Mac equivalent to .Net SerialPort.ReadExisting() - objective-c

Question

I'm working on porting a VB program to the PC. It uses serial communication to interact with a physical device. I have a version up and running on the Mac using ORSSerialPort. However, once piece of the VB library that is great is the SerialPort.ReadExisting() function. This essentially reads any messages and discards them.
Has anyone built something similar on the Mac side? I've tried pulling out the ORSSerialPort into a function to directly read values (see below). However, unless I send a message I receive a null response. The readExisting function would be great for a scenario when things get a little out of alignment such as:
I send a message "Message1" to the device and nothing happens (was expecting Response1).
I send a message "Message2" to the device and receive: "Response1" instead of "Response2"
I'd like to detect this, call an equivalent to SerialPort.readExisting() since Response2 is the next to be found if I continue.
My read function:
-(NSString *) directRead
{
// Read Directly
int localPortFD = self.fileDescriptor;
struct timeval timeout;
int result=0;
fd_set localReadFDSet;
FD_ZERO(&localReadFDSet);
FD_SET(localPortFD, &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) return nil; // Port closed while select call was waiting
if (result < 0)
{
NSLog(#"No Data To Read");
}
if (result == 0 || !FD_ISSET(localPortFD, &localReadFDSet)) return nil;
// Data is available
char buf[1024];
long lengthRead = read(localPortFD, buf, sizeof(buf));
if (lengthRead>0)
{
NSData *readData = [NSData dataWithBytes:buf length:lengthRead];
if (readData != nil)
return [[NSString alloc] initWithData:readData encoding:NSUTF8StringEncoding];
}
return nil;
}
You would think just doing:
NSString *result = nil;
do
{
result = [serialPort directRead];
NSLog(#"Past Message is: %#", result);
} while(result != nil);
Would flush out the messages. However, it acts as if there aren't any messages. However, if I call sendData:Message1 again I'd still see Response2 show up (in the above scenario).
Thanks for any and all help.

Answer 1

I'm having a little trouble following exactly what you're trying to do here, but in any case, it's something you'll have to implement at a higher level than ORSSerialPort or the POSIX serial read functions. ORSSerialPort will tell you any time bytes are available, as reported by the underlying standard POSIX file APIs it uses. So, there's nothing to "flush" from its perspective. If you get a response you were not expecting, e.g. a response to an "old" request, there's no way at all for the serial port to know that, it's up to your code to figure it out.
Something like this:
- (void)serialPort:(ORSSerialPort *)serialPort didReceiveData:(NSData *)data
{
if (![self dataIsResponseToLatestRequest:data]) return; // Try again next time around
// Process a valid (expected response)
}
This raises a couple other issues. You can't be guaranteed that data will come in "whole packets". The serial hardware, low level serial APIs, and ORSSerialPort have no way of knowing what a whole packet looks like, and therefore just deliver data a bit at a time as it arrives. See this answer for more on that. So, it's up to you to buffer incoming data and assemble it into packets.
If you need to match requests you sent with responses coming in, you have to do that yourself. (sounds like you're really already doing this).
Lastly, if you're only receiving "Response1" after sending "Message2", this leads me to think there's a problem with the device on the other side of the connection not responding to requests properly. It would probably be worth fixing that, if you can (assuming it's your hardware/software).