Question

I’m writing a command line application in Mac using Objective-c

At the start of the application, i want to check if another instance of the same application is already running. If it is, then i should be either wait for it to finish or exit the current instance or quit the other instance etc.

Is there any way of doing this?

Answer 1

The standard Unix solution for this is to create a “run file”. When you start up, you try to create that file and write your pid to it if it doesn’t exist; if it does exist, read the pid out of it, and if there’s a running program with that pid and your process name, wait/exit/whatever.

The question is, where do you put that file, and what do you call it?

Well, first, you have to decide what exactly “already running” means. Obviously not “anywhere in the world”, but it could be anything from “anywhere on the current machine” to “in the current desktop session”. (For example, if User A starts your program, pauses it, then User B comes along and takes over the computer via Fast User Switching, should she be able to run the program, or not?)

For pretty much any reasonable answer to that question, there’s an obvious pathname pattern. For example, on a Mac, /tmp is shared system-wide, while $TMPDIR is specific to a given session, so, e.g., /tmp/${ARGV[0]}.pid is a good way to say “only one copy on the machine, period”, while ${TMPDIR}/${ARGV[0]}.pid is a good way to say “only one copy per session”.