on Fedora 19. Coding in c++
my /dev directory there are many tty designators listed. How do I know which one will use the serial port on my PC.

For my code I chose tty50.

fd = open("/dev/tty50", O_RDWR | O_NOCTTY | O_NDELAY);
It opens successfully, at least the open returns a non-negative, non-zero number, usually a 9.

cfsetispeed(&attribs, B38400);
cfsetospeed(&attribs, B38400);
When configuring the input baud and output baud there are no errors thrown.

char bytestosend[] = "hello\n";
nbytes = TEMP_FAILURE_RETRY (write (fd, bytestosend, 6));
When I go to write I get errno 11 "Resource temporarily unavailable"

Is there something else I need to do?
Can the physical serial port be left "unattached" (nothing plugged into it) just to run through this code

Thanks for any help

emp1952

I'd try ttyS0 instead of tty50.

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I have been changing to su root to chmod the ttyXX designators in /dev

I tried ttyS0 (hopefully that's a zero not a capital 'O')

I tried tty5.

same results no errors thrown for the open and baud rate assignments but for write "Resource temporarily unavailable"

Do I have to do something in the system to assign the port?

I know in Windows I had to do something like that.

Thanks

emp1952

You have to use ioctl to control tty

see: man tty_ioctl, short example in C at the end of page

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Also dmesg(1) might be useful:

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I am porting a Visual Basic 6 test equipment application over to Linux.
I am using Qt 4.8 and QtCreator on Fedora 19.
Qt decided not to include serial port support in version 4.8. And the version of gcc included with the Fedora installation was the wrong version for QtCreator.
I could not get the QSerialPort add on to install and work so I'm going this route with primitive serial port controls using termios.
I tried upgrading Qt to version 5+ but it corrupted my system badly. I actually had to start over from scratch.
I formatted the HD, installed a later version of Fedora and a 5+ version of Qt.
Nothing worked right, QtCreator said it couldn't work with a 5+ version of Qt. (Creator was part of the Qt 5+ install package)
I wasted 2 weeks trying to make it all work together. Finally I just reformatted again, and reinstalled Fedora 19 and Qt 4.8. Now all
is working again. I also have another computer with Red Hat 5 on it and cannot get the QSerialPort add on to install or work with that.

Thank you for your advice

emp1952

I am using qextserialport with Qt 4.8.0: https://github.com/qextserialport/qextserialport

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Is there a loop back capability with the serial port like there is with an internal socket?

It requires a piece of hardware called loopback adapter.
Eg: https://support.microfocus.com/techdocs/1368.html

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I can make that with what I've got here, thanks a bunch