Non-blocking Linux server socket - linux

Question

I want to create a server socket that always prints to screen "tick" , and if one client sends data to this server, the server will print that data. I use non-blocking socket, but the it does not work, server prints to screen "tick" but cannot receives data from client.
Server
int main(int argc, char *argv[]) {
int server_sockfd, client_sockfd;
sockaddr_un server_address;
sockaddr_un client_address;
int client_len;
int res;
/* remove old socket and create unnamed socket */
unlink("server_socket");
server_sockfd = socket(AF_UNIX, SOCK_STREAM, 0);
/* non-blocking socket */
fcntl(server_sockfd, F_SETFL, O_NONBLOCK);
/* name the socket */
server_address.sun_family = AF_UNIX;
strcpy(server_address.sun_path, "server_socket");
bind(server_sockfd, (sockaddr*)&server_address, sizeof(server_address));
/* listen client */
printf("server_waiting\n");
listen(server_sockfd, 5);
client_len = sizeof(client_address);
client_sockfd = accept(server_sockfd, (sockaddr*)&client_address, (socklen_t*)&client_len);
while(1) {
char ch;
res = recv(client_sockfd, &ch, 1, 0);
if (res == -1) {
printf("tick\n");
}
else {
printf("received: %c\n", ch);
}
}
}
client
int main(int argc, char *argv[]) {
int sock_fd;
struct sockaddr_un address;
int result;
char ch = 'A';
/* create socket for client */
sock_fd = socket(AF_UNIX, SOCK_STREAM, 0);
/* name of socket as agreed with server */
address.sun_family = AF_UNIX;
strcpy(address.sun_path, "server_socket");
result = connect(sock_fd, (sockaddr*) &address, sizeof(address));
if (result == -1) {
perror("fail\n");
exit(1);
}
/* write via socket */
send(sock_fd, &ch, 1, 0);
close(sock_fd);
exit(0);
}

Answer 1

You are setting the listing socket to be non-blocking instead of the accepted socket.
Following your code logic, you DO want to wait on the accept call, but not the recv call
Instead of
/* non-blocking socket */
fcntl(server_sockfd, F_SETFL, O_NONBLOCK);
Delete it and instead add the fcntl call to the socket you are getting back from the accept call, like
client_sockfd = accept(....);
int flags = fcntl(client_sockfd, F_GETFL, 0);
fcntl(client_sockfd, F_SETFL, flags | O_NONBLOCK);
accept and fcntl can fail so you should check for failures in production code.