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000-834 demur Oriented Analysis and Design - share 2 (Design)

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000-834 exam Dumps Source : Object Oriented Analysis and Design - share 2 (Design)

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IBM demur Oriented Analysis and

Object-Oriented analysis and Design | killexams.com real Questions and Pass4sure dumps

This chapter is from the booklet 

evaluation emphasizes an investigation of the hardship and necessities, in preference to a solution. as an example, if a recent on-line trading system is preferred, how will or not it's used? What are its services?

"analysis" is a huge time period, foremost certified, as in requirements evaluation (an investigation of the requirements) or object-oriented evaluation (an investigation of the region objects).

Design emphasizes a conceptual concede (in application and hardware) that fulfills the necessities, rather than its implementation. for example, an profile of a database schema and software objects. Design ideas frequently exclude low-level or "obvious" details—evident to the supposititious buyers. in the end, designs will also breathe carried out, and the implementation (similar to code) expresses the actual and comprehensive realized design.

As with evaluation, the term is most fulfilling qualified, as in object-oriented design or database design.

constructive analysis and design were summarized in the phrase finish the perquisite thing (evaluation), and finish the factor revise (design).


Python GUI Programming tasks using Tkinter and Python three - Simpliv | killexams.com real Questions and Pass4sure dumps

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Object-Oriented analysis And Design — Introduction (part 1) | killexams.com real Questions and Pass4sure dumps

The understanding Of Object-Orientation

Object-orientation is what’s referred to as a programming paradigm. It’s not a language itself but a set of ideas that's supported by course of many languages.

if you aren’t regular with the ideas of object-orientation, you might also purchase a glance at the account of Object-Oriented Programming.

If every Little thing they finish in these languages is object-oriented, it skill, they are oriented or focused around objects.

Now in an object-oriented language, this one huge application will as an alternative breathe split apart into self contained objects, very nearly infatuation having several mini-classes, each demur representing a different a share of the software.

and every demur carries its own records and its own common sense, and they communicate between themselves.

These objects aren’t random. They picture the course you speak and suppose about the hardship you try to clear up for your real existence.

They characterize issues infatuation employees, photographs, bank money owed, spaceships, asteroids, video phase, audio files, or whatever exists in your program.

Object-Oriented analysis And Design (OOAD)

It’s a structured system for inspecting, designing a outfit via making employ of the thing-oriented ideas, and ameliorate a collection of graphical gadget fashions full through the structure life cycle of the software.

OOAD within the SDLC

The application life cycle is customarily divided up into levels going from summary descriptions of the hardship to designs then to code and trying out and at terminal to deployment.

The earliest levels of this system are evaluation (requirements) and design.

The inequity between evaluation and design is frequently described as “what Vs how”.

In analysis developers work with clients and domain consultants to profile what the outfit is supposititious to do. Implementation particulars are presupposed to breathe by and big or completely omitted at this phase.

The level of the evaluation section is to create a model of the device even with constraints equivalent to acceptable expertise. this is customarily finished by the employ of employ cases and summary definition of essentially the most captious objects using conceptual mannequin.

The design section refines the analysis mannequin and applies the vital technology and different implementation constrains.

It specializes in describing the objects, their attributes, habits, and interactions. The design model may quiet acquire full the particulars required so that programmers can invoke the design in code.

They’re most excellent carried out in an iterative and incremental application methodologies. So, the actions of OOAD and the developed fashions aren’t accomplished as soon as, they will revisit and refine these steps consistently.

Object-Oriented analysis

within the object-oriented analysis, we …

  • Elicit necessities: define what does the application deserve to do, and what’s the hardship the software making an attempt to clear up.
  • Specify requirements: portray the necessities, continually, the usage of employ instances (and eventualities) or consumer stories.
  • Conceptual model: determine the captious objects, refine them, and define their relationships and conduct and draw them in an simple diagram.
  • We’re now not going to cowl the first two activities, just the ultimate one. These are already defined in aspect in requirements Engineering.

    Object-Oriented Design

    The analysis side identifies the objects, their relationship, and conduct using the conceptual model (an summary definition for the objects).

    while in design part, they portray these objects (by means of creating ilk diagram from conceptual diagram — usually mapping conceptual mannequin to classification diagram), their attributes, behavior, and interactions.

    in addition to making employ of the application design concepts and patterns which should breathe lined in later tutorials.

    The input for object-oriented design is provided by using the output of object-oriented evaluation. however, evaluation and design might also revolve up in parallel, and the consequences of one activity can breathe used by using the different.

    in the object-oriented design, we …

  • Describe the classes and their relationships the usage of class diagram.
  • Describe the interplay between the objects using sequence diagram.
  • follow software design ideas and design patterns.
  • a class diagram gives a visible illustration of the classes you want. And perquisite here is where you glean to breathe definitely particular about object-oriented principles infatuation inheritance and polymorphism.

