リストを使ったechoサーバの作成がうまくいきません

現在、echoサーバを作成しています。
要件は、長い文字列が送られていた時も、リストを用いてちゃんと返すというものです。
自分でここまでプログラムを書いたのですが、以下のようにエラーが起きます。
どこに原因があるのでしょうか。

master@LAPTOP-4FABUBT7:~$ ./a.out 3875
free(): invalid pointer
Aborted (core dumped)

master@LAPTOP-4FABUBT7:~$ telnet localhost 3875
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
hello
OK: hello

 helooooooooooooooooooooooooo
Connection closed by foreign host.

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <errno.h>
#include <err.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <arpa/inet.h>

#define SIZE 2048
#define L_SIZE 6//リストサイズ

/* リスト */
struct List {
    char*str;
    char *save;
    int len;
    int head;
};

/*リスト初期化*/
void init_list(struct List*list) {
    list->len = L_SIZE;
    list->save = (char*)malloc(sizeof(char)*L_SIZE);
    list->head = 0;
    list->str=(char*)malloc(sizeof(char)*L_SIZE);
}

/*ソケット準備*/
int socket_set(char*argv, struct addrinfo *hints, struct addrinfo **res) {

    struct sockaddr_storage from;
    socklen_t fromlen;
    int error;

    hints->ai_socktype = SOCK_STREAM;
    hints->ai_family = AF_INET;
    hints->ai_flags = AI_PASSIVE;

    error = getaddrinfo(NULL, argv, hints, res);
    if (error) {
        errx(1, "%s", gai_strerror(error));
        return 1;
    }

    return socket((*res)->ai_family, (*res)->ai_socktype, (*res)->ai_protocol);

}

/*拡張*/
int expand(struct List**list) {

    char*new_cell;
    int i;
    (*list)->len *= 2;
    if ((new_cell = (char*)malloc(sizeof(char)*((*list)->len))) == NULL) {
        return 0;
    }
    for (i = 0; i < (*list)->head; i++) {
        new_cell[i] = (*list)->save[i];
    }
    for (i = 0; i < (*list)->len; i++) {
        new_cell[(*list)->head + i] = (*list)->str[i];
    }
    (*list)->head = (*list)->head * 2 + L_SIZE;
    free((*list)->str);
    free((*list)->save);
    (*list)->save = new_cell;
    (*list)->str = (char*)malloc(sizeof(char)*((*list)->len));

    return 1;
}
void Read_Write(int *i, int *n, struct List*buf0, struct List*buf1, fd_set rfd0) {

    int k;
    int l;
    while ((*n = read(*i, buf0->str, buf0->len)) >= buf0->len) {
        if ((buf0->str[*n - 2] == 13) || (buf0->str[*n - 2] == 10)) {
            break;
        }
        expand(&buf0);
        expand(&buf1);
    }
    if (*n <= L_SIZE)
    {
        if (*n >= 2) {
            buf0->str[*n - 2] = '\0';
            for (k = 0; k < L_SIZE; k++) {
                buf0->save[k + buf0->head] = buf0->str[k];
            }
        }
        else if (*n == 1) {
            buf0->save[buf0->head] = buf0->str[k];
        }
        else {
            buf0->str[*n - 2] = '\0';
            expand(&buf0);
            expand(&buf1);
        }
    }
    if (strncmp("quit", buf0->str,4) == 0) {
        close(*i);
        FD_CLR(*i, &rfd0);
    }
    else {
        sprintf(buf1->str, "OK: %s\n", buf0->save);
        l = write(*i, buf1->str, strlen(buf1->str) + 1);
        if (l < 0) {
            perror("write");
            exit(1);
        }
    }
}
int main(int argc, char *argv[]) {

    struct addrinfo hints, *res;
    struct sockaddr_storage from;
    socklen_t fromlen;
    int i, n, ls, s, test;
    i = n = ls = s = test = 0;

    fd_set rfd, rfd0;
    char buf[2048], buf2[2048];

    struct List list0, list1;

    if (argc != 2) {
        fprintf(stderr, "usage: %s  port\n", argv[0]);
        exit(1);
    }
    memset(&hints, 0, sizeof(hints));
    init_list(&list0);
    init_list(&list1);

    while (res != 0) {
        s = socket_set(argv[1], &hints, &res);
            if (s < 0) {
                fprintf(stderr,"socket_erorr\n");
                res = res->ai_next;
            }
            else {
                break;
            }
    }

    if (bind(s, res->ai_addr, res->ai_addrlen) < 0) {
        perror("bind");
        close(s);
        return -1;
    };

    if (listen(s, 5) < 0) {
        perror("listen");
        close(s);
        return -1;
    }

    FD_ZERO(&rfd0);
    FD_SET(s, &rfd0);
    while (1) {
        rfd = rfd0;
        if((select(FD_SETSIZE,&rfd,NULL,NULL,NULL))<0){
            perror("select");
            exit(1);
        }
        for (i = 0; i < FD_SETSIZE; i++) {
            if (FD_ISSET(i, &rfd)) {
                if ((ls = accept(i, NULL, NULL)) > 0) {
                    FD_SET(ls, &rfd0);
                }
                else {
                    Read_Write(&i,&n,&list0,&list1,rfd0);
                    free(list0.str);

            free(list0.save);
                    free(list1.str);
                    free(list1.save);
                    init_list(&list0);
                    init_list(&list1);
                }
            }
        }
    }
    close(s);
}