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IBM kick Oriented Analysis and

Object-Oriented evaluation and Design | true Questions and Pass4sure dumps

This chapter is from the ebook 

analysis emphasizes an investigation of the problem and requirements, as opposed to a solution. for example, if a novel on-line trading system is preferred, how will it exist used? What are its functions?

"evaluation" is a wide time period, most desirable certified, as in requirements evaluation (an investigation of the requirements) or object-oriented evaluation (an investigation of the district objects).

Design emphasizes a conceptual reply (in utility and hardware) that fulfills the necessities, rather than its implementation. as an example, a description of a database schema and application objects. Design concepts often exclude low-level or "evident" details—obtrusive to the supposititious patrons. finally, designs can also exist applied, and the implementation (equivalent to code) expresses the actual and complete realized design.

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

valuable analysis and design had been summarized in the phrase finish the remedy factor (analysis), and finish the issue arrogate (design).

UNICOM Nabs Versant for Object-Oriented Databases | true Questions and Pass4sure dumps

UNICOM accelerated into the kick oriented database information realm this month when it announced plans to purchase Versant, a publicly traded developer of a pair of object-oriented databases, for roughly $32 million.

situated in 1988 as kick Sciences Corp., Versant become an early proponent of kick oriented databases, which store facts as objects. kick database management techniques (ODBMS) deliver efficiency merits over relational database management programs (RDBMS) in powering purposes that are written in kick oriented languages and require storing of tangled objects.

Versant’s first product become the Versant kick Database (VOD), which started to profit steam within the early 1990s and is still actively developed nowadays. VOD runs on Unix, Linux, and home windows operating methods, and excels at running applications with extravagant concurrency and efficiency necessities such as buying and selling structures, telecom community management systems, airline and inn reservation techniques, and risk evaluation for banks, based on the Wikipedia entry on Versant.

Versant changed its denomination to Versant kick know-how in 1993, went public on the NASDAQ trade in 1996, and altered its identify to Versant Corp. in 1999. In 2004, it acquired bard application, the German developer of an ODBMS called FastObjects. In 2008 Versant bought a further ODBMS, known as db40, which is an open supply ODBMS assignment centered in 2000 by using Carl Rosenberger.

these days, Versant continues to sell and augment VOD, FastObjects, and db40. The company has struggled to generate sales over the remaining a few years, as revenues hold dropped from $25.3 million in 2008 to about $16.3 million ultimate yr. The company, which is based mostly in Redwood metropolis, California, has remained profitable, youngsters, and continues to enhance novel utility and are looking for novel shoppers and OEM partnerships with ISVs.

Versant’s board approved the sale to UNICOM. The deal requires Versant shareholders to exist paid $11.50 per objective share, a 17 % top class on Versant’s closing expense on September 28. The transaction is anticipated to complete earlier than the conclusion of Versant’s current quarter ending January 31, 2013. Versant is free to are seeking option offers through October 29.

“This merger will deliver premium value and liquidity to Versant’s shareholders. Versant is worked up to associate with UNICOM to proceed to expand the enterprise’s market management place,” cited Versant president and CEO Bernhard Woebker in a statement.

The sentiment became echoed via Corry Hong, founder, president, and CEO of Mission Hills, California-primarily based UNICOM, who mentioned that Versant’s Redwood city and Hamburg places of drudgery “will boost their existing presence in each California and Germany.”

UNICOM has acquired a few utility organizations over the final a number of years, including SoftLanding methods, a developer of change management software for the IBM i server.

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Frenemies: a brief historical past of Apple and IBM Partnerships | true Questions and Pass4sure dumps

notwithstanding Apple and IBM hold had their participate of fights over the years, this week's deal isn't their first pairing.

IBM Apple Merger

Steve Jobs battled a few tech competitors, however within the early 80s, his ire became largely directed at IBM. The professional squabble dates returned to at least 1981 with Apple's "Welcome, IBM. critically" ad, and seemingly ended simply yesterday with a joint agreement to convey IBM's commercial enterprise software to iOS contraptions later this yr. but while the organizations hold certainly clashed over the ultimate 30 years, the day prior to this's pairing became not the first time these two behemoths hold teamed up.

