Question

Could someone help identify why my server cannot accept more than one message from the client?

I am attempting to have the flow be like the following:
1. Client sends size of message to server
2. Server receives the size and sends a response back. In this case 0.
3. Client checks response and then writes message to server.
4. Server reads message and prints it out.

The problem I am getting is that the accept() at step 4 is never unblocking.

CLIENT

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

int main(int argc, char *argv[])
{
int sock = socket(AF_INET, SOCK_STREAM, 0);

struct sockaddr_in s_address;
s_address.sin_family = AF_INET;
s_address.sin_port = htons(51717);
s_address.sin_addr.s_addr = INADDR_ANY;
if (connect(sock, (struct sockaddr *) &s_address, sizeof(s_address)) < 0) {
    printf("ERROR: Cannot connect()\n");
    exit(0);
}

char *org_msg = "Hello";

printf("Writing size of Hello\n");
char msg1[1];
msg1[0] = sizeof(org_msg);
write(sock, msg1, sizeof(msg1));

printf("Waiting for response from server\n");
struct sockaddr_in c_address;
socklen_t c_length = sizeof(c_address);
int new_sock = accept(sock, (struct sockaddr *) &c_address, &c_length);

printf("Reading response from server\n");
char stat[1];
read(new_sock, stat, 1);

if (atoi(stat) == 0) {
    printf("Writing Hello to server\n");
    write(sock, org_msg, sizeof(org_msg));
}
close(sock);
close(new_sock);
}

SERVER

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

int main(int argc, char *argv[])
{
int sock = socket(AF_INET, SOCK_STREAM, 0);

struct sockaddr_in s_address;
s_address.sin_family = AF_INET;
s_address.sin_port = htons(51717);
s_address.sin_addr.s_addr = INADDR_ANY;
if (bind(sock, (struct sockaddr *) &s_address, sizeof(s_address)) < 0) {
    printf("ERROR: Cannot bind()\n");
    exit(0);
}

listen(sock, 3);

printf("Waiting for client message\n");
struct sockaddr_in c_address;
socklen_t c_length = sizeof(c_address);
int new_sock = accept(sock, (struct sockaddr *) &c_address, &c_length);

printf("Reading client message\n");
char msg[1];
read(new_sock, msg, 1);

printf("Writing response to client\n");
char stat[1];
stat[0] = '0';
write(new_sock, stat, sizeof(stat));

printf("Waiting for client message\n");
int new_sock2 = accept(sock, (struct sockaddr *) &c_address, &c_length);

printf("Reading client message\n");
char msg2[atoi(msg)];
read(new_sock2, msg2, sizeof(msg2));

printf("MESSAGE: %s\n", msg2);

close(sock);
close(new_sock);
close(new_sock2);
}

Answer 1

You should not call accept() on an already-connected socket. Once you have a connected socket in the server (the socket returned by accept()) you should just keep reading and writing that socket until the connection is closed. The steps for the server should be similar to:

listen_socket = socket(...);
listen(listen_socket, ...);
connected_socket = accept(listen_socket, ...);
read(connected_socket, ...)
write(connected_socket, ...)
read(connected_socket, ...)
write(connected_socket, ...)
...

Similarly the client should just keep reading and writing the socket once it has been connected successfully – the steps for the client should be:

connected_socket = socket(...);
connect(connected_socket, ...);
write(connected_socket, ...);
read(connected_socket, ...);
write(connected_socket, ...);
read(connected_socket, ...);
...