Cpp socket - TCP transmission (send and then receive with the same port)

Question

I am writing a socket programming, one part is first send then receive, one part is receive then send.

The program work like this:

ThreadA: Send to Port 8000-> close socket -> get myport (portA) that i send -> listen to my port(portA)

ThreadB: Listen port 8000 -> Get the port when receive (portA)-> close socket -> send pack to same port (port A)

std::thread ThreadA,ThreadB;
long share_port=8000;
char share_addr[INET_ADDRSTRLEN]="127.0.0.1";

void send_then_receive();
void receive_then_send();


void send_then_receive(){
    long myport = -1, receiver_port = share_port;
    SOCKET receive_then_send_A_Socket,receive_then_send_A_Socket_b,receive_then_send_B_Socket;

    //- Send to who i want
    struct sockaddr_in receive_then_send_B_Addr;
    receive_then_send_B_Addr.sin_family = AF_INET;
    inet_pton(AF_INET, share_addr, &receive_then_send_B_Addr.sin_addr.s_addr);
    receive_then_send_B_Addr.sin_port = htons((u_short)receiver_port);
    receive_then_send_B_Socket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    while (connect(receive_then_send_B_Socket, (struct sockaddr *)&receive_then_send_B_Addr, sizeof(sockaddr_in)) == SOCKET_ERROR){
        printf("\nConnect Fail in first response| Fail Code:%i\n", WSAGetLastError());
    }
    send(receive_then_send_B_Socket, "4321", 5, 0);
    printf("[1]The port I want to send to:%ld\n",receiver_port);

    //- Check My Port
    socklen_t checkport_adr_len = sizeof(receive_then_send_B_Addr);
    if (getsockname(receive_then_send_B_Socket, (struct sockaddr *)&receive_then_send_B_Addr, &checkport_adr_len) == 0){
        myport = ntohs(receive_then_send_B_Addr.sin_port);
        printf("[1]MyPort Check:%ld\n", myport);
    }
    close(receive_then_send_B_Socket);



    //- Then Recive
    sockaddr_in *receive_then_send_A_Addr = new sockaddr_in;
    receive_then_send_A_Addr->sin_family = AF_INET;
    receive_then_send_A_Addr->sin_addr.s_addr = INADDR_ANY;
    receive_then_send_A_Addr->sin_port = htons((u_short)myport);
    receive_then_send_A_Socket = socket(AF_INET, SOCK_STREAM, 0);
    bind(receive_then_send_A_Socket, (struct sockaddr *)receive_then_send_A_Addr, sizeof(struct sockaddr_in));
    listen(receive_then_send_A_Socket, 1);
    std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    receive_then_send_A_Socket_b = accept(receive_then_send_A_Socket, 0, 0);
    char* buffer_A= new char[100];
    recv(receive_then_send_A_Socket_b, buffer_A, 100, 0);
    printf("[1]What I then receive: %s\n",buffer_A);

    close(receive_then_send_A_Socket);
    close(receive_then_send_A_Socket_b);


}
void receive_then_send(){
    long myport = -1, receiver_port = -1;
    SOCKET receive_then_send_A_Socket,receive_then_send_A_Socket_b,receive_then_send_B_Socket;

    //- First Recive
    sockaddr_in *receive_then_send_A_Addr = new sockaddr_in;
    receive_then_send_A_Addr->sin_family = AF_INET;
    receive_then_send_A_Addr->sin_addr.s_addr = INADDR_ANY;
    receive_then_send_A_Addr->sin_port = htons((u_short)share_port);
    receive_then_send_A_Socket = socket(AF_INET, SOCK_STREAM, 0);
    bind(receive_then_send_A_Socket, (struct sockaddr *)receive_then_send_A_Addr, sizeof(struct sockaddr_in));
    listen(receive_then_send_A_Socket, 1);
    receive_then_send_A_Socket_b = accept(receive_then_send_A_Socket, 0, 0);
    char* buffer_A= new char[100];
    recv(receive_then_send_A_Socket_b, buffer_A, 100, 0);
    printf("[2]What I first receive: %s\n",buffer_A);

    //- Check receiver Port
    struct sockaddr_storage receiver_addr;
    socklen_t receiver_addr_len = (socklen_t)sizeof receiver_addr;
    getpeername(receive_then_send_A_Socket_b, (struct sockaddr*)&receiver_addr, &receiver_addr_len);


    struct sockaddr_in *s = (struct sockaddr_in *)&receiver_addr;
    inet_ntop(AF_INET, &s->sin_addr, share_addr, sizeof share_addr);
    receiver_port = ntohs(s->sin_port); 
    printf("[2]The port I want to send to:%ld\n",receiver_port);
    close(receive_then_send_A_Socket);
    close(receive_then_send_A_Socket_b);

    //- Send Back to the port that sender send to me
    struct sockaddr_in receive_then_send_B_Addr;
    receive_then_send_B_Addr.sin_family = AF_INET;
    inet_pton(AF_INET, share_addr, &receive_then_send_B_Addr.sin_addr.s_addr);
    receive_then_send_B_Addr.sin_port = htons((u_short)receiver_port);
    receive_then_send_B_Socket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    while (connect(receive_then_send_B_Socket, (struct sockaddr *)&receive_then_send_B_Addr, sizeof(sockaddr_in)) == SOCKET_ERROR){
        printf("\nFail Code:%i\n", WSAGetLastError());
        break;
    }
    send(receive_then_send_B_Socket, "1234", 5, 0);
    close(receive_then_send_B_Socket);
    printf("[2]The port I want to send to:%ld\n",receiver_port);
}

int main(int argc, char *argv[])
{
    ThreadB = std::thread(receive_then_send);
    ThreadA = std::thread(send_then_receive);
    while(1);
    return 0;
}

However, it works well in first transmission, but it doesnt work in second transmission, whats wrong???

It shows like this:

Connect Fail in first response| Fail Code:111

Connect Fail in first response| Fail Code:111 [1]The port I want to send to:8000 [1]MyPort Check:34004 [2]What I first receive: 4321 [2]The port I want to send to:34004

Fail Code:111

Note that: I use Linux (2nd transmission not work), but all functions well when i modify into WinSock version. Why this happen?

Note that Before Bind, i have tired // set SO_REUSEADDR on a socket to true (1): int optval= 1; setsockopt(receive_then_send_A_Socket, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(int));

YSC YSC · Accepted Answer · 2015-11-05T13:32:46

One liner answer: SO_REUSEADDR.

The system forbids to reuse an IP address on a given port for, as I recall, 60 seconds after it has been freed. Except if the SO_REUSEADDR flag has been set.

Here is a comprehensive answer on SO about why and how to set this option flag.

Cpp socket - TCP transmission (send and then receive with the same port)

1 Answers