    Describing the interactions between those objects permits you to more advantageous breathe mindful the obligations of the distinct objects, the behaviors they should have.

    — other diagrams

    there are many other diagrams they can employ to model the gadget from distinctive views; interactions between objects, structure of the equipment, or the conduct of the system and how it responds to pursuits.

    It’s at full times about deciding upon the commandeer diagram for the commandeer want. you should realize which diagrams can breathe effective when pondering or discussing a situation that isn’t clear.

    gadget modeling and the diverse fashions they will employ can breathe mentioned next.

    gadget Modeling

    gadget modeling is the process of setting up models of the system, with every model representing a unique perspectives of that system.

    essentially the most essential component a few system mannequin is that it leaves out aspect; It’s an summary representation of the device.

    The fashions are continually in keeping with graphical notation, which is almost always according to the notations in the Unified Modeling Language (UML). other fashions of the gadget infatuation mathematical model; an in depth outfit description.

    fashions are used perquisite through the analysis procedure to advocate to elicit the requirements, full the course through the design process to clarify the device to engineers, and after implementation to doc the device constitution and operation.

    distinctive perspectives

    We may additionally boost a mannequin to symbolize the outfit from different perspectives.

  • external, where you model the context or the atmosphere of the gadget.
  • interplay, where you model the interaction between accessories of a system, or between a outfit and other programs.
  • Structural, the location you mannequin the corporation of the system, or the structure of the statistics being processed by course of the gadget.
  • Behavioral, the location you mannequin the dynamic conduct of the outfit and how it reply to hobbies.
  • Unified Modeling Language (UML)

    The unified modeling language revolve into the typical modeling language for object-oriented modeling. It has many diagrams, although, the most diagrams that are conventional are:

  • Use case diagram: It shows the interplay between a outfit and it’s ambiance (users or programs) within a selected condition.
  • category diagram: It suggests the distinctive objects, their relationship, their behaviors, and attributes.
  • Sequence diagram: It indicates the interactions between the different objects within the equipment, and between actors and the objects in a gadget.
  • State machine diagram: It shows how the outfit reply to exterior and internal routine.
  • exercise diagram: It indicates the movement of the statistics between the procedures in the device.
  • you can finish diagramming work on paper or on a whiteboard, at least within the initial degrees of a task. but there are some diagramming outfit with a view to attend you to draw these UML diagrams.


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    Object Oriented Analysis and Design - share 2 (Design)

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    Object-Oriented Analysis & Design | killexams.com real questions and Pass4sure dumps

    Object-Oriented Analysis & DesignJune 2, 3, 9, 10Worcester status CollegeTaught by Jan Bergandy, Computer Science, UMass, Dartmouth

    Seminar Objectives:

  • To develop an in-depth understanding of object-oriented paradigm
  • To obtain a working knowledge of object-oriented analysis & design techniques
  • To learn object-oriented modeling using Unified Modeling Language (UML)
  • To learn about basic design patterns and the role of patterns is software development
  • To understand the repercussion of demur paradigm on software development activities
  • To explore synergy between object-oriented design and object-oriented programming
  • To learn about key demur technologies
  • Who should attend:This workshop is addressed to faculty involved in teaching programming, software design, and other courses related to software development. It is addressed to those who puss a transition to demur technology and want to learn about challenges and benefits of this transition. The workshop does not require any prior knowledge of object-oriented programming or knowledge of demur paradigm. universal computer fluency and universal knowledge of issues associated with software and software development are expected.

    Seminar Organization:The course will breathe conducted as a project with instructor giving short presentations pertaining to a specific stage of the analysis and design process. During this course the participants will construct an analysis model for a selected problem. This model will breathe refined in to the circumstantial design level providing an break for discussion about the relationship between object-oriented design and object-oriented programming. Each student will receive a copy of the course materials and the textbook.

    Tools & Platforms:Rational-Rose CASE toolThe CASE utensil is used exclusively to expedite the process of model construction. The students expend no more than half an hour of their time during the entire class on learning how to employ the tool. Not using the CASE toll will figure it almost impossible to suffer hands-on full the elements of the object-oriented analysis and design process.

    Textbooks:M. Fowler, ÒUML DistilledÓ, Addison-Wesley, ISBN 0-201-32563-2 (additional/optional )

    E. Gamma, R. Helm, R. Johnson, J. Vlissides, ÒDesign PatternÓ, Addison-Wesley, ISBN 0-201-63361-2

    Outline:

    June 2, 2001, 9:00 - 5:00Topics to breathe addressed:Object paradigm top-down - analysis & design perspectiveObject paradigm bottom-up - programming perspectiveBasic concepts: abstraction, encapsulation, information hiding, modularityResponsibility view of the requirementsClasses and objects emerging from responsibilitiesComparison of procedural and object-oriented paradigmsClasses and relationships as the structure blocks of software architectureCriteria of class qualityIntroduction to Unified Modeling Language (UML)Static & dynamic modelActors and employ casesTransitioning from functional requirements to objects - introduction

