf( "Printing a character: %c\n", c );
       break;
     }
     // integer
     else if( *format == 'd' ) {
       int d = va_arg( argptr, int );
       printf( "Printing an integer: %d\n", d );
     }          

     *format++;
   }            

   va_end( argptr );
 }              
                

 int main( void ) {             

   my_printf( "sdc", "This is a string", 29, 'X' );         

   return( 0 );
 }              

This code displays the following output when run:

 Printing a string: This is a string
 Printing an integer: 29
 Printing a character: X