NSStream TCP Keep-alive iOS - ios

Question

I have written this code to setup a stream with a server:
-(void)streamOpenWithIp:(NSString *)ip withPortNumber:(int)portNumber;
{
CFReadStreamRef readStream;
CFWriteStreamRef writeStream;
CFStreamCreatePairWithSocketToHost(kCFAllocatorDefault, (__bridge CFStringRef)ip, portNumber, &readStream, &writeStream);
if(readStream && writeStream)
{
CFReadStreamSetProperty(readStream, kCFStreamPropertyShouldCloseNativeSocket, kCFBooleanTrue);
CFWriteStreamSetProperty(writeStream, kCFStreamPropertyShouldCloseNativeSocket, kCFBooleanTrue);
//Setup inpustream
inputStream = (__bridge NSInputStream *)readStream;
[inputStream setDelegate:self];
[inputStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
[inputStream open];
//Setup outputstream
outputStream = (__bridge NSOutputStream *)writeStream;
[outputStream setDelegate:self];
[outputStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
[outputStream open];
}
}
I am able to connect to the server and send & receive data. But I want to check if the connection is still there. If I disconnect a cable from the wifi router, I still can send data in the stream and no error occurred.
You can solve this on application level by using a timer to send some message and check if you receive something back. But you can solve this also with TCP Keep-Alive technique on a lower level.
How do I implement this with NSStream? How can I set the checking interval?
I assume that you get a NSStreamEventErrorOcurred when the stream is down by checking it with TCP Keep-Alive?
I have checked this post, but I can't figure it out:
Keeping a socket connection alive in iOS
Thanks for your help!

Answer 1

You can get the native socket handle with
CFDataRef socketData = CFReadStreamCopyProperty((__bridge CFReadStreamRef)(inputStream), kCFStreamPropertySocketNativeHandle);
CFSocketNativeHandle socket;
CFDataGetBytes(socketData, CFRangeMake(0, sizeof(CFSocketNativeHandle)), (UInt8 *)&socket);
CFRelease(socketData);
and then set socket options (you need to #include <sys/socket.h> for this):
int on = 1;
if (setsockopt(socket, SOL_SOCKET, SO_KEEPALIVE, &on, sizeof(on)) == -1) {
NSLog(#"setsockopt failed: %s", strerror(errno));
}
You could put this code in the event handler function for the kCFStreamEventOpenCompleted event:
- (void)stream:(NSStream *)stream handleEvent:(NSStreamEvent)event {
switch (event) {
case kCFStreamEventOpenCompleted:
if (stream == self.inputStream) {
CFDataRef socketData = CFReadStreamCopyProperty((__bridge CFReadStreamRef)(stream), kCFStreamPropertySocketNativeHandle);
CFSocketNativeHandle socket;
CFDataGetBytes(socketData, CFRangeMake(0, sizeof(CFSocketNativeHandle)), (UInt8 *)&socket);
CFRelease(socketData);
int on = 1;
if (setsockopt(socket, SOL_SOCKET, SO_KEEPALIVE, &on, sizeof(on)) == -1) {
NSLog(#"setsockopt failed: %s", strerror(errno));
}
}
break;
/* ... other cases ... */;
}
}

Answer 2

There is a more complete answer to a similar question.
For the example of the app that starts sending keepalive after 10 seconds, sends keppalive every 2 seconds and closes the stream after 4 keepalives with no reply, take a look at this post:
Is it possible to activate TCP keepalive on Apple iOS devices
It also shows how to set retransmission timeout after which the connection is closed.