Take AIMJust 10 years after the first salvo become fired between Apple and IBM, the two joined with Motorola in 1991 to strengthen a touchstone for the PowerPC architecture that might ebb against the Microsoft-Intel Wintel alliance. The Apple-Intel-Motorola (purpose) crew took IBM's vigour lead set architecture and grew to become it birthright into a buyer-friendly version that turned into manufactured with the aid of IBM and Motorola and used in Apple's Macintosh line from 1994 to 2006, when Apple - to which Jobs returned in 1997 - transitioned to Intel-based machines.

"it's been ten years due to the fact that their transition to the PowerPC, and they reckon Intel's expertise will advocate us create the top-quality personal computer systems for the next ten years," Jobs talked about in announcing the circulation.

From OS to Oh NoIn the late 80s, Apple changed into working on an object-oriented operating equipment, codenamed pink. but it surely changed into having quandary pulling it every collectively. in the wake of the goal alliance, Apple turned to IBM for tips, and pink become reborn as a joint challenge referred to as Taligent with IBM's Joe Guglielmi on the helm.

The Taligent construction crew worked away for 2 years, ultimately switching from an object-oriented OS to an object-oriented programming ambiance that might drudgery on any synchronous OS. but despite advocate from Hewlett-Packard, Taligent every but fizzled by means of the mid-90s. Guglielmi left the company for Motorola in 1995, and after his successor, Dick Guarino, died of a heart assault later that 12 months, Taligent grew to become simply an extra IBM subsidiary. It became formally dissolved in 1998.

Bullet and a TargetAfter being spurned by Microsoft on an kick linking and embedding undertaking, Apple approached IBM about engaged on it collectively in 1992. It resulted in the OpenDoc word-processing application and browser however didn't hold what it took to ebb against Java. When he again to Apple, Jobs "put a bullet via [OpenDoc's] head," as he noted.

The Enemy of My EnemyIn the closing two decades, Apple and IBM hold taken sever paths. IBM jettisoned its computer traffic and concentrated firmly on the business, making headlines alongside the route for its artificial intelligence drudgery on Watson. Apple, meanwhile, grew to exist a consumer darling, with consumers lining up to nab the enterprise's newest phones and tablets. by means of coming lower back collectively, although, Apple delves further into the enterprise with a companion it is widespread in that space, whereas IBM gets to present up items with a determined "cool" constituent and tap into the Apple mystique.

For extra, acquire a study at Apple and IBM Bury the Hammer and IBM and Apple spell company.

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Object Oriented Analysis and Design - participate 2 (Design)

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Object-oriented design patterns in the kernel, participate 2 | true questions and Pass4sure dumps

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June 7, 2011

This article was contributed by Neil Brown

In the first participate 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 manner dispatch, looked at the different forms that vtables could take, and the circumstances where sever vtables were eschewed in preference for storing function pointers directly in objects. In this conclusion they will explore a second vital 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" 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 exist said to inherit from that data structure. There are three different approaches to this sharing and extending that can exist found in the Linux kernel, and every can exist 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 constituent 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 blank 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 when ext3 was added.

This approach to data inheritance is simple and straightforward, but is also significantly clumsy. There are two obvious problems. Firstly, every novel filesystem implementation needs to add an extra domain to the union "u". With 3 fields this may not appear 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 exist the very size and will exist 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 note shortly. The second is a true problem and the generic ugliness of the design encouraged change. Early in the 2.5 development 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 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 allocate recollection 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 exist solved, but the solutions were not costly.

The first rigor 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 spot 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 order fields in the inode to glean optimal performance - as some kernel developers are wont to do.

