Serial Port Access Denied using open("/dev/ttyS0", O_RDWR | O_NOCTTY

morty346 · 02-10-2009, 10:46 AM

I am creating a program that opens a serial port for communication but it gets denied when I open it
I have installed a board that has two ports and have verified they are working by using the terminal:
dmesg | grep tty

Code:

console [tty0] enabled
0000:02:00.0: ttyS0 at I/O 0xdf00 (irq = 21) is a 16550A
0000:02:00.0: ttyS1 at I/O 0xde00 (irq = 21) is a 16550A

Code:

 int open_port(void)
    {
      int fd; /* File descriptor for the port */


      fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
      if (fd == -1)
      {
       /*
	* Could not open the port.
	*/
	perror("open_port: Unable to open /dev/ttyS0 - ");
      }
      else
	fcntl(fd, F_SETFL, 0);

      return (fd);
    }

Code:

open_port: Unable to open /dev/ttyS0 -: Permission denied
return code -1

Thank you
Greg

morty346 · 02-10-2009, 11:04 AM

Solved!

chmod o+rw /dev/ttyS0
chmod o+rw /dev/ttyS1

Thank you anyways!
Greg

mwasif · 12-27-2010, 02:41 PM

I also faces that problem.
when I write this...
[Shaheen@Shaheen ~]$ chmod o+rw/dev/ttyS0
It return this...
chmod: missing operand after `o+rw/dev/ttyS0'
Try `chmod --help' for more information.
how can I solve this problem?

harry edwards · 12-27-2010, 03:27 PM

Permissions on serial ports have caused me headaches a few times, so if you reboot and the permissions are reset, you may need to modify the default permissions applied to serial ports by udev.

phil.d.g · 12-27-2010, 03:52 PM

Quote:

Originally Posted by mwasif

Code:

chmod o+rw/dev/ttyS0

There should be a space between 'w' and '/'

You might also find that all tty devices have permissions 0660, and that by adding yourself to the group that owns the tty device you have permission to open it, rather than manually going in and altering permissions.

PS For future reference you should create a new thread, rather than resurrect a dead one

redhatstand · 12-28-2010, 11:04 AM

For posterity: this can also be caused by running xen virtualization: the host machine loses access to the first serial port because xen grabs it (for very sound reasons)

If you encounter this problem you can add a kernel option before the grub boot loader runs with defaults (hit escape when prompted)

google xencons=tty for more info.

Andy T

mwasif · 12-30-2010, 12:32 PM

Thanks everybody

How can I set the properties (baud rate, parity bit etc)?
what are the default values of baud rate, parity bit and data bits?

redhatstand · 12-30-2010, 12:54 PM

you might find

man setserial

a useful manpage to try?

You could also scan the source code for the setserial command for pointers

If that doesn't help (and you don't get a better answer than this one) maybe start another thread asking how to get from knowing a manpage name to the source code for the command...

Regards,

Andy T

theNbomr · 01-01-2011, 02:05 PM

Quote:

Originally Posted by mwasif

Thanks everybody

How can I set the properties (baud rate, parity bit etc)?
what are the default values of baud rate, parity bit and data bits?

Are you trying to do that in some programming language, or by using existing tools? If the latter, then redhatstand has already pointed out the setserial command. All of the common tools for doing serial communications will have a facility to set comm's paramters.
To build such capability into you own programs, you would use some form of the termios API. You can read about the generalities of termios and it's C language interface via the termios manpage.

--- rod.

mwasif · 01-02-2011, 11:25 AM

Quote:

Originally Posted by theNbomr

Are you trying to do that in some programming language, or by using existing tools? If the latter, then redhatstand has already pointed out the setserial command. All of the common tools for doing serial communications will have a facility to set comm's paramters.
To build such capability into you own programs, you would use some form of the termios API. You can read about the generalities of termios and it's C language interface via the termios manpage.

--- rod.

I doing work on C code. My code is almost same as given above. I don't understand how to set the communicating properties (baudrate etc). So please tell me how I set the properties?

Thanks in advance.

redhatstand · 01-02-2011, 12:43 PM

I googled for

simple termios example

and got 14,600 results - you should find something useful in that lot...

theNbomr · 01-02-2011, 05:23 PM

These should get you all of that, and more:

Serial-HOWTO
Serial-Programming-HOWTO
Serial Programming Guide for POSIX Operating Systems

--- rod.