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Test Code : 000-633
Test title : Object Oriented Analysis and Design - participate 1
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June 1, 2011
this text became contributed by Neil Brownhowever that the Linux Kernel is generally written in C, it makes wide exhaust of some thoughts from the sphere of object-oriented programming. developers desperate to exhaust these object-oriented options receive runt assist or assistance from the language and so are left to fend for themselves. As is commonly the case, here's a double-edged sword. The developer has ample flexibility to carryout in fact frigid issues, and equally the flexibility to carryout basically dilatory issues, and it is rarely at any times pellucid at first peruse which is which, or greater precisely: where on the spectrum a specific way sits.
instead of trying to the language to supply assistance, a software engineer ought to look to relative follow to ascertain what works neatly and what's most efficient avoided. interpreting relative rehearse isn't any the time as handy as one could enjoy and the effort, as soon as made, is worth protecting. To preserve that trouble in your creator's half, this text brings yet another installment in an occasional collection on Linux Kernel Design Patterns and attempts to set out - with examples - the design patterns within the Linux Kernel which repercussion an object-oriented style of programming.
as opposed to providing a brief introduction to the article-oriented fashion, tempting though it really is, they are able to assume the reader has a simple edge of objects, courses, methods, inheritance, and identical phrases. For those as yet unfamiliar with these, there are loads of resources to subsist discovered somewhere else on the web.
Over two weeks they can hunt patterns in just two areas: formula dispatch and information inheritance. despite their patent simplicity they lead to some moneyed veins for investigation. this primary article will focal point on formulation dispatch.formulation Dispatch
The gigantic variety of styles of inheritance and guidelines for its usage in languages today looks to insinuate that there is not any uniform realizing of what "object-oriented" definitely potential. The term is a bit enjoy "love": any and sundry thinks they know what it potential however if you net down to particulars individuals can ascertain they hold very several ideas. while what it potential to subsist "oriented" might no longer subsist clear, what they insinuate with the aid of an "object" does appear to subsist uniformly agreed upon. it's conveniently an abstraction comprising both condition and habits. An expostulate is enjoy a record (Pascal) or struct (C), apart from that some of the names of contributors discuss with features which act on the different fields within the object. These duty participants are every now and then talked about a "strategies".
the most evident option to enforce objects in C is to declare a "struct" the spot some fields are tips to services which bewitch a pointer to the struct itself as their first argument. The calling conference for system "foo" in expostulate "bar" would readily be: bar->foo(bar, ...args); while this pattern is used in the Linux kernel it is not the paramount pattern so they can depart discussion of it except a runt later.
As strategies (in contrast to state) aren't at any times modified on a per-object foundation, a greater typical and simplest just a runt less obtrusive approach is to bring together any the methods for a specific classification of objects into a sever structure, occasionally known as a "virtual characteristic table" or vtable. the article then has a sole pointer to this desk in spot of a sever pointer for every system, and consequently makes exhaust of less memory.
This then ends up in their first pattern - a sheer vtable being a constitution which contains best feature pointers the spot the primary dispute of each is a pointer to any other structure (the article type) which itself carries a pointer to this vtable. Some essential examples of this within the Linux kernel are the file_lock_operations constitution which includes two duty pointers each of which bewitch a pointer to a struct file_lock, and the seq_operations vtable which includes 4 duty pointers which each operate on a struct seq_file. These two examples disclose an obtrusive naming pattern - the constitution protecting a vtable is named for the constitution maintaining the article (probably abbreviated) followed by "_operations". whereas this sample is yardstick it is by using no capability prevalent. around the time of two.6.39 there are about 30 "*_operations" structures along with well over 100 "*_ops" constructions, most if no longer any of that are vtables of some sort. There are additionally several structs akin to struct mdk_personality which are well-nigh vtables but shouldn't hold exceptionally useful names.
