Question

I have many POSIX threads, two reader that read from serial port and others write to same port using a file descriptor. How can I share same descriptor between them? I have synchronized read/write and write/write actions between all threads by semaphores.

Note: I’m supposing a file descriptor should be shared between threads of same process but my code fails to run with a EBUSY error when second reader tries to read from port. (asked a question before)

Update

This is a little weird situation, even if only one thread is present at runtime, any call to read() after write() return -l with EBUSY error. Maybe I’m asking wrong question. There should be a some kind of flush after each write() to make sure that device is free? or somehow force write() to block?

Answer 1

Clearly, the EBUSY return code signals that the port is in use, and should be queried again later. Your threads should just wait a little bit and try again, until the command passes.

You sort of mention in one of your comments that the system behind the port is a mechanical one, which would explain why it could take a little while for a command to get processed.

I think the “one thread to handle IO” is the best approach. Each read/write would block the thread and avoid the EBUSY problem you are witnessing. All you would have left to do is implement a command queue (very easy with std::queue or similar and just just one mutex to sync all accesses).

UPDATE: reading your update, I guess that EBUSY are just the sign that commands are really slow to execute, and finish a little while after the system call returned, to the point that even when one single thread is doing IO, it may experience it. As I said at the beginning of my answer, have the thread wait a bit before reissuing its command, and that should do it.