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Showing content from https://cplusplus.com/reference/cstdarg/va_start/ below:

macro

va_start

void va_start (va_list ap, paramN);

Initialize a variable argument list

Initializes ap to retrieve the additional arguments after parameter paramN.

A function that invokes va_start, shall also invoke va_end before it returns.

Parameters

ap

Uninitialized object of type va_list.
After the call, it carries the information needed to retrieve the additional arguments using va_arg.
If ap has already been passed as first argument to a previous call to va_start or va_copy, it shall be passed to va_end before calling this function.

paramN

Name of the last named parameter in the function definition. The arguments extracted by subsequent calls to va_arg are those after paramN.

The parameter shall not be a parameter declared with register storage class, with function or array type, or with a type that is not compatible with the type that results after application of the default argument promotions.

The parameter shall not be of a reference type, or of a type that is not compatible with the type that results when passing an argument for which there is no parameter.

Return Value none

Example

/* va_start example */
#include <stdio.h>      /* printf */
#include <stdarg.h>     /* va_list, va_start, va_arg, va_end */

void PrintFloats (int n, ...)
{
  int i;
  double val;
  printf ("Printing floats:");
  va_list vl;
  va_start(vl,n);
  for (i=0;i<n;i++)
  {
    val=va_arg(vl,double);
    printf (" [%.2f]",val);
  }
  va_end(vl);
  printf ("\n");
}

int main ()
{
  PrintFloats (3,3.14159,2.71828,1.41421);
  return 0;
}

The function PrintFloats takes the number of additional arguments as first argument (n), which are then sequentially read using the cstdarg macro and printed out with a specific format.

Output:

Printing floats: [3.14] [2.72] [1.41]