The solution was to utilize the list_entry() macro to execute 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 utilize 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 drill lasted until 2.5.28 when a novel 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 rigor was that the filesystem had to exist answerable for allocating the inode - it could no longer exist allocated by common code as the common code did not hold enough information to allocate the remedy 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 illustrious earlier, the union pattern was not an impenetrable barrier to adding novel filesystems independently. This is because the union u had one more domain that was not an "inode_info" structure. A generic pointer domain 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 allocate a sever 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 novel 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 recollection 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 recollection 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 recollection 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 utilize of the generic_ip pointer. It was certainly used, but not by the major, high-performance filesystems.

Though this pattern has problems it is still 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 route 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 exist converted to using the embedded-structure pattern.

Beyond inodes

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

A survey of the utilize 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 miniature expectation of more being added. A simple illustration of this is struct nfs_fattr which stores file ascribe 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 very information as NFSv3 this is very unlikely to ever exist 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 exist 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 lucid whether these examples should exist 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 every except through subtype inheritance so there is clearly a school of thought that would want to handle every usages of union as a shape of subtyping. Many other languages, such as C++, provide both inheritance and unions allowing the programmer to construct a choice. So the reply is not clear.

For their purposes it doesn't really matter what they convene it as long as they know where to utilize each pattern. The examples in the kernel fairly clearly point to that when every of the variants are understood by a solitary module, then a union is a very arrogate 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 sever pieces of code, then one of the other mechanisms is preferred. The utilize of unions for this case has almost completely disappeared with only struct cycx_device remaining as an illustration of a deprecated pattern.

Problems with void pointers

Void pointers are not quite so smooth to classify. It would probably exist unbiased to notify that void pointers are the modern equivalent of "goto" statements. They can exist very useful but they can also lead to very convoluted designs. A particular problem is that when you study 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 purposeless without reading lots of context.

Examining every the different uses that void pointers can exist set to would exist well beyond the scope of this article. Instead they will restrict their attention to just one novel usage which relates to data inheritance and illustrates how the untamed nature of void pointers makes it hard to recognize their utilize in data inheritance. The illustration they will utilize to account for this usage is struct seq_file used by the seq_file library which makes it smooth to synthesize simple text files fancy some of those in /proc. The "seq" participate 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 novel seq_file it allocates a struct seq_file and assigns it to the private_data domain of the struct file which is being opened. This is a straightforward illustration 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 domain which is a void pointer and it can exist used by clients of seq_file to add extra condition to the file. For illustration 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 domain 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 mention to from the private field, svc_pool_stats_open simply stores that pointer directly in the private domain itself. This certainly seems fancy a sensible optimization - performing an allocation to store a solitary pointer would exist a consume - but it highlights exactly the source of confusion that was suggested earlier: that when you study at a void pointer you don't really know what is it pointing at, or why.

To construct 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 exist stored is a pointer, it can exist 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 note a void pointer being used it may not exist 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 exist embedded in a larger structure which contains extra state. However this structure still has not just one but two void pointers, both with names suggesting that they are for private utilize by subtypes:

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

It is common that a v4l sub-device (a sensor, usually) will exist realized by, for example, an I2C device (much as a block device which stores your filesystem might exist 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 exist 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 utilize this field, one appears to follow that goal while the other two utilize it to point to an allocated extension structure. So both of these pointers are intended to exist 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 lucid 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 lucid is that an apparently "private" void pointer can exist intended for various qualitatively different uses and, as they hold seen in two different circumstances, they may not exist 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 study at struct page may exist interesting. This structure uses both of these patterns, though they are hidden significantly due to historical baggage. This illustration is particularly instructive because it is one case where struct embedding simply is not an option.

In Linux recollection is divided into pages, and these pages are set 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 reply kmalloc() requests. Others are simply participate of a multi-page allocation or maybe are on a free list waiting to exist used. Each of these different utilize cases could exist seen as a subtype of the generic 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 very struct page describing it, so if the effectual ilk of the page is to change - as it must as the demands for different uses of recollection 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 kick is immutable, they find here that the kernel has a very true exigency for ilk mutability. Both unions and void pointers allow types to change and as noted, struct page uses both.

