But the whole point is that they behave differently on open(). Afterwards
they’re the same.
- /dev/tty initially checks the state of DTR on the port
- /dev/cu initially ignores the state of DTR on the port
This has the practical effect of:
- /dev/tty blocks on open() until DTR transitions high
- /dev/cu allows open() and write() regardless of initial DTR
Once a connection has been established with the device both behave
identically.
Once DTR has been dropped (after being asserted), both types
disconnect.
> On Jun 8, 2015, at 6:01 PM, Jack Brindle <[email protected]> wrote:
>
> Actually it is probably the other way. The cu (call unix) device was
> primarily for UUCP communications. The tty port is mostly used these days.
> Both do the exact same thing, it is
> the defaults when they are opened that are different. You should immediately
> set the port for whatever handshaking and control you need after opening
> which tends to negate the defaults and give you the desired environment.
>
> See: http://lists.apple.com/archives/darwin-dev/2009/Nov/msg00099.html
> <http://lists.apple.com/archives/darwin-dev/2009/Nov/msg00099.html>
> for some interesting discussion.
>
> - Jack
>
>> On Jun 8, 2015, at 5:54 PM, Carl Hoefs <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>>
>>> On Jun 8, 2015, at 5:25 PM, Roland King <[email protected]
>>> <mailto:[email protected]>> wrote:
>>>
>>> The difference between the tty device and the cu device is that ttys were
>>> traditionally used to connect a terminal to, so opening a tty should block
>>> until the device on the other end raises DTR (ie you turn it on). getty
>>> opens tty devices and blocks in this way. The cu devices were designed for
>>> callups where you needed to connect to say a modem first, then dial, and
>>> only when the connection was established does DTR go high; so open doesn’t
>>> block.
>>>
>> Okay, that sounds like in general /dev/cu would almost always be called for
>> (unless you’re doing actual terminal or modem work!) The way USB/serial
>> devices seem to work on OS X is that they don’t even show up with a mount
>> point unless they’re ready to communicate (perhaps at some level simulating
>> DTR high?). In practice it hasn’t made any difference which I’ve used, cu or
>> tty, but this info is good to know regardless.
>> -Carl
>>
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