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Concurrent read / write of serial port

Posted on 2006-07-14
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I have a serial port access class which wraps the termios API. This serial port in some situations is used in a multithreaded environment. In this case it will be shared by 2 threads but I can guarantee that 1 will perform output only, and the other will perform input only. Does this require a mutex to access the object?

I can't find any documentation on the thread-safety of read(2) and write(2), or termios(3). I guess it really depends on the implementation, but given that read, write and the termios API are POSIX-compliant, does anyone know if POSIX specifies the thread-safety of these functions?

--Rob
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Question by:boycy
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by:manish_regmi
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> does anyone know if POSIX specifies the thread-safety of these functions?

No the POSIX does not say anything about thread safety of those functions. This means that thread safety is not guranteed and one should assume that they are not thread safe.

> Does this require a mutex to access the object?
At the hardware level, Serial ports have seperate pins for data transmission and reception.
So, you may not need to protect read from writes.

regards
Manish Regmi
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by:boycy
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Granted the hardware level is fine for concurrent read/write - after all, it is full duplex. I would *assume* that providing an API which removes this full duplex ability would be a really silly idea; however assumptions are not good things. If there is any doubt at all as to whether these functions are thread-safe then I cannot use them. I'm having real trouble finding any information on this subject.
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manish_regmi earned 125 total points
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A discussion on red hat forum also suggests it is safe.

http://sources.redhat.com/ml/ecos-discuss/2006-04/msg00219.html
So under Linux it may be safe but may not be for other OS.

Also since POSIX does not confirm thread safety. You can protect them with mutex for safety.

regards
Manish Regmi
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by:boycy
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Manish, thanks :)
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