TCP套接字，“无法分配请求的地址”和“传输端点已连接”错误

Question

I am new to working with TCP sockets and I do not understand the errors I am getting when running my programs. I have two programs that when run simultaneous I would like to be able to pass messages between. The initial message gets through but then when trying to return another message I get two errors. When I run my first program the output is:

Input Message: hello

Waiting for incoming connections...

Connection accepted

Message Sent

Connection error: Transport endpoint is already connected 

Now running the other program simultaneously gives:

Input Message: Hello Back

Connected

Message received

hello

Bind error: Cannot assign requested address

If anyone could explain these "cannot assign requested address" and " Transport endpoint is already connected" errors I would greatly appreciate it!! Thanks!

The first program is:

int main(int argc, char *argv[]) {

    int socket_info, new_socket;
    struct sockaddr_in server, client;
    char message[100];
    char incoming_message[100];

printf("Input Message: ");

fgets(message, 100, stdin);

        //create socket
        socket_info = socket(AF_INET, SOCK_STREAM, 0);
        if (socket_info == -1) {
        printf("Could not create socket");
        }

        //assign values
        server.sin_addr.s_addr = INADDR_ANY;
        server.sin_family = AF_INET;
        server.sin_port = htons( 1100 );

    int y=1;
        if(setsockopt(socket_info, SOL_SOCKET, SO_REUSEADDR, (char*)&y, sizeof(y)) == -1) {
    perror("set reuseaddr");
    return -1;
        }

        //binds socket
        if (bind(socket_info, (struct sockaddr *)&server, sizeof(server)) < 0) {
        perror("Bind error");
        return 1;
        }

        //listen
        listen(socket_info , 5);

        //waiting for connection
        puts("Waiting for incoming connections...");
        int c = sizeof(struct sockaddr_in);

        //accept connection
        new_socket = accept(socket_info, (struct sockaddr *)&client, (socklen_t*)&c);
        if (new_socket < 0){
        perror("accept failed");
        return 1;
        }
        puts("Connection accepted");

        //send message
        if( send(new_socket , message , strlen(message) , 0) < 0) {
        perror("Send failed");
        return 1;
        }
        puts("Message Sent");

        //connects
        if (connect(socket_info, (struct sockaddr *)&server, sizeof(server)) < 0) {
        perror("Connection error");
        return 1;
        }
        puts("Connected");

        //Receive an incoming message
        if( recv(socket_info, incoming_message , sizeof(incoming_message) , 0) < 0) {
        perror("Received failed");
        return 1;
        }
        puts("Message received");
    incoming_message[strlen(incoming_message)-1]=0;
        puts(incoming_message);

close(socket_info);
}

The second program is:

int main(int argc, char *argv[]) {

    int socket_info, new_socket;
    struct sockaddr_in server, client;
    char incoming_message[100];
    char message[100];

printf("Input Message: ");

fgets(message, 100, stdin);

        //create socket
        socket_info = socket(AF_INET, SOCK_STREAM, 0);
        if (socket_info == -1) {
        printf("Could not create socket");
        }

        //assign values
        server.sin_addr.s_addr = inet_addr("172.21.8.178");
        server.sin_family = AF_INET;
        server.sin_port = htons( 1100 );

        //connects
        if (connect(socket_info, (struct sockaddr *)&server, sizeof(server)) < 0) {
        perror("Connection error");
           return 1;
        }
        puts("Connected");

        //Receive an incoming message
        if( recv(socket_info, incoming_message , sizeof(incoming_message) , 0) < 0) {
        perror("Received failed");
        return 1;
        }
        puts("Message received");
    incoming_message[strlen(incoming_message)-1]=0;
        puts(incoming_message);

    int y=1;
       if(setsockopt(socket_info, SOL_SOCKET, SO_REUSEADDR, (char*)&y, sizeof(y)) == -1) {
    perror("set reuseaddr");
    return -1;
        }

        //binds socket
        if (bind(socket_info, (struct sockaddr *)&server, sizeof(server)) < 0) {
        perror("Bind error");
        return 1;
        }

        //listen
        listen(socket_info , 5);

        //waiting for connection
        puts("Waiting for incoming connections...");
        int c = sizeof(struct sockaddr_in);

        //accept connection
        new_socket = accept(socket_info, (struct sockaddr *)&client, (socklen_t*)&c);
        if (new_socket < 0){
        perror("accept failed");
        return 1;
        }
        puts("Connection accepted");

    //send message
        if( send(new_socket , message , strlen(message) , 0) < 0) {
        perror("Send failed");
        return 1;
        }
        puts("Message Sent");

close(socket_info);
}

Answer 1

  if (connect(socket_info, (struct sockaddr *)&server, sizeof(server)) < 0) {
    perror("Connection error");
    return 1;
    }

Connection error: Transport endpoint is already connected 

I asssume socket_info above should be new_socket ?

You can't connect a listening socket.

You don't need to connect the listening socket. You have just accepted a socket from a client connection. You should do your I/O to that client with that socket.

In your second program:

if(setsockopt(socket_info, SOL_SOCKET, SO_REUSEADDR, (char*)&y, sizeof(y)) == -1) {
perror("set reuseaddr");
return -1;
    }

This is futile. The socket is already bound, implicitly, via the connect() call preceding.

    //binds socket
    if (bind(socket_info, (struct sockaddr *)&server, sizeof(server)) < 0) {
    perror("Bind error");
    return 1;
    }

Bind error: Cannot assign requested address

You can't bind a socket that is already connected.

Nor can you bind a socket to a remote address.

Nor can you listen on a connected socket.

Nor can you accept from it. You don't need to bind, or listen, or accept from it. You are already connected to the peer.

In short your code doesn't make any sense whatsoever. You need to find a proper tutorial and study it.

incoming_message[strlen(incoming_message)-1] = 0;

This makes even less sense. Here you are searching for a trailing null byte (which may not even be there) and replacing the byte before it by ... a null byte. Why?