Here s one approach: Consider how you would multiply these numbers by hand, on paper. Implement this method in C. You will have to discover:

• how to break up an integer (represented as a string) into digits
• how to convert each digit back to an integer 0 <= d < 10
• how to manage arrays of digits (ie. how big should you make the arrays?)
• how to write the loop(s) you might need to implement multiplication
• how to manage carrying products from one digit to the next
• how to convert those digits back to characters for output

usually big integers represented as byte arrays. You can look at Microsoft s BigInteger implementation in DLR. I think they ve used algorithms developed by Knuth

Check this BigInteger library and a very basic sample code from World of Seven.

If you are interested in some of my home cooked codes in C (only multiplication) :

////////////////////////////////////////////////////////////////////////////////

Code removed after I checked the home-work tag ;)

///////////////////////////////////////////////////////////////////////////////////////

This works in some of the earlier programming contests I had participated ;)But if you are looking for even faster multiplication algorithm you can Implement Karatsuba algorithm,I personally use this now in real time contest.

Hey man,check this out,i just completed it yester day as a part of my homework:

#include<stdio.h>
#include<string.h>

int main()
{
    char one[195];
    char two[195];
    char temp[195];
    int a[195],c[195],b[195];
    int x,i,j,k,l,p,add;

    for(;;)/*determining the larger number...*/
    {
        printf("Input A:");
            gets(one);
        printf("Input B:");
            gets(two);

        k=strlen(one);
        l=strlen(two);
        if(l>k)
        {
            strcpy(temp,one);
            strcpy(one,two);
            strcpy(two,temp);
            break;
        }
        else
        {
            break;
        }
    }
        k=strlen(one);
        l=strlen(two);
    for(p=0;p<195;p++)/*assigning all initial values to 0*/
    {
        a[p]=0;
        b[p]=0;
        c[p]=0;
    }

    for(i=0;one[i];i++)/*converting char to integer(note:1,as a character assigned as 49.)*/
    {
        a[i]=((one[--k])-48);
    }

    for(i=0;i<two[i];i++)
    {
        b[i]=((two[--l])-48);
    }


    for(i=0;i<=strlen(two);i++)/*main algorithm*/
    {
        add=0;
        p=0;
        for(j=i;j<=(2*strlen(one)-1);j++)
        {
            x=c[j]+b[i]*a[p]+add;
            c[j]=x%10;
            add=x/10;
            p++;
        }
    }

    printf("
Multiplication:");
    for(p=(2*strlen(one)-1);p>=0;p--)
    {
        if(p>strlen(one)&&c[p]==0)
        {
            continue;
        }
        printf("%d",c[p]);
    }
    printf("
");
}

You can use a library for large integer arithmetik, Wikipedia has a list here.

Another approach would be to multiply the numbers as float/double and stip off the decimal when displaying the results.