At the first flat of subtyping there are only a petite number of different subtypes as listed above; these are every known to the core recollection management code, so a union would exist pattern here. Unfortunately struct page has three unions with fields for some subtypes spread over every three, thus hiding the true structure somewhat.

When the primary subtype in utilize 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 domain that can exist 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 domain actually store a pointer here and hold 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 illustration 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 exist used, and where the list of viable subtypes is not known in advance, it seems to exist increasingly the preferred choice. To gain a plenary understanding of it they will again exigency to explore a miniature 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 snoopy 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 utilize is the data inheritance embedding that they hold already discussed. The third is fancy it but importantly different. This third utilize is typified by struct list_head and other structs used as an embedded anchor when creating abstract data types.

The utilize of an embedded anchor fancy struct list_head can exist 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 kick can hold several struct list_heads embedded - struct inode has six (if they embrace 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 exist mixed-in to other objects, an whimsical 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 exist shared by systems that used, for example, garbage collection to manage lifetimes.

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

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

It also means that, when designing a data structure, it is vital to mediate about lifetime issues and whether this data structure should hold its own reference counter or whether it should depend on something else for its lifetime management. That is, whether it is an kick in its own right, or simply a service provided to other objects. These issues are not really novel and apply equally to void pointer inheritance. However an vital incompatibility with void pointers is that it is relatively smooth to change your mind later and switch an extension structure to exist a fully independent object. Structure embedding requires the discipline of thinking clearly about the problem up front and making the birthright 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 novel 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() function is called which can optionally allocate and initialize any extension object. By contrast when structure embedding is used the structure needs to exist allocated by the lowest flat device driver which then initializes its own fields and calls in to common code to initialize the common fields.

Continuing the struct inode illustration from above which has an alloc_inode() manner 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 utilize of a piece of recollection is unknown, the second is enough by itself when they know that the recollection was previously used for some other inode. They note this very pattern of an initializer function sever 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 exist 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.


In exploring the utilize of manner dispatch (last week) and data inheritance (this week) in the Linux kernel they find that while some patterns appear to dominate they are by no means universal. While almost every data inheritance could exist implemented using structure embedding, unions provide true value in a few specific cases. Similarly while simple vtables are common, mixin vtables are very vital and the competence to delegate methods to a related kick can exist valuable.

We also find that there are patterns in utilize with miniature to recommend them. Using void pointers for inheritance may hold an initial simplicity, but causes longer term wastage, can cause confusion, and could nearly always exist replaced by embedded inheritance. Using NULL pointers to testify default conduct is similarly a destitute option - when the default is vital 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 true problems, and some less elegant solutions. The willing student can pursue the former to advocate ameliorate their mind, and pursue the latter to advocate ameliorate the kernel itself. With that in mind, the following exercises might exist of interest to some.

  • As inodes now utilize structure embedding for inheritance, void pointers should not exist 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 illustrious in the text, struct seq_file can exist extended both through "void pointer" and a limited shape of "union" data inheritance. account for how seq_open_private() allows this structure to also exist extended through "embedded structure" data inheritance and give an illustration by converting one usage in the kernel from "void pointer" to "embedded structure". reckon submitting a patch if this appears to exist 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 kick to accommodate fields that not every users are interested in. This can testify that more fine grained subtyping is possible. As very many completely different things can exist represented by a "file descriptor", it is likely that struct file could exist 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 generic utilize of the proposed open() manner for inodes might advocate here.

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

  • (Log in to post comments)

    Design patterns, the sizable picture, participate 2: Gang-of-four classics revisited | true questions and Pass4sure dumps

    The Gang of Four's Design Patterns: Elements of Reusable Object-Oriented Software was a milestone in design pattern history. In participate 2 of his design patterns overview, Jeff Friesen focuses on the book and its 23 patterns. He first revisits the book's Strategy and Visitor patterns from a Java developer's perspective, then offers tips for learning every of the GoF patterns by successfully using Design Patterns as a reference manual. The article concludes with a summary of critiques of the GoF book and a warning about the perils of over-reliance on design patterns generally, which may exist especially apposite for software developers novel to programming.

