Why is the client's file descriptor used while calling the recv and send funtions on both server and client sides? - sockets

Question

TCPServer
#include<stdio.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/ip.h>
#include<netinet/in.h>
#include<string.h>
#include<stdlib.h>
int main()
{
int fd = socket(AF_INET, SOCK_STREAM, 0);
if(fd == -1)
{
printf("socket failed!\n");
exit(0);
}
printf("Enter port: ");
int port;
scanf("%d",&port);
struct sockaddr_in server;
server.sin_family = AF_INET;
server.sin_port = htons(port);
server.sin_addr.s_addr = INADDR_ANY;
int bind_ret = bind(fd, (struct sockaddr*)(&server), sizeof(server));
if(bind_ret == -1)
{
printf("bind failed!\n");
exit(0);
}
int listen_ret = listen(fd, 10);
if(listen_ret == -1)
{
printf("listen failed!\n");
exit(0);
}
struct sockaddr_in client;
int l = sizeof(client);
int client_fd = accept(fd, (struct sockaddr*)(&client), &l);
if(client_fd == -1)
{
printf("accept failed!\n");
exit(0);
}
while(1)
{
char msg_recv[50];
int recv_ret = recv(client_fd, msg_recv, sizeof(msg_recv),0);
if(recv_ret == -1)
{
printf("recv failed!\n");
exit(0);
}
msg_recv[recv_ret]='\0';
if(strcmp("bye",msg_recv)==0)
{
exit(0);
}
printf("Message recieved: %s\n",msg_recv);
char msg_send[50];
printf("Enter message: ");
scanf(" %s",msg_send);
int send_ret = send(client_fd, msg_send, strlen(msg_send),0);
if(send_ret == 0)
{
printf("send failed!\n");
}
if(strcmp("bye",msg_send) == 0)
exit(0);
}
}
TCPClient
#include<stdio.h>
#include<sys/socket.h>
#include<sys/types.h>
#include<netinet/ip.h>
#include<netinet/in.h>
#include<string.h>
#include<stdlib.h>
int main()
{ int fd = socket(AF_INET, SOCK_STREAM, 0);
if(fd == -1)
{
printf("socket failed!\n");
exit(0);
}
int port;
printf("Enter port number: ");
scanf("%d",&port);
struct sockaddr_in client;
client.sin_family = AF_INET;
client.sin_port = htons(port);
client.sin_addr.s_addr = INADDR_ANY;
int connect_ret = connect(fd, (struct sockaddr*)(&client), sizeof(client));
if(connect_ret == -1)
{
printf("connect failed!\n");
exit(0);
}
while(1)
{
printf("Enter message: ");
char msg_send[50];
scanf("%s",msg_send);
int send_ret = send(fd, msg_send, strlen(msg_send), 0);
if(send_ret == -1)
{
printf("send failed!\n");
exit(0);
}
if(strcmp("bye", msg_send)==0)
{
exit(0);
}
char msg_recv[50];
int recv_ret = recv(fd, msg_recv, sizeof(msg_recv), 0);
if(recv_ret == -1)
{
printf("recv failed!\n");
exit(0);
}
msg_recv[recv_ret]= '\0';
if(strcmp("bye", msg_recv) == 0)
exit(0);
printf("Message recieved: %s \n",msg_recv);
}
}
In the above program for Server, recv and send were called by passing client_fd as the argument, while in the program for Client, recv and send were called by passing fd as the argument. I wanted to know why on the server side we did not use its own socket file descriptor like we did on the client side?

Answer 1

The server’s fd descriptor is a listen()’ing socket. It can’t perform any I/O, only receive incoming client connections. accept() pulls a pending client connection from fd’s queue and returns a new socket descriptor that can perform I/O with that client.
The client’s fd descriptor is a connect()‘ing socket. It can perform I/O with the server once its connection has been accepted.