How to connect a TCP server to another TCP server - c

Question

I'm working with TCP servers. Let's say I have a server running with a specific port, but then I want to connect a client to it, I would simply go through the typical procedure of socket, bind, listen, accept for the server and then socket, connect for the client. So let's say our server port is 4000, and our client port 4001. Now, I want to create a new client that will connect to my client on port 4001, but to my limited understanding, I cannot do this as a client. Port 4001 would have to pertain to a server and not a client (i.e. it would have to be listening). The issue arises because I don't think you can use the same port for both the server and client.
I've decided to attempt this through the sample code I've provided below. I call the program on the command line as follows:
If this is the first call of the server, then I simply call the program without any arguments and it will automatically run on port 3000. i.e. ./serverprogram
If I would like to connect a client on port 3001 to our server on port 3000. Then I would call the command line with two arguments, the first being 3001 and the second being 3000. i.e. ./serverprogram 3001 3000
#define PORT 3000
int main (int argc, char * argv[]){
int sfd = socket(AF_INET, SOCK_STREAM, 0);
int my_port = (argc == 3) ? atoi(argv[1]) : PORT;
if (argc > 2){
struct sockaddr_in c_addr;
c_addr.sin_family = AF_INET;
memset(&c_addr.sin_zero, 0, 8);
c_addr.sin_port = htons(atoi(argv[2]));
struct addrinfo *result = NULL;
getaddrinfo("AcaciaLinux", NULL, NULL, &result);
struct sockaddr_in *x = (struct sockaddr_in*) result->ai_addr;
c_addr.sin_addr = x->sin_addr;
freeaddrinfo(result);
if(connect(sfd, (struct sockaddr *) &c_addr, sizeof(struct sockaddr_in)) == -1){
perror("connect");
exit(1);
}
printf("We have connected to a server.");
}
if (sfd == -1){
perror("socket");
exit(1);
}
struct sockaddr_in saddr;
saddr.sin_family = AF_INET;
saddr.sin_port = htons(my_port);
saddr.sin_addr.s_addr = INADDR_ANY;
memset(&(saddr.sin_zero), 0, 8);
if(bind(sfd, (struct sockaddr*) &saddr, sizeof(struct sockaddr_in)) == -1){
perror("bind");
close(sfd);
exit(1);
}
if (listen(sfd, 5) < 0){
perror("listen");
exit(1);
}
struct sockaddr_in caddr;
saddr.sin_family = AF_INET;
int cfd;
unsigned int c_len = sizeof(struct sockaddr_in);
if ((cfd = accept(sfd, (struct sockaddr*) &caddr, &c_len)) == -1){
perror("accept");
exit(1);
}
printf("Alas, we have finally connected to a client.");
return 0;
}
Upon running the second instance of the program I receive the error "bind: Invalid argument". I am assuming that this is due to the fact that the port is already in use. Is there any way to bypass this, or is there any way to connect a server to a client, and allow the client to also act as a server using the same port

Answer 1

You cannot open a socket which can do the both listen and connect.

Answer 2

A TCP connection is identified by its two endpoints. Each of those, in turn, is identified by an (IP address, port) pair. Therefore, you cannot simultaneously have two distinct connections between the same two IP addresses with the same ports on each end -- if all of those properties are the same, then they are the same connection.
From the perspective of system interfaces, you cannot create that situation because the system will not allow you to bind an address / port pair that is already in use to any socket (a stronger constraint than is strictly required). This means that one machine cannot use the same port simultaneously for both a client socket and a server socket, even for different remote endpoints.
You can, however, have any number of simultaneous TCP connections that each differ from all the others in at least one of those parameters. In particular, you can have any number of connections between the same two machines, with the same port on one side, and different ports on the other. This is extremely common, in fact, as web browsers often open multiple simultaneous connections to a web server to download multiple resources concurrently. All of those connections have the same server address, server port, and client address, but different client port.
If you want to have multiple simultaneous connections that are associated with one another in some way that goes beyond IP addresses, then you'll need to develop a protocol for it that involves multiple ports at at least one end. If the machines make reciprocal connections, with A connecting to B and then B connecting, separately, to A, then you'll need different ports on both sides. The port numbers to use might be fixed by the protocol or negotiated in some way, at your discretion, but the specifics described in the question are not an option.