    Project:Analysis of the requirements for the selected projectIdentifying actors and employ casesConstructing employ case diagrams

    June 3, 2001, 9:00 - 2:00Topics to breathe addressed:Transitioning from functional requirements to objectsIdentifying the first group of classesClass specificationClass as an encapsulation of a responsibilityClass, Utility Class, Parameterized Class and its instantiationClass diagram - introductionIdentifying relationships between classesAssociation relationshipsAssociation classesRepresenting relationships with cardinalityAggregation versus compositionRepresenting aggregation and composition relationshipsRepresenting generalization/ specialization (inheritance)PolymorphismAbstract classes and interfacesSpecification of relationshipsImplementing classes & relationships (bottom-up view of relationships)Class diagram

    Project:Identifying first group of classes based on responsibilitiesPreliminary class diagramIdentifying relationships between classesDefining cardinalitiesClass diagram

    June 10, 2001, 9:00 - 5:00 (part I)Topics to breathe addressed:Static versus dynamic modelIdentifying scenarios through refinement of employ casesModeling scenarios using object-interaction and sequence diagrams

    Project:Refining employ casesDeveloping and modeling scenariosIdentifying methodsRefining class specifications

    June 10, 2001 (part II)Topics to breathe addressed:Events, states and actionsState diagramCriteria for using status diagramsConcurrency, active objectsMutual exclusion problemSequential, guarded, and synchronous objectsModeling concurrencyConcurrent status diagramsActivity diagrams

    Project:Evaluating classes for the necessity of status diagramsConstructing status diagrams for selected classes(Constructing activity diagrams)Refining class specifications

    June 10, 2001, 9:00 - 2:00Topics to breathe addressed:Introduction to design patterns: Creational patterns, Abstract Factory, Builder, Prototype, Singleton, Virtual Constructor

    Structural Patterns: Adapter, Bridge, Composite, Decorator, Façade, Proxy

    Behavioral Patterns: Chain of Responsibility, Command, Iterator, Mediator, Memento

    Other captious topics to breathe covered in this course:What to expect from an object-oriented languageDynamic nature of object-oriented systems and the issues of garbage collectionEffective employ of inheritance and polymorphism and their repercussion on software qualitySingle versus multiple inheritancePolymorphism versus genericsClass design and data normalization (attribute dependence issues)


    Object-Oriented Analysis And Design — Introduction (Part 1) | killexams.com real questions and Pass4sure dumps

    The Concept Of Object-Orientation

    Object-orientation is what’s referred to as a programming paradigm. It’s not a language itself but a set of concepts that is supported by many languages.

    If you aren’t confidential with the concepts of object-orientation, you may purchase a see at The account of Object-Oriented Programming.

    If everything they finish in these languages is object-oriented, it means, they are oriented or focused around objects.

    Now in an object-oriented language, this one big program will instead breathe split apart into self contained objects, almost infatuation having several mini-programs, each demur representing a different share of the application.

    And each demur contains its own data and its own logic, and they communicate between themselves.

    These objects aren’t random. They picture the course you talk and deem about the problem you are trying to decipher in your real life.

    They picture things infatuation employees, images, bank accounts, spaceships, asteroids, video segment, audio files, or whatever exists in your program.

    Object-Oriented Analysis And Design (OOAD)

    It’s a structured fashion for analyzing, designing a system by applying the object-orientated concepts, and develop a set of graphical system models during the development life cycle of the software.

    OOAD In The SDLC

    The software life cycle is typically divided up into stages going from abstract descriptions of the problem to designs then to code and testing and finally to deployment.

    The earliest stages of this process are analysis (requirements) and design.

    The distinction between analysis and design is often described as “what Vs how”.

    In analysis developers work with users and domain experts to define what the system is supposititious to do. Implementation details are supposititious to breathe mostly or totally ignored at this phase.

    The goal of the analysis side is to create a model of the system regardless of constraints such as commandeer technology. This is typically done via employ cases and abstract definition of the most captious objects using conceptual model.

    The design side refines the analysis model and applies the needed technology and other implementation constrains.

    It focuses on describing the objects, their attributes, behavior, and interactions. The design model should acquire full the details required so that programmers can implement the design in code.

    They’re best conducted in an iterative and incremental software methodologies. So, the activities of OOAD and the developed models aren’t done once, they will revisit and refine these steps continually.

    Object-Oriented Analysis

    In the object-oriented analysis, we …

  • Elicit requirements: Define what does the software necessity to do, and what’s the problem the software trying to solve.
  • Specify requirements: portray the requirements, usually, using employ cases (and scenarios) or user stories.
  • Conceptual model: Identify the captious objects, refine them, and define their relationships and conduct and draw them in a simple diagram.
  • We’re not going to cover the first two activities, just the terminal one. These are already explained in detail in Requirements Engineering.

