(4）用C语言写出TCP有限状态机的实现函数

TCP有限状态机的实现可以使用switch语句和枚举类型来实现。以下是一个简单的TCP有限状态机实现函数的示例代码：

#include <stdio.h>
#include <stdlib.h>

enum state {
    CLOSED,
    LISTEN,
    SYN_SENT,
    SYN_RECEIVED,
    ESTABLISHED,
    FIN_WAIT_1,
    FIN_WAIT_2,
    CLOSE_WAIT,
    CLOSING,
    LAST_ACK,
    TIME_WAIT
};

void tcp_fsm(enum state *state, int event) {
    switch(*state) {
        case CLOSED:
            if(event == 0) {
                *state = LISTEN;
                printf("State: LISTEN\n");
            }
            break;
        case LISTEN:
            if(event == 1) {
                *state = SYN_RECEIVED;
                printf("State: SYN_RECEIVED\n");
            } else if(event == 0) {
                *state = CLOSED;
                printf("State: CLOSED\n");
            }
            break;
        case SYN_SENT:
            if(event == 2) {
                *state = ESTABLISHED;
                printf("State: ESTABLISHED\n");
            } else if(event == 3) {
                *state = CLOSED;
                printf("State: CLOSED\n");
            }
            break;
        case SYN_RECEIVED:
            if(event == 4) {
                *state = ESTABLISHED;
                printf("State: ESTABLISHED\n");
            } else if(event == 3) {
                *state = CLOSED;
                printf("State: CLOSED\n");
            } else if(event == 5) {
                *state = SYN_SENT;
                printf("State: SYN_SENT\n");
            }
            break;
        case ESTABLISHED:
            if(event == 6) {
                *state = FIN_WAIT_1;
                printf("State: FIN_WAIT_1\n");
            } else if(event == 3) {
                *state = CLOSED;
                printf("State: CLOSED\n");
            }
            break;
        case FIN_WAIT_1:
            if(event == 7) {
                *state = FIN_WAIT_2;
                printf("State: FIN_WAIT_2\n");
            } else if(event == 3) {
                *state = CLOSED;
                printf("State: CLOSED\n");
            } else if(event == 4) {
                *state = CLOSING;
                printf("State: CLOSING\n");
            }
            break;
        case FIN_WAIT_2:
            if(event == 7) {
                *state = TIME_WAIT;
                printf("State: TIME_WAIT\n");
            } else if(event == 3) {
                *state = CLOSED;
                printf("State: CLOSED\n");
            }
            break;
        case CLOSE_WAIT:
            if(event == 6) {
                *state = LAST_ACK;
                printf("State: LAST_ACK\n");
            } else if(event == 3) {
                *state = CLOSED;
                printf("State: CLOSED\n");
            }
            break;
        case CLOSING:
            if(event == 7) {
                *state = TIME_WAIT;
                printf("State: TIME_WAIT\n");
            } else if(event == 3) {
                *state = CLOSED;
                printf("State: CLOSED\n");
            }
            break;
        case LAST_ACK:
            if(event == 3) {
                *state = CLOSED;
                printf("State: CLOSED\n");
            }
            break;
        case TIME_WAIT:
            if(event == 8) {
                *state = CLOSED;
                printf("State: CLOSED\n");
            }
            break;
    }
}

int main() {
    enum state tcp_state = CLOSED;
    tcp_fsm(&tcp_state, 0); // event: 0 => Passive Open
    tcp_fsm(&tcp_state, 1); // event: 1 => SYN Received
    tcp_fsm(&tcp_state, 2); // event: 2 => SYN ACK Received
    tcp_fsm(&tcp_state, 6); // event: 6 => FIN Received
    tcp_fsm(&tcp_state, 7); // event: 7 => ACK Received
    tcp_fsm(&tcp_state, 8); // event: 8 => TIME WAIT Timeout
    return 0;
}

以上代码中，事件的含义如下：

- 0: Passive Open
- 1: SYN Received
- 2: SYN ACK Received
- 3: Close Connection
- 4: FIN Received
- 5: SYN Retransmission
- 6: Close Request Sent
- 7: ACK Received
- 8: Timeout

该代码使用枚举类型表示TCP状态，并使用switch语句实现有限状态机。在tcp_fsm函数中，根据当前的状态和事件来计算出下一个状态，并打印出状态的转移。在main函数中，我们模拟了一些TCP连接的场景，例如建立连接和关闭连接。