amongst these basically 200 vtable buildings there's lots of variability and so a lot of scope to search for exciting patterns. In specific they are able to peruse for typical diversifications from the "pure vtable" pattern described above and investigate how these variations manufacture a contribution to their realizing of expostulate exhaust in Linux.NULL feature pointers
the primary observation is that some characteristic pointers in some vtables are allowed to subsist NULL. clearly attempting to summon any such characteristic can subsist futile, so the code that calls into these strategies commonly consists of an categorical check for the pointer being NULL. There are a couple of several explanations for these NULL pointers. probably easiest to warrant is the incremental construction reason. because of the style vtable buildings are initialized, including a new characteristic pointer to the structure definition explanations any existing desk declarations to initialise that pointer to NULL. as a result it is viable to add a caller of the brand new components earlier than any illustration supports that components, and hold it assess for NULL and duty a default behavior. Then as incremental construction continues these vtable instances which requisite it could actually net non-default methods.
A coincident illustration is consign 77af1b2641faf4 including set_voltage_time_sel() to struct regulator_ops which acts on struct regulator_dev. Subsequent consign 42ab616afe8844 defines that formula for a specific machine. here's readily probably the most coincident illustration of a very common theme.
one other commonplace purpose is that inescapable strategies aren't primarily meaningful in Definite circumstances so the calling code without rigor checks for NULL and returns an preempt oversight when discovered. There are multiple examples of this within the digital filesystem (VFS) layer. for instance, the create() feature in inode_operations is simply meaningful if the inode in question is a listing. So inode_operations structures for non-directories usually hold NULL for the create() characteristic (and a lot of others) and the calling code in vfs_create() tests for NULL and returns -EACCES.
A final rationale that vtables sometimes comprehend NULL is that a component of functionality should subsist would becould very well subsist being transitioned from one interface to a different. a superb illustration of this is the ioctl() operation in file_operations. In 2.6.11, a brand new formulation, unlocked_ioctl() was introduced which became referred to as devoid of the huge kernel lock held. In 2.6.36, when any drivers and filesystems had been converted to manufacture exhaust of unlocked_ioctl(), the original ioctl() became ultimately removed. during this transition a file gadget would usually define only one of two, leaving the other defaulting to NULL.
A a bit of greater refined instance of here is study() and aio_read(), besides in file_operations, and the corresponding write() and aio_write(). aio_read() turned into added to aid asynchronous IO, and whether it is supplied the typical synchronous read() is not obligatory (it is effected the exhaust of do_sync_read() which calls the aio_read() components). in this case there seems to subsist no goal of ever disposing of examine() - it'll continue to subsist for situations where async IO isn't central comparable to particular filesystems enjoy procfs and sysfs. So it is soundless the case that only one of each and every pair requisite subsist defined by a filesystem, nonetheless it is not with ease a transition, it's a long-time term state.
notwithstanding there appear to subsist a couple of different reasons for a NULL duty pointer, virtually every case is an instance of one essential sample - that of featuring a default implementation for the components. in the "incremental construction" examples and the non-meaningful way case, here's pretty straightforward. e.g. the default for inode->create() is without rigor to further back an error. in the interface transition case it is just a bit of less obvious. The default for unlocked_ioctl() would subsist to bewitch the kernel lock after which title the ioctl() method. The default for read() is precisely do_sync_read() and some filesystems similar to ext3 truly give this expense explicitly in spot of using "NULL" to betoken a default.
With that in intellect, a bit reflection means that if the actual train is to give a default, then perhaps the best way would subsist to explicitly provide a default as opposed to the usage of the circuitous route of the usage of a default of NULL and deciphering it especially.
