Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761820AbXHBX14 (ORCPT ); Thu, 2 Aug 2007 19:27:56 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757322AbXHBX1s (ORCPT ); Thu, 2 Aug 2007 19:27:48 -0400 Received: from einhorn.in-berlin.de ([192.109.42.8]:48530 "EHLO einhorn.in-berlin.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756841AbXHBX1r (ORCPT ); Thu, 2 Aug 2007 19:27:47 -0400 X-Envelope-From: stefanr@s5r6.in-berlin.de Message-ID: <46B26860.2000705@s5r6.in-berlin.de> Date: Fri, 03 Aug 2007 01:27:28 +0200 From: Stefan Richter User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.4) Gecko/20070609 SeaMonkey/1.1.2 MIME-Version: 1.0 To: Al Viro CC: Guennadi Liakhovetski , Andi Kleen , linux-kernel@vger.kernel.org Subject: Re: gcc fixed size char array initialization bug - known? References: <46B25B5E.30406@s5r6.in-berlin.de> <20070802230914.GT21089@ftp.linux.org.uk> In-Reply-To: <20070802230914.GT21089@ftp.linux.org.uk> X-Enigmail-Version: 0.94.1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1935 Lines: 60 Al Viro wrote: > On Fri, Aug 03, 2007 at 12:51:16AM +0200, Guennadi Liakhovetski wrote: >> On Fri, 3 Aug 2007, Stefan Richter wrote: >> >>> Guennadi Liakhovetski wrote: >>>> with >>>> >>>> char c[4] = "012345"; >>>> >>>> the compiler warns, but actually allocates a 6-byte long array... >>> Off-topic here, but: sizeof c / sizeof *c == 4. >> Don't think it is OT here - kernel depends on gcc. And, what I meant, is, >> that gcc places all 7 (sorry, not 6 as I said above) characters in the >> .rodata section of the compiled object file. Of course, it doesn't mean, >> that c is 7 characters long. > > So gcc does that kind of recovery, after having warned you. Makes sense, > as long as it's for ordinary variables (and not, say it, struct fields) - > you get less likely runtime breakage on the undefined behaviour (e.g. > passing c to string functions). So gcc has generated some padding between > the global variables, that's all. No, the fact that the full 012345\0 ends up in the object file is apparently unrelated to what happens to the variable c... > It doesn't change the fact that use of c[4] or strlen(c) or strcpy(..., c) > means nasal demon country for you. > > Now, if gcc does that for similar situation with struct fields, you'd have > a cause to complain. ...since only 0123 will get into c at runtime, i.e. a 4 bytes long array without \0 appendix or other extraordinary padding. #include #include int main() { char c[4] = "012345"; printf("%d %d _%s_\n", sizeof c / sizeof *c, strlen(c), c); return 0; } $ ./a.out 4 8 _01230??_ $ strings a.out |grep 0123 012345 -- Stefan Richter -=====-=-=== =--- ---== http://arcgraph.de/sr/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/