    In participate 1 of this three-part progression introducing design patterns, I referred to Design Patterns: Elements of Reusable Object-Oriented Design. This classic was written by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, who were collectively known as the Gang of Four. As most readers will know, Design Patterns presents 23 software design patterns that fitting into the categories discussed in participate 1: Creational, structural, and behavioral.

    GoF patterns on JavaWorld

    David Geary's Java design patterns progression introduced many of the GoF patterns in Java code. note the Resources section at the cease of this article for a listing of Geary's articles on JavaWorld.

    Design Patterns is canonical reading for software developers, but many novel programmers are challenged by its reference format and scope. Each of the 23 patterns is described in detail, in a template format consisting of 13 sections, which can exist a lot to digest. Another challenge for novel Java developers is that the Gang of Four patterns spring from object-oriented programming, with examples based on C++ and Smalltalk, not Java code.

    In this article I'll unpack two of the commonly used patterns -- Strategy and Visitor -- from a Java developer's perspective. Strategy is a fairly simple pattern that serves as an illustration of how to glean your feet damp with the GoF design patterns generally; Visitor is more tangled and intermediate in scope. I'll start with an illustration that should demystify double dispatch, which is an vital participate of the Visitor pattern. Then I'll demonstrate the Visitor pattern in a compiler utilize case.

    Following my examples here should advocate you explore and utilize the other GoF patterns for yourself. In addition, I'll tender tips for getting the most out of the Gang of Four book and conclude with a summary of critiques of using design patterns in software development. That discussion could exist especially apposite to developers novel to programming.


    Unpacking Strategy

    The Strategy pattern lets you define a family of algorithms such as those used for sorting, text composition, or layout management. Strategy also lets you encapsulate each algorithm in its own class and construct them interchangeable. Each encapsulated algorithm is known as a strategy. At runtime, a client chooses the arrogate algorithm for its requirements.

    What is a client?

    A client is any piece of software that interacts with a design pattern. Although typically an object, a client could also exist code within an application's public static void main(String[] args) method.

    Unlike the Decorator pattern, which focuses on changing an object's skin, or appearance, Strategy focuses on changing the object's guts, significance its changeable behaviors. Strategy lets you avoid using multiple conditional statements by poignant conditional branches into their own strategy classes. These classes often derive from an abstract superclass, which the client references and uses to interact with a specific strategy.

    From an abstract perspective, Strategy involves Strategy, ConcreteStrategyx, and Context types.


    Strategy provides a common interface to every supported algorithms. Listing 1 presents the Strategy interface.

    Listing 1. void execute(int x) must exist implemented by every concrete strategies public interface Strategy { public void execute(int x); }

    Where concrete strategies are not parameterized with common data, you can implement them via Java's interface feature. Where they are parameterized, you would instead declare an abstract class. For example, right-align, center-align, and justify text alignment strategies participate the concept of a width in which to execute text alignment. So you would declare this width in the abstract class.


    Each ConcreteStrategyx implements the common interface and provides an algorithm implementation. Listing 2 implements Listing 1's Strategy interface to recount a specific concrete strategy.

    Listing 2. ConcreteStrategyA executes one algorithm public class ConcreteStrategyA implements Strategy { @Override public void execute(int x) { System.out.println("executing strategy A: x = "+x); } }

    The void execute(int x) manner in Listing 2 identifies a specific strategy. mediate of this manner as an abstraction for something more useful, fancy a specific benign of sorting algorithm (e.g., Bubble Sort, Insertion Sort, or Quick Sort), or a specific benign of layout manager (e.g., current Layout, border Layout, or Grid Layout).

    Listing 3 presents a second Strategy implementation.

    Listing 3. ConcreteStrategyB executes another algorithm public class ConcreteStrategyB implements Strategy { @Override public void execute(int x) { System.out.println("executing strategy B: x = "+x); } } Context

    Context provides the context in which the concrete strategy is invoked. Listings 2 and 3 point to data being passed from a context to a strategy via a manner parameter. Because a generic strategy interface is shared by every concrete strategies, some of them may not require every parameters. To avoid wasted parameters (especially when passing many different kinds of arguments to only a few concrete strategies), you could pass a reference to the context instead.