    Object-Oriented Design

    The analysis side identifies the objects, their relationship, and conduct using the conceptual model (an abstract definition for the objects).

    While in design phase, they portray these objects (by creating class diagram from conceptual diagram — usually mapping conceptual model to class diagram), their attributes, behavior, and interactions.

    In addition to applying the software design principles and patterns which will breathe covered in later tutorials.

    The input for object-oriented design is provided by the output of object-oriented analysis. But, analysis and design may occur in parallel, and the results of one activity can breathe used by the other.

    In the object-oriented design, we …

  • Describe the classes and their relationships using class diagram.
  • Describe the interaction between the objects using sequence diagram.
  • Apply software design principles and design patterns.
  • A class diagram gives a visual representation of the classes you need. And here is where you glean to breathe really specific about object-oriented principles infatuation inheritance and polymorphism.

    Describing the interactions between those objects lets you better understand the responsibilities of the different objects, the behaviors they necessity to have.

    — Other diagrams

    There are many other diagrams they can employ to model the system from different perspectives; interactions between objects, structure of the system, or the conduct of the system and how it responds to events.

    It’s always about selecting the perquisite diagram for the perquisite need. You should realize which diagrams will breathe useful when thinking about or discussing a situation that isn’t clear.

    System modeling and the different models they can employ will breathe discussed next.

    System Modeling

    System modeling is the process of developing models of the system, with each model representing a different perspectives of that system.

    The most captious aspect about a system model is that it leaves out detail; It’s an abstract representation of the system.

    The models are usually based on graphical notation, which is almost always based on the notations in the Unified Modeling Language (UML). Other models of the system infatuation mathematical model; a circumstantial system description.

    Models are used during the analysis process to attend to elicit the requirements, during the design process to portray the system to engineers, and after implementation to document the system structure and operation.

    Different Perspectives

    We may develop a model to picture the system from different perspectives.

  • External, where you model the context or the environment of the system.
  • Interaction, where you model the interaction between components of a system, or between a system and other systems.
  • Structural, where you model the organization of the system, or the structure of the data being processed by the system.
  • Behavioral, where you model the dynamic conduct of the system and how it respond to events.
  • Unified Modeling Language (UML)

    The unified modeling language become the standard modeling language for object-oriented modeling. It has many diagrams, however, the most diagrams that are commonly used are:

  • Use case diagram: It shows the interaction between a system and it’s environment (users or systems) within a particular situation.
  • Class diagram: It shows the different objects, their relationship, their behaviors, and attributes.
  • Sequence diagram: It shows the interactions between the different objects in the system, and between actors and the objects in a system.
  • State machine diagram: It shows how the system respond to external and internal events.
  • Activity diagram: It shows the rush of the data between the processes in the system.
  • You can finish diagramming work on paper or on a whiteboard, at least in the initial stages of a project. But there are some diagramming tools that will attend you to draw these UML diagrams.


    Object-oriented design patterns in the kernel, share 2 | killexams.com real questions and Pass4sure dumps

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    June 7, 2011

    This article was contributed by Neil Brown

    In the first share of this analysis they looked at how the polymorphic side of object-oriented programming was implemented in the Linux kernel using regular C constructs. In particular they examined fashion dispatch, looked at the different forms that vtables could take, and the circumstances where part vtables were eschewed in preference for storing office pointers directly in objects. In this conclusion they will explore a second captious aspect of object-oriented programming - inheritance, and in particular data inheritance.

    Data inheritance

    Inheritance is a core concept of object-oriented programming, though it comes in many forms, whether prototype inheritance, mixin inheritance, subtype inheritance, interface inheritance etc., some of which overlap. The figure that is of interest when exploring the Linux kernel is most infatuation subtype inheritance, where a concrete or "final" ilk inherits some data fields from a "virtual" parent type. They will convene this "data inheritance" to emphasize the fact that it is the data rather than the conduct that is being inherited.

    Put another way, a number of different implementations of a particular interface share, and separately extend, a common data structure. They can breathe said to inherit from that data structure. There are three different approaches to this sharing and extending that can breathe organize in the Linux kernel, and full can breathe seen by exploring the struct inode structure and its history, though they are widely used elsewhere.

    Extension through unions

    The first approach, which is probably the most obvious but also the least flexible, is to declare a union as one component of the common structure and, for each implementation, to declare an entry in that union with extra fields that the particular implementation needs. This approach was introduced to struct inode in Linux-0.97.2 (August 1992) when

    union { struct minix_inode_info minix_i; struct ext_inode_info ext_i; struct msdos_inode_info msdos_i; } u;

    was added to struct inode. Each of these structures remained vacant until 0.97.5 when i_data was moved from struct inode to struct ext_inode_info. Over the years several more "inode_info" fields were added for different filesystems, peaking at 28 different "inode_info" structures in 2.4.14.2 when ext3 was added.

