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000-834 remonstrate Oriented Analysis and Design - Part 2 (Design)

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

Test Code : 000-834
Test designation : Object Oriented Analysis and Design - Part 2 (Design)
Vendor designation : IBM
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IBM remonstrate 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 difficulty and necessities, in preference to a solution. as an example, if a current 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 retort (in application and hardware) that fulfills the necessities, rather than its implementation. for example, an silhouette of a database schema and software objects. Design ideas frequently exclude low-level or "obvious" details—evident to the suppositious buyers. in the end, designs will too live 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 rectify (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 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 ideas that's supported by course of many languages.

if you aren’t regular with the ideas of object-orientation, you might too win a glance at the narrative of Object-Oriented Programming.

If every minute 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 live split apart into self contained objects, very nearly fancy having several mini-classes, each remonstrate representing a different a Part of the software.

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

These objects aren’t random. They depict the course you discourse and suppose about the difficulty you try to limpid up for your real existence.

They characterize issues fancy 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 apparatus via making expend of the thing-oriented ideas, and help a collection of graphical gadget fashions barnone through the edifice 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 difficulty to designs then to code and trying out and at final to deployment.

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

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

In analysis developers toil with clients and domain consultants to silhouette what the apparatus is suppositious to do. Implementation particulars are presupposed to live by and great or completely omitted at this phase.

The point 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 expend of expend cases and summary definition of essentially the most critical 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 silent believe barnone the particulars required so that programmers can enforce 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 difficulty the software making an attempt to limpid up.
  • Specify requirements: picture the necessities, continually, the usage of expend instances (and eventualities) or consumer stories.
  • Conceptual model: determine the critical objects, refine them, and define their relationships and conduct and draw them in an light 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 phase identifies the objects, their relationship, and conduct using the conceptual model (an summary definition for the objects).

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

    in addition to making expend of the application design concepts and patterns which should live 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 too circle up in parallel, and the consequences of one activity can live 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 win to live definitely particular about object-oriented principles fancy inheritance and polymorphism.

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

    — other diagrams

    there are many other diagrams they can expend 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 barnone times about deciding upon the appropriate diagram for the appropriate want. you should realize which diagrams can live efficient when pondering or discussing a situation that isn’t clear.

    gadget modeling and the diverse fashions they will expend can live 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 ingredient 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 fancy mathematical model; an in depth apparatus description.

    fashions are used perquisite through the analysis procedure to support to elicit the requirements, barnone the course through the design process to define 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 apparatus 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 apparatus and other programs.
  • Structural, the site you mannequin the corporation of the system, or the structure of the statistics being processed by course of the gadget.
  • Behavioral, the site you mannequin the dynamic conduct of the apparatus and how it reply to hobbies.
  • Unified Modeling Language (UML)

    The unified modeling language circle 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 apparatus 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 apparatus 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 toil on paper or on a whiteboard, at least within the initial degrees of a task. but there are some diagramming apparatus with a view to mitigate you to draw these UML diagrams.


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    Object Oriented Analysis and Design - Part 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 information 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 remonstrate paradigm on software progress activities
  • To explore synergy between object-oriented design and object-oriented programming
  • To learn about key remonstrate 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 physiognomy a transition to remonstrate technology and want to learn about challenges and benefits of this transition. The workshop does not require any prior information of object-oriented programming or information of remonstrate paradigm. universal computer fluency and universal information of issues associated with software and software progress are expected.

    Seminar Organization:The course will live 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 live refined in to the particular design flush providing an opportunity 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 disburse no more than half an hour of their time during the entire class on learning how to expend the tool. Not using the CASE toll will manufacture it almost impossible to tang hands-on barnone 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 live 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 edifice blocks of software architectureCriteria of class qualityIntroduction to Unified Modeling Language (UML)Static & dynamic modelActors and expend casesTransitioning from functional requirements to objects - introduction

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

    June 3, 2001, 9:00 - 2:00Topics to live 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 live addressed:Static versus dynamic modelIdentifying scenarios through refinement of expend casesModeling scenarios using object-interaction and sequence diagrams

    Project:Refining expend casesDeveloping and modeling scenariosIdentifying methodsRefining class specifications

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

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

    June 10, 2001, 9:00 - 2:00Topics to live 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 notable topics to live covered in this course:What to await from an object-oriented languageDynamic nature of object-oriented systems and the issues of garbage collectionEffective expend 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 close with the concepts of object-orientation, you may win a explore at The narrative 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 great program will instead live split apart into self contained objects, almost fancy having several mini-programs, each remonstrate representing a different Part of the application.