while NULL is definitely the easiest value to supply as a default - as the C regular assures us that uninitialized contributors of a structure carryout net set to NULL - it isn't very a lot tougher to set a more significant default. i am indebted to LWN reader wahern for the statement that C99 enables fields in a structure to subsist initialized varied instances with simplest the final value taking result and that this allows convenient surroundings of default values comparable to via following the fundamental model:#outline FOO_DEFAULTS .bar = default_bar, .baz = default_baz struct foo_operations my_foo = FOO_DEFAULTS, .bar = my_bar, ;
this may declare my_foo with a predefined default expense for baz and a localized expense for bar. as a consequence for the miniature freight of defining a few "default" services and including a "_DEFAULTS" entry to each and every statement, the default value for any box can easily subsist chosen when the sphere is first created, and automatically blanketed in each exhaust of the structure.
no longer best are meaningful defaults effortless to setaside into effect, they could result in a greater efficient implementation. In those circumstances the spot the characteristic pointer basically is NULL it's probably faster to peruse at various and department rather than to manufacture an circuitous feature name. however the NULL case is awfully regularly the exception instead of the rule, and optimizing for an exception is not ordinary practice. within the greater accustomed case when the characteristic pointer is not NULL, the peruse at various for NULL is with no peril a fritter of code house and a fritter of execution time. If they disallow NULLs they can manufacture any summon websites a runt bit smaller and less difficult.
In conventional, any testing performed by using the caller before calling a way can subsist seen for instance of the "mid-layer mistake" mentioned in a outdated article. It suggests that the mid-layer is making assumptions concerning the habits of the lessen degree driver rather than readily giving the driver freedom to behave in whatever approach is most relevant. This may no longer at any times subsist a pricey mistake, however continues to subsist foremost prevented where viable. on the other hand there's a pellucid pattern within the Linux kernel that pointers in vtables can every so often subsist NULLable, typically even though now not always to permit a transition, and the title websites may soundless in these instances examine for NULL before continuing with the name.
The observant reader could hold seen a pocket within the above profitable judgment denouncing the exhaust NULL pointers for defaults. in the case where the default is the yardstick case and where efficiency is paramount, the reasoning doesn't hang and a NULL pointer could neatly subsist justified. Naturally the Linux kernel provides an illustration of one of these case for their examination.
one of the most data buildings used by the VFS for caching filesystem information is the "dentry". A "dentry" represents a reputation in the filesystem, and so every "dentry" has a father or mother, being the directory containing it, and an "inode" representing the named file. The dentry is sever from the inode as a result of a sole file can hold several names (so an "inode" can hold varied "dentry"s). there's a dentry_operations vtable with a few operations together with, for instance, "d_compare" on the artery to compare two names and "d_hash" in order to generate a hash for the title to book the storage of the "dentry" in a hash table. Most filesystems carryout not hold this pliability. They deal with names as uninterpreted strings of bytes so the default evaluate and hash features are the accustomed case. a couple of filesystems profile these to deal with case-insensitive names however that is not the norm.
additional, filename peruse up is a yardstick operation in Linux and so optimizing it is a priority. consequently these two operations appear to subsist respectable candidates the spot a verify for NULL and an inlined default operation might subsist appropriate. What they locate even though is that after such an optimization is warranted it is not by itself sufficient. The code that calls d_compare() and d_hash() (and a couple of other dentry operations) does not examine these functions for NULL directly. rather they require that a number of flag bits (DCACHE_OP_HASH, DCACHE_OP_COMPARE) within the "dentry" are set up to point out no matter if the generic default should soundless subsist used, or whether the feature may soundless subsist known as. because the flag sphere is likely to subsist in cache anyway, and the dentry_operations constitution will often subsist not necessary at all, this avoids a reminiscence fetch in a sharptangy route.
So they find that the one case where the usage of a NULL characteristic pointer to point out a default may subsist justified, it isn't definitely used; instead, a distinct, greater effective, mechanism is used to point out that the default way is requested.individuals aside from characteristic pointers
whereas most vtable-like buildings in the kernel contain exclusively feature pointers, there are a significant minority which hold non-feature-pointer fields. a lot of these look on the surface fairly whimsical and just a few nearer inspections betoken that some of them influence of unfavorable design or bit-rot and their removal would simplest enrich the code.
there is one exception to the "features simplest" pattern that happens again and again and provides precise value, and so is value exploring. This pattern is seen in its most yardstick shape in struct mdk_personality which provides operations for a particular software RAID level. In selected this constitution includes an "owner", a "name", and a "record". The "proprietor" is the module that provides the implementation. The "name" is a simple identifier: some vtables hold string names, some hold numeric names, and it's always referred to as whatever distinctive enjoy "edition", "family", "drvname", or "stage". but conceptually it remains a reputation. within the current instance there are two names, a string and a numeric "level".