    This approach to data inheritance is simple and straightforward, but is also kindhearted of clumsy. There are two obvious problems. Firstly, every recent filesystem implementation needs to add an extra territory to the union "u". With 3 fields this may not look infatuation a problem, with 28 it was well past "ugly". Requiring every filesystem to update this one structure is a barrier to adding filesystems that is unnecessary. Secondly, every inode allocated will breathe the very size and will breathe big enough to store the data for any filesystem. So a filesystem that wants lots of space in its "inode_info" structure will impose that space cost on every other filesystem.

    The first of these issues is not an impenetrable barrier as they will see shortly. The second is a real problem and the universal ugliness of the design encouraged change. Early in the 2.5 development series this change began; it was completed by 2.5.7 when there were no "inode_info" structures left in union u (though the union itself remained until 2.6.19).

    Embedded structures

    The change that happened to inodes in early 2.5 was effectively an inversion. The change which removed ext3_i from struct inode.u also added a struct inode, called vfs_inode, to struct ext3_inode_info. So instead of the private structure being embedded in the common data structure, the common data structure is now embedded in the private one. This neatly avoids the two problems with unions; now each filesystem needs to only commandeer memory to store its own structure without any necessity to know anything about what other filesystems might need. Of course nothing ever comes for free and this change brought with it other issues that needed to breathe solved, but the solutions were not costly.

    The first hardship is the fact that when the common filesystem code - the VFS layer - calls into a specific filesystem it passes a pointer to the common data structure, the struct inode. Using this pointer, the filesystem needs to find a pointer to its own private data structure. An obvious approach is to always location the struct inode at the top of the private inode structure and simply cast a pointer to one into a pointer to the other. While this can work, it lacks any semblance of ilk safety and makes it harder to systematize fields in the inode to glean optimal performance - as some kernel developers are wont to do.

    The solution was to employ the list_entry() macro to fulfill the necessary pointer arithmetic, subtracting from the address of the struct inode its offset in the private data structure and then casting this appropriately. The macro for this was called list_entry() simply because the "list.h lists" implementation was the first to employ this pattern of data structure embedding. The list_entry() macro did exactly what was needed and so it was used despite the peculiar name. This exercise lasted until 2.5.28 when a recent container_of() macro was added which implemented the very functionality as list_entry(), though with slightly more ilk safety and a more meaningful name. With container_of() it is a simple matter to map from an embedded data structure to the structure in which it is embedded.

    The second hardship was that the filesystem had to breathe amenable for allocating the inode - it could no longer breathe allocated by common code as the common code did not acquire enough information to commandeer the revise amount of space. This simply involved adding alloc_inode() and destroy_inode() methods to the super_operations structure and calling them as appropriate.

    Void pointers

    As preeminent earlier, the union pattern was not an impenetrable barrier to adding recent filesystems independently. This is because the union u had one more territory that was not an "inode_info" structure. A generic pointer territory called generic_ip was added in Linux-1.0.5, but it was not used until 1.3.7. Any file system that does not own a structure in struct inode itself could define and commandeer a part structure and link it to the inode through u.generic_ip. This approach addressed both of the problems with unions as no changes are needed to shared declarations and each filesystem only uses the space that it needs. However it again introduced recent problems of its own.

    Using generic_ip, each filesystem required two allocations for each inode instead of one and this could lead to more wastage depending on how the structure size was rounded up for allocation; it also required writing more error-handling code. also there was memory used for the generic_ip pointer and often for a back pointer from the private structure to the common struct inode. Both of these are wasted space compared with the union approach or the embedding approach.

    Worse than this though, an extra memory dereference was needed to access the private structure from the common structure; such dereferences are best avoided. Filesystem code will often necessity to access both the common and the private structures. This either requires lots of extra memory dereferences, or it requires holding the address of the private structure in a register which increases register pressure. It was largely these concerns that stopped struct inode from ever migrating to broad employ of the generic_ip pointer. It was certainly used, but not by the major, high-performance filesystems.

    Though this pattern has problems it is quiet in wide use. struct super_block has an s_fs_info pointer which serves the very purpose as u.generic_ip (which has since been renamed to i_private when the u union was finally removed - why it was not completely removed is left as an exercise for the reader). This is the only course to store filesystem-private data in a super_block. A simple search in the Linux embrace files shows quite a collection of fields which are void pointers named "private" or something similar. Many of these are examples of the pattern of extending a data ilk by using a pointer to a private extension, and most of these could breathe converted to using the embedded-structure pattern.

    Beyond inodes

    While inodes serve as an effective vehicle to introduce these three patterns they finish not display the full scope of any of them so it is useful to see further afield and see what else they can learn.

