From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 37661 invoked by alias); 19 Jun 2017 15:44:50 -0000 Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org Received: (qmail 33853 invoked by uid 89); 19 Jun 2017 15:44:47 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=BAYES_00,SPF_HELO_PASS,T_RP_MATCHES_RCVD autolearn=ham version=3.3.2 spammy=Hx-languages-length:2460, BAR X-HELO: mx1.redhat.com Received: from mx1.redhat.com (HELO mx1.redhat.com) (209.132.183.28) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Mon, 19 Jun 2017 15:44:46 +0000 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id B47BC4E4FC; Mon, 19 Jun 2017 15:44:49 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com B47BC4E4FC Authentication-Results: ext-mx09.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx09.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=palves@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com B47BC4E4FC Received: from [127.0.0.1] (ovpn04.gateway.prod.ext.ams2.redhat.com [10.39.146.4]) by smtp.corp.redhat.com (Postfix) with ESMTP id 283F282754; Mon, 19 Jun 2017 15:44:45 +0000 (UTC) Subject: Re: [PATCH v6] C++ify gdb/common/environ.c To: Simon Marchi References: <20170413040455.23996-1-sergiodj@redhat.com> <20170619043531.32394-1-sergiodj@redhat.com> <87k248y3zp.fsf@redhat.com> <8aabc6fabb04f4e3e8b08e6fa1b0eacc@polymtl.ca> <816a5744-b3b4-855c-5f2e-4c9f0d255512@redhat.com> <1cff1a8055c0d770fef7171b8394e86d@polymtl.ca> Cc: Sergio Durigan Junior , GDB Patches From: Pedro Alves Message-ID: <7cf7b53f-600a-32f5-c9d0-2f45a8bb2b46@redhat.com> Date: Mon, 19 Jun 2017 15:44:00 -0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: <1cff1a8055c0d770fef7171b8394e86d@polymtl.ca> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-SW-Source: 2017-06/txt/msg00519.txt.bz2 On 06/19/2017 04:30 PM, Simon Marchi wrote: > On 2017-06-19 16:26, Pedro Alves wrote: >> Right, m_environ_vector.clear() is not necessary. >> >> Note that this move assignment (and likewise the move ctor) leaves the >> source vector empty, which violates the "there's always a NULL entry >> at the end" invariant. That's OK if the only thing we want to support >> of moved-from gdb_environ objects is destroying them, but please do >> document that. >> >> Otherwise, people assuming the standard library's rule, may be >> confused/surprised, into thinking that this, e.g., should work: >> >> gdb_environ env1; >> env1.set ("VAR1", "value1"); >> gdb_environ env2; >> env2 = std::move (env1); // env1 has no NULL terminator after this. >> env1.set ("VAR1", "value2); // whoops. >> >> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ >> 17.6.5.15 Moved-from state of library types >> [lib.types.movedfrom] >> >> Objects of types defined in the C++ standard library may be moved >> from (12.8). >> Move operations may be explicitly specified or implicitly >> generated. Unless >> otherwise specified, such moved-from objects shall be placed in a >> valid >> but unspecified state. >> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > That's a good point. We should definitely not let the environ object > get in an invalid state. > > Whatever the rule we choose for the terminating NULL, there exists some > valid vector states which result in invalid environ states. For > example, an environ whose vector contains { NULL, NULL } is not valid. > Trying to set an env var in it would give { NULL, "FOO=BAR", NULL }, and > that results in an unexpected environment array in the end. > > Does that mean that after the vector move, we should make sure to leave > the moved-from vector in a known state (i.e. clear it, and possible add > a NULL), to make sure that we leave our environ object in a valid state? If we take the "always push a NULL on construction" approach, and we want moved-from gdb_environs to be valid, then yes. Note how this results in extra heap allocations when e.g., returning a gdb_environ from functions by value, and makes std::vector much less efficient when it decides it needs to reallocate/move elements. Representing the empty state with a cleared internal vector would avoid this. Note BTW, that we need to be careful with self-move leaving the *this object in a valid state. Thanks, Pedro Alves