void uip_appcall(void)
{
    uint8_t send = 0;
    uint16_t a, b;

    if (ntohs(uip_conn->lport) == 8000)
    {
        if (uip_connected())
        {
            uip_conn->appstate.step = 0;
            send = 1;

            sprintf(uip_conn->appstate.buf, "Connected! id=%d, port=%d\n", uip_conn - uip_conns, ntohs(uip_conn->lport)); // 显示所用的conn连接数组下标号, 以及端口号
        }

        if (uip_rexmit())
            send = 1;

        if (uip_newdata())
        {
            uip_conn->appstate.step++;
            send = 1;

            if (uip_conn->appstate.step == 1)
            {
                ((char *)uip_appdata)[uip_len] = '\0';
                sscanf(uip_appdata, "%d, %d", &a, &b);
                sprintf(uip_conn->appstate.buf, "sum=%d\n", a + b);
            }
        }

        if (send)
        {
            switch (uip_conn->appstate.step)
            {
            case 0:
            case 1:
                uip_send(uip_conn->appstate.buf, strlen(uip_conn->appstate.buf));
                break;
            case 2:
                uip_send_P(PSTR("Ignored!\n"));
                break;
            }
        }
    }
    else if (ntohs(uip_conn->lport) == 3000)
    {
        if (uip_connected())
        {
            uip_conn->appstate.step = 0;
            send = 1;
        }

        if (uip_rexmit())
            send = 1;

        if (uip_acked())
        {
            send = 1;
            uip_conn->appstate.step++;
        }

        if (send)
        {
            switch (uip_conn->appstate.step)
            {
            case 0:
                uip_send_P(PSTR("The closesocket function closes a socket. Use it to release the socket descriptor passed in the s parameter. Note that the socket descriptor passed in the s parameter may immediately be reused by the system as soon as closesocket function is issued. "));
                break;
            case 1:
                uip_send_P(PSTR("As a result, it is not reliable to expect further references to the socket descriptor passed in the s parameter to fail with the error WSAENOTSOCK. A Winsock client must never issue closesocket on s concurrently with another Winsock function call.\n"));
                break;
            case 2:
                uip_send_P(PSTR("Any pending overlapped send and receive operations ( WSASend/ WSASendTo/ WSARecv/ WSARecvFrom with an overlapped socket) issued by any thread in this process are also canceled. "));
                break;
            case 3:
                uip_send_P(PSTR("Any event, completion routine, or completion port action specified for these overlapped operations is performed. The pending overlapped operations fail with the error status WSA_OPERATION_ABORTED.\n"));
                break;
            case 4:
                uip_send_P(PSTR("An application should not assume that any outstanding I/O operations on a socket will all be guaranteed to completed when closesocket returns. "));
                break;
            case 5:
                uip_send_P(PSTR("The closesocket function will initiate cancellation on the outstanding I/O operations, but that does not mean that an application will receive I/O completion for these I/O operations by the time the closesocket function returns. "));
                break;
            case 6:
                uip_send_P(PSTR("Thus, an application should not cleanup any resources (WSAOVERLAPPED structures, for example) referenced by the outstanding I/O requests until the I/O requests are indeed completed.\n"));
                break;
            case 7:
                uip_send_P(PSTR("An application should always have a matching call to closesocket for each successful call to socket to return any socket resources to the system.\n"));
                break;
            case 8:
                uip_send_P(PSTR("The linger structure maintains information about a specific socket that specifies how that socket should behave when data is queued to be sent and the closesocket function is called on the socket.\n"));
                break;
            case 9:
                uip_send_P(PSTR("The l_onoff member of the linger structure determines whether a socket should remain open for a specified amount of time after a closesocket function call to enable queued data to be sent. This member can be modified in two ways.\n"));
                break;
            }
        }
    }
}

void uip_udp_appcall(void)
{
    
}

void myapp_init(void)
{
    uip_listen(HTONS(3000));
    uip_listen(HTONS(8000));
}

状态结构体的定义：
typedef struct
{
    uint8_t step;
    char buf[40];
} uip_tcp_appstate_t;

AVR存储空间的使用情况：
Program Memory Usage     :    13974 bytes   85.3 % Full
Data Memory Usage         :    997 bytes   97.4 % Full

【PC端程序1】
#include <stdio.h>
#include <WinSock2.h>

#pragma comment(lib, "ws2_32.lib")

void display(SOCKET sock, char *buf, int len)
{
    len = recv(sock, buf, len, (int)NULL);
    buf[len] = '\0';
    printf("%s", buf);
}

int main(void)
{
    char buf[100];
    int a, b;
    SOCKADDR_IN addr;
    SOCKET sock;
    WSADATA wsadata;
   
    WSAStartup(MAKEWORD(2, 2), &wsadata);
    addr.sin_addr.S_un.S_addr = inet_addr("192.168.1.50");
    addr.sin_family = AF_INET;
    addr.sin_port = htons(8000);
    sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

    if (connect(sock, (SOCKADDR *)&addr, sizeof(SOCKADDR)) == 0)
    {
        display(sock, buf, sizeof(buf));

        printf("Please input two numbers: ");
        scanf_s("%d%d", &a, &b);
        sprintf_s(buf, sizeof(buf), "%d, %d", a, b);
        send(sock, buf, strlen(buf), (int)NULL);
        display(sock, buf, sizeof(buf));
   
        send(sock, "sth", 3, (int)NULL);
        display(sock, buf, sizeof(buf));

        closesocket(sock);
    }
    else
        printf("Connection failed!\n");
   
    WSACleanup();
    return 0;
}

【PC端程序2】
#include <stdio.h>
#include <WinSock2.h>

#pragma comment(lib, "ws2_32.lib")

void display(SOCKET sock)
{
    char buf[500];
    int len = recv(sock, buf, sizeof(buf), (int)NULL);
    buf[len] = '\0';
    printf("%s", buf);
}

int main(void)
{
    int i;
    SOCKADDR_IN addr;
    SOCKET sock;
    WSADATA wsadata;
   
    WSAStartup(MAKEWORD(2, 2), &wsadata);
    addr.sin_addr.S_un.S_addr = inet_addr("192.168.1.50");
    addr.sin_family = AF_INET;
    addr.sin_port = htons(3000);
    sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

    if (connect(sock, (SOCKADDR *)&addr, sizeof(SOCKADDR)) == 0)
    {
        for (i = 0; i <= 9; i++)
            display(sock);

        closesocket(sock);
    }
    else
        printf("Connection failed!\n");
   
    WSACleanup();
    return 0;
}

在AVR Studio中，为了节约SRAM，sprintf和scanf最好改成sprintf_P和scanf_P，从Flash中读取格式字符串。
例如：
strcpy_P(uip_appdata, PSTR("This is a string."));
sscanf_P(uip_appdata, PSTR("%d, %d"), &a, &b);
sprintf_P(uip_conn->appstate.buf, PSTR("sum=%d\n"), a + b);