Someone

void insert_char(char *s, char c, int pos) {
    int s_len, i;
    char *new_s;
    
    s_len = strlen(s);
    new_s = (char*)malloc(sizeof(char) * (s_len + 1));
    strncpy(old_s, s, s_len);
    new_s[s_len] = '\0';

    if (pos < s_len) {
        i = pos;
        new_s[i] = c;
        for (i++; i <= s_len; i++) {
            new_s[i] = s[i-1];
        }
        s[i] = '\0';
    }
    else {
        sprintf(new_s, "%s%c", s, c);
    }

    free(s);
    s = new_s;
}