    A survey of the employ of unions elsewhere in the kernel shows that they are widely used though in very different circumstances than in struct inode. The particular aspect of inodes that is missing elsewhere is that a wide ambit of different modules (different filesystems) each wanted to extend an inode in different ways. In most places where unions are used there are a petite fixed number of subtypes of the foundation ilk and there is Little expectation of more being added. A simple example of this is struct nfs_fattr which stores file mention information decoded out of an NFS reply. The details of these attributes are slightly different for NFSv2 and NFSv3 so there are effectively two subtypes of this structure with the inequity encoded in a union. As NFSv4 uses the very information as NFSv3 this is very unlikely to ever breathe extended further.

    A very common pattern in other uses of unions in Linux is for encoding messages that are passed around, typically between the kernel and user-space. struct siginfo is used to convey extra information with a signal delivery. Each signal ilk has a different ilk of ancillary information, so struct siginfo has a union to encode six different subtypes. union inputArgs appears to breathe the largest current union with 22 different subtypes. It is used by the "coda" network file system to pass requests between the kernel module and a user-space daemon which handles the network communication.

    It is not clear whether these examples should breathe considered as the very pattern as the original struct inode. finish they really picture different subtypes of a foundation type, or is it just one ilk with internal variants? The Eiffel object-oriented programming language does not advocate variant types at full except through subtype inheritance so there is clearly a school of thought that would want to deal full usages of union as a figure of subtyping. Many other languages, such as C++, provide both inheritance and unions allowing the programmer to figure a choice. So the concede is not clear.

    For their purposes it doesn't really matter what they convene it as long as they know where to employ each pattern. The examples in the kernel fairly clearly exhibit that when full of the variants are understood by a separate module, then a union is a very commandeer mechanism for variants structures, whether you want to mention to them as using data inheritance or not. When different subtypes are managed by different modules, or at least widely part pieces of code, then one of the other mechanisms is preferred. The employ of unions for this case has almost completely disappeared with only struct cycx_device remaining as an example of a deprecated pattern.

    Problems with void pointers

    Void pointers are not quite so simple to classify. It would probably breathe honest to Say that void pointers are the modern equivalent of "goto" statements. They can breathe very useful but they can also lead to very convoluted designs. A particular problem is that when you see at a void pointer, infatuation looking at a goto, you don't really know what it is pointing at. A void pointer called private is even worse - it is infatuation a "goto destination" command - almost mindless without reading lots of context.

    Examining full the different uses that void pointers can breathe Put to would breathe well beyond the scope of this article. Instead they will restrict their attention to just one recent usage which relates to data inheritance and illustrates how the untamed nature of void pointers makes it hard to recognize their employ in data inheritance. The example they will employ to clarify this usage is struct seq_file used by the seq_file library which makes it simple to synthesize simple text files infatuation some of those in /proc. The "seq" share of seq_file simply indicates that the file contains a sequence of lines corresponding to a sequence of items of information in the kernel, so /proc/mounts is a seq_file which walks through the mount table reporting each mount on a separate line.

    When seq_open() is used to create a recent seq_file it allocates a struct seq_file and assigns it to the private_data territory of the struct file which is being opened. This is a straightforward example of void pointer based data inheritance where the struct file is the foundation ilk and the struct seq_file is a simple extension to that type. It is a structure that never exists by itself but is always the private_data for some file. struct seq_file itself has a private territory which is a void pointer and it can breathe used by clients of seq_file to add extra status to the file. For example md_seq_open() allocates a struct mdstat_info structure and attaches it via this private field, using it to meet md's internal needs. Again, this is simple data inheritance following the described pattern.

    However the private territory of struct seq_file is used by svc_pool_stats_open() in a subtly but importantly different way. In this case the extra data needed is just a separate pointer. So rather than allocating a local data structure to mention to from the private field, svc_pool_stats_open simply stores that pointer directly in the private territory itself. This certainly seems infatuation a sensible optimization - performing an allocation to store a separate pointer would breathe a squander - but it highlights exactly the source of confusion that was suggested earlier: that when you see at a void pointer you don't really know what is it pointing at, or why.

    To figure it a bit clearer what is happening here, it is helpful to imagine "void *private" as being infatuation a union of every different practicable pointer type. If the value that needs to breathe stored is a pointer, it can breathe stored in this union following the "unions for data inheritance" pattern. If the value is not a separate pointer, then it gets stored in allocated space following the "void pointers for data inheritance" pattern. Thus when they see a void pointer being used it may not breathe obvious whether it is being used to point to an extension structure for data inheritance, or being used as an extension for data inheritance (or being used as something else altogether).

