From: Tom Lane <tgl@sss.pgh.pa.us>
To: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
        pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: GCC 7 warnings
In-reply-to: <c4e7f361-358b-45f9-bbe4-e167fa4de16c@2ndquadrant.com>
References: <20170411175708.coshtfzejgaqhar2@alvherre.pgsql>
 <c4e7f361-358b-45f9-bbe4-e167fa4de16c@2ndquadrant.com>
Comments: In-reply-to Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
	message dated "Tue, 11 Apr 2017 23:46:30 -0400"
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Date: Wed, 12 Apr 2017 00:12:10 -0400
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Peter Eisentraut <peter.eisentraut@2ndquadrant.com> writes:
> Attached is a more refined patch that I propose for PG10 now.  Compared
> to the previous rushed version, this one uses some more precise
> arithmetic to size some of the buffers.

Generally +1 for switching the snprintf calls to use sizeof() rather
than repeating the declared sizes of the arrays.

The change in setup_formatted_log_time seems a bit weird:

-	char		msbuf[8];
+	char		msbuf[10];

The associated call is

	sprintf(msbuf, ".%03d", (int) (saved_timeval.tv_usec / 1000));

Now a human can see that saved_timeval.tv_usec must be 0..999999, so
that the %d format item must always emit exactly 3 characters, which
means that really 5 bytes would be enough.  I wouldn't expect a
compiler to know that, but if it's making a generic assumption about
the worst-case width of %d, shouldn't it conclude that we might need
as many as 13 bytes for the buffer?  Why does msbuf[10] satisfy it
if msbuf[8] doesn't?

IOW, if we're going to touch this at all, I'd be inclined to go with
msbuf[16] or so, as being more likely to satisfy compilers that have
decided to try to warn about this.  And maybe we should use snprintf,
just for luck.

			regards, tom lane


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