Fibonacci sequence

//
// https://www.programiz.com/c-programming/online-compiler/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>

char* add_strings(char *a, char *b);

int main(int argc, char *argv[]) {

    char x[1001]="0";
    char y[1001]="1";
    char *z;
    int i;
    int seq;
    if (argc != 2) return 1;
    if ((seq=atoi(argv[1])) == 0) return 1;
    if (seq > 1000) return 1;
    seq += 1;

    printf("1:\t1\n");
    for (i=2; i < seq; i++) {
        z=add_strings(x,y);
        strcpy(x,y);
        strcpy(y,z);
        printf("%d:\t%s\n", i,z);
    }

    return 0;
}


char* add_strings(char *a, char *b) {
    char *r;
    char tmp,carry;
    int la=strlen(a);
    int lb=strlen(b);

   // swab a - b , so a is always bigger
    if (la < lb) {
        r = a;
        a = b;
        b = r;
        la = strlen(a);
        lb = strlen(b);
    }

    //  result string is 1 bigger than a for carry bit
    r = (char*)malloc((la+2) * (sizeof(char)));
    r[la+1] ='\0';

    carry=0;
    int j = lb-1;
    for (int i=la-1; i >=0; i--, j--) {
        if (j >= 0) {
            tmp = a[i] + b[j] - 96 + carry;
        } else {
            tmp = a[i] - 48 + carry;
        }
        r[i+1] = (tmp % 10) + 48;
        if (tmp / 10 ) carry = 1; else carry = 0;
    }
    if (carry) r[0] = 49; else r = r + 1;

    return r;
}