LPT programming example in C for ubuntu linux

LED with LPT port A while back I wanted to try out LPT printer port programming for upcoming project. After hours of learning, testing and searching, I came up with fallowing piece of code what finally worked for me. This example code makes LED blink in rate of 2 blinks per second wich is soldered between ground and any of the data pins.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/io.h>
#include <sys/types.h>
#include <fcntl.h>

#define BASEPORT 0x378 /* lp1 */

int main() {
	char c;
	int n, tem;
	printf("Hit any key to stop\n");
	//set permissions to access port
	if (ioperm(BASEPORT, 3, 1)) {perror("ioperm"); exit(1);}
	tem = fcntl(0, F_GETFL, 0);
	fcntl (0, F_SETFL, (tem | O_NDELAY));
	//main loop where actual blinking is done
	while (1) {
		//if some key is pressed, break out from loop
		n = read(0, &c, 1);
		if (n > 0) break;
		//write 'on' bit on all data pins and wait 1/4 second
		outb(255, BASEPORT);
		//write 'off' bit on all data pins and wait 1/4 second
		outb(0, BASEPORT);
	fcntl(0, F_SETFL, tem);
	outb(0, BASEPORT);
	//take away permissions to access port
	if (ioperm(BASEPORT, 3, 0)) {perror("ioperm"); exit(1);}

I compiled it with command where lpt.c is source code and lpt is program name compiled.

gcc lpt.c -O2 -o lpt

After that check that program has excecute permissions and run it with command.


I used LED with 550 ohms resistor in series and soldered them between pin 2 and 20 to test out this program. It worked fine.

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Magnus #
Thanks a lot man! I was looking for this but everything seemed to be wrong for some reason until I found this. This worked for me. Thanks.
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Riccardo #
Hello, could you add some comments to you code so that we can understand it? I'd like to learn to open it and read data...what documentation do i need?
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david #
Could I use your code to writing my bachelor of science dissertation? I'm looking forward your respond.
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pk #
Work well under ubuntu 9.10 thanks
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Joe L #
This looks like just what I need! I've noticed that my lpt port seems to be /dev/lpt0 though. Do you know the BASEPORT value that should be used for this (how do you find out?)... or is this still the same value?
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Joe #
I've tried running your example and watching the voltage over pins 2, 3 and 4 (data0, data1, data2) using a multimeter. Unfortunately the voltage just stays at 5v (surely it should be dropping to low voltage a couple of times per second??)
Janar #
I think it should, but maybe it is changing too fast for multimeter to register the voltage drop? With LED would be much better to see it.
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Bijoy #
There is no error, no warning
but my light is still blind
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leo #
bijoy.. check your circuit.....
the code works..
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Nadeera #
Ya this works.tnx lot
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J.W. #
It is wonderful ! Thank you a lot ! I would like make a lot of lpt port program and electronic machine and I would like process a lot of thing with lpt port. Have a nice day !
J.W. from Hungary.
R.S. #
thank you so much.
This saved me tons of time.

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shailesh tatware #
what can i do for my desired output on databit pins?
Janar #
Maybe this thread can help you- http://cboard.cprogramming.com/tech-board/92965-parallel-port-sending-signals-binary.html#post666615

My article is old and I can't remember much of the stuff going on here.
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sesil #
thanks dude that was very useful to me ....
i was trying to test the port for two days finally i got your post.....i'm happy
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Omni Tech Support #
I know the basics of Linux. What you have written is beyond my knowledge. The device looks quite cool. I tried to make one before few months. But it never worked properly. Anyway I will try it again and let me see how it is going.
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vlad #
Thanks a lot!
I had to run the code with
sudo ./ltp
to aviod ioperm error.
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george #
Work well under wheezy thanks,Thanks a lot man! There is no error, no warning. excellent to do. It is wonderful !
George Sariev #
sudo gcc lpt.c -O2 -o lpt
sudo ./lpt
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Susanne #
I'm using your code and I'm fascinated from the simplicity and functionality but can you give me a short explanation what the file operation commands {fcntl (0, F_SETFL, (tem | O_NDELAY))....} are good for, since no file is beeing opened ?
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nattakiet #
it is work, thank you.
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Vinod #
how to confirm in program whether data is written or not to parallel port. I am doing as follows:

printf("writing data");
outb(); // write some data
printf("write complete");

Output : writing data write complete.
OVH Studio #
You are a Genius it works, Thanks a lot
George Sariev #
in IBM have problem just go to bios and put not
#define BASEPORT 0x378 /* lp1 */
put in code
#define BASEPORT 0x278 /* lp1 */
just see bios setings
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Cristian #
Very nice and useful! It's work well under Puppy Linux. Thank you!
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