C Tutorial/Function/Variable Number of Arguments

va_arg, va_copy, va_start, and va_end

Item Value Header file stdarg.h Declartion type va_arg(va_list argptr, type);void va_copy(va_list target, va_list source);void va_end(va_list argptr);void va_start(va_list argptr, last_parm);

Use va_arg(), va_start(), and va_end() macros together to pass a variable number of arguments to a

function. For example, printf(). The type va_list is defined by <stdarg.h>.

<source lang="cpp">#include <stdio.h>

 #include <stdarg.h>
 double sum_series(int num, ...);
 /* Variable length argument example - sum a series. */
 int main(void)
 {
   double d;
   d = sum_series(5, 0.5, 0.25, 0.125, 0.0625, 0.03125);
   printf("Sum of series is %f.\n", d);
   return 0;
 }
 double sum_series(int num, ...)
 {
   double sum=0.0, t;
   va_list argptr;
   /* initialize argptr */
   va_start(argptr, num);
   /* sum the series */
   for( ; num; num--) {
     t = va_arg(argptr, double); /* get next argument */
     sum += t;
   }
   /* do orderly shutdown */
   va_end(argptr);
   return sum;
 }</source>