What is Ellipsis or … in c ?

Ellipsis is there consecutive period (.) with no white space. With the help of ellipsis we can write a function of variable number of parameter. For example:

#include<stdio.h>
int number(int a,...){
   return a;
}

int main() {
   int i;
   i=number(5,4,3,2,1);
   printf("%d",i);

   return 0;
}

Output: 5

Ellipsis can be used as a variable number of parameters or a variable which stores variable number of arguments. For example:

#include<stdio.h>

int dynamic(int,...);

int main(){

    int x,y;

    x=dynamic(2,4,6,8,10,12,14);

    y=dynamic(3,6,9,12);

   

    printf("%d %d ",x,y);

    return 0;

}

int dynamic(int s,...){

    void *ptr;

    ptr=...;

    (int *)ptr+=2;

    s=*(int *)ptr;

    return s;

}

Here dynamic function is example of variable number of arguments while pointer ptr is example of variable which is storing variable number of arguments.

In header file stdarg.h has three macro va_arg, va_end, va_start, with help of this macro we can know other parameter of a function in case of variable number of parameters. Syntax:

void va_start (va_list ap,lastfix)

type va_arg(va_list ap,type)

void va_end(va_list ap);

va_list is list of data .

lastfix is a last fixed parameter used in a function of variable number of parameters.

type is used to know which data type are using in function of variable number of parameters .

va_arg always returns next parameter from right to left.

va_start set the pointer ap to the first parameter from right side of a function of variable number of parameters.

va_end help in the normal return. Example:

#include<stdio.h>

#include <stdarg.h>

void sum(char *msg, ...){

int total = 0;

va_list p;

int arg;

va_start(p, msg);

while ((arg = va_arg(p,int)) != 0) {

total += arg;

}

printf(msg, total);

va_end(p);

}

int main() {

sum("The total sum is %d\n", 5,7,11,8);

return 0;

}

Output: 31

Properties of variable number of arguments:

1. First parameter must be any other data type in case of variable number of arguments.

Invalid declaration of function:

int (...);

Valid declaration of function:

int (char c,...);

void(int,float,...);

2.

We cannot pass any other data type after the variable number of arguments.

Invalid declaration of function:

int (int a,...,int b);

3. We cannot give any blank space between any two periods (dot) in the ellipsis.

Invalid declaration:

int (float b,. . .);

4. In place of ellipsis we can pass the data either of same type or different type.

#include<stdio.h>

#include <stdarg.h>

int flue(char c,...);

int main(){

    int x,y;

    x=flue('A',1,2,3);

    y=flue('1',1.0,1,'1',1.0f,1l);

   

    printf("%d %d",x,y);

return 0;

}

int flue(char c,...){

    return c;

}

Output: 65 49