How to close a serial communication in Cocoa background thread - cocoa

Question

I'm trying to run a serial communication example, in order to send data from an Arduino to a Cocoa application following the provided code in http://playground.arduino.cc/Interfacing/Cocoa ( IOKit/ioctl method ). It works, but I cannot stop the receiver thread once started.
I've implemented a switch button ( Start/Stop ) which at start time opens the serial port and launches the receiver thread:
- (IBAction) startButton: (NSButton *) btn {
(…)
error = [self openSerialPort: [SelectPort titleOfSelectedItem] baud:[Baud intValue]];
(…)
[self refreshSerialList:[SelectPort titleOfSelectedItem]];
[self performSelectorInBackground:#selector(incomingTextUpdateThread:) withObject:[NSThread currentThread]];
(…)
}
The thread code is practically the same as in the example, excepting that I've included code to rebuild the serial packet from received buffers and save it to a SQLite database:
- (void)incomingTextUpdateThread: (NSThread *) parentThread {
// mark that the thread is running
readThreadRunning = TRUE;
const int BUFFER_SIZE = 100;
char byte_buffer[BUFFER_SIZE]; // buffer for holding incoming data
int numBytes=0; // number of bytes read during read
(…)
// assign a high priority to this thread
[NSThread setThreadPriority:1.0];
// this will loop until the serial port closes
while(TRUE) {
// read() blocks until some data is available or the port is closed
numBytes = (int) read(serialFileDescriptor, byte_buffer, BUFFER_SIZE); // read up to the size of the buffer
if(numBytes>0) {
// format serial data into packets, but first append at start the end of last read
buffer = [[NSMutableString alloc] initWithBytes:byte_buffer length:numBytes encoding:NSASCIIStringEncoding];
if (status == 1 && [ipacket length] != 0) {
[buffer insertString:ipacket atIndex:0];
numBytes = (int) [buffer length];
}
ipacket = [self processSerialData:buffer length:numBytes]; // Recompose data and save to database.
} 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;
}
I try to close the port in the main thread with this code, also part of the startButton selector, following the provided example:
if (serialFileDescriptor != -1) {
[self appendToIncomingText:#"Trying to close the serial port...\n"];
close(serialFileDescriptor);
serialFileDescriptor = -1;
// Revisar... crec que el thread no s'adona que s'ha tancat el file descriptor...
// wait for the reading thread to die
while(readThreadRunning);
// re-opening the same port REALLY fast will fail spectacularly... better to sleep a sec
sleep(0.5);
//[btn setTitle:#"Start"];
[Start setTitle:#"Start"];
}
But it seems that the receiver thread is not aware of the status change in global variable serialFileDescriptor.

Answer 1

So, startButton: opens the port, spawns off a thread to start reading from it, and then immediately closes the port? That's not going to turn out well.
startButton: should not close the port. Leave that for the reading thread to do when it's done, and do it on the main thread only when you need to close the port for some other reason (e.g., quitting).
Global variables are, by definition, visible throughout the program, and this includes across thread boundaries. If readThreadRunning is not getting set to FALSE (which assumes that FALSE hasn't been defined to something exotic), then your read thread's loop must still be running. Either it is still reading data, or read is blocked (it is waiting for more data).
Note that read has no way to know whether there will be more data. As your comment in the code says, it will block until either it has some data to return or the port gets closed. You should either work out a way to know ahead of time how much data you'll need to read, and stop when you've read that much, or see if you can close the port at the opposite end when everything has been sent and received.