    To highlight this issue from a slightly different perspective it is instructive to examine struct v4l2_subdev which represents a sub-device in a video4linux device, such as a sensor or camera controller within a webcam. According to the (rather helpful) documentation it is expected that this structure will normally breathe embedded in a larger structure which contains extra state. However this structure quiet has not just one but two void pointers, both with names suggesting that they are for private employ by subtypes:

    /* pointer to private data */ void *dev_priv; void *host_priv;

    It is common that a v4l sub-device (a sensor, usually) will breathe realized by, for example, an I2C device (much as a cloak device which stores your filesystem might breathe realized by an ATA or SCSI device). To allow for this common occurrence, struct v4l2_subdev provides a void pointer (dev_priv), so that the driver itself doesn't necessity to define a more specific pointer in the larger structure which struct v4l2_subdev would breathe embedded in. host_priv is intended to point back to a "parent" device such as a controller which acquires video data from the sensor. Of the three drivers which employ this field, one appears to result that end while the other two employ it to point to an allocated extension structure. So both of these pointers are intended to breathe used following the "unions for data inheritance" pattern, where a void pointer is playing the role of a union of many other pointer types, but they are not always used that way.

    It is not immediately clear that defining this void pointer in case it is useful is actually a valuable service to provide given that the device driver could easily enough define its own (type safe) pointer in its extension structure. What is clear is that an apparently "private" void pointer can breathe intended for various qualitatively different uses and, as they acquire seen in two different circumstances, they may not breathe used exactly as expected.

    In short, recognizing the "data inheritance through void pointers" pattern is not easy. A fairly abysmal examination of the code is needed to determine the exact purpose and usage of void pointers.

    A diversion into struct page

    Before they leave unions and void pointers behind a see at struct page may breathe interesting. This structure uses both of these patterns, though they are hidden kindhearted of due to historical baggage. This example is particularly instructive because it is one case where struct embedding simply is not an option.

    In Linux memory is divided into pages, and these pages are Put to a variety of different uses. Some are in the "page cache" used to store the contents of files. Some are "anonymous pages" holding data used by applications. Some are used as "slabs" and divided into pieces to concede kmalloc() requests. Others are simply share of a multi-page allocation or maybe are on a free list waiting to breathe used. Each of these different employ cases could breathe seen as a subtype of the universal class of "page", and in most cases necessity some dedicated fields in struct page, such as a struct address_space pointer and index when used in the page cache, or struct kmem_cache and freelist pointers when used as a slab.

    Each page always has the very struct page describing it, so if the effective ilk of the page is to change - as it must as the demands for different uses of memory change over time - the ilk of the struct page must change within the lifetime of that structure. While many ilk systems are designed assuming that the ilk of an demur is immutable, they find here that the kernel has a very real necessity for ilk mutability. Both unions and void pointers allow types to change and as noted, struct page uses both.

    At the first level of subtyping there are only a petite number of different subtypes as listed above; these are full known to the core memory management code, so a union would breathe ideal here. Unfortunately struct page has three unions with fields for some subtypes spread over full three, thus hiding the real structure somewhat.

    When the primary subtype in employ has the page being used in the page cache, the particular address_space that it belongs to may want to extend the data structure further. For this purpose there is a private territory that can breathe used. However it is not a void pointer but is an unsigned long. Many places in the kernel assume an unsigned long and a void * are the very size and this is one of them. Most users of this territory actually store a pointer here and acquire to cast it back and forth. The "buffer_head" library provides macros attach_page_buffers and page_buffers to set and glean this field.

    So while struct page is not the most elegant example, it is an informative example of a case where unions and void pointers are the only option for providing data inheritance.

    The details of structure embedding

    Where structure embedding can breathe used, and where the list of practicable subtypes is not known in advance, it seems to breathe increasingly the preferred choice. To gain a full understanding of it they will again necessity to explore a Little bit further than inodes and contrast data inheritance with other uses of structure embedding.

    There are essentially three uses for structure embedding - three reasons for including a structure within another structure. Sometimes there is nothing particularly tantalizing going on. Data items are collected together into structures and structures within structures simply to highlight the closeness of the relationships between the different items. In this case the address of the embedded structure is rarely taken, and it is never mapped back to the containing structure using container_of().

    The second employ is the data inheritance embedding that they acquire already discussed. The third is infatuation it but importantly different. This third employ is typified by struct list_head and other structs used as an embedded anchor when creating abstract data types.

    The employ of an embedded anchor infatuation struct list_head can breathe seen as a style of inheritance as the structure containing it "is-a" member of a list by virtue of inheriting from struct list_head. However it is not a strict subtype as a separate demur can acquire several struct list_heads embedded - struct inode has six (if they embrace the similar hlist_node). So it is probably best to deem of this sort of embedding more infatuation a "mixin" style of inheritance. The struct list_head provides a service - that of being included in a list - that can breathe mixed-in to other objects, an capricious number of times.

    A key aspect of data inheritance structure embedding that differentiates it from each of the other two is the existence of a reference counter in the inner-most structure. This is an observation that is tied directly to the fact that the Linux kernel uses reference counting as the primary means of lifetime management and so would not breathe shared by systems that used, for example, garbage collection to manage lifetimes.

