TCP server/client how to keep connections alive? - c

Question

I wrote a simple TCP echo server to handle multiple clients. It uses select() to get multiple connections.
Server Code:
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <string.h>
int create_listener(uint16_t port) {
int listen_fd;
struct sockaddr_in name;
listen_fd = socket(AF_INET, SOCK_STREAM, 0);
if (listen_fd < 0) {
perror ("socket");
exit(EXIT_FAILURE);
}
bzero(&name, sizeof(name));
name.sin_family = AF_INET;
name.sin_addr.s_addr = htonl(INADDR_ANY);
name.sin_port = htons(port);
if (bind(listen_fd, (struct sockaddr *) &name, sizeof(name)) < 0) {
perror ("bind");
exit(EXIT_FAILURE);
}
return listen_fd;
}
int read_from_client(int fd) {
char buffer[100];
int nbytes;
nbytes = read(fd, buffer, 100);
if (nbytes < 0) {
perror("read");
exit(EXIT_FAILURE);
}
else if (nbytes == 0) {
return -1;
}
else {
fprintf(stderr, "Server: got message: %s\n", buffer);
write(fd, buffer, strlen(buffer) + 1);
return 0;
}
}
int main(int argc, char *argv[]) {
int listen_fd;
uint16_t port = 22000;
fd_set active_fd_set, read_fd_set;
int i;
struct sockaddr_in servaddr;
/* Create the socket and set it up to accept connections. */
listen_fd = create_listener(port);
if (listen(listen_fd, 10) < 0) {
perror("listen");
exit(EXIT_FAILURE);
}
/* Initialize the set of active sockets. */
FD_ZERO(&active_fd_set);
FD_SET(listen_fd, &active_fd_set);
while (1) {
/* Block until input arrives on one or more active sockets. */
read_fd_set = active_fd_set;
if (select(FD_SETSIZE, &read_fd_set, NULL, NULL, 0) < 0) {
perror("select");
exit(EXIT_FAILURE);
}
/* Service all the sockets with input pending. */
for (i = 0; i < FD_SETSIZE; ++i) {
if (FD_ISSET(i, &read_fd_set)) {
if (i == listen_fd) {
/* Connection request on original socket. */
int new_fd;
new_fd = accept(listen_fd, (struct sockaddr *) NULL, NULL);
if (new_fd < 0) {
perror ("accept");
exit(EXIT_FAILURE);
}
FD_SET(new_fd, &active_fd_set);
}
else {
/* Data arriving on an already-connected socket. */
if (read_from_client(i) < 0) {
close(i);
FD_CLR(i, &active_fd_set);
}
}
}
}
}
return 0;
}
Client code:
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
int main(int argc, char *argv[]) {
int sockfd, n;
char sendline[100];
char recvline[100];
struct sockaddr_in servaddr;
sockfd = socket(AF_INET, SOCK_STREAM, 0);
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(22000);
inet_pton(AF_INET, "127.0.0.1", &(servaddr.sin_addr));
connect(sockfd, (struct sockaddr *)&servaddr, sizeof(servaddr));
while (1) {
bzero(sendline, 100);
bzero(recvline, 100);
fgets(sendline, 100, stdin);
write(sockfd, sendline, strlen(sendline) + 1);
read(sockfd, recvline, 100);
printf("%s", recvline);
}
return 0;
}
The problem is when I run server in one terminal and run two clients in another two terminals. If I use Ctrl+C to terminate one client, the server automatically terminates. I'm wondering why the server acts this way. What I'm expecting is the server runs forever. When client 1 terminates, server should still has a live connection with client 2.

Answer 1

Looks like you're hitting the exit in read_from_client. In general, in a server that serves multiple clients, you probably don't want to exit when you have a failure with one of the client connections.