The "record", whereas participate of the identical functionality, is less regular. The mdk_personality structure has a struct list_head, as does struct ts_ops. struct file_system_type has a simple pointer to the subsequent struct file_system_type. The underlying thought perquisite here is that for any particular implementation of an interface (or "closing" definition of a category) to subsist usable, it ought to subsist registered further what may so that it can besides subsist found. additional, once it has been found it hold to subsist viable to manufacture confident that the module maintaining the implementation isn't eliminated while it's in use.
There appear to subsist practically as many forms of registration towards an interface in Linux as there are interfaces to register towards, so finding fantastic patterns there would subsist a difficult task. despite the fact it's pretty typical for a "vtable" to subsist handled because the fundamental deal with on a particular implementation of an interface and to hold an "owner" pointer which can besides subsist used to net a reference on the module which offers the implementation.
So the sample they ascertain perquisite here is that a constitution of characteristic pointers used as a "vtable" for expostulate components dispatch should soundless at any times comprehend handiest characteristic pointers. Exceptions require pellucid justification. a yardstick exception makes it viable for a module pointer and viable other fields equivalent to a title and a list pointer. These fields are used to aid the registration protocol for the particular interface. When there isn't any listing pointer it is very probably that the total vtable can subsist treated as study-simplest. during this case the vtable will frequently subsist declared as a const constitution and so could even subsist saved in examine-most effective reminiscence.Combining methods for distinctive objects
A closing regular divergence from the "pure vtable" pattern that they see in the Linux kernel happens when the first dispute to the duty isn't at any times the equal expostulate category. In a sheer vtable which is referenced by using a pointer in a particular records constitution, the primary dispute of each characteristic is exactly that statistics structure. What reason could there subsist for deviating from that pattern? It seems that there are few, some more engaging than others.
The easiest and least exciting clarification is that, for no obvious motive, the target records structure is listed in different places in the dispute record. as an illustration any services in struct fb_ops bewitch a struct fb_info. while in 18 situations that structure is the first argument, in five cases it is the closing. there's nothing most likely wrong with this preference and it's unlikely to fuddle builders. It is simply an issue for records miners enjoy your writer who requisite to filter it out as an inappropriate sample.
A mild divergence on this pattern is viewed in struct rfkill_ops the spot two features bewitch a struct rkfill but the third - set_block() - takes a void *records. additional investigation suggests that this opaque facts is exactly that which is stored in rfkill->information, so set_block() might quite simply subsist described to bewitch a struct rfkill and simply to follow the ->information hyperlink itself. This divergence is sufficiently non-obvious that it might conceivably fuddle developers as well as records miners and so should subsist averted.
The next divergence in seen for illustration in platform_suspend_ops, oprofile_operations, security_operations and a number of others. These bewitch an peculiar assortment of arguments with no glaring sample. although these are actually very different types of vtable structures in that the expostulate they belong to are singletons. There is just one active platform, just one profiler, only one safety coverage. consequently the "object" on which these operations act is a component of the international condition and so doesn't deserve to subsist protected in the arguments of any functions.
Having filtered these two patterns out as now not being very wonderful we're left with two that carryout serve to inform us anything about expostulate exhaust in the kernel.
quota_format_ops and export_operations are two diverse operations constructions that duty on a lot of diverse statistics structures. In each case the obvious fundamental expostulate (e.g. a struct super_block or a struct dentry) already has a vtable constitution dedicated to it (equivalent to super_operations or dentry_operations) and these new structures add new operations. In each and every case the brand new operations benign a cohesive unit presenting a related set of functionality - no matter if aiding disk quotas or NFS export. They don't any act on the identical expostulate with ease since the performance in question is relative upon a variety of objects.
