ABOUT

sysmond is a daemon program that periodically (by default once a second)
reads system status.  It listens to a UDP port (default 12686) and
when it receaves a message, returns it's current readings.

The program is written in C to minimize execution time and size.
Since the program runs periodically (once a second by default), it
only takes about 2 milliseconds on one core for each cycle. It's 
only dependency is glibc.

One purpose of the program is to demonstrate some moderately advanced
techniques in a Linux envronment.  It demonstrates the following:

- Parsing command line options
- Parsing a configuration file
- Reading from the /proc file system
- Reading from the /sys file system
- Using UDP to receive requests and to reply to the sender
- Using the fork command
- Creating a thread 
- Handling signals
- Raising process priority
- Attaching a process affinity to a CPU core
- Using a mutex (mutual exclusion token)

When the Linux kernel runs, it frequently updates files in the /proc
and /sys virtual file systems.  For the purpose of this program we
read:
/proc/loadavg
/proc/uptime
/proc/stat
/proc/meminfo
/sys/class/hwmon/hwmon<number>
  name  (interface name)
  temp<number>_label (sensor name)
  temp<number>_input (sensor value)

The program starts by parsing the command line and configuration file.
It then sets itself up as a daemon by 

1. Changing to the / directory
2. Checking to see if it is already running
   - Terminate if it is
3. Fork the process
   - The original process:
     - Gets the pid of the forked process and writes it to /run/sysmond.pid
     - Terminates successfully
   - The forked process:
     - Closes stdin, stdout, and stderr
     - Sets up signal handlers
     - Creates a separate thread to handle udp input and output
     - Runs an infinite loop reading system data and sleeping
       between scans

The scanning process alternates writing it's output to two different
locations, active and inactive. It writes to the inactive string and
then makes it active.

The udp process, in a separate thread, waits for udp input (but ignores the
content) and then immediatey responds by sending the active string to the
sender.  The length of the output depends on the number of temperature sensing
devices and the number of sensors on each device. Typically the length is in the 
range of 500 to 600 bytes.

The format of the output is as follows:

<hostname>;time:<time string>;uptime:<in seconds>;load:<three values>;cpu%:<value>;mem%:<value>

For each sensor device:
interface;<device name>,<sensor name>:<value>,[<sensor name>:<value>...]

These values are used by the client, sysmon3, for display.

Note:  
  The temperature values in /sys/class/hwmon are written by kernel drivers.
  These may be built into the kernel or as modules, but must be selected
  when configuring the kernel. To help decide which drivers are needed for
  a specific system, the lm-sensors package has a program, sensors-detect, 
  that can be used. This program is a Perl script that walks through the 
  process of scanning a system for various hardware monitoring chips (sensors).