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

    These objects aren’t random. They depict the course you talk and mediate about the problem you are trying to resolve in your real life.

    They depict things fancy 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 progress 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 toil with users and domain experts to define what the system is suppositious to do. Implementation details are suppositious to live mostly or totally ignored at this phase.

    The goal of the analysis phase is to create a model of the system regardless of constraints such as appropriate technology. This is typically done via expend cases and abstract definition of the most notable objects using conceptual model.

    The design phase 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 believe barnone 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 exigency to do, and what’s the problem the software trying to solve.
  • Specify requirements: picture the requirements, usually, using expend cases (and scenarios) or user stories.
  • Conceptual model: Identify the notable 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 final one. These are already explained in detail in Requirements Engineering.

    Object-Oriented Design

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

    While in design phase, they picture 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 live 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 live 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 win to live really specific about object-oriented principles fancy inheritance and polymorphism.

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

    — Other diagrams

    There are many other diagrams they can expend 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 live useful when thinking about or discussing a situation that isn’t clear.

    System modeling and the different models they can expend will live 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 notable 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 fancy mathematical model; a particular system description.

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

    Different Perspectives

    We may develop a model to depict 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 gauge 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 flood of the data between the processes in the system.
  • You can finish diagramming toil on paper or on a whiteboard, at least in the initial stages of a project. But there are some diagramming tools that will mitigate you to draw these UML diagrams.


    Object-oriented design patterns in the kernel, Part 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 Part 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 divorce vtables were eschewed in preference for storing role pointers directly in objects. In this conclusion they will explore a second notable 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 shape that is of interest when exploring the Linux kernel is most fancy subtype inheritance, where a concrete or "final" sort inherits some data fields from a "virtual" parent type. They will muster 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 live said to inherit from that data structure. There are three different approaches to this sharing and extending that can live found in the Linux kernel, and barnone can live 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 too the least flexible, is to declare a union as one ingredient 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 bare 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 too tosomeextent clumsy. There are two obvious problems. Firstly, every current filesystem implementation needs to add an extra field to the union "u". With 3 fields this may not look fancy 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 live the same size and will live great 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 observe shortly. The second is a real problem and the universal ugliness of the design encouraged change. Early in the 2.5 progress progression 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 too 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 appropriate reminiscence to store its own structure without any exigency 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 live solved, but the solutions were not costly.

    The first difficulty 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 site 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 sort safety and makes it harder to sort fields in the inode to win optimal performance - as some kernel developers are wont to do.

    The solution was to expend 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 expend this pattern of data structure embedding. The list_entry() macro did exactly what was needed and so it was used despite the atypical name. This exercise lasted until 2.5.28 when a current container_of() macro was added which implemented the same functionality as list_entry(), though with slightly more sort 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 difficulty was that the filesystem had to live amenable for allocating the inode - it could no longer live allocated by common code as the common code did not believe enough information to appropriate the rectify 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 current filesystems independently. This is because the union u had one more field that was not an "inode_info" structure. A generic pointer field 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 appropriate a divorce 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 current 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 too required writing more error-handling code. too there was reminiscence 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 reminiscence dereference was needed to access the private structure from the common structure; such dereferences are best avoided. Filesystem code will often exigency to access both the common and the private structures. This either requires lots of extra reminiscence 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 expend of the generic_ip pointer. It was certainly used, but not by the major, high-performance filesystems.

    Though this pattern has problems it is silent in wide use. struct super_block has an s_fs_info pointer which serves the same 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 comprehend 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 sort by using a pointer to a private extension, and most of these could live converted to using the embedded-structure pattern.

    Beyond inodes

    While inodes serve as an efficient vehicle to interlard these three patterns they finish not display the plenary scope of any of them so it is useful to explore further afield and observe what else they can learn.

