character I/O basics Post: 44842

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Top Forums UNIX for Dummies Questions & Answers character I/O basics Post 44842 by Perderabo on Thursday 11th of December 2003 10:44:23 AM

12-11-2003

Administrator Emeritus

starless, you really need to mention that you're using QNX. I guess that I shouldn't have remembered that you use QNX, but I didn't.

Your /dev/ser may indeed behave in ways that I don't understand. Or it may be very close to the unix tty. I'm not sure.

I'm not a c++ programmer nor do I know QNX. But I think that I understand this last character repeating business...maybe. But I'll have to explain unix system calls invoked from c.

I guessing that you're doing the equivalent of
read(fd, buffer, len)
and fd is now one side of a fifo. You need to change that to:
iret=read(fd, buffer, len)
if(iret == 0) { /* arrange to stop reading */ }
if (iret == -1) { /* error routine */ }

The read is failing, but you're not noticing this. Your buffer is not being touched. You're just assuming that it got a character. On unix, this situation would cause read to simply return a zero. In your case, you may being getting an error of some kind.

You can prove that you're not simply getting the last character... put some other character in the buffer just prior to the read. That will become the character that you seem to be reading in a loop.

Perderabo

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LEARN ABOUT ULTRIX

dc

dc(4) Kernel Interfaces Manual dc(4)

Name
dc - serial line/mouse/keyboard

Syntax
device dc0 at ibus? vector dcintr

Description
The serial line controller provides four ports, with modem control on two of the ports. The DECstation 3100 and DECstation 2100 only pro-
vide partial modem control. The DECstation 5000 provides full modem control. The ports are used as follows:

Port Usage
0 Graphics device keyboard at 4800 BPS
1 Mouse or tablet at 4800 BPS
2 Communications port 1 (w/modem control)/local terminal
3 Communications port 2 (w/modem control)/local terminal

Each communication port from the serial line controller behaves as described in and can be set to run at any of 16 speeds. For the encod-
ing, see

When a graphics device is not being used as the system console, communications port 2 becomes the system console. In this configuration,
the port can only be used at 9600 BPS and no modem control is supported.

The serial line driver operates in interrupt-per-character mode (all pending characters are flushed from the silo on each interrupt).

Restrictions
Speed must be set to 9600 BPS on the console port and 4800 BPS on ports used by graphics devices. The serial line driver enforces this
restriction; that is, changing speeds with the command may not always work on these ports.

Files
console terminal

local terminal

See Also
console(4), devio(4), tty(4), ttys(5), MAKEDEV(8)

RISC dc(4)

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