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

[Jamon Camisso]

On 16/01/14 07:55 PM, Jamon Camisso 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,

Ah and note: I know that the inline variable assignment is bad form, I'm
just trying to save some space. I also know my rounding is off here:
float temp_f = floorf((atof(temp_raw)/1000 + 0.05)*100)/100;

But that I can figure out on my own.

Cheers, Jamon
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