title: Getting a unique id from a unix-like system
body: <p>I want to get from any Unix-like system (if this is possible) a unique id that will be persistent every time my application runs in the same machine. If it is possible, I want to get the same id from Linux or FreeBSD or Solaris, etc... I don't want to generate a new id for each machine, but get an already existent id, and I prefer this id to come from the operating system and I don't prefer to use something like the MAC address.</p>
  
  <p>If there is no other option available, I can use MAC in combination with something else, for example the id can be the md5 hash of the combination of the MAC address and something else.</p>
  
  <p>I would like to listen to your suggestions.</p>
  
  <p>If it is useful, my application is written in C/C++.</p>
  
  <p>The aim of all this is to prevent a user to run my application for two or more times. I want to run just once.</p>