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Mon, 15 Mar 2021 09:53:23 -0700 (PDT) Received: from zen.linaroharston ([51.148.130.216]) by smtp.gmail.com with ESMTPSA id p3sm7665885ejd.7.2021.03.15.09.53.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 15 Mar 2021 09:53:22 -0700 (PDT) Received: from zen.lan (localhost [127.0.0.1]) by zen.linaroharston (Postfix) with ESMTP id EA3901FF7E; Mon, 15 Mar 2021 16:53:21 +0000 (GMT) From: =?UTF-8?q?Alex=20Benn=C3=A9e?= To: qemu-devel@nongnu.org Subject: [RFC PATCH] docs/devel: expand style section of memory management Date: Mon, 15 Mar 2021 16:53:12 +0000 Message-Id: <20210315165312.22453-1-alex.bennee@linaro.org> X-Mailer: git-send-email 2.20.1 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2a00:1450:4864:20::531; envelope-from=alex.bennee@linaro.org; helo=mail-ed1-x531.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action 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?= , peter.maydell@linaro.org, thuth@redhat.com, berrange@redhat.com, stefanha@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This aims to provide a bit more guidance for those who take on one of our "clean up memory allocation" bite-sized tasks. Signed-off-by: Alex Bennée --- docs/devel/style.rst | 40 ++++++++++++++++++++++++++++++---------- 1 file changed, 30 insertions(+), 10 deletions(-) diff --git a/docs/devel/style.rst b/docs/devel/style.rst index 8b0bdb3570..823fa6f209 100644 --- a/docs/devel/style.rst +++ b/docs/devel/style.rst @@ -385,17 +385,35 @@ avoided. Low level memory management =========================== -Use of the malloc/free/realloc/calloc/valloc/memalign/posix_memalign +Use of the ``malloc/free/realloc/calloc/valloc/memalign/posix_memalign`` APIs is not allowed in the QEMU codebase. Instead of these routines, -use the GLib memory allocation routines g_malloc/g_malloc0/g_new/ -g_new0/g_realloc/g_free or QEMU's qemu_memalign/qemu_blockalign/qemu_vfree -APIs. +use the GLib memory allocation routines +``g_malloc/g_malloc0/g_new/g_new0/g_realloc/g_free`` +or QEMU's ``qemu_memalign/qemu_blockalign/qemu_vfree`` APIs. -Please note that g_malloc will exit on allocation failure, so there -is no need to test for failure (as you would have to with malloc). -Calling g_malloc with a zero size is valid and will return NULL. +Please note that ``g_malloc`` will exit on allocation failure, so there +is no need to test for failure (as you would have to with ``malloc``). -Prefer g_new(T, n) instead of g_malloc(sizeof(T) ``*`` n) for the following +Care should be taken to avoid introducing places where the guest could +trigger an exit. For example using ``g_malloc`` on start-up is fine +if the result of a failure is going to be a fatal exit anyway. There +may be some start-up cases where failing is unreasonable (for example +speculatively loading debug symbols). + +However if we are doing an allocation because of something the guest +has done we should never trigger an exit. The code may deal with this +by trying to allocate less memory and continue or re-designed to allocate +buffers on start-up. + +If the lifetime of the allocation is within the function and there are +multiple exist paths you can also improve the readability of the code +by using ``g_autofree`` and related annotations. See :ref:`autofree-ref` +for more details. + + +Calling ``g_malloc`` with a zero size is valid and will return NULL. + +Prefer ``g_new(T, n)`` instead of ``g_malloc(sizeof(T) * n)`` for the following reasons: * It catches multiplication overflowing size_t; @@ -409,8 +427,8 @@ Declarations like are acceptable, though. -Memory allocated by qemu_memalign or qemu_blockalign must be freed with -qemu_vfree, since breaking this will cause problems on Win32. +Memory allocated by ``qemu_memalign`` or ``qemu_blockalign`` must be freed with +``qemu_vfree``, since breaking this will cause problems on Win32. String manipulation =================== @@ -485,6 +503,8 @@ 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. +.. _autofree-ref: + Automatic memory deallocation ============================= -- 2.20.1