overlap.c

#include <stdio.h>

int overlap_count(const char *s1, const char *s2)
{
    const char *s1_begin = s1;
    const char *p1, *p2;

    while(*s1_begin != '\0') {
        p1 = s1_begin;
        p2 = s2;

        while(*p1 != '\0' && *p2 != '\0' && *p1 == *p2) {
            p1++;
            p2++;
        }

        if(*p1 == '\0')
            return p1 - s1_begin;

        s1_begin++;
    }

    return 0;
}

int main(int argc, char **argv) {
    if(argc < 3) {
        printf("Not enough input.\n");
        return 1;
    }

    printf("Overlapping size = %d\n", overlap_count(argv[1], argv[2]));
    return 0;
}