sven sven - 1 month ago 9
C Question

TCP Server doesn't receive data correctly

The below code has a TCP client in C and TCP Server in Qt C++. My problem is that I am using TCP for reliability, but it has data losses (not packet). In my main code, if I run tcp client to send data, TCP server receives only one packet. if I add

sleep(1);
to the client between each packet transfer, then TCP server receives data. Both client and server runs on the same computer.

To simplify the question and can't put too huge code here, I have the below code that performs faster, but it losses last 10-15 bytes of the packet.

TCP C client

main.c

#include "socket_handler.h" //I didn't put the all includes here
#define PORT 22208
//tcp server
int main(void)
{
int sockfd;
uint32_t senderAddress = 2130706433; //127.0.0.1
if( connect_to_server_w_uint( &sockfd, senderAddress, PORT ) < 0 ){
printf("error at line 454\n");
exit(1);
}

char data[] = "124b00068c158f$321$52712304$13.212779$0$O$0$0$b4$1$0$3$0$0$0$0$11$0$7$0$1$fe$f1$aaa9fffffffffd80$2132b00$eb460b5e$1$1$2016-02-22 03:01:00$0000-00-00 00:00:00$321$24754$321$13132$1$98$0$5.1$0$3c$64$1$96$4d$3e8$38$2$46$dc$4$3$f6$e6$17$0$e6$d3$1$0$e6$d3$2$0£";
char buffer[512];

int i=0;
for(i=0; i<1000; i++){
bzero(buffer, 512);
sprintf(buffer, "%d***%s -----",i,data);
send_data_to_server(&sockfd, buffer, strlen(data) +1 );
printf("[%d]: data is sent\n", i);
}
close_connection(&sockfd);

return 0;
}


socket_handler.c

int connect_to_server(int *sockfd , struct in_addr senderAddress, uint16_t destPort){
struct sockaddr_in serv_addr;

*sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (*sockfd < 0)
//error("ERROR opening socket");

bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr = senderAddress;
serv_addr.sin_port = htons(destPort);

if (connect( *sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0){
printf("connection error line 1413\n");
close( *sockfd );
return -1;
}

return 0;
}


int connect_to_server_w_uint(int *sockfd, uint32_t senderAddress, uint16_t destPort){
struct sockaddr_in serv_addr;

*sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (*sockfd < 0){
printf("ERROR opening socket");
close(*sockfd);
return -1;
}

bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = htonl(senderAddress);

serv_addr.sin_port = htons(destPort);
if (connect(*sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0)
{
printf("ERROR connecting");
close(*sockfd);
return -1;
}

return 0;
}

int send_data_to_server(int *sockfd, char *message, uint16_t msgLength){
int n = write(*sockfd, message, msgLength);
if (n < 0){
printf("ERROR writing to socket");
return -1;
}
return 0;
}

int close_connection(int *sockfd){
close( *sockfd );
return 0;
}


Qt C++ TCP Server

MainWindow.cpp

class MainWindow;
}

class MainWindow : public QMainWindow
{
Q_OBJECT

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();

private slots:
void ParseThePacket(QByteArray data);

private:
Ui::MainWindow *ui;

Server *server;
};


Client.h

class Client : public QObject
{
Q_OBJECT
public:
explicit Client(QObject *parent = 0);

public slots:
bool connectToHost(QString host);
bool writeData(QByteArray data);

private:
QTcpSocket *socket;

};


Server.cpp

Server::Server(QObject *parent) : QObject(parent)
{
server = new QTcpServer(this);
connect(server, SIGNAL(newConnection()), this, SLOT(newConnection()));
if( server->listen(QHostAddress::Any, PORT) ){
qDebug() << "tcp server started!";
}else{
qDebug() << "tcp server couldn't start listening";
}
}

void Server::newConnection()
{
qDebug() << "new connection";
while (server->hasPendingConnections())
{
socket = server->nextPendingConnection();
connect(socket, SIGNAL(readyRead()), this, SLOT(readyRead()));
connect(socket, SIGNAL(disconnected()), this, SLOT(disconnected()));
}
}

void Server::disconnected()
{
qDebug() << "disconnected";
socket->deleteLater();
}

void Server::readyRead()
{
qDebug() << "readyRead";

