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I want only one copy of my program in the system. How can I look for another copies in the system from C-code? I want something like that
# program &
[1] 12586
# program &
Program is already running
The best idea I have is making .lock-files. But I didn’t find any guildlines about them.
Thank you.
One daemon I wrote opened a UNIX domain socket for regular client-daemon communication. Other instances then checked whether they could connect to that socket. If they could, another instance was currently running.
Edit: As noted by @psmears, there’s a race condition. The other instances should just try to create that same listening socket. That will fail if it is already in use.
Lock files work more often than that special case. You may create an (empty) file in a well known location and then use file locks, say with
fcntl(2)andF_SETLKandF_GETLKto set a lock on that file or determine whether a lock is held. May not work over NFS. Locks are cleared when your process dies, so this should work, and is portable (at least to HP-UX). Some daemons like to dump their pid into that file if they determine that no other instance is currently running.