Simple posix server can't set UILabel from loop - ios

Question

I'm trying to add a posix socket server to my iOS app that will allow a TCP connection and writes the buffer to a UILabel object as a test.
I can get it to work...once. Then it finishes and closes the connection. Ok, easy fix, I'll just put it in a loop. Now whenever I put the exact same code in a loop, it won't update the UILabel for some reason. I don't actually need it to be able to update the UILabel, it was just a test to make sure the server was working.......but it's making me nervous. I take it out of the while loop, it works, I put it back in, and everything but the UILabel setText call work.
Also, two other small questions: I'm having trouble figuring out how to exit the loop after a client disconnects, and I'm not sure how to correctly close the ports when I exit, I have to keep changing the port number because it can't bind.
-(void)viewDidLoad
NSThread *listenThread = [[NSThread alloc] initWithTarget:self selector:#selector(createPosixServer) object:nil];
[listenThread start];
-(void)createPosixServer
//declarations
int sockfd, newsockfd, portno;
socklen_t clilen;
char buffer[256];
struct sockaddr_in serv_addr, cli_addr;
int n;
NSString *nsbuffer;
//bind and listen on socket
sockfd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (sockfd < 0) {
NSLog(#"Error while calling socket()");
}
bzero((char *) &serv_addr, sizeof(serv_addr));
portno = 1818;
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = INADDR_ANY;
serv_addr.sin_port = htons(portno);
if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
NSLog(#"ERROR on binding");
}
listen(sockfd, 5);
NSLog(#"Begin listen loop");
clilen = sizeof(cli_addr);
newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
if (newsockfd < 0) {
NSLog(#"ERROR on accept");
}
while(true) {
bzero(buffer,256);
n = read(newsockfd,buffer,255);
if (n < 0) {
NSLog(#"ERROR reading from socket");
}
if (n > 0) {
nsbuffer = [NSString stringWithCString:buffer encoding:NSASCIIStringEncoding];
[_lblStatus setText:nsbuffer];
NSLog(#"You sent %#", nsbuffer);
}
NSLog(#"Finished listen loop");
sleep(1);
}
close(newsockfd);
close(sockfd);
NSLog(#"Socket closed");

Answer 1

createPosixServer is running on a background thread. It is never safe to update the UI from a background thread. UIKit will sometimes work, sometimes just ignore you. You need to dispatch the call to update the label onto the main thread, something like this:
typeof(self) __weak weakSelf = self;
dispatch_async(dispatch_get_main_queue(), ^{
weakSelf.lblStatus.text = nsbuffer;
});
To answer the side question: when a client disconnects you want to close the socket that you accepted for that connection (your newsockfd) but you don't want to close your listener sockfd until you are tearing down the whole service.
To exit the loop, simply do this:
if (n < 0) {
NSLog(#"ERROR reading from socket");
break;
}
Though you probably want to check errno in that block too because you're probably going to want different behavior depending on the error.
Remove the sleep(1), that is doing nothing good for you. The read call will block, there's no need to sleep.