The top-quality term from the language of object-oriented programming for this is likely the "mixin". although the fitting can besides not subsist ultimate - reckoning on what your accurate figuring out of mixin is - the theory of bringing in a group of functionality devoid of the exhaust of strict hierarchical inheritance is terribly near the purpose of quota_format_ops and export_operations.
once they comprehend to subsist searching for mixins enjoy these they are able to ascertain rather a couple of extra examples. The pattern to subsist alert for is not the one that led us perquisite here - an operations structure that operates on plenty of distinctive objects - however reasonably the one they discovered the spot the services in an "operations" structure operate on objects that hold already got their personal "operations" structure. When an expostulate has a huge number of operations which are central and these operations naturally community into subsets, it makes a lot of sense to divide them into sever vtable-like structures. There are a few examples of this in the networking code the spot for instance both tcp_congestion_ops and inet_connection_sock_af_ops operate (primarily) on a struct sock, which itself has already received a miniature set of committed operations.
So the pattern of a "mixin" - at the least as defined as a set of operations which celebrate to at least one or more objects with out being the fundamental operations for these objects - is a sample it's frequently found in the kernel and looks to subsist rather valuable in enabling improved modularization of code.
The remaining sample which explains non-uniform feature targets is likely the most interesting, in particular in its distinction to the evident utility of object-oriented programming style. Examples of this sample abound with ata_port_operations, tty_operations, nfs_rpc_ops and atmdev_ops any performing as helpful examples. although they will focus primarily on some examples from the filesystem layer, peculiarly super_operations and inode_operations.
there is a powerful hierarchy of objects in the implementation of a filesystem where the filesystem - represented by using a "super_block" - has a few data (struct inode) which might besides hold a few names or links (struct dentry). further every file may support statistics in the page cache (struct address_space) which comprises a couple of particular person pages (struct page). there is a artery in which any of those several objects belong to the filesystem as a whole. If a page has to subsist loaded with information from a file, the filesystem is vigilant of the artery to carryout that, and it's probably the equal mechanism for every web page in each file. the spot it is rarely any the time the equal, the filesystem is vigilant of that too. So they could conceivably deliver every operation on every one of these objects in the struct super_block, as it represents the filesystem and could recognize what to carryout in each case.
In rehearse that extreme isn't a profitable option. it is quite probably that while there are similarities between the storage of an everyday file and a listing, there are additionally Important alterations and being capable of encode those changes in sever vtables may besides subsist effective. from time to time miniature symbolic hyperlinks are stored directly within the inode while better hyperlinks are kept just enjoy the contents of a regular file. Having different readlink() operations for both instances can manufacture the code much more readable.
while the fierce of every operation attached to the one significant constitution is not gold standard, it is equally proper that the wayward austere is not model either. The struct web page in Linux does not hold a vtable pointer at any - partially as a result of they are looking to hold the structure as miniature as viable since it is so populous. rather the address_space_operations structure includes the operations that act on a web page. in a similar artery the super_operations structure consists of some operations that rehearse to inodes, and inode_operations carries some operations that apply to dentries.
it's evidently viable to hold operations constructions attached to a guardian of the goal expostulate - presenting the target holds a reference to the guardian, which it perpetually does - notwithstanding it is not fairly so pellucid that it is at any times really useful. within the case of struct web page which avoids having a vtable pointer altogether the capitalize is obvious. within the case of struct inode which has its own vtable pointer, the improvement of getting some operations (similar to destroy_inode() or write_inode()) attached to the super_block is less clear.
