Need multiple connections in C socket - c

Question

I have written the following code as an intermediate to connect two programs. There is a server program running and a client program on two different systems. This code is expected to act as an intermediate between these two programs.
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
//Connect with program on server side
char * serv_con(char app_data[50])
{
int sock, bytes_recieved;
char send_data[1024];
char *recv_data = malloc(1024);
struct hostent *host;
struct sockaddr_in server_addr;
host = gethostbyname("10.47.3.249");
if ((sock = socket(AF_INET, SOCK_STREAM, 0)) == -1)
{
perror("Socket");
exit(1);
}
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(3128);
server_addr.sin_addr = *((struct in_addr *)host->h_addr);
bzero(&(server_addr.sin_zero),8);
if (connect(sock, (struct sockaddr *)&server_addr, sizeof(struct sockaddr)) == -1)
{
perror("Connect");
exit(1);
}
bytes_recieved=recv(sock,recv_data,1024,0);
recv_data[bytes_recieved] = '\0';
send(sock, app_data, 50, 0);
return recv_data;
//close(sock);
}
//Connect with client app
char * cli_con(char ser_data[50])
{
int sock, connected, bytes_recieved , true = 1;
char send_data [1024];
char *recv_data = malloc(1024);
struct sockaddr_in server_addr,client_addr;
int sin_size;
if ((sock = socket(AF_INET, SOCK_STREAM, 0)) == -1)
{
perror("Socket");
exit(1);
}
if (setsockopt(sock,SOL_SOCKET,SO_REUSEADDR,&true,sizeof(int)) == -1)
{
perror("Setsockopt");
exit(1);
}
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(5000);
server_addr.sin_addr.s_addr = INADDR_ANY;
bzero(&(server_addr.sin_zero),8);
if (bind(sock, (struct sockaddr *)&server_addr, sizeof(struct sockaddr)) == -1)
{
perror("Unable to bind1");
exit(1);
}
if (listen(sock, 5) == -1)
{
perror("Listen");
exit(1);
}
sin_size = sizeof(struct sockaddr_in);
connected = accept(sock, (struct sockaddr *)&client_addr,&sin_size1);
printf("\n I got a connection from (%s , %d)",inet_ntoa(client_addr.sin_addr),ntohs(client_addr.sin_port));
bytes_recieved = recv(connected,recv_data,1024,0);
recv_data[bytes_recieved] = '\0';
send(connected, ser_data,50, 0);
//close(sock);
}
int main()
{
char *ser_data, *app_data;
int pid = fork();
while(1)
{
if(pid == 0)
app_data = serv_con(ser_data);
else
ser_data = cli_con(app_data);
}
}
It works fine until the client side app runs. But as soon as the client side app runs, the code exit giving the error:
Unable to bind: Address already in use
I got a connection from (192.168.0.3 , 45691)
What modification should I make in the code to rectify this error? I am working on linux. Thanks in advance.
EDIT:
I have removved the comment from close(sock)and added close(connect) in the function cli_con. The code on the client side is given below:
int sock, bytes_recieved;
char send_data[1024],recv_data[1024];
struct hostent *host;
struct sockaddr_in server_addr;
host = gethostbyname("192.168.0.2");
if ((sock = socket(AF_INET, SOCK_STREAM, 0)) == -1)
{
perror("Socket");
exit(1);
}
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(5555);
server_addr.sin_addr = *((struct in_addr *)host->h_addr);
bzero(&(server_addr.sin_zero),8);
if (connect(sock, (struct sockaddr *)&server_addr,
sizeof(struct sockaddr)) == -1)
{
perror("Connect");
exit(1);
}
while(1)
{
//necessary codes
if (connect(sock, (struct sockaddr *)&server_addr, sizeof(struct sockaddr)) == -1)
{
close(sock);
goto connct;
}
}
But now on running, the first program doesnot exit but doesnot even print
I got a connection from (192.168.0.3 , 45691)
But just keeps on running without printing ANY messages. But on the other hand, the client exits showing the error:
Connect: Connection reset by peer
What should I do now?

Answer 1

When a client disconnects you create a new server socket and bind it to the same port. If the server side socket was not closed the port is still in use, so bind fails.
Usually the server side of a socket program has a loop around accept to allow it process connections from many clients. This way bind and listen are called only once.
while (connected = accept(sock, (struct sockaddr *)&client_addr,&sin_size1)) {
printf("\n I got a connection from (%s , %d)",inet_ntoa(client_addr.sin_addr),ntohs(client_addr.sin_port));
bytes_recieved = recv(connected,recv_data,1024,0);
recv_data[bytes_recieved] = '\0';
send(connected, ser_data,50, 0);
close(connected);
}

Answer 2

It means you're trying to listen on port 5000, but there is already a program listening on that port (possibly an earlier version of your program which didn't close the port properly.) Change the port number to another value, or close any application listening on the port.
Since you're under Linux, you can use "netstat -nlp" as root to see what programs have which ports open.

Answer 3

One pointer for you is that, you should close both the listen socket and the socket which you obtain from the accept call.
close(connected);
close(sock);
Also, the socket takes sometime to be freed by the OS after it's usage and so, it might fail with an "Address already in use" error. You can check again properly in your code if your SO_REUSEADDR part of the code is executing properly.
Also, since you can try to add the listen socket creation code in your main function itself and pass it as an argument to cli_con function. Commonly followed mechanism is to create one listen socket and use it to accept multiple connections from client.
I hope in your original code you have proper memory allocation, initialization etc for ser_data & app_data.