(TCP)Echo client: Automatic disconnect after message sent

Question

So here is the code for a TCP echo client. When I have the echo server listening on port 5000, the client connects but then once I type a message and hit enter, it disconnects automatically. What's the problem here?

source code(tcp_ec.c):

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <time.h>
#include <netdb.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>



#define MAX_BUFFER 1024

void die(char *s)
{
      perror(s);
      exit(1);
}


int main()

{

  int    connector,flags,r;
  int  port;
  struct hostent*        host;
  struct in_addr         in;
  struct sockaddr_in     rmaddr;
  bool   connected = false;   
  char   sndbuffer[MAX_BUFFER];
  char   rcvbuffer[MAX_BUFFER];
  char   hostname[INET_ADDRSTRLEN];
  char*  exit = "quit";



  if((connector = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP))<0){
      perror("socket");
      return -1;
   }


  printf("\n");
  printf("Enter the remote hostname(URL/IP4 address): ");
  scanf("%s", hostname);
  printf("\n");



  printf("Enter the port number you wish to connect(on): ");
  scanf("%u", &port);
  printf("\n");

  if(port==0){
              printf("ERR0R: Port number must be between 1 & 65,535\n");
              printf("\n");
              printf("Enter the port number you wish to connect(on): ");
              scanf("%u", &port);
              printf("\n");        
  }

  host = gethostbyname(hostname);

  if(host==NULL){
         perror("hostname");
         return -1;
  }


  bzero(&rmaddr,sizeof(rmaddr));   
  rmaddr.sin_family = AF_INET;
  rmaddr.sin_port = htons(port);
  bcopy((char*)host->h_addr, (char*)&rmaddr.sin_addr.s_addr, host->h_length);  


  if(connect(connector,(struct sockaddr*)&rmaddr,sizeof(rmaddr))<0){
       perror("connect");
       return -1;
  }else{
       connected=true;
       printf("\n");
       printf("Connected to host: %s",hostname,"on port %u",port);
       printf("     type 'quit' to disconnect\n");
       printf("\n");
  }

  while(connected==true){

       printf(">");
       scanf("%s",sndbuffer);
       printf("\n");

       if(sndbuffer==exit){
           close(connector);
           connected = false;
           return 0;
       }

       if(send(connector,(void*)sndbuffer,sizeof(sndbuffer),MSG_EOR||MSG_NOSIGNAL)<0){
            perror("send");
            close(connector);
            return -1;
       }

       if(recv(connector,(void*)rcvbuffer,sizeof(rcvbuffer),MSG_EOR||MSG_NOSIGNAL)<0){
            perror("recv");
            close(connector);
            return -1;
       }else{
        printf(">>");
            printf("%s\n",rcvbuffer);
            printf("\n");
       }




  }




}

The console output(program output file is "tc"):

zermacr0yd@DALEK /usr/lib/gcc/x86_64-linux-gnu/4.7.3/include $ ./tec

Enter the remote hostname(URL/IP4 address): 127.0.0.1

Enter the port number you wish to connect(on): 5000


Connected to host: 127.0.0.1     type 'quit' to disconnect

>asssr

sendmsg: Broken pipe
zermacr0yd@DALEK /usr/lib/gcc/x86_64-linux-gnu/4.7.3/include $ 

This is done when the echo_server is running on port 5000.

Answer 1

Replace:

if(send(connector,(void*)sndbuffer,sizeof(sndbuffer),MSG_EOR||MSG_NOSIGNAL)<0){
... and
if(recv(connector,(void*)rcvbuffer,sizeof(rcvbuffer),MSG_EOR||MSG_NOSIGNAL)<0){

By:

if(send(connector,(void*)sndbuffer,sizeof(sndbuffer),MSG_EOR|MSG_NOSIGNAL)<0){
... and
if(recv(connector,(void*)rcvbuffer,sizeof(rcvbuffer),MSG_EOR|MSG_NOSIGNAL)<0){

... plus some minor errors.

