From patchwork Fri Jan 18 23:26:14 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Laurent Pinchart X-Patchwork-Id: 275 Return-Path: Received: from perceval.ideasonboard.com (perceval.ideasonboard.com [IPv6:2001:4b98:dc2:55:216:3eff:fef7:d647]) by lancelot.ideasonboard.com (Postfix) with ESMTPS id 80A9960C78 for ; Sat, 19 Jan 2019 00:26:21 +0100 (CET) Received: from pendragon.bb.dnainternet.fi (dfj612yhrgyx302h3jwwy-3.rev.dnainternet.fi [IPv6:2001:14ba:21f5:5b00:ce28:277f:58d7:3ca4]) by perceval.ideasonboard.com (Postfix) with ESMTPSA id 0B8DA53F for ; Sat, 19 Jan 2019 00:26:21 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1547853981; bh=zAKRI7atiW0LPy9DC4Rg7GRvlrl1BUFgEErdv6hVTkc=; h=From:To:Subject:Date:In-Reply-To:References:From; b=rcdte6FJ9aplpEqeddJaVx6db218BtHDYqhrRW4bCMAOH9y0+epZ8tZGj5ztJql+o xIi2mf2h/AdoeBbO2ImN2Doy3ObTkciQyiIZOdtzjNpa8xfzDK8ssvhy2mi0E5Ra7h U26Ehgu3gVFrYObqg0Hl3+7TprPSQsn3zyQVHkbA= From: Laurent Pinchart To: libcamera-devel@lists.libcamera.org Date: Sat, 19 Jan 2019 01:26:14 +0200 Message-Id: <20190118232617.14631-2-laurent.pinchart@ideasonboard.com> X-Mailer: git-send-email 2.19.2 In-Reply-To: <20190118232617.14631-1-laurent.pinchart@ideasonboard.com> References: <20190118232617.14631-1-laurent.pinchart@ideasonboard.com> MIME-Version: 1.0 Subject: [libcamera-devel] [PATCH v2 1/4] Documentation: coding_style: Add object ownership rules X-BeenThere: libcamera-devel@lists.libcamera.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 18 Jan 2019 23:26:21 -0000 Object ownership is a complex topic that can lead to many issues, from memory leak to crashes. Document the rules that libcamera enforces to make object ownership tracking explicit. This is a first version of the rules and is expected to be expanded as the library is developed. Signed-off-by: Laurent Pinchart Reviewed-by: Niklas Söderlund Reviewed-by: Jacopo Mondi --- Changes since v1: - Clarify documentation - Reference the object ownership rules from the Chromium C++ style guide --- Documentation/coding-style.rst | 84 ++++++++++++++++++++++++++++++++++ 1 file changed, 84 insertions(+) diff --git a/Documentation/coding-style.rst b/Documentation/coding-style.rst index f8d2fdfeda8e..66db3cebe132 100644 --- a/Documentation/coding-style.rst +++ b/Documentation/coding-style.rst @@ -81,6 +81,90 @@ C++-11-specific features: overused. * Variadic class and function templates +Object Ownership +~~~~~~~~~~~~~~~~ + +libcamera creates and destroys many objects at runtime, for both objects +internal to the library and objects exposed to the user. To guarantee proper +operation without use after free, double free or memory leaks, knowing who owns +each object at any time is crucial. The project has enacted a set of rules to +make object ownership tracking as explicit and fool-proof as possible. + +In the context of this section, the terms object and instance are used +interchangeably and both refer to an instance of a class. The term reference +refers to both C++ references and C++ pointers in their capacity to refer to an +object. Passing a reference means offering a way to a callee to obtain a +reference to an object that the caller has a valid reference to. Borrowing a +reference means using a reference passed by a caller without ownership transfer +based on the assumption that the caller guarantees the validity of the +reference for the duration of the operation that borrows it. + +#. Single Owner Objects + + * By default an object has a single owner at any time. + * Storage of single owner objects varies depending on how the object + ownership will evolve through the lifetime of the object. + + * Objects whose ownership needs to be transferred shall be stored as + std::unique_ptr<> as much as possible to emphasize the single ownership. + * Objects whose owner doesn't change may be embedded in other objects, or + stored as pointer or references. They may be stored as std::unique_ptr<> + for automatic deletion if desired. + + * Ownership is transferred by passing the reference as a std::unique_ptr<> + and using std::move(). After ownership transfer the former owner has no + valid reference to the object anymore and shall not access it without first + obtaining a valid reference. + * Objects may be borrowed by passing an object reference from the owner to + the borrower, providing that + + * the owner guarantees the validity of the reference for the whole duration + of the borrowing, and + * the borrower doesn't access the reference after the end of the borrowing. + + When borrowing from caller to callee for the duration of a function call, + this implies that the callee shall not keep any stored reference after it + returns. These rules apply to the callee and all the functions it calls, + directly or indirectly. + + When the object is stored in a std::unique_ptr<>, borrowing passes a + reference to the object, not to the std::unique_ptr<>, as + + * a 'const &' when the object doesn't need to be modified and may not be + null. + * a pointer when the object may be modified or may be null. Unless + otherwise specified, pointers passed to functions are considered as + borrowed references valid for the duration of the function only. + +#. Shared Objects + + * Objects that may have multiple owners at a given time are called shared + objects. They are reference-counted and live as long as any references to + the object exist. + * Shared objects are created with std::make_shared<> or + std::allocate_shared<> and stored in an std::shared_ptr<>. + * Ownership is shared by creating and passing copies of any valid + std::shared_ptr<>. Ownership is released by destroying the corresponding + std::shared_ptr<>. + * When passed to a function, std::shared_ptr<> are always passed by value, + never by reference. The caller can decide whether to transfer its ownership + of the std::shared_ptr<> with std::move() or retain it. The callee shall + use std::move() if it needs to store the shared pointer. + * Borrowed references to shared objects are passed as references to the + objects themselves, not to the std::shared_ptr<>, with the same rules as + for single owner objects. + +These rules match the `object ownership rules from the Chromium C++ Style Guide`_. + +.. _object ownership rules from the Chromium C++ Style Guide: https://chromium.googlesource.com/chromium/src/+/master/styleguide/c++/c++.md#object-ownership-and-calling-conventions + +.. attention:: Long term borrowing of single owner objects is allowed. Example + use cases are implementation of the singleton pattern (where the singleton + guarantees the validity of the reference forever), or returning references + to global objects whose lifetime matches the lifetime of the application. As + long term borrowing isn't marked through language constructs, it shall be + documented explicitly in details in the API. + Tools -----