Sockets: premature disconnect - c

Question

So I have a simple TCP echo server program here(IPv4) that seems to disconnect immediately after it receives a connection from a new client before the client sends a FIN packet.
Here's the echo server code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <time.h>
#include <sys/types.h>
#include <netdb.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#define MAX_BUFFER 1024
#define PORT 4000
int main()
{
int lsock,csock, ret, in , i;
int yes = 1;
char buffer[MAX_BUFFER];
char* c;
struct sockaddr_in servaddr;
struct sockaddr_in cliaddr; // connector's address information
if((lsock = socket(AF_INET,SOCK_STREAM,0))<0){
perror("socket");
return -1;
}
bzero(&servaddr,sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(PORT);
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
if(bind(lsock,(struct sockaddr*)&servaddr,sizeof(servaddr))==-1) {
perror("bind");
close(lsock);
return -1;
}
if(listen(lsock,5)==-1){
perror("listen");
close(lsock);
return -1;
}else{
printf("Server listening on port %i\n",PORT);
system("gnome-terminal");
}
while(1){
int len = sizeof(cliaddr);
bzero(&cliaddr,len);
if(csock = accept(lsock,(struct sockaddr*)&cliaddr,&len)==-1){
perror("accept");
exit(0);
}
printf("New client connected....\n");
in = recv(csock,(void*)&buffer,sizeof(buffer),0);
if(in==-1){
perror("recv");
close(csock);
exit(0);
}else if(in==0){
printf("client disconnected\n");
close(csock);
}else{
if(send(csock,(void*)buffer,sizeof(buffer),0)==-1){
perror("send");
close(csock);
}
}
}
return 0;
}
And for the the echo client application:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <unistd.h>
#include <fcntl.h>
#include <time.h>
#include <netdb.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <linux/ioctl.h>
#define MAX_BUFFER 1024
void die(char *s)
{
perror(s);
exit(1);
}
int main()
{
int connector,flags,r;
int port;
int set = 1;
struct hostent* host;
struct in_addr in;
struct sockaddr_in rmaddr;
char sendbuffer[MAX_BUFFER];
char recvbuffer[MAX_BUFFER];
char hostname[INET_ADDRSTRLEN];
char* exit = "quit";
if((connector = socket(AF_INET,SOCK_STREAM,0))<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");
close(connector);
return -1;
}else{
printf("\n");
printf("Connected to host: %s",hostname,"on port %u",port);
printf(" type 'quit' to disconnect\n");
printf("\n");
}
while(1){
int nbr,nbs;
nbr = 0;
printf(">");
scanf("%s",sendbuffer);
printf("\n");
if(sendbuffer==exit){
close(connector);
return 0;
}
nbs = send(connector,(void*)&sendbuffer,strlen(sendbuffer),MSG_NOSIGNAL);
printf("\n");
printf("bytes sent: %i\n",nbs);
printf("\n");
if(nbs < 0){
perror("send() failed");
close(connector);
return -1;
}
while(nbr < nbs){
nbr = recv(connector,(void*)&recvbuffer,strlen(recvbuffer),MSG_NOSIGNAL);
if(nbr < 0){
perror("recv() failed");
close(connector);
return -1;
}else if(nbr==0){
printf("recv(): connection closed prematurely");
close(connector);
return -1;
}else if(nbr > 0){
printf("bytes received: %i\n",nbr);
printf("\n");
printf(">>");
printf("%s",recvbuffer);
printf("\n");
}
}
}
EXIT:
close(connector);
return 0;
}
Now when I compile the code and then execute it, this is the output(after I connect using the echo client):
zermacr0yd#DALEK /usr/lib/gcc/x86_64-linux-gnu/4.7/include $ ./ES
Server listening on port 4000
New client connected....
recvmsg: Socket operation on non-socket
zermacr0yd#DALEK /usr/lib/gcc/x86_64-linux-gnu/4.7/include $
Now I've tried connecting the client to other hosts, like www.google.com on port 80 and the output is the same. Which is:
zermacr0yd#DALEK /usr/lib/gcc/x86_64-linux-gnu/4.7.3/include $ ./EC
Enter the remote hostname(URL/IP4 address): www.google.com
Enter the port number you wish to connect(on): 80
Connected to host: www.google.com type 'quit' to disconnect
>hi
bytes sent: 2
recv(): connection closed prematurelyzermacr0yd#DALEK /usr/lib/gcc/x86_64-linux-gnu/4.7.3/include $
So it's clear that the connection is getting through(that is, the server receives the SYN packet)but immediately afterwards it closes the connection. So it appears to be a problem with the recv() function but it might very well be a connect() issue. But when I try to connect to the loopback address when the server isn't running I get:
zermacr0yd#DALEK /usr/lib/gcc/x86_64-linux-gnu/4.7.3/include $ ./EC
Enter the remote hostname(URL/IP4 address): 127.0.0.1
Enter the port number you wish to connect(on): 5000
connect: Connection refused
zermacr0yd#DALEK /usr/lib/gcc/x86_64-linux-gnu/4.7.3/include $
So I'm confoozed: Is this a server side error or a client side error? I'm thinking that the recv() function on the server side fails, the server closes the connection and shuts down, and then the client doesn't know that the server isn't running until the user on the client side tries to send a message and no bytes are received. It might very well be the client disconnecting but that doesn't look likely.

