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To: "xsun" <xu2002261@163.com>
cc: pgsql-admin@postgresql.org
Subject: Re: The function of lock file named ".s.PGSQL.<port>.lock" on Linux? 
In-reply-to: <i399mq$28vn$1@news.hub.org> 
References: <i399mq$28vn$1@news.hub.org>
Comments: In-reply-to "xsun" <xu2002261@163.com>
	message dated "Tue, 03 Aug 2010 22:39:29 +0800"
Date: Thu, 05 Aug 2010 12:41:14 -0400
Message-ID: <16464.1281026474@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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"xsun" <xu2002261@163.com> writes:
> When the psql server is started under Linux platform, in /tmp directory 
> there is a lock files named ".s.PGSQL.<port>.lock". So I want to know what 
> is the function of this lock file?

It prevents multiple postmasters from trying to use the same socket file.

> And does the server work well without 
> this lock file?

No.  (Well, it will run, but you have no protection against accidentally
starting another postmaster on the same port number.)

> Additionally, I know that the file ".s.PGSQL.<port>" is used as the unix 
> domain socket file. But the file ".s.PGSQL.<port>.lock" makes me still 
> pazzled. Could you explain that? Thanks.

We can't portably lock the socket file itself, so we make a separate
ordinary file for locking purposes.

			regards, tom lane