How to prevent a Linux program from running more than once?

Question

What is the best way to prevent a Linux program/daemon from being executed more than once at a given time?

Answer 1

The most common way is to create a PID file: define a location where the file will go (inside /var/run is common). On successful startup, you'll write your PID to this file. When deciding whether to start up, read the file and check to make sure that the referenced process doesn't exist (or if it does, that it's not an instance of your daemon: on Linux, you can look at /proc/$PID/exe). On shutdown, you may remove the file but it's not strictly necessary.

There are scripts to help you do this, you may find start-stop-daemon to be useful: it can use PID files or even just check globally for the existence of an executable. It's designed precisely for this task and was written to help people get it right.

Answer 2

Use the boost interprocess library to create a memory block that will be created by the process. If it already exists, it means that there is another instance of the process. Exit.

The more precise link to what you need would be this one.

#include <boost/interprocess/shared_memory_object.hpp>
#include <boost/scoped_ptr.hpp>

int main()
{
  using boost::interprocess;
  boost::scoped_ptr<shared_memory_object> createSharedMemoryOrDie;
  try
  {
     createSharedMemoryOrDie.reset( 
       new shared_memory_object(create_only, "shared_memory", read_write));
  } catch(...)
  {
     // executable is already running
     return 1; 
  }

  // do your thing here
}