Preventing multiple process instances on Linux

Question

What is the best way on Linux platform for the process (C++ application) to check its instance is not already running?

Answer 1

You can use files and file locks to accomplish this, but, beware it isn't perfect and don't copy the infamous Firefox bug where it refuses to start sometimes even if it isn't already running.

The basic logic of it is:

Invariant:
    File xxxxx will exist if and only if the program is running, and the
    contents of the file will contain the PID of that program.

On startup:
    If file xxxxx exists:
        If there is a process with the PID contained in the file:
            Assume there is some instance of the program, and exit
        Else:
            Assume that the program terminated abnormally, and
            overwrite file xxxx with the PID of this program
    Else:
        Create file xxxx, and save the current PID to that file.

On termination (typically registered via atexit):
    Delete file xxxxx

In addition to the logic above, you should also use a second file that you lock in order to synchronize access to the PID file (i.e. to act as a mutex to make it safe in terms of process-level concurrency).