How to monitor a device? A Linux Programing question.

randomsel · 09-25-2007, 05:29 PM

Greetings to all,

I've just started a little project that needs to monitor a character device in Linux for changes (specifically, a Gumstix computer monitoring /dev/gpio-event).

Quote:

Once pins are setup to be monitored, any program can read /dev/gpio-event to get notified of the events. By default, an ASCII string of the form: "pin edge timestamp" followed by a newline will be delivered to /devb/gpio-event for each event that occurs.
...
For C programs, you can opt to use a binary interface instead. The binary versus ASCII setting is maintained on a per-file basis, so each program could each open /dev/gpio-event with a different ASCII/binary setting.

I don't have much experience programing in Linux, so this could (hopefully) come down to a yes/no answer. Here goes:

If programing in C, would the correct way of getting notified of an event from the device, without using polling, be pipe()? If not, please help with pointers to the correct answer/s.

Thanks in advance.

maroonbaboon · 09-25-2007, 07:58 PM

You might get a better informed answer in the programming or embedded forums.

In C you can just use 'read' on a device file. The call will block until there is something to read, and you would have to loop until you got a newline. Is that the behaviour you want?

Alternatively you can use Asynchronous_I/O (Wikipedia), but that is really a form of polling.

I'm not sure what other alternatives there are.

randomsel · 09-25-2007, 08:23 PM

Thanks for the quick reply!

The blocking behavior is exactly what I wanted, but the books I read about programming said reading a file from at it's end would return 0, and the only mention about blocking was in pipes. Thanks for clearing that up!

jtshaw · 09-26-2007, 08:08 PM

Moved: This thread is more suitable in Programming and has been moved accordingly to help your thread/question get the exposure it deserves.

jdiggitydogg · 09-26-2007, 10:34 PM

other functions such as fgets and fread should also work after the device is opened with fopen. if you really, really wanted to use pipes, you could execute 'cat /dev/xxxx' with popen. you'd then read from the popen file handle. i'm just throwing this out there, although i can't think of a good reason to use pipes/popen in this scenario.