I often see that programs specify pid and lock files. And I'm not quite sure what they do.
For example, when compiling nginx:
--pid-path=/var/run/nginx.pid \
--lock-path=/var/lock/nginx.lock \
Can somebody shed some light on this one?
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pid files are written by some programs to record their process ID while they are starting. This has multiple purposes:
- It's a signal to other processes and users of the system that that particular program is running, or at least started successfully.
- It allows one to write a script really easy to check if it's running and issue a plain
killcommand if one wants to end it. - It's a cheap way for a program to see if a previous running instance of it did not exit successfully.
Mere presence of a pid file doesn't guarantee that that particular process id is running, of course, so this method isn't 100% foolproof but "good enough" in a lot of instances. Checking if a particular PID exists in the process table isn't totally portable across UNIX-like operating systems unless you want to depend on the ps utility, which may not be desirable to call in all instances (and I believe some UNIX-like operating systems implement ps differently anyway).
The idea with lock files is the following: the purpose is for two (well-behaved) separate instances of a program, which may be running concurrently on one system, don't access something else at the same time. The idea is before the program accesses its resource, it checks for presence of a lock file, and if the lock file exists, either error out or wait for it to go away. When it doesn't exist, the program wanting to "acquire" the resource creates the file, and then other instances that might come across later will wait for this process to be done with it. Of course, this assumes the program "acquiring" the lock does in fact release it and doesn't forget to delete the lock file.
This works because the filesystem under all UNIX-like operating systems enforces serialization, which means only one change to the filesystem actually happens at any given time. Sort of like locks with databases and such.
Operating systems or runtime platforms typically offer synchronization primitives and it's usually a better decision to use those instead. There may be situations such as writing something that's meant to run on a wide variety of operating systems past and future without a reliable library to abstract the calls (such as possibly sh or bash based scripts meant to work in a wide variety of unix flavors) - in that case this scheme may be a good compromise.
LawrenceC
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These files are often used by daemons that should only be run once on a system. The PID file usually contains the process ID number of the already launched and running program if one exists. Also, when it starts up, it creates the lock file. As long as the lock file exists, it won't start another one without user intervention. If the lock file exists and the process id mentioned in the pid file isn't running, the daemon is considered to be in a "dead" state, meaning it's supposed to be running but isn't probably due to a crash or improper shutdown. This might initiate a special startup / restart scenario for some programs. Properly shutting it down will remove the lock file.
Caleb
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1+1 Explaining the use of having both the lock file and the pid file. – Kyle Krull Oct 06 '15 at 02:00
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2@Caleb - Please explain why both a PID file and a lock file would be used. It seems a PID file would be sufficient. If the PID file exists, the PID could be checked to see if the process is running, just takes less steps than check for a lockfile, checking for a PID file, and then verifying the existence of the process. – MVaughan Jan 25 '17 at 16:57
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2@MVaughan To avoid race conditions if nothing else. Some apps have uses for times when the still need the PID but can relinquish the lock. But at a more fundamental level if you overload one file for both operations you open the door to failures such as a crash leaving an inconsistent state on the system. – Caleb Jan 25 '17 at 21:06
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A PID file will contain the Process ID of a running process. This has various uses; you can read it and check that the process is still running and take appropriate action or read it and kill the process.
A lock file is most likely application specific. Lock files are used to indicate that some resource is in use and that the process wanting access should wait until the resource is freed before continuing.
.lckfile in the VM's directory otherwise it will tell you it's in use when you try to start it. – LawrenceC Jun 11 '14 at 14:04/procfilesystem, where there is a directory per process. If my program would want to know whether a particular program is running, examining/procfilesystem sounds like a reasonable choice, and undermines the need for pid files. Granted, it might not be available on systems other than Linux, such as Mac OS. – Sergiy Kolodyazhnyy Jan 25 '17 at 17:15/procis a Linux invention IIRC - so that will work on Linux but might not other UNIX like OS's (though doesn't OpenBSD or similar have /proc too?) - Shells are pretty portable and you have things like AIX, Tru64, HP-UX, Solaris, etc. There's a lot of Unices. Many of the traditional Unix shells predate Linux. – LawrenceC Jan 25 '17 at 17:36fcntlorflockto grab an actual lock on their lockfile, and that this real lock is automatically released by the operating system when the process holding the lock ends, thereby preventing the liveness problem with simply relying on the existence or non-existence of the lockfile. (This omission is big enough to be misleading and dangerous, since any number of people might be getting their impressions of how lockfiles Are Done from this answer.) – mtraceur Mar 11 '21 at 23:24