Answer 1

This line is wrong. It is going to evaluate to TRUE or FALSE (value 0 or 1) every time.
if (csock = accept(lsock, (struct sockaddr*) & cliaddr, &len) == -1)
{
perror("accept");
exit(0);
}
Consequently when you try a socket operation on file descriptor 0 or 1 - normally stdin and stdout - you are going to get the error that you are seeing: recvmsg: Socket operation on non-socket
Change the line to this:
if ((csock = accept(lsock, (struct sockaddr*) & cliaddr, &len)) == -1)

Answer 2

In your orignal server code, you had:
if(csock = accept(lsock,(struct sockaddr*)&cliaddr,&len)<0){
perror("accept");
exit(0);
}
which you have now changed to:
if(csock = accept(lsock,(struct sockaddr*)&cliaddr,&len)==-1){
perror("accept");
exit(0);
} 
But the problem is the same. Here, csock is probably getting the value 0, which would correspond to the standard input, and would thus not be a socket. (I assume 0, because if it becomes 1, your server would have exited early with an accept error message from perror().) This is because the < and == operators have higher precedence that =. You can fix this by adding a set of parentheses around the assignment (I stated this clearly before Duck provided his answer), or placing the assignment before the check. Since it seems you did not actually attempt either of my suggestions until you saw Duck's answer, I will illustrate the second suggestion:
csock = accept(lsock,(struct sockaddr*)&cliaddr,&len);
if (csock == -1) {
perror("accept");
exit(0);
} 
Your client test is invalid since you are connecting to an HTTP port, expecting ECHO behavior. You cannot draw any conclusions other than web servers do not accept hi as input.
Your server code is not well designed. It is a single threaded iterative server, but it does not properly clean up the existing client connection before iterating to perform another blocking accept() call. Instead, the client should handle the client connection until the connection is terminated before looping back to accept(). There are alternatives (have a separate thread handle the client connection, or use event driven loop with select()/poll()/epoll()). However, given that this seems to be a learning project, just handle one connection at a time is fine. However, you need an inner loop to completely handle the client connection before you accept() again. In pseudo-code:
while (not done)
new_conn = accept()
while (new_conn != -1)
result = read(new_conn, buf, bufsize)
switch (result)
case -1:
perror("read") /* FALLTHROUGH */
case 0:
close(new_conn)
new_conn = -1
break
default:
write(new_conn, buf, result)
break
The illustration above fixes another issue with your server, in that you are writing the complete buffer on the new connection, when instead you should only be writing out the number of bytes read, which in your case is stored in the variable in. So, instead of:
if(send(csock,(void*)buffer,sizeof(buffer),0)==-1){
Do this instead:
if(send(csock,(void*)buffer,in,0)==-1){

Answer 3

You had an operator precedence problem in the line that calls accept(), as noted by Duck. That accounts for 'socket operation on non-socket'.
'Connection refused' occurs at the client when the server cannot be found. It therefore has nothing to do with the server code whatsoever.
In your client you need to call perror() immediately after detecting an error, not three printf()'s later. Otherwise you corrupt errno and print the wrong error.
There is no 'premature disconnect' here. Just a disconnect, and a very misleading error message of your own devising. You're reading in a loop. Somebody has to disconnect some time. If you connect that client to a real echo server, it will echo one response and it may well then close the connection.

Answer 4

So in addition to following Ducks suggestion and adding a receiving loop to the server code, I realized that part of the problem with the echo_client was the following lines:
while(1){
int nbr,nbs;
nbr = 0;
When I removed the line that initialized nbr(number-of bytes received) to zero, the client does not disconnect and it actually works as I intended it to.