#include <stdio.h>

void repstr(char *str, char *substr, char *repstr) {
    char *pb, *ps;
    while (*str) {
        pb = str;
        ps = substr;
        while (*ps) {
            if (*pb == *ps) {
                pb++;
                ps++;
            } else {
                break;
            }
        }
        if (*ps == 0) {
            ps = repstr;
            while (*ps)
                *str++ = *ps++;
            str = pb;
        } else {
            str++;
        }
    }
}

void main() {
    char str[100] = "abcdefabcdefabcdef";
    repstr(str, "abc", "XYZ");
    puts(str);
}