From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from conssluserg-01.nifty.com (conssluserg-01.nifty.com [210.131.2.80]) by sourceware.org (Postfix) with ESMTPS id 1E6993858405 for ; Tue, 23 Nov 2021 08:34:23 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 1E6993858405 Received: from Express5800-S70 (z221123.dynamic.ppp.asahi-net.or.jp [110.4.221.123]) (authenticated) by conssluserg-01.nifty.com with ESMTP id 1AN8Y9uA010331 for ; Tue, 23 Nov 2021 17:34:09 +0900 DKIM-Filter: OpenDKIM Filter v2.10.3 conssluserg-01.nifty.com 1AN8Y9uA010331 X-Nifty-SrcIP: [110.4.221.123] Date: Tue, 23 Nov 2021 17:34:09 +0900 From: Takashi Yano To: cygwin@cygwin.com Subject: Re: possible snprintf() regression in 3.3.2 Message-Id: <20211123173409.0db4d5ccd94501ce1b8f69ea@nifty.ne.jp> In-Reply-To: <20211122232302.GI10332@venus.tony.develop-help.com> References: <20211118000649.GG10332@venus.tony.develop-help.com> <20211118203538.a049809d57731fe375801c15@nifty.ne.jp> <7545bb24-43de-cd7d-0764-55c85f1af957@gmx.com> <20211121001613.GH10332@venus.tony.develop-help.com> <20211122232302.GI10332@venus.tony.develop-help.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.30; i686-pc-mingw32) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-5.8 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, KAM_NUMSUBJECT, NICE_REPLY_A, RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H2, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: cygwin@cygwin.com X-Mailman-Version: 2.1.29 Precedence: list List-Id: General Cygwin discussions and problem reports List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 23 Nov 2021 08:34:28 -0000 On Tue, 23 Nov 2021 10:23:02 +1100 Tony Cook wrote: > On Mon, Nov 22, 2021 at 02:04:06PM +0100, Corinna Vinschen via Cygwin wrote: > > On Nov 22 11:34, Corinna Vinschen via Cygwin wrote: > > > On Nov 21 11:16, Tony Cook wrote: > > > > On Thu, Nov 18, 2021 at 09:08:40PM +0000, Sam Edge via Cygwin wrote: > > > > > I use newlib on embedded with threading libs that have predetermined > > > > > fixed thread stack sizes. While we tend to have more RAM than in former > > > > > times we also have multiple thread stacks. Use of alloca() or variable > > > > > length automatic arrays makes me wince especially in code I might not be > > > > > able to avoid calling which is often the case with XXXprintf() in > > > > > third-party libraries' debug output. I'd usually rather take the > > > > > performance hit from using heap instead of having to make all my stacks > > > > > bigger. > > > > > > > > A simple option would be to use an small auto fixed buffer for most > > > > conversions, but use malloc() for %f formats for numbers greater in > > > > magnitude than some limit, though it would also need to be adjusted > > > > for the precision (ndigits here), since they take extra space. > > > > > > > > This would avoid using the optional-to-implement VLA feature too. > > > > > > Good idea. I guess I create a simple fix doing just that. > > > > I created a patch: > > https://sourceware.org/git/?p=newlib-cygwin.git;a=commitdiff;h=68faeef4be71 > > > > Please test the latest developer snapshot from http://cygwin.com/snapshots/ > > I don't think this solves the fundamental problem. > > Simply looking at ndigits isn't enough for %f. > > For %f with a large number (like 9e99), the buffer size required is > ndigits plus (roughly) log10(n), which we can further estimate > with log2(n)*146/485 (log2(10) is 3.32 ~== 485/146) > > I think something more like: > > size_t outsize; > if (mode == 3) { /* %f */ > int expon = (e[NI-1] & 0x7fff) - (EXONE - 1); /* exponent part of float */ > /* log2(10) approximately 485/146 */ > outsize = expon * 146 / 485 + ndigits + 10; > } > else { /* %g/%e */ > outsize = ndigits + MAX_EXP_DIGITS + 10; > } > if (outsize > NDEC_SML) { > outbuf = (char *)_malloc_r(ptr, outsize); > } > > You'll probably need to pass outsize into etoasc() rather than > calculating it. > > See https://github.com/Perl/perl5/blob/blead/sv.c#L13295 for code in > perl that calculates the buffer size needed for %f (precis aka ndigits > is added at line 13385). I guess Corinna thinks that 'ndigits' keeps the total number of digits to be printed. However, in reality, for example in the case: snprintf(buf, sizeof(buf), "%.3f", 1234567890123456.789); 'ndigits' is only 3 even though total digits will be 20. So, Tony thinks current code does not correct. However, I think something is wrong with interpretation of 'ndigits' in dltoa.c. printf("%.3f\n", sqrt(2)*1e70); printf("%.50f\n", sqrt(2)*1e70); outputs 14142135623730951759073108307330633613786387000000000000000000000000000.000 14142135623730951759073108307330633613786386978891021459448717416650727.13402790000888758223149296720949629080194006476078 Is this as intended? -- Takashi Yano