Date: Wed, 13 May 2020 17:23:39 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Fujii Masao <masao.fujii@oss.nttdata.com>,
	Alvaro Herrera <alvherre@2ndquadrant.com>,
	Masahiko Sawada <masahiko.sawada@2ndquadrant.com>,
	"tsunakawa.takay@fujitsu.com" <tsunakawa.takay@fujitsu.com>,
	"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: SLRU statistics
Message-ID: <20200513152339.s7ossibhr6wai4p7@development>
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On Wed, May 13, 2020 at 11:01:47AM -0400, Tom Lane wrote:
>Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
>> On Wed, May 13, 2020 at 10:26:39AM -0400, Tom Lane wrote:
>>> Why is that necessary?  A static variable is defined by C to start off
>>> as zeroes.
>
>> But is it a static variable? It's not declared as 'static' but maybe we
>> can assume it inits to zeroes anyway? I see we do that for
>> BgWriterStats.
>
>Sorry, by "static" I meant "statically allocated", not "private to
>this module".  I'm sure the C standard has some more precise terminology
>for this distinction, but I forget what it is.
>

Ah, I see. I'm no expert in reading C standard (or any other standard),
but a quick google search yielded this section of C99 standard:

-------------------------------------------------------------------------
If an object that has static storage duration is not initialized
explicitly, then:

- if it has pointer type, it is initialized to a null pointer;

- if it has arithmetic type, it is initialized to (positive or unsigned)
   zero;

- if it is an aggregate, every member is initialized (recursively)
   according to these rules;

- if it is au nion, the first named member is initialized (recursively)
   according to these rules
-------------------------------------------------------------------------

I assume the SLRU variable counts as aggregate, with members having
arithmetic types. In which case it really should be initialized to 0.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services