QByteArray buffer = socket->readAll();
emit dataReceived(buffer);
}


Here is an example output from the TCP server(the end of qDebug() output):



00:00:00$321$24754$321$13132$1$98$0$5.1$0$3c$64$1$96$4d$3e8$38$2$46$dc$4$3$f6$e6$17$0$e6$d3$1$0$e6$d3$996***124b00068c158f$321$52712304$13.212779$0$O$0$0$b4$1$0$3$0$0$0$0$11$0$7$0$1$fe$f1$aaa9fffffffffd80$2132b00$eb460b5e$1$1$2016-02-22
03:01:00$0000-00-00
00:00:00$321$24754$321$13132$1$98$0$5.1$0$3c$64$1$96$4d$3e8$38$2$46$dc$4$3$f6$e6$17$0$e6$d3$1$0$e6$d3$997***124b00068c158f$321$52712304$13.212779$0$O$0$0$b4$1$0$3$0$0$0$0$11$0$7$0$1$fe$f1$aaa9fffffffffd80$2132b00$eb460b5e$1$1$2016-02-22
03:01:00$0000-00-00
00:00:00$321$24754$321$13132$1$98$0$5.1$0$3c$64$1$96$4d$3e8$38$2$46$dc$4$3$f6$e6$17$0$e6$d3$1$0$e6$d3$998***124b00068c158f$321$52712304$13.212779$0$O$0$0$b4$1$0$3$0$0$0$0$11$0$7$0$1$fe$f1$aaa9fffffffffd80$2132b00$eb460b5e$1$1$2016-02-22
03:01:00$0000-00-00
00:00:00$321$24754$321$13132$1$98$0$5.1$0$3c$64$1$96$4d$3e8$38$2$46$dc$4$3$f6$e6$17$0$e6$d3$1$0$e6$d3$999***124b00068c158f$321$52712304$13.212779$0$O$0$0$b4$1$0$3$0$0$0$0$11$0$7$0$1$fe$f1$aaa9fffffffffd80$2132b00$eb460b5e$1$1$2016-02-22
03:01:00$0000-00-00
00:00:00$321$24754$321$13132$1$98$0$5.1$0$3c$64$1$96$4d$3e8$38$2$46$dc$4$3$f6$e6$17$0$e6$d3$1$0$e6$d3$"
disconnected



Question 1
Comparing to the original message, it misses "1$0$e6$d3$2$0£" part of the (14 byte) sent data. What is the reason of the missing message? How to fix the code that the TCP server receives the complete data.

Question 2
As I mentioned in the beginning that I am using the same code as a part of a large code and TCP server receives packets when I put sleep(1) between each packet transmission (otherwise it receives only the first packet). What is the reason of it and how to solve it?

I observed the packet transmission on Wireshark that all the TCP packets are sent successfully, but it seems like the receive part has an issue.

I am using Ubuntu 15.04, kernel 3.19.0-69-generic, gcc version 4.9.2

Answer
int n = write(*sockfd, message, msgLength);

if (n < 0){

You are only checking that write() did not return a negative value, indicating an error.

However, a write() to a socket does not guarantee that all requested bytes will be written. write() on a socket may return a positive value, fewer than msgLength here, indicating that fewer than the requested bytes have been written. This is documented, in detail, in write()'s manual page.

You are ignoring this possibility, and that's the likely reason you're missing your data. It's up to you to figure out what to do, in this case. The usual approach is to simply go back and attempt to write the remaining bytes (which, again, may not be written in their entirety).

Similarly, when reading from a socket, you are not guaranteed that everything that was written to the socket, by the sender, will be read in one gulp. It is up to you to verify that your reader has read everything that there is to read, and if the reader expects more data, continue reading from the socket until it is received.