This is a simple c program to sum two big number using c programming language.

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<ctype.h>
int chrtoint(char a){
int i;
for (i = 48; i<=57; i++)
if (toascii(i)==a) return i-48;
return 0;
}

void main(){
char n1[80];
char n2[80];
int rs[80];
int c1, c2;

int i,j,m, cmax, sum;

printf("Enter First Number:");
scanf("%s", &n1);
printf("\nEnter Second Number:");
scanf("%s", &n2);
c1 = strlen(n1);
c2 = strlen(n2);

strrev(n1);
strrev(n2);

cmax = c1;
if(c1<c2){
cmax = c2;
}

m=0;
for(i=0; i< cmax; i++){
if(c1==c2 || (i < c1 && i < c2)){
sum = m+chrtoint(n1[i])+chrtoint(n2[i]);
}else if(i >=c1){
sum = m+chrtoint(n2[i]);
}else if(i >=c2){
sum = m+chrtoint(n1[i]);
}
rs[i] = sum%10;
m = sum/10;
}

if(m){
rs[i]=m;
i++;
}

printf("\nResult: ");
for(j=0; j < i; j++){
printf("%d", rs[i-j-1]);
}

}

Hope I will solve the multiplication problem soon.