EDIT: complete corrected source

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <time.h>
#include <netdb.h>
#include <stdbool.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>

#define MAX_BUFFER 1024

void die(char *s)
{
      perror(s);
      exit(1);
}

int main() {
  int    connector,flags,r;
  int  port;
  struct hostent*        host;
  struct in_addr         in;
  struct sockaddr_in     rmaddr;
  bool   connected = false;
  char   sndbuffer[MAX_BUFFER];
  char   rcvbuffer[MAX_BUFFER];
  char   hostname[INET_ADDRSTRLEN];
  char*  exit = "quit";


  if((connector = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP))<0){
      perror("socket");
      return -1;
   }

  printf("\n");
  printf("Enter the remote hostname(URL/IP4 address): ");
  scanf("%s", hostname);
  printf("\n");

  printf("Enter the port number you wish to connect(on): ");
  scanf("%u", &port);
  printf("\n");

  if(port==0){
              printf("ERR0R: Port number must be between 1 & 65,535\n");
              printf("\n");
              printf("Enter the port number you wish to connect(on): ");
              scanf("%u", &port);
              printf("\n");
  }
  host = gethostbyname(hostname);

  if(host==NULL){
         perror("hostname");
         return -1;
  }


  bzero(&rmaddr,sizeof(rmaddr));
  rmaddr.sin_family = AF_INET;
  rmaddr.sin_port = htons(port);
  bcopy((char*)host->h_addr, (char*)&rmaddr.sin_addr.s_addr, host->h_length);


  if(connect(connector,(struct sockaddr*)&rmaddr,sizeof(rmaddr))<0){
       perror("connect");
       return -1;
  }else{
       // connected=true;
       printf("\n");
       printf("Connected to host: %son port %u", hostname, port);
       printf("     type 'quit' to disconnect\n");
       printf("\n");
  }

  // while(connected==true){
  while(1){

       printf(">");
       scanf("%s",sndbuffer);
       printf("\n");

       if(sndbuffer==exit){
           close(connector);
           // connected = false;
           return 0;
       }

       if(send(connector,(void*)sndbuffer,sizeof(sndbuffer),MSG_EOR|MSG_NOSIGNAL)<0){
            perror("send");
            close(connector);
            return -1;
       }

       if(recv(connector,(void*)rcvbuffer,sizeof(rcvbuffer),MSG_EOR|MSG_NOSIGNAL)<0){
            perror("recv");
            close(connector);
            return -1;
       }else{
        printf(">>");
            printf("%s\n",rcvbuffer);
            printf("\n");
       }
  }

return 0;
}

OUTPUT:

~/src/usenet$ ./a.out

Enter the remote hostname(URL/IP4 address): 127.0.0.1

Enter the port number you wish to connect(on): 7


Connected to host: 127.0.0.1on port 7     type 'quit' to disconnect

>aap

>>aap

>noot

>>noot

>mies

>>mies

>quit

>>quit

>^C

[the not reacting to "quit" is caused by the lack of strncmp() // memcmp(), plus "quit is actually "quit\n" (and not nul-terminated) ]

Answer 2

I thought the problem had to do with the Byte Ordering functions because I noticed that adding the header produces compiler warnings.But it turns out that what is causing the problem in the first place, along with the fact that calling the bind() function in the echo server application gives an EONOTSUPP error, is that the socket is not binding to a specific address! It is binding to all addresses and this GCC bug has been documented Here. So now the task at hand is to get the patch downloaded and installed and I'll see how it works from there.

Answer 3

I've seen before that connect() to a non-existent port on 127.0.0.1 can return successfully, but it's decades ago and not on Linux. The technique I used then was to do a zero-length write, which would fail if the connect didn't really work. I never got to the bottom of it and I haven't seen the problem for over 20 years.

But you code has other problems.

You're ignoring the count returned by 'recv()' unless it is negative. This has two consequences:

1. You may be printing junk afterwards.
2. You aren't detecting end of stream from recv(), so you don't know when the peer has disconnected, so your loop continues and you send to a connection that has already been closed by the peer, which produces your 'broken pipe' problem. It is never correct to call recv() without storing the result into a variable.

You're also sending junk. Instead of 'sizeof sndbuffer', the count you supply to send() should be 'strlen(sndbuffer)' in this case, as you've just read a null-terminated string with 'fgets()'.