Variadic functions allow a function to take a variable number of arguments. They are declared with an ellipsis after the last named parameter. The stdarg.h header provides macros for accessing the variable arguments: va_start initializes argument parsing, va_arg returns the next argument of a given type, and va_end cleans up. An example sums all integer arguments by iterating with va_arg, illustrating how variadic functions work.

1. Variadic Functions

2. Introduction
 A function which takes a variable number or type of
arguments is called variadic function.
 Also called as Varargs function (or) Variable
Argument Lists.
 variadic functions are declared using a partial
parameter list followed by the ellipsis notation.
 A variadic function is invoked simply by specifying
the desired number of arguments in the function call.
E.g. average(3, 5, 8, -1).

3.  Most common variable argument
functions are
 void printf(const char* fmt, …);
 void scanf(const char* fmt, …);
 Parameters include
 One named argument usually a format
string.
 Ellipsis indicating the function may
take any number of arguments and their
types.

4. Creating a Variadic Function
 Use standard header stdarg.h
 4 macros (not prototypes) needed from this
header file
 va_list
 va_start
 va_args
 va_end

5. Variable arguments
 va_list
 Stores the list of arguments (argument pointer) and be
defined for variables used to traverse the list.
 Declared like any other variable.
 Ex. va_list ap
 va_start
 initializes the list, point to the first argument.
 Accepts two arguments.
o va_list.
o Name of the variable that directly precedes the
ellipsis.
 Ex. va_start(ap, fmt)

6.  va_args
 returns the next argument of whatever type it is
told.
 moves down to the next argument in the list.
 Accepts two arguments
o va_list
o Variable argument type
 Ex. va_args(ap, int) //returns next int value in the
argument
 va_end
 Cleans up the variable argument list.
 Ex. va_end(ap)

7. Example1
#include <stdarg.h>
#include <stdio.h>
int add_em_up (int count,...)
{
va_list ap;
int i, sum;
va_start (ap, count); /* Initialize the argument list. */
sum = 0;
for (i = 0; i < count; i++)
sum += va_arg (ap, int); /* Get the next argument value. */
va_end (ap); /* Clean up. */
return sum;
}
int main (void)
{
printf ("%dn", add_em_up (3,5,5,6));
printf ("%dn", add_em_up (10,1,2,3,4,5,6,7,8,9,10));
return 0;
}

8. Example2
#include <stdio.h>
#include <stdarg.h>
void simple_printf(const char *fmt, ...)
{
va_list args;
va_start(args, fmt);
while (*fmt != '0') {
if (*fmt == 'd') {
int i = va_arg(args, int);
printf( "%dn", i ) ;
} else if (*fmt == 'c') {
int c = va_arg(args, int);
printf( "%cn", (char)c ) ;
} else if (*fmt == 'f') {
double d = va_arg(args, double);
printf( "%fn", d ) ;
}
++fmt;
}
va_end(args);
}
int main()
{
simple_printf("dcff", 3, 'a', 1.999, 42.5);
}