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Urgent: ttyS0 is my stdin. Now I want to use ttyS0 for something else... HOW?

Posted on 2006-07-11
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Last Modified: 2010-08-05
I have a micro Linux board (SSV DIL-PC) http://www.dilnetpc.com/.

Since it has no keyboard nor screen, it is set up to use /dev/ttyS0 as keyboard and console. But I need all the serial ports for my own application!

What happens now is that I can open the port and write to it without a problem in my own C++ program, but when receive a single byte, the port is "taken" from me, and first of all I did not receive the data, but also from then on I can not send anything anymore. It's like the Kernel claimed the port again (set ttyS0 back to 115200 instead of my 19200) etc...

How do I set /dev/null as system stdin and stdout so I can have my ttyS0 back for myself. (Linux 2.4 micro kernel with busybox compiled as an image)
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Question by:De_Wagter
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Expert Comment

by:manish_regmi
ID: 17088792
I thnik you can solve your problem using freopen

FILE *fp;
...
fp = freopen ("/dev/null", "w", stdout);


similarly do it for stdin

see man freopen

regards
Manish Regmi
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Author Comment

by:De_Wagter
ID: 17093249
The command you suggest does exactly what it is supposed to do. I start the program from a telnet window. All stdin/stdout (=stdin/stdout from that telnet window) is redirected to /dev/null.

Unfortunately my problem is not solved with this, while I do like the trick though! Might be usefull later on...

I can still login via the serial port... this means the kernel still runs something that uses ttyS0... How do I get rid of this?
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Expert Comment

by:manish_regmi
ID: 17096175
Kernel does not use it until it is told to do so.
Some demons might be using it.
If you dont need those daemons terminate all daemons/programs that uses it.

like telnetd, pppd might be using your serial port.

also replace /dev/ttys0 to something else in your inittab.

regards
Manish Regmi

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Accepted Solution

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noci earned 500 total points
ID: 17097303
You probably need to disable a getty (or agetty etc.) command in your inittab and a "kill -HUP 1"
to reread the inittab.

Or better setup runlevels.

runlevel 0 = shutdown
runlevel 1 = single user
runlevel 2 = multiuser, no network
runlevel 3 = normal use
runlevel 4= application use
runlevel 6 = reboot.

Run level 4 is your special own mode.
then a line like

s0:12345:respawn:/sbin/getty ......

should be changed to

s0:1235:respawn:/sbin/getty

Not the missing 4...,
an on another place you could say:

ap:4:respawn:/usr/loca/myapp

and run myapp when in runlevel 4

The line
id:3:initdefault:
specifies the initial mode it starts in,
telinit 4
could be used to move on to level 4 manualy,
or replace the 3 by 4 and it will do it by default.
During boot you probably also can tell at what level to start after loading the kernel,
I have to refer you to the Micro Linux documentation for this.

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Author Comment

by:De_Wagter
ID: 17155313
the inittab had a getty on ttyS0 which runs a login server that screws up my application.

but:

the inittab is loaded from ROM that I am not allowed to change, so I still had the problem. But the comment from noci with the "kill -HUP 1" finally solved the trouble:

I made a good inittab in the flash that I am allowed to change. Then the board boots the bad inittab but runs the bootscript, which I am allowed to change. So in this script I copy the good inittabs with the runlevels as above to the /etc directory which is mounted in RAM. And thanks to the kill -HUP 1 command I was able to re-execute (this time the good) initab without having the bootloader overwriting my RAM again.

I would also lik to thank manish_regmi for his ideas and fast responses. You idea to replace the ttyS0 in inittabs would have worked if I were able to change it. Thanks for helping to identify the problem
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