From patchwork Thu Aug 29 16:50:35 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: =?utf-8?q?Daniel_P=2E_Berrang=C3=A9?= X-Patchwork-Id: 1155404 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Authentication-Results: ozlabs.org; spf=pass (mailfrom) smtp.mailfrom=nongnu.org (client-ip=209.51.188.17; helo=lists.gnu.org; envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org; receiver=) Authentication-Results: ozlabs.org; dmarc=fail (p=none dis=none) header.from=redhat.com 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 ozlabs.org (Postfix) with ESMTPS id 46K7yp102tz9s4Y for ; Fri, 30 Aug 2019 02:55:30 +1000 (AEST) Received: from localhost ([::1]:52086 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i3Nhn-0005bH-Lh for incoming@patchwork.ozlabs.org; Thu, 29 Aug 2019 12:55:27 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:47508) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i3NdJ-00013X-UN for qemu-devel@nongnu.org; Thu, 29 Aug 2019 12:50:51 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1i3NdI-0007vJ-Hg for qemu-devel@nongnu.org; Thu, 29 Aug 2019 12:50:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:56594) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1i3NdI-0007uw-A0 for qemu-devel@nongnu.org; Thu, 29 Aug 2019 12:50:48 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 8CCB630842B0; Thu, 29 Aug 2019 16:50:47 +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 C2E30100197A; Thu, 29 Aug 2019 16:50:45 +0000 (UTC) From: =?utf-8?q?Daniel_P=2E_Berrang=C3=A9?= To: qemu-devel@nongnu.org Date: Thu, 29 Aug 2019 17:50:35 +0100 Message-Id: <20190829165036.9773-4-berrange@redhat.com> In-Reply-To: <20190829165036.9773-1-berrange@redhat.com> References: <20190829165036.9773-1-berrange@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.40]); Thu, 29 Aug 2019 16:50:47 +0000 (UTC) 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 v3 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_Benn=C3=A9e?= , =?utf-8?q?Daniel_P=2E_Berrang=C3=A9?= , =?utf-8?q?Marc-Andr=C3=A9_Lureau?= , Stefan Hajnoczi Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" Document the use of g_autofree and g_autoptr in glib for automatic freeing of memory. Signed-off-by: Daniel P. Berrangé Reviewed-by: Eric Blake --- CODING_STYLE.rst | 85 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 85 insertions(+) diff --git a/CODING_STYLE.rst b/CODING_STYLE.rst index 4501d87352..39397f0f6f 100644 --- a/CODING_STYLE.rst +++ b/CODING_STYLE.rst @@ -441,6 +441,91 @@ In addition, QEMU assumes that the compiler does not use the latitude given in C99 and C11 to treat aspects of signed '<<' as undefined, as documented in the GNU Compiler Collection manual starting at version 4.0. +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: + + ``_ + +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 created + by a previous use of G_DEFINE_AUTOPTR_CLEANUP_FUNC. This is + supported for most GLib data types and GObjects + +For example, instead of + +.. code-block:: c + + int somefunc(void) { + int ret = -1; + char *foo = g_strdup_printf("foo%", "wibble"); + GList *bar = ..... + + if (eek) { + goto cleanup; + } + + ret = 0; + + cleanup: + g_free(foo); + g_list_free(bar); + return ret; + } + +Using g_autofree/g_autoptr enables the code to be written as: + +.. code-block:: c + + int somefunc(void) { + g_autofree char *foo = g_strdup_printf("foo%", "wibble"); + g_autoptr (GList) bar = ..... + + 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 + + +.. code-block:: c + + char *somefunc(void) { + g_autofree char *foo = g_strdup_printf("foo%", "wibble"); + g_autoptr (GList) bar = ..... + + if (eek) { + return NULL; + } + + return g_steal_pointer(&foo); + } + + Error handling and reporting ============================