    A survey of the expend 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 sweep of different modules (different filesystems) each wanted to extend an inode in different ways. In most places where unions are used there are a tiny fixed number of subtypes of the base sort and there is minute expectation of more being added. A simple specimen of this is struct nfs_fattr which stores file credit 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 incompatibility encoded in a union. As NFSv4 uses the same information as NFSv3 this is very unlikely to ever live 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 sort has a different sort of ancillary information, so struct siginfo has a union to encode six different subtypes. union inputArgs appears to live 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 limpid whether these examples should live considered as the same pattern as the original struct inode. finish they really depict different subtypes of a base type, or is it just one sort with internal variants? The Eiffel object-oriented programming language does not support variant types at barnone except through subtype inheritance so there is clearly a school of thought that would want to treat barnone usages of union as a shape of subtyping. Many other languages, such as C++, provide both inheritance and unions allowing the programmer to manufacture a choice. So the retort is not clear.

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

    Problems with void pointers

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

    Examining barnone the different uses that void pointers can live Put to would live well beyond the scope of this article. Instead they will restrict their attention to just one current usage which relates to data inheritance and illustrates how the untamed nature of void pointers makes it arduous to recognize their expend in data inheritance. The specimen they will expend to define this usage is struct seq_file used by the seq_file library which makes it light to synthesize simple text files fancy some of those in /proc. The "seq" Part 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 solitary line.

    When seq_open() is used to create a current seq_file it allocates a struct seq_file and assigns it to the private_data field of the struct file which is being opened. This is a straightforward specimen of void pointer based data inheritance where the struct file is the base sort 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 field which is a void pointer and it can live used by clients of seq_file to add extra status to the file. For specimen 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 field 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 solitary pointer. So rather than allocating a local data structure to advert to from the private field, svc_pool_stats_open simply stores that pointer directly in the private field itself. This certainly seems fancy a sensible optimization - performing an allocation to store a solitary pointer would live a fritter - but it highlights exactly the source of confusion that was suggested earlier: that when you explore at a void pointer you don't really know what is it pointing at, or why.

    To manufacture it a bit clearer what is happening here, it is helpful to imagine "void *private" as being fancy a union of every different viable pointer type. If the value that needs to live stored is a pointer, it can live stored in this union following the "unions for data inheritance" pattern. If the value is not a solitary pointer, then it gets stored in allocated space following the "void pointers for data inheritance" pattern. Thus when they observe a void pointer being used it may not live 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 live embedded in a larger structure which contains extra state. However this structure silent has not just one but two void pointers, both with names suggesting that they are for private expend by subtypes:

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

    It is common that a v4l sub-device (a sensor, usually) will live realized by, for example, an I2C device (much as a cloak device which stores your filesystem might live 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 exigency to define a more specific pointer in the larger structure which struct v4l2_subdev would live 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 expend this field, one appears to succeed that goal while the other two expend it to point to an allocated extension structure. So both of these pointers are intended to live 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 limpid 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 limpid is that an apparently "private" void pointer can live intended for various qualitatively different uses and, as they believe seen in two different circumstances, they may not live 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 explore at struct page may live interesting. This structure uses both of these patterns, though they are hidden tosomeextent due to historical baggage. This specimen is particularly instructive because it is one case where struct embedding simply is not an option.

    In Linux reminiscence 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 retort kmalloc() requests. Others are simply Part of a multi-page allocation or maybe are on a free list waiting to live used. Each of these different expend cases could live seen as a subtype of the universal class of "page", and in most cases exigency 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 same struct page describing it, so if the efficient sort of the page is to change - as it must as the demands for different uses of reminiscence change over time - the sort of the struct page must change within the lifetime of that structure. While many sort systems are designed assuming that the sort of an remonstrate is immutable, they find here that the kernel has a very real exigency for sort mutability. Both unions and void pointers allow types to change and as noted, struct page uses both.

    At the first flush of subtyping there are only a tiny number of different subtypes as listed above; these are barnone known to the core reminiscence management code, so a union would live exemplar here. Unfortunately struct page has three unions with fields for some subtypes spread over barnone three, thus hiding the real structure somewhat.