    Instead of passing a context reference to the method, you could store it in the abstract class, making your manner calls parameterless. However, the context would exigency to specify a more extensive interface that would embrace the constrict for accessing context data in a uniform manner. The result, as shown in Listing 4, is a tighter coupling between strategies and their context.

    Listing 4. Context is configured with a ConcreteStrategyx instance class Context { private Strategy strategy; public Context(Strategy strategy) { setStrategy(strategy); } public void executeStrategy(int x) { strategy.execute(x); } public void setStrategy(Strategy strategy) { this.strategy = strategy; } }

    The Context class in Listing 4 stores a strategy when it is created, provides a manner to subsequently change the strategy, and provides another manner to execute the current strategy. Except for passing a strategy to the constructor, this pattern can exist seen in the java.awt .Container class, whose void setLayout(LayoutManager mgr) and void doLayout() methods specify and execute the layout manager strategy.


    We exigency a client to demonstrate the previous types. Listing 5 presents a StrategyDemo client class.

    Listing 5. StrategyDemo public class StrategyDemo { public static void main(String[] args) { Context context = novel Context(new ConcreteStrategyA()); context.executeStrategy(1); context.setStrategy(new ConcreteStrategyB()); context.executeStrategy(2); } }

    A concrete strategy is associated with a Context instance when the context is created. The strategy can exist subsequently changed via a context manner call.

    If you compile these classes and bustle StrategyDemo, you should commemorate the following output:

    executing strategy A: x = 1 executing strategy B: x = 2


    Revisiting the Visitor pattern

    Visitor is the final software design pattern to materialize in Design Patterns. Although this behavioral pattern is presented ultimate in the book for alphabetical reasons, some believe that it should near ultimate due to its complexity. Newcomers to Visitor often struggle with this software design pattern.

    As explained in Design Patterns, a visitor lets you add operations to classes without changing them, a bit of magic that is facilitated by the so-called double dispatch technique. In order to understand the Visitor pattern, they exigency first to digest double dispatch.


    What is double dispatch?

    Java and many other languages advocate polymorphism (many shapes) via a technique known as dynamic dispatch, in which a message is mapped to a specific sequence of code at runtime. Dynamic dispatch is classified as either solitary dispatch or multiple dispatch:

  • Single dispatch: Given a class hierarchy where each class implements the very manner (that is, each subclass overrides the previous class's version of the method), and given a variable that's assigned an instance of one of these classes, the ilk can exist figured out only at runtime. For example, suppose each class implements manner print(). Suppose too that one of these classes is instantiated at runtime and its variable assigned to variable a. When the Java compiler encounters a.print();, it can only verify that a's ilk contains a print() method. It doesn't know which manner to call. At runtime, the virtual machine examines the reference in variable a and figures out the actual ilk in order to convene the birthright method. This situation, in which an implementation is based on a solitary ilk (the ilk of the instance), is known as solitary dispatch.
  • Multiple dispatch: Unlike in solitary dispatch, where a solitary controversy determines which manner of that denomination to invoke, multiple dispatch uses every of its arguments. In other words, it generalizes dynamic dispatch to drudgery with two or more objects. (Note that the controversy in solitary dispatch is typically specified with a epoch separator to the left of the manner denomination being called, such as the a in a.print().)
  • Finally, double dispatch is a special case of multiple dispatch in which the runtime types of two objects are involved in the call. Although Java supports solitary dispatch, it doesn't advocate double dispatch directly. But they can simulate it.

    Do they over-rely on double dispatch?

    Blogger Derek Greer believes that using double dispatch may testify a design issue, which could impact an application's maintainability. Read Greer's "Double dispatch is a code smell" blog post and associated comments for details.

