High CPU consumption and latency while reading serial data - objective-c

Question

I have two functions in my software that cause important latency problems. The software is written in Objective-C. I receive serial data from an usb device and my goal is to encapsulate them and then to send them to another object which will process the data.
This section of the program causes large cpu and latency issues and I don't know how to solve this at all. The device only sends data when its state changes, thus when lots of changes occur well everything becomes laggy..
- (void)getSerialData {
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
[self getSerialDataLoop];
});
}
- (void)getSerialDataLoop {
readThreadRunning = YES;
char byte_buffer[2]; // buffer for holding incoming data
int numBytes=0; // number of bytes read during read
NSString *text;
// this will loop untilthe serial port closes
while(TRUE) {
// read() blocks until some data is available or the port is closed
numBytes = (int)read(serialFileDescriptor, byte_buffer, 1); // read up to the size of the buffer
if(numBytes>0) {
///text = [NSString stringWithCString:byte_buffer encoding:NSSymbolStringEncoding];
if(![text isEqualToString:#""]){
text = [NSString stringWithUTF8String:byte_buffer];
[self performSelectorOnMainThread:#selector(processNSStringData:) withObject:text waitUntilDone:YES];
}
} else {
break; // Stop the thread if there is an error
}
}
// make sure the serial port is closed
if (serialFileDescriptor != -1) {
close(serialFileDescriptor);
serialFileDescriptor = -1;
}
// mark that the thread has quit
readThreadRunning = FALSE;
}
Do you have any ideas or pointers?

Answer 1

You've basically reinvented NSStream here. I would first recommend that you investigate this already available solution that ties into the run loop.
You also could easily be overwhelming yourself with calls to getSerialData. Nothing in your system prevents multiple calls to this routine, and if you make multiple calls, you'll get dueling concurrent operations. Using NSStream would address that. In any case, though, you shouldn't keep creating new read blocks if one is already running.
You're also reading one byte at time and processing it. This is likely your biggest impact. Calling back to the main thread for every byte is likely quite expensive. If nothing else you're creating a new NSString object for every byte.
Note that your code is very dangerous and could crash. You never initialize byte_buffer, and you only read one byte into it. When you call stringWithUTF8String:, you're assuming that the second byte is \0, which depends on the current state of the stack.