TCP IP socket communication in C. During the client exit, intimating server? - c

Question

I am checking the communication between 2 entities (A and B) for their presence. A is a server and B is a client. When the server is up and running, it waits for connections and when B starts, it sends a message "Available" every one second. Now, the problem is, when I terminate the B program( ctrl+c or press the close button of the terminal), the server A does not recognise and still continues its operation. I would like for it to display message like "B is no more available" or "communicating partner is off". I understand TCP would be a best fit for the connection oriented communication. Please suggest me the changes to incorporate the display message on server A, when B is closed.
PS: amateur in socket programming
EDIT1: Managed to display the Message. However, since the server is designed to listen to many connections, I would like to make it accept the connections and not end the loop. Any guidance here would be useful.
SERVER(A) TCP SERVER
#include <stdio.h>
#include <stdlib.h>
#include <netdb.h>
#include <netinet/in.h>
#include <string.h>
int main( int argc, char *argv[] ) {
int sockfd, newsockfd, portno, clilen;
char buffer[256];
struct sockaddr_in serv_addr, cli_addr;
int n;
/* Initialize socket structure */
bzero((char *) &serv_addr, sizeof(serv_addr));
portno = 5001;
// create socket and get file descriptor
sockfd = socket(AF_INET, SOCK_STREAM, 0);
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = INADDR_ANY;
serv_addr.sin_port = htons(portno);
clilen = sizeof(cli_addr);
// bind the host address using bind() call
if (bind(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0){
perror("ERROR on binding\n");
exit(1);
}
// start listening for the clients,
// here process will go in sleep mode and will wait for the incoming connection
listen(sockfd, 5);
// accept actual connection from the client
newsockfd = accept(sockfd, (struct sockaddr *)&cli_addr, &clilen);
// inside this while loop, implemented communication with read/write or send/recv function
//printf("start");
while (1) {
bzero(buffer,256);
n = read(newsockfd, buffer, 255);
if (n < 0){
perror("ERROR in reading from socket");
exit(1);
}
if (n == 0){
perror("Client has abruptly ended\n");
close(sockfd);
exit(1);
}
printf("client said: %s \n", buffer);
n = write(newsockfd, buffer, strlen(buffer));
if (n < 0){
perror("ERROR in writing to socket");
exit(1);
}
// escape this loop, if the client sends message "quit"
// if (!bcmp(buffer, "quit", 4))
// break;
}
return 0;
}
CLIENT(B) TCP CLIENT
#include <stdio.h>
#include <stdlib.h>
#include <netdb.h>
#include <netinet/in.h>
#include <string.h>
int main(int argc, char *argv[]) {
int sockfd, portno, n;
struct sockaddr_in serv_addr;
struct hostent *server;
char buffer[256];
portno = 5001;
// create socket and get file descriptor
sockfd = socket(AF_INET, SOCK_STREAM, 0);
server = gethostbyname("127.0.0.1");
if (server == NULL) {
fprintf(stderr,"ERROR, no such host\n");
exit(0);
}
bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
bcopy((char *)server->h_addr, (char *)&serv_addr.sin_addr.s_addr, server->h_length);
serv_addr.sin_port = htons(portno);
// connect to server with server address which is set above (serv_addr)
if (connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
perror("ERROR while connecting");
exit(1);
}
// inside this while loop, implement communicating with read/write or send/recv function
while (1) {
strcpy(buffer,"Available");
n = write(sockfd,buffer,strlen(buffer));
if (n < 0){
perror("ERROR while writing to socket");
exit(1);
}
bzero(buffer,256);
n = read(sockfd, buffer, 255);
if (n < 0){
perror("ERROR while reading from socket");
exit(1);
}
printf("server replied: %s \n", buffer);
// escape this loop, if the server sends message "quit"
//if (!bcmp(buffer, "quit", 4))
// break;
sleep(1);
}
return 0;
}
Please anybody tell me how to handle that issue?

Answer 1

When the client closes the TCP connection, the server's call to read(newsockfd) will return 0 to indicate that the connection has closed. At that point, the server should close(newsockfd), print your "Client has gone away message", and not use newsockfd anymore (i.e. either the server program should exit, or, more usefully, it should just break out of its while(1)-loop and go back to calling accept() again, so that the next time a client runs it too can connect to the server)