As there are a few vtable buildings where any given feature pointer can subsist kept, the precise alternative is in many circumstances runt more than historical accident. certainly the proliferation of struct dentry operations in inode_operations looks to subsist largely because of the incontrovertible fact that a few of them used to act at once on the inode, however adjustments in the VFS finally required this to alternate. as an illustration in 2.1.seventy eight-pre1, each and every of link(), readlink(), followlink() (and a few others which at the instant are defunct) were changed from taking a struct inode to bewitch a struct dentry instead. This set the scene for "dentry" operations to subsist in inode_operations, so when setattr and getattr had been introduced for two.3.48, it doubtless appeared completely herbal to encompass them in inode_operations however that they acted essentially on a dentry.
might subsist they might simplify issues through doing away with dentry_operations altogether. Some operations that act on dentries are already in inode_operations and super_operations - why not flux them any there? whereas dentries are not as populous as struct web page there are nonetheless lots of them and casting off the "d_op" sphere might deliver 5% of the reminiscence used by means of that structure (on x86-64).
With two exceptions, each lively filesystem only has a sole dentry operations constitution in effect. Some filesystem implementations enjoy "vfat" define two - e.g. one with case-sensitive matching and one with case-insensitive matching - but there is only one energetic per tremendous-block. So it might look to subsist that the operations in dentry_operations could subsist moved to super_operations, or at the least accessed via "s_d_op". both exceptions are ceph and procfs. These filesystems exhaust several d_revalidate() operations in diverse parts of the filesystem and - in the case of procfs - distinctive d_release() operations. The necessary distinctions may quite simply subsist made in per-superblock versions of those operations. carryout these cases warrant the 5% house charge? Arguably no longer.directly embedded characteristic pointers
finally it's acceptable to contemplate the alternate sample outlined at the beginning, where duty pointers are kept without delay within the expostulate in spot of in a sever vtable structure. This pattern may besides subsist seen in struct request_queue which has nine characteristic pointers, struct efi which has ten feature pointers, and struct sock which has six characteristic pointers.
The can freight of embedded pointers is absolutely house. When vtables are used, there is only one reproduction of the vtable and multiple copies of an expostulate (in most situations) so if more than one characteristic pointer is needed, a vtable would support area. The can freight of a vtable is a further recollection reference, though cache may gash back much of this freight in some situations. A vtable additionally has a value of flexibility. When each expostulate wants the exact selfsame set of operations a vtable is decent, but when there's a requisite to personally tailor one of the operations for each object, then embedded characteristic pointer can supply that flexibility. here is illustrated quite properly with the aid of the observation with "zoom_video" in struct pcmcia_socket/* Zoom video behaviour is so chip inescapable its not value adding this to _ops */
So the spot objects aren't very populous, the spot the list of duty pointers is small, and the spot dissimilar mixins are needed, embedded duty pointers are used instead of a sever vtable.formulation Dispatch abstract
If they mix any of the sample elements that we've found in Linux they ascertain that:
system pointers that duty on a particular ilk of expostulate are at any times accumulated in a vtable associated at once with that object, even though they could additionally seem:
These vtables infrequently contain the ease apart from feature pointers, although fields mandatory to register the item ilk can subsist appropriate. allowing these feature pointers to subsist NULL is a common but not always choicest way for handling defaults.
So in exploring the Linux Kernel code they hold discovered that notwithstanding it isn't written in an object-oriented language, it definitely carries objects, courses (represented as vtables), and even mixins. It additionally incorporates ideas no longer perpetually found in object-oriented languages comparable to delegating expostulate the perquisite artery to a "dad or mum" object.
hopefully understanding these different patterns and the factors for making a preference on between them can result in extra uniform application of the patterns throughout the kernel, and therefore manufacture it simpler for a newcomer to bewitch into account which sample is being followed. within the second a participate of their examination of expostulate oriented patterns they are able to ascertain the a lot of ways in which data inheritance is done in the Linux kernel and focus on the strengths and weaknesses of every way with a purpose to see where each is most acceptable.(Log in to setaside up comments)
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