Long Fibonacci numbers

#include <stdlib.h>
#include <stdio.h>

#define MAXLENGTH 600

char a[MAXLENGTH], b[MAXLENGTH];
long int length;

void add (char *x, char *y){
    long i;
    for(i=0;i<length;i++){
        x[i]+=y[i];
        if(x[i]>9){
            x[i]-=10;
            x[i+1]++;
        }
    }
    if(x[length]) ++length;
}

void output(char *x){
    long i;
    for(i=length-1;i>=0;i--){
        putchar(x[i]+'0');
    }
    putchar('\n');
}

int main(void){
    a[0]=1, length=1;
    while(length<MAXLENGTH){
        output(b);
        add(a,b);
        output(a);
        add(b,a);
    }
    exit(0);
}