[C++知识库]how to use ellipses in c function

1.Define a function with its last parameter as ellipses and the one just before the ellipses is always an int which will represent the number of arguments.

2.Create a va_list type variable in the function definition. This type is defined in stdarg.h header file.

3.Use int parameter and va_start macro to initialize the va_list variable to an argument list. The macro va_start is defined in stdarg.h header file.

3.Use va_arg macro and va_list variable to access each item in argument list.

4.Use a macro va_end to clean up the memory assigned to va_list variable.

example following:

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

double average(int num,...) {

   va_list valist;
   
   double sum = 0.0;
   int i;

/* initialize valist for num number of arguments */   
// void va_start(va_list ap, last_arg);
//   The last_arg is the last known fixed argument being passed to the function i.e. the argument before the ellipsis.
   va_start(valist, num);

/* access all the arguments assigned to valist */
   for (i = 0; i < num; i++) {
//   This macro returns the next additional argument as an expression of type type.
      sum += va_arg(valist, int);
   }
	
   /* clean memory reserved for valist */
   va_end(valist);

   return sum/num;
}

int main() {
   printf("Average of 2, 3, 4, 5 = %f\n", average(4, 2,3,4,5));
   printf("Average of 5, 10, 15 = %f\n", average(3, 5,10,15));
}

/*
 * syslog, vsyslog --
 *	print message on log file; output is intended for syslogd(8).
 */
void
__syslog(int pri, const char *fmt, ...)
{
	va_list ap;

	va_start(ap, fmt);
	__vsyslog_chk(pri, -1, fmt, ap);
	va_end(ap);
}