How do I know whether my device is attached to tty?

I'm using AIX Ver 4.1 on IBM server.
In a C program, how do I know whether my device is attached to tty?  My device is asynchronous.

If my device is not attached, my program will hang on the following sentence : read(port, buf, 22);

I've tried using the following sentences but to no avail:
      port = open(PORT, O_RDWR | O_NODELAY);
and
     port = open(PORT, O_RDWR | O_NONBLOCK);


Thanks!


Rita060297Asked:
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remboCommented:


What sort of device is it?  Take a look at the man page
for the specific device and there should be an ioctl()
that you can use to determine device readiness.  The open()
system call often just sets a flag and never accesses
the device.  
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Rita060297Author Commented:
The device is a digital clock which will send me the current time when
I send it a certain character.

I've tried ioctl with FIONREAD but I receive 0 for the number of bytes available to be read on the file descriptor when device is attached and device not attached.

Are there any alternatives?
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ozoCommented:
So it's a tty device?
Can you try an ioctl TIOCMGET?
Can you do a select with a timeout?

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aditya070797Commented:
I don't understand the question completely, but I think you can
try using:
----------------------------------
struct termios *termios_ptr;
...
rv = tcgetattr(port, termios_ptr);
----------------------------------
If you get an error (rv == -1) and errno is set to ENOTTY,
then the port is not a tty.
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Rita060297Author Commented:
I tried ioctl(port, TIOCMGET, &nbyte) and received the following results :

nbyte = 70 when device attached to port
nbyte = 6 when device not attached to port

I've found out that TIOCMGET defines the following :
Gets all modem bits.  The argument to this command is a pointer to an
integer where the current state of the modem status lines is stored.
Which modem status and modem control lines are supported depends on the capabilities of the hardware and the hardware's device driver.

Question :
(1) How do I know what 70 or 6 means?
(2) Can I use 70 and 6 as a guide for deteching the attached or non-attached device?  It seems to be consistent during my numerous testing.


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ozoCommented:
You should be able to find the definition of the bits in /usr/include/sys/termios.h
It looks like 6 is DTR+RTS, and 0106 is DTR+RTS+CD (carrier detect)


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