OSX Serial read freeze / hang - objective-c

Question

I'm writing a serial communication wrapper class in Objective-C. To list all serial available modems and setup the connection I'm using pretty much the same code as used in this example project by Apple.
I could read and write the ways apple does it. But I want to implement a loop on a second thread and write to the stream if a NSString *writeString longer 0 and read after write if bytes are available.
I got writing working quite straight forward. I just used the write function declared in unistd.h.
Reading will not work. Whenever I call read(), the function hangs and my loop does not proceed.
Here is the code used in my loop:
- (void)runInCOMLoop {
do {
// write
} while (bytesWritten < strlen([_writeString UTF8String]));
NSMutableString *readString = [NSMutableString string];
ssize_t bytesRead = 0;
ssize_t readB = 0;
char buffer[256];
do {
readB = read(_fileDescriptor, &buffer, sizeof(buffer));
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this function hangs
bytesRead += readB;
if (readB == -1 {
// error
}
else if (readB > 0) {
if(buffer[bytesRead - 1] == '\r' ]] buffer[bytesRead - 1] == '\n') {
break;
}
[readString appendString:[NSString stringWithUTF8String:buffer]];
}
} while (readB > 0);
What am I doing wrong here?

Answer 1

read() will block if there is nothing to read. Apple probably has their own of doing things, but you can use select() to see if there is anything to read on _fileDescriptor. Google around for examples on how to use select.
Here's one link on StackOverflow:
Can someone give me an example of how select() is alerted to an fd becoming "ready"
This excerpt from the select man is pertains:
To effect a poll, the timeout argument should be
non-nil, pointing to a zero-valued timeval structure. Timeout is not
changed by select(), and may be reused on subsequent calls, however it is
good style to re-initialize it before each invocation of select().

Answer 2

You can set the non-blocking flag (O_NONBLOCK) on the file descriptor using fcntl() to keep read() from waiting for data, but if you do that, you have to continuously poll looking for data, which is obviously bad from a CPU usage standpoint. As Charlie Burns' answer explains, the best solution is to use select() which will allow your program to efficiently wait until there is some data to be read on the port's file descriptor. Here's some example code taken from my own Objective-C serial port class, ORSSerialPort (slightly modified):
fd_set localReadFDSet;
FD_ZERO(&localReadFDSet);
FD_SET(self.fileDescriptor, &localReadFDSet);
timeout.tv_sec = 0;
timeout.tv_usec = 100000; // Check to see if port closed every 100ms
result = select(localPortFD+1, &localReadFDSet, NULL, NULL, &timeout);
if (!self.isOpen) break; // Port closed while select call was waiting
if (result < 0) {
// Handle error
}
if (result == 0 || !FD_ISSET(localPortFD, &localReadFDSet)) continue;
// Data is available
char buf[1024];
long lengthRead = read(localPortFD, buf, sizeof(buf));
NSData *readData = nil;
if (lengthRead>0) readData = [NSData dataWithBytes:buf length:lengthRead];
Note that select() indicates that data is available by returning. So, your program will sit suspended at the select() call while no data is available. The program is not hung, that's how it's supposed to work. If you need to do other things while select() is waiting, you should put the select() call on a different queue/thread from the other work you need to do. ORSSerialPort does this.