c++ socket closing the first connection attempt

Question

I'm trying to make a server sample in c++ using <sys/socket.h> and Qt Creator gui builder but two weird behaviors are going on at socket layer of the program. First of, I run the server but at the first attempt I make to connect to it using telnet is imediately closed

Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
Connection closed by foreign host.

When I attempt connection for the second time, it works and the terminal waits for my input. The second thing is when I close the connection. If I rerun right after, in a matter of minutes, the program halts on bind exiting and returning:

ERROR on binding: Address already in use

So I suppose maybe a connection is being held after I break it using function call onCortarConexao() or just stopping the debugger. Anyway, what am I missing?

My code:

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

On MainWindow.cpp:

void MainWindow::on_pushButton_clicked()
{
    socket1 = new MSocket();
    socket1->start();
}
void MainWindow::on_pushButton_3_clicked()
{
    socket1->onCortarConexao();
}

Socket class:

#ifndef MSOCKET_H
#define MSOCKET_H
#include <QString>
#include <QObject>
#include <QThread>
#include <QList>
#include <string.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
#define SERVER_BUFFER 4096
#define PORTRUN 15000


class MSocket : public QThread
{
public:
    MSocket();
    void error(char *msg);
    void onCortarConexao();

private:
    int sockfd;
    int newsockfd;
    int portno;
    int clilen;
    int n;
    char buffer[SERVER_BUFFER];
    struct sockaddr_in serv_addr, cli_addr;
    u_short port;
    void run();
};

#endif // MSOCKET_H

Socket implementation:

void MSocket::run()
{
sockfd = socket(AF_INET, SOCK_STREAM, 0);

if (sockfd < 0){
    error("ERROR opening socket");
    exit(-1);
}
bzero((char *) &serv_addr, sizeof(serv_addr));

portno = PORTRUN;
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(portno);
serv_addr.sin_addr.s_addr = INADDR_ANY;

fprintf(stdout,"Iniciando servidor..");
if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0){

    error("ERROR on binding");
    exit(-1);
}
listen(sockfd,5);

newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, (socklen_t*) sizeof(cli_addr));
if (newsockfd < 0){
    error("ERROR on accept");
    exit(-1);
}

bzero(buffer,SERVER_BUFFER);
n = read(newsockfd,buffer,SERVER_BUFFER-1);
if (n < 0)
     error("ERROR reading from socket");

printf("Here is the message: %s",buffer);

n = write(newsockfd,"I got your message",18);
if (n < 0)
     error("ERROR writing to socket");




}

void MSocket::onCortarConexao(){
    printf("Encerrando socket");
    close(newsockfd);
    close(sockfd);

}

The complete code is at: https://github.com/FabioNevesRezende/BasicCppServer

Edit 1:

So, this is the list of packets of the communication between telnet and my Qt Server application it can be graphicaly seen in WireShark (.pcapng file). It contains 11 frames. The 6 first ones are from the first telnet, when it is imediately closed. As it seems on frame 4 and 5 where the application sends [FIN, ACK] and the server responds to it by closing the connection. The frames 7,8,9 are the second attempt to connect and frames 10 and 11 is when I send the abc to the server. As in the print screen:

The problem is I don't know why the application is sending this FIN and where in the code it is.

Answer 1

Use SO_REUSEADDR with setsockopt :

optval = 1;
setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof optval);

This allows the port to be reused by other sockets and gets around the address already in use issue you're facing.

Answer 2

The previous answers have indicate that how to handle the read correctly and how to use SO_RESUEADDR. I want to point out why your program can just receive one message. because your run function doesn't have a loop, so it can be only performed once, then the thread exits. after accept a client's request, then the connection is establish, then you should enter a loop to wait the client's input, and write them back once the read function returns.

Answer 3

Here's the first serious problem I found:

n = read(newsockfd,buffer,SERVER_BUFFER-1);
if (n < 0)
     error("ERROR reading from socket");

printf("Here is the message: %s",buffer);

There's no handling for read returning zero. The %s format specifier is only for strings. You can't use it for arbitrary, unchecked data.

Answer 4

On unix operating systems, system calls can sometimes fail with an errorno set to EINTR. This means that a signal was delivered to the process which interrupted the system call and you should retry it. Try something like this:

while  ((n = read(newsockfd,buffer,SERVER_BUFFER-1) < 0) {
 if (errno != EINTR)
 break;
 }
if (n < 0)
     error("ERROR reading from socket");

The same should be done for the call to write.