    In Linux, every demur with an independent existence will acquire a reference counter, sometimes a simple atomic_t or even an int, though often a more express struct kref. When an demur is created using several levels of inheritance the reference counter could breathe buried quite deeply. For example a struct usb_device embeds a struct device which embeds struct kobject which has a struct kref. So usb_device (which might in revolve breathe embedded in a structure for some specific device) does acquire a reference counter, but it is contained several levels down in the nest of structure embedding. This contrasts quite nicely with a list_head and similar structures. These acquire no reference counter, acquire no independent existence and simply provide a service to other data structures.

    Though it seems obvious when Put this way, it is useful to bethink that a separate demur cannot acquire two reference counters - at least not two lifetime reference counters (It is fine to acquire two counters infatuation s_active and s_count in struct super_block which count different things). This means that multiple inheritance in the "data inheritance" style is not possible. The only figure of multiple inheritance that can work is the mixin style used by list_head as mentioned above.

    It also means that, when designing a data structure, it is captious to deem about lifetime issues and whether this data structure should acquire its own reference counter or whether it should depend on something else for its lifetime management. That is, whether it is an demur in its own right, or simply a service provided to other objects. These issues are not really recent and apply equally to void pointer inheritance. However an captious inequity with void pointers is that it is relatively simple to change your intellect later and switch an extension structure to breathe a fully independent object. Structure embedding requires the discipline of thinking clearly about the problem up front and making the perquisite determination early - a discipline that is worth encouraging.

    The other key telltale for data inheritance structure embedding is the set of rules for allocating and initializing recent instances of a structure, as has already been hinted at. When union or void pointer inheritance is used the main structure is usually allocated and initialized by common code (the mid-layer) and then a device specific open() or create() office is called which can optionally commandeer and initialize any extension object. By contrast when structure embedding is used the structure needs to breathe allocated by the lowest level device driver which then initializes its own fields and calls in to common code to initialize the common fields.

    Continuing the struct inode example from above which has an alloc_inode() fashion in the super_block to request allocation, they find that initialization is provided for with inode_init_once() and inode_init_always() advocate functions. The first of these is used when the previous employ of a piece of memory is unknown, the second is enough by itself when they know that the memory was previously used for some other inode. They see this very pattern of an initializer office part from allocation in kobject_init(), kref_init(), and device_initialize().

    So apart from the obvious embedding of structures, the pattern of "data inheritance through structure embedding" can breathe recognized by the presence of a reference counter in the innermost structure, by the delegation of structure allocation to the final user of the structure, and by the provision of initializing functions which initialize a previously allocated structure.

    Conclusion

    In exploring the employ of fashion dispatch (last week) and data inheritance (this week) in the Linux kernel they find that while some patterns look to dominate they are by no means universal. While almost full data inheritance could breathe implemented using structure embedding, unions provide real value in a few specific cases. Similarly while simple vtables are common, mixin vtables are very captious and the aptitude to delegate methods to a related demur can breathe valuable.

    We also find that there are patterns in employ with Little to recommend them. Using void pointers for inheritance may acquire an initial simplicity, but causes longer term wastage, can cause confusion, and could nearly always breathe replaced by embedded inheritance. Using NULL pointers to indicate default conduct is similarly a impecunious altenative - when the default is captious there are better ways to provide for it.

    But maybe the most valuable lesson is that the Linux kernel is not only a useful program to run, it is also a useful document to study. Such study can find elegant practical solutions to real problems, and some less elegant solutions. The willing student can pursue the former to attend ameliorate their mind, and pursue the latter to attend ameliorate the kernel itself. With that in mind, the following exercises might breathe of interest to some.

    Exercises
  • As inodes now employ structure embedding for inheritance, void pointers should not breathe necessary. Examine the consequences and wisdom of removing "i_private" from "struct inode".

  • Rearrange the three unions in struct page to just one union so that the enumeration of different subtypes is more explicit.

  • As was preeminent in the text, struct seq_file can breathe extended both through "void pointer" and a limited figure of "union" data inheritance. clarify how seq_open_private() allows this structure to also breathe extended through "embedded structure" data inheritance and give an example by converting one usage in the kernel from "void pointer" to "embedded structure". reckon submitting a patch if this appears to breathe an improvement. Contrast this implementation of embedded structure inheritance with the mechanism used for inodes.

  • Though subtyping is widely used in the kernel, it is not uncommon for a demur to hold fields that not full users are interested in. This can indicate that more fine grained subtyping is possible. As very many completely different things can breathe represented by a "file descriptor", it is likely that struct file could breathe a candidate for further subtyping.

    Identify the smallest set of fields that could serve as a generic struct file and explore the implications of embedding that in different structures to implement regular files, socket files, event files, and other file types. Exploring more universal employ of the proposed open() fashion for inodes might attend here.

  • Identify an "object-oriented" language which has an demur model that would meet full the needs of the Linux kernel as identified in these two articles.

  • (Log in to post comments)


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