How to read a 1-wire sensor with C on a Raspberry Pi

[Bill Thanis]

I'm not sure if the exercise is to learn C or to read the device.

If it is simply to read the device,

tail -f <device> should work fine.

Bill



On Thu, Jan 16, 2014 at 7:55 PM, Jamon Camisso <jamon.camisso-H217xnMUJC0sA/PxXw9srA at public.gmane.org>wrote:

> So I've managed to get a temperature sensor wired up and outputting data
> from my Raspberry Pi. I can poll the /sys/bus/w1/devices file for the
> sensor and see output like the following:
>
> 1a 01 4b 46 7f ff 06 10 ea : crc=ea YES
> 1a 01 4b 46 7f ff 06 10 ea t=17625
>
> I've given myself the exercise of learning some C and so far I've done a
> reasonable job with a C function to read it and get the t=17625 value as
> a temperature, the function's output: temp is : 17.7C
>
> const char file[] = "/sys/bus/w1/devices/device-id-here/w1_slave";
> float get_temp() {
>   FILE *fp;
>   char line[40], temp_raw[5];
>   fp = fopen(file, "r");
>   if (fp == NULL) {
>     fprintf(stderr, "File %s not found\n", file);
>     exit(1);
>   }
>   else {
>     while (fgets(line, 40, fp) != NULL) {
>       // pass, just want the last line of the file
>     }
>     fclose(fp);
>   }
>   int len = strlen(line);
>   strncpy(temp_raw, line+len-6, 6);
>   float temp_f = floorf((atof(temp_raw)/1000 + 0.05)*100)/100;
>   printf("temp is : %.1f\n", temp_f );
>   return temp_f;
> }
>
> So ok, great, job done right? Well I don't think so. I think I'm going
> about this in a very non-reusable manner. I can't tell if the device is
> returning lines one at a time to stdout first of all, hence the while loop.
>
> Question 1) Would fread( )or fscanf() be better here? Can I skip the
> while loop? What if this was 500,000 lines long? Any better way? I was
> thinking of looking at how head and tail in coreutils handle this but
> figured to ask here first.
>
> Second, I'm not sure how to extract the t=17625 value - it is always at
> position len-6 (\0 line termination needs to be accounted for). It is
> always the same length which is great. But:
>
> Question 2) Is there a better way than strncpy?
>
> Thanks for any tips,
>
> Jamon
> --
> The Toronto Linux Users Group.      Meetings: http://gtalug.org/
> TLUG requests: Linux topics, No HTML, wrap text below 80 columns
> How to UNSUBSCRIBE: http://gtalug.org/wiki/Mailing_lists
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://gtalug.org/pipermail/legacy/attachments/20140117/f8cb3148/attachment.html>