Running multiple programs accessing GPIO simultaneously?

[pjc123]

Mike,

 

Question. There may be a better way to do this, but here goes. I am using your library and I have a program that polls a switch every 3 seconds to check if it has been pressed, at which point I run the Linux shutdown command. The reason I need this is because I am running the pi headless, and I want to be able to reboot in case I get myself in a situation where for instance the wifi signal is lost, or there is a system lockup, etc, etc, because I have no keyboard or monitor attached. I would like to have that program run continuously, so I will set it up in the init scripts. Now, I want to run a second program to do all the input/output GPIO interaction I need as part of the project I am working on, and this would use different GPIO pins. Is it possible to run two programs at the same time to access the GPIO ports? If so, do I perform a single !bcm2835_init() in just one program? What about bcm2835_close() or setting ports as inputs and outputs, or other commands? Ideally, I would like to do all this in a single program, but I suspect interrupts of some sort would be required.

 

Thanks,

 

PJC123

[Mike McCauley]

Hello,

Hmmm, I expect you might see problems (rarely) if you have multiple processes
doing GPIO access at the same time. There is the possibility that 2 processes
might overlap.
But, Im bot sure and Ive never tried it.
You could use a semaphore to control access to the GPIO from multiple
processes?

My bcm library does not support interrupts at all.

Cheers.

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Mike McCauley mi...@open.com.au
Open System Consultants Pty. Ltd
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[pjc123]

 

 

Mike,

 

     Thanks;  I suspected that might cause a problem.

 

pjc123 

[Arjan]

Hi,

It seems to me that it would be doable running multiple programs accessing GPIO pins simultaneously.
You can have a bcm2835_init / bcm2835_close for each program.
When your programs are accessing the same GPIO pins, you could work with semaphores.

Regards, Arjan

[pjc123]

Am I reading this as stating that I do not need semaphores as long as I am not accessing the same exact GPIO pins in each program, because that is my case, as I will be accessing different pins in each program?  I have never worked with semaphores, but if I do need them for every situation, then I will Google around for some example programs.

[Arjan]

Correct. When there is no pin state dependency across different programs (or threads) , then there is no need to use semaphores.

Op woensdag 5 december 2012 12:48:18 UTC+1 schreef pjc123 het volgende:

[pjc123]

Thanks to everyone.  I will give it a try when I get the chance.

[pjc123]

So, it has been a while, but I finally fabricated my project and was able to give this a test today, and just wanted to report back that it works great.  I only define and access the reboot switch GPIO port in the first executable and run it at boot in the background using rc.local (Later on I might move it to the init scripts).  It will indeed still work if I push the reset button while in the middle of accessing the GPIO ports in my second program, initiating a clean shutdown.  The only minor problem is that the second program isn't exiting very cleanly for obvious reasons, so i hope that does not lead to some sort of corruption.  I suspect that there must be a way around that though.