From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8FBADC3A5A2 for ; Fri, 23 Aug 2019 16:43:19 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 6847B21848 for ; Fri, 23 Aug 2019 16:43:19 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6847B21848 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:59406 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i1Cek-0008T7-4O for qemu-devel@archiver.kernel.org; Fri, 23 Aug 2019 12:43:18 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:46837) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i1CbG-0006aX-LT for qemu-devel@nongnu.org; Fri, 23 Aug 2019 12:39:43 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1i1CbF-0002uU-Ce for qemu-devel@nongnu.org; Fri, 23 Aug 2019 12:39:42 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41522) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1i1CbF-0002u7-4x for qemu-devel@nongnu.org; Fri, 23 Aug 2019 12:39:41 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 7889A18DE76B; Fri, 23 Aug 2019 16:39:40 +0000 (UTC) Received: from localhost.localdomain.com (ovpn-112-60.ams2.redhat.com [10.36.112.60]) by smtp.corp.redhat.com (Postfix) with ESMTP id 2D418BA75; Fri, 23 Aug 2019 16:39:38 +0000 (UTC) From: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= To: qemu-devel@nongnu.org Date: Fri, 23 Aug 2019 17:39:30 +0100 Message-Id: <20190823163931.7442-4-berrange@redhat.com> In-Reply-To: <20190823163931.7442-1-berrange@redhat.com> References: <20190823163931.7442-1-berrange@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.63]); Fri, 23 Aug 2019 16:39:40 +0000 (UTC) Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: [Qemu-devel] [PATCH 3/4] docs: document use of automatic cleanup functions in glib X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: =?UTF-8?q?Alex=20Benn=C3=A9e?= , =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Document the use of g_autofree and g_autoptr in glib for automatic freeing of memory, or other resource cleanup (eg mutex unlocking). Signed-off-by: Daniel P. Berrang=C3=A9 --- CODING_STYLE.md | 101 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 101 insertions(+) diff --git a/CODING_STYLE.md b/CODING_STYLE.md index 9f4fc9dc77..f37b6c2d01 100644 --- a/CODING_STYLE.md +++ b/CODING_STYLE.md @@ -479,3 +479,104 @@ terminate QEMU. =20 Note that &error_fatal is just another way to exit(1), and &error_abort is just another way to abort(). + + +## Automatic memory deallocation + +QEMU has a mandatory dependency either the GCC or CLang compiler. As +such it has the freedom to make use of a C language extension for +automatically running a cleanup function when a stack variable goes +out of scope. This can be used to simplify function cleanup paths, +often allowing many goto jumps to be eliminated, through automatic +free'ing of memory. + +The GLib2 library provides a number of functions/macros for enabling +automatic cleanup: + + https://developer.gnome.org/glib/stable/glib-Miscellaneous-Macros.html + +Most notably: + + - g_autofree - will invoke g_free() on the variable going out of scope + + - g_autoptr - for structs / objects, will invoke the cleanup func creat= ed + by a previous use of G_DEFINE_AUTOPTR_CLEANUP_FUNC. This = is + supported for most GLib data types and GObjects + +For example, instead of + + int somefunc(void) { + int ret =3D -1; + char *foo =3D g_strdup_printf("foo%", "wibble"); + GList *bar =3D ..... + + if (eek) { + goto cleanup; + } + + ret =3D 0; + + cleanup: + g_free(foo); + g_list_free(bar); + return ret; + } + +Using g_autofree/g_autoptr enables the code to be written as: + + int somefunc(void) { + g_autofree char *foo =3D g_strdup_printf("foo%", "wibble"); + g_autoptr (GList) bar =3D ..... + + if (eek) { + return -1; + } + + return 0; + } + +While this generally results in simpler, less leak-prone code, there +are still some caveats to beware of + + * Variables declared with g_auto* MUST always be initialized, + otherwise the cleanup function will use uninitialized stack memory + + * If a variable declared with g_auto* holds a value which must + live beyond the life of the function, that value must be saved + and the original variable NULL'd out. This can be simpler using + g_steal_pointer + + + char *somefunc(void) { + g_autofree char *foo =3D g_strdup_printf("foo%", "wibble"); + g_autoptr (GList) bar =3D ..... + + if (eek) { + return NULL; + } + + return g_steal_pointer(&foo); + } + +The cleanup functions are not restricted to simply free'ing memory. The +GMutexLocker class is a variant of GMutex that has automatic locking and +unlocking at start and end of the enclosing scope + +In the following example, the `lock` in `MyObj` will be held for the +precise duration of the `somefunc` function + + typedef struct { + GMutex lock; + } MyObj; + + char *somefunc(MyObj *obj) { + g_autofree GMutexLocker *locker =3D g_mutex_locker_new(&obj->loc= k) + g_autofree char *foo =3D g_strdup_printf("foo%", "wibble"); + g_autoptr (GList) bar =3D ..... + + if (eek) { + return NULL; + } + + return g_steal_pointer(&foo); + } --=20 2.21.0