Thursday, December 15, 2016

109: RPi Linux vs. Particle-Agent
(there's a culture clash)

When I installed particle-agent on 2 of my RPis I didn't realize that they would run in the background as "root" (superuser). I suppose the rationale is so an Arduino-like sketch program could access the GPIO pins. However, there is a better way: google "raspberry pi /dev/gpiomem".

I had no problem installing their particle-agent package. I did on 2 RPis: a Bv2 (40 pins) and a Bv3 (quad processor). I thought I could just use gcc to compile, but how to get to Particle libraries? Turns out you can just use the "Web IDE" -- like for a Particle Photon. Once you install and run particle-agent your RPi devices will show up on the web page like other devices. Anyway, I then wrote the following simple program:

#include <iostream>
#include <fstream>
using namespace std;

char Rstr[100];
int Ct = 0;

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

void loop() {

int pifunc(String cmd) {
    int arg = cmd.toInt();
    sprintf(Rstr, "Call count = %d, Arg = %d\n", Ct, arg);
    ofstream pifile; ("/home/pi/pi_out.txt"); // belongs to "root"
    pifile << Rstr;
    return Ct;

Note the fully qualified path for the output file. Particle-agent doesn't know about your home directory.  Here's my shell script to get results (from my Mac Terminal app):

echo Call Pi function:
curl$id/$fun -d access_token=$ac -d args=$arg
echo Get Pi variable:

Shell results:
$ sh pitest
Call Pi function:
  "id": "???",
  "last_app": "",
  "connected": true,
  "return_value": 6
Get Pi variable:
  "cmd": "VarReturn",
  "name": "dhpi1v",
  "error": null,
  "result": "Call count = 6, Arg = 123",
  "coreInfo": {
    "last_app": "",
    "last_heard": "2016-12-13T16:35:58.241Z",
    "connected": true,
    "last_handshake_at": "2016-12-13T16:03:52.588Z",
    "deviceID": "???",
    "product_id": 31

Note: I don't think much of Particle's inconsistently formatted output. And yipe! As I mentioned above, the file I wrote should not belong to root.

$ ls -l:

-rw-r--r--  1 root root     26 Dec 13 17:08 pi_out.txt

So I added the following to my sketch:

#include <sys/types.h>
#include <unistd.h>

#define SAFE_UID 1000 // i.e., the "pi" user built into RPi Linux
. . .
int Euid;
int SetRet;

void setup() {
    SetRet = setuid(SAFE_UID); // don't be ROOT!
    Euid = geteuid();
. . .
// in pifunc

sprintf(Rstr, "SetRet = %d, Euid = %d, Call count = %d, Arg = %d\n", SetRet, Euid, Ct, arg);

Anyway, you'd think that setuid() would have fixed the permissions problem. But no such luck. Still belongs to root.

Two other problems (yes, I know the "Particle Pi" is beta):
1. When I changed the source program (as above), downloaded it and ran it the output message didn't change. After much useless retrying I went looking for the files the Particle installed (good old Unix/Linux, everything is a file). So I found a directory whose name is my Pi's ID# and that contains a file named firmware.bin.


And the directory permissions were rwx------.

So, apparently, the downloader can install a firmware.bin but can't over-write it afterwards. So I unilaterally changed them to rwxrwxrwx (probably not optimal). Now I could recompile.

2. While flailing about with the above problem I decided to download the sample program on my RPi Brev3. I got the same results except the up-to-date output was displayed. But then it got interesting. This RPi is 50 miles away at the moment. I use it to monitor Particle Photons installed at my granddaughter's farm. It runs off UPS and hasn't been down in months. But I try to be careful so I have a crontab task that checks the RPi's on-chip temperature sensor every few minutes. The sustem generally runs at about 110F. If it gets to 130F the RPi sends me an email warning. And at 140F it executes halt. Guess what? 20 minutes after running my test program I got the "RPi HOT!" email and it has been down ever since. I can't blame particle-agent just yet, but one thing I know: CPU load raises the sensor's temperature. I have a favorite test program that I have used for 30 years. It is now in Python and it computes PI to as many places as you specify. So PI to 10,000 places raises the temp a few degrees. That only uses a single processor (probably). Anyway, this weekend I will visit this RPi and find out.

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