110: More about Particle-Agent

Particle got back to me. You can see the conversation at 

https://community.particle.io/t/linux-versus-particle-agent/27984/7
and later--
https://community.particle.io/t/feedback-raspberry-pi-particle-agent-structure-complaint/28124

Turns out an empty loop() function loops as fast as it can -- 100% of one core (in a 4-core RPi Bv3). And also, my RPi might have been fried except for the code below.

This file is started at system boot by this line in crontab:
@reboot sh bin/cputemp.sh
==================

while true ;
do
 a=`/opt/vc/bin/vcgencmd measure_temp` # returns Celsius
 b=`expr "$a" : '.*=\([0-9][0-9]*\)'`
 if [ $b -gt "70" ] ; # 160F
 then
  echo 'CPU HOT!' | mail -s 'DicksRpi4-HOT' myemail@gmail.com
  sudo halt # NOTE: have to restrart manually
 fi
 if [ $b -lt "55" ] ; # 130F
 then
  sleep 300 # sleep longer if under 130F
 else
  sleep 60
 fi
done

===============

Also, I changed the Particle code, mainly to add a delay inside the loop function. Plus I got rid of most of the C++ crap.

// Danger! This thing is running as super-user (root)

#include <stdio.h>
#include <unistd.h>

#define SAFE_UID 1000 // i.e., the "pi" user built into RPi Linux
#define MY_PATH "/home/pi"
char Rstr[100];
int Ct = 0;

void setup() {
    Particle.variable("dhpi1v", Rstr, STRING);
    Particle.function("dhpi1f", pifunc);
}

void loop() {
    delay(10); // kludge to keep the empty loop from being a cpu hog
}

int pifunc(String cmd) {
    int arg = cmd.toInt();
    char file[100];
    FILE *fp;
    
    ++Ct;
    sprintf(Rstr, "Call count = %d, Arg = %d\n", Ct, arg);
    sprintf(file, "%s/pi_out.txt", MY_PATH);
    fp = fopen(file, "w");
    fputs(Rstr, fp);
    fclose(fp);
    chown(file, SAFE_UID, SAFE_UID); // otherwise belongs to root
    return Ct;
}

The delay(10) was supposed to reduce CPU load to about "1%" but my tests show 12%.

Before I added the delay statement particle-agent was raising my RPi's temperature about 1C every 3 minutes. Maybe I need heat sinks (I had some but can't find the tiny package). And maybe when an RPi gets too hot it halts by itself. Anyone know?

NOTE: I have since changed the loop delay to 25ms. Even so, my (modified) sketch above has used 192 minutes of CPU time in ~3 days.