Serial port communcation, issue with write()

Question

I wrote a program in C on a linux machine that reads and writes to a serial port. It's connected to a Windows XP machine running some emulator that is sending my program data.

In my program I have 2 threads, one for reading the input from the serial port and processing it. And a thread for that connects to a server socket and listens for data that is procesed and then written back on the serial port. Both of these threads are continually looping during execution.

The problem I am having is with the write command.

 void writeToPort(unsigned char* buf, int length)
 {
     int w = 0;
     if(fd > 0)
         w = write(fd, buf, length);

      printf("wrote %d bytes\n", w);
 }

and I open the port like this

fd = open(MODEMDEVICE, O_RDWR | O_NOCTTY | O_NONBLOCK);

For the first few seconds it is functioning correctly. I've verfied in the hyperterminal on Windows that the bytes I expect are being sent properly. However, at some point the write() call will hang. The other input reading thread continues and reading from the serial port works fine.

Does anyone know why this might be happening? I've tried using mutexes whenever a read or write is done on fd but that didn't seem to make a difference. It also does not seem to matter if the other thread is running/ reading from the port. Any advice or suggestions are welcome, thanks

Check the return value from write. It might have written fewer bytes than requested. (and in case of O_NONBLOCK, it could even return -1/E_WOULDBLOCK) — wildplasser
woops, I forgot to assign w when copying this over. The return value (w) is always correct when it is running, but when it hangs, w is never printed. — Fajak
In that case sombody else messed up the port. Either the guys from Redmond, or another process, or maybe even your other thread. Maybe the other thread is blocking on read(), after iocntls ? — wildplasser
hm, well I still have the issue if I don't create the thread that read()s. So that makes me think it might be on the windows computer since no other process should be using the port. Any idea why that might happen? — Fajak

blearn blearn · Accepted Answer · 2012-10-18T22:40:26

If I were to approach your problem from a high-level, I would implement a basic fork server model, where the main thread remains the same as your design (listens for input via serial). Once it has a new message, fork a new process to do your processing and then echo the reply back...

Take a look at Steven's UNIX Network Programming for the de-facto bible on Unix socket programming.

Serial port communcation, issue with write()

1 Answers