    When the primary subtype in expend 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 field that can live 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 same size and this is one of them. Most users of this field actually store a pointer here and believe to cast it back and forth. The "buffer_head" library provides macros attach_page_buffers and page_buffers to set and win this field.

    So while struct page is not the most elegant example, it is an informative specimen 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 live used, and where the list of viable subtypes is not known in advance, it seems to live increasingly the preferred choice. To gain a plenary understanding of it they will again exigency to explore a minute 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 captivating 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 expend is the data inheritance embedding that they believe already discussed. The third is fancy it but importantly different. This third expend is typified by struct list_head and other structs used as an embedded anchor when creating abstract data types.

    The expend of an embedded anchor fancy struct list_head can live 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 solitary remonstrate can believe several struct list_heads embedded - struct inode has six (if they comprehend the similar hlist_node). So it is probably best to mediate of this sort of embedding more fancy a "mixin" style of inheritance. The struct list_head provides a service - that of being included in a list - that can live 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 actuality 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 live shared by systems that used, for example, garbage collection to manage lifetimes.

    In Linux, every remonstrate with an independent actuality will believe a reference counter, sometimes a simple atomic_t or even an int, though often a more explicit struct kref. When an remonstrate is created using several levels of inheritance the reference counter could live buried quite deeply. For specimen a struct usb_device embeds a struct device which embeds struct kobject which has a struct kref. So usb_device (which might in circle live embedded in a structure for some specific device) does believe 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 believe no reference counter, believe no independent actuality and simply provide a service to other data structures.

    Though it seems obvious when Put this way, it is useful to remember that a solitary remonstrate cannot believe two reference counters - at least not two lifetime reference counters (It is fine to believe two counters fancy s_active and s_count in struct super_block which weigh different things). This means that multiple inheritance in the "data inheritance" style is not possible. The only shape of multiple inheritance that can toil is the mixin style used by list_head as mentioned above.

    It too means that, when designing a data structure, it is notable to mediate about lifetime issues and whether this data structure should believe its own reference counter or whether it should depend on something else for its lifetime management. That is, whether it is an remonstrate in its own right, or simply a service provided to other objects. These issues are not really current and apply equally to void pointer inheritance. However an notable incompatibility with void pointers is that it is relatively light to change your intellect later and switch an extension structure to live a fully independent object. Structure embedding requires the discipline of thinking clearly about the problem up front and making the perquisite conclusion 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 current 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() role is called which can optionally appropriate and initialize any extension object. By contrast when structure embedding is used the structure needs to live allocated by the lowest flush device driver which then initializes its own fields and calls in to common code to initialize the common fields.

    Continuing the struct inode specimen 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() support functions. The first of these is used when the previous expend of a piece of reminiscence is unknown, the second is sufficient by itself when they know that the reminiscence was previously used for some other inode. They observe this same pattern of an initializer role divorce 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 live 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 expend 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 barnone data inheritance could live implemented using structure embedding, unions provide real value in a few specific cases. Similarly while simple vtables are common, mixin vtables are very notable and the skill to delegate methods to a related remonstrate can live valuable.

    We too find that there are patterns in expend with minute to recommend them. Using void pointers for inheritance may believe an initial simplicity, but causes longer term wastage, can antecedent confusion, and could nearly always live replaced by embedded inheritance. Using NULL pointers to argue default conduct is similarly a needy option - when the default is notable 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 too 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 mitigate help their mind, and pursue the latter to mitigate help the kernel itself. With that in mind, the following exercises might live of interest to some.

    Exercises
  • As inodes now expend structure embedding for inheritance, void pointers should not live 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 live extended both through "void pointer" and a limited shape of "union" data inheritance. define how seq_open_private() allows this structure to too live extended through "embedded structure" data inheritance and give an specimen by converting one usage in the kernel from "void pointer" to "embedded structure". admiration submitting a patch if this appears to live 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 remonstrate to contain fields that not barnone users are interested in. This can argue that more fine grained subtyping is possible. As very many completely different things can live represented by a "file descriptor", it is likely that struct file could live 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 expend of the proposed open() fashion for inodes might mitigate here.

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

  • (Log in to post comments)


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