    Simulating double dispatch in Java code

    Wikipedia's entry on double dispatch provides a C++-based illustration that shows it to exist more than function overloading. In Listing 6, I present the Java equivalent.

    Listing 6. Double dispatch in Java code public class DDDemo { public static void main(String[] args) { Asteroid theAsteroid = novel Asteroid(); SpaceShip theSpaceShip = novel SpaceShip(); ApolloSpacecraft theApolloSpacecraft = novel ApolloSpacecraft(); theAsteroid.collideWith(theSpaceShip); theAsteroid.collideWith(theApolloSpacecraft); System.out.println(); ExplodingAsteroid theExplodingAsteroid = novel ExplodingAsteroid(); theExplodingAsteroid.collideWith(theSpaceShip); theExplodingAsteroid.collideWith(theApolloSpacecraft); System.out.println(); Asteroid theAsteroidReference = theExplodingAsteroid; theAsteroidReference.collideWith(theSpaceShip); theAsteroidReference.collideWith(theApolloSpacecraft); System.out.println(); SpaceShip theSpaceShipReference = theApolloSpacecraft; theAsteroid.collideWith(theSpaceShipReference); theAsteroidReference.collideWith(theSpaceShipReference); System.out.println(); theSpaceShipReference = theApolloSpacecraft; theAsteroidReference = theExplodingAsteroid; theSpaceShipReference.collideWith(theAsteroid); theSpaceShipReference.collideWith(theAsteroidReference); } } class SpaceShip { void collideWith(Asteroid inAsteroid) { inAsteroid.collideWith(this); } } class ApolloSpacecraft extends SpaceShip { void collideWith(Asteroid inAsteroid) { inAsteroid.collideWith(this); } } class Asteroid { void collideWith(SpaceShip s) { System.out.println("Asteroid hit a SpaceShip"); } void collideWith(ApolloSpacecraft as) { System.out.println("Asteroid hit an ApolloSpacecraft"); } } class ExplodingAsteroid extends Asteroid { void collideWith(SpaceShip s) { System.out.println("ExplodingAsteroid hit a SpaceShip"); } void collideWith(ApolloSpacecraft as) { System.out.println("ExplodingAsteroid hit an ApolloSpacecraft"); } }

    Listing 6 follows its C++ counterpart as closely as possible. The final four lines in the main() manner along with the void collideWith(Asteroid inAsteroid) methods in SpaceShip and ApolloSpacecraft demonstrate and simulate double dispatch.

    Consider the following excerpt from the cease of main():

    theSpaceShipReference = theApolloSpacecraft; theAsteroidReference = theExplodingAsteroid; theSpaceShipReference.collideWith(theAsteroid); theSpaceShipReference.collideWith(theAsteroidReference);

    The third and fourth lines utilize solitary dispatch to figure out the remedy collideWith() manner (in SpaceShip or ApolloSpacecraft) to invoke. This determination is made by the virtual machine based on the ilk of the reference stored in theSpaceShipReference.

    Object-Oriented Analysis and Design | true questions and Pass4sure dumps

    This chapter is from the book 

    There are many viable activities and artifacts in introductory OOA/D, and a wealth of principles and guidelines. Suppose they must elect a solitary practical skill from every the topics discussed here—a "desert island" skill. What would it be?

    A critical competence in OO development is to skillfully allocate responsibilities to software objects.

    Why? Because it is one activity that must exist performed—either while drawing a UML diagram or programming—and it strongly influences the robustness, maintainability, and reusability of software components.

    Of course, there are other vital skills in OOA/D, but responsibility assignment is emphasized in this introduction because it tends to exist a challenging skill to master (with many "degrees of freedom" or alternatives), and yet is vitally important. On a true project, a developer might not hold the opening to execute any other modeling activities—the "rush to code" development process. Yet even in this situation, assigning responsibilities is inevitable.

    Consequently, the design steps in this book emphasize principles of responsibility assignment.

    Nine fundamental principles in kick design and responsibility assignment are presented and applied. They are organized in a learning aid called GRASP of principles with names such as Information Expert and Creator.

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