Subject: Re: GCC 7 warnings
To: Tom Lane <tgl@sss.pgh.pa.us>
References: <20170411175708.coshtfzejgaqhar2@alvherre.pgsql>
 <c4e7f361-358b-45f9-bbe4-e167fa4de16c@2ndquadrant.com>
 <24498.1491970330@sss.pgh.pa.us>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
From: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Organization: 2ndQuadrant
Message-ID: <600b3625-603a-e400-7881-075830e98ba3@2ndquadrant.com>
Date: Wed, 12 Apr 2017 11:20:42 -0400
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:45.0)
 Gecko/20100101 Thunderbird/45.8.0
MIME-Version: 1.0
In-Reply-To: <24498.1491970330@sss.pgh.pa.us>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org

On 4/12/17 00:12, Tom Lane wrote:
> 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?

Because the /1000 takes off three digits?

The full message from an isolated test case is

test.c: In function 'f':
test.c:11:15: warning: '%03d' directive writing between 3 and 8 bytes
into a region of size 7 [-Wformat-overflow=]
  sprintf(buf, ".%03d", (int) (tv.tv_usec / 1000));
               ^
test.c:11:15: note: directive argument in the range [-2147483, 2147483]
test.c:11:2: note: '__builtin___sprintf_chk' output between 5 and 10
bytes into a destination of size 8
  sprintf(buf, ".%03d", (int) (tv.tv_usec / 1000));
  ^

(This is with -O2.  With -O0 it only asks for 5 bytes.)

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


-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers