Bit Fields In Structures Pdf Free [REPACK]
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Structures and unions must derive from the Structure and Unionbase classes which are defined in the ctypes module. Each subclass mustdefine a _fields_ attribute. _fields_ must be a list of2-tuples, containing a field name and a field type.
By default, Structure and Union fields are aligned in the same way the Ccompiler does it. It is possible to override this behavior by specifying a_pack_ class attribute in the subclass definition. This must be set to apositive integer and specifies the maximum alignment for the fields. This iswhat #pragma pack(n) also does in MSVC.
ctypes uses the native byte order for Structures and Unions. To buildstructures with non-native byte order, you can use one of theBigEndianStructure, LittleEndianStructure,BigEndianUnion, and LittleEndianUnion base classes. Theseclasses cannot contain pointer fields.
If the errcheck function returns the argument tuple it receivesunchanged, ctypes continues the normal processing it does on the outputparameters. If you want to return a tuple of window coordinates instead of aRECT instance, you can retrieve the fields in the function and return theminstead, the normal processing will no longer take place:
Concrete structure and union types must be created by subclassing one of thesetypes, and at least define a _fields_ class variable. ctypes willcreate descriptors which allow reading and writing the fields by directattribute accesses. These are the
A sequence defining the structure fields. The items must be 2-tuples or3-tuples. The first item is the name of the field, the second itemspecifies the type of the field; it can be any ctypes data type.
The _fields_ class variable must, however, be defined before thetype is first used (an instance is created, sizeof() is called on it,and so on). Later assignments to the _fields_ class variable willraise an AttributeError.
The fields listed in this variable must be structure or union type fields.ctypes will create descriptors in the structure type that allowsaccessing the nested fields directly, without the need to create thestructure or union field.
The TYPEDESC structure describes a COM data type, the vt fieldspecifies which one of the union fields is valid. Since the u fieldis defined as anonymous field, it is now possible to access the membersdirectly off the TYPEDESC instance. td.lptdesc and td.u.lptdescare equivalent, but the former is faster since it does not need to createa temporary union instance:
It is possible to define sub-subclasses of structures, they inherit thefields of the base class. If the subclass definition has a separate_fields_ variable, the fields specified in this are appended to thefields of the base class.
Structure and union constructors accept both positional and keywordarguments. Positional arguments are used to initialize member fields in thesame order as they are appear in _fields_. Keyword arguments in theconstructor are interpreted as attribute assignments, so they will initialize_fields_ with the same name, or create new attributes for names notpresent in _fields_.
Within CPUs and other logic devices, collections of bit fields called flags are commonly used to control or to indicate the outcome of particular operations.[4] Processors have a status register that is composed of flags. For example if the result of an addition cannot be represented in the destination an arithmetic overflow is set. The flags can be used to decide subsequent operations, such as conditional jump instructions. For example, a JE ... (Jump if Equal) instruction in the x86 assembly language will result in a jump if the Z (zero) flag was set by some previous operation.
A bit field is distinguished from a bit array in that the latter is used to store a large set of bits indexed by integers and is often wider than any integral type supported by the language.[citation needed] Bit fields, on the other hand, typically fit within a machine word,[3] and the denotation of bits is independent of their numerical index.[2]
Bit fields can be used to reduce memory consumption when a program requires a number of integer variables which always will have low values. For example, in many systems storing an integer value requires two bytes (16-bits) of memory; sometimes the values to be stored actually need only one or two bits. Having a number of these tiny variables share a bit field allows efficient packaging of data in the memory.[5]
In C and C++, native implementation-defined bit fields can be created using unsigned int, signed int, or (in C99) _Bool. In this case, the programmer can declare a structure for a bit field which labels and determines the width of several subfields.[6] Adjacently declared bit fields of the same type can then be packed by the compiler into a reduced number of words, compared with the memory used if each 'field' were to be declared separately.
For languages lacking native bit fields, or where the programmer wants control over the resulting bit representation, it is possible to manually manipulate bits within a larger word type. In this case, the programmer can set, test, and change the bits in the field using combinations of masking and bitwise operations.[7]
The layout of bit fields in a C struct is implementation-defined. For behavior that remains predictable across compilers, it may be preferable to emulate bit fields with a primitive and bit operators:
In C, we can specify the size (in bits) of the structure and union members. The idea of bit-field is to use memory efficiently when we know that the value of a field or group of fields will never exceed a limit or is within a small range. Bit fields are used when the storage of our program is limited. Need of bit fields in C programming language:
However, if the same code is written using signed int and the value of the fields goes beyond the bits allocated to the variable and something interesting can happen. For example, consider the same code but with signed integers:
A bitfield of the wrong length is considered an error. Clients should drop the connection if they receive bitfields that are not of the correct size, or if the bitfield has any of the spare bits set.
A better strategy is to download pieces in rarest first order. The client can determine this by keeping the initial bitfield from each peer, and updating it with every have message. Then, the client can download the pieces that appear least frequently in these peer bitfields. Note that any Rarest First strategy should include randomization among at least several of the least common pieces, as having many clients all attempting to jump on the same "least common" piece would be counter productive.
Structures (and unions) may contain instances of other structures andunions, but of course not themselves. It is possible for a structureor union type to contain a field which is a pointer to the same type(see Incomplete Types).
That example defines a structure type with two bit fields, suit andface_value, which take up 2 bits and 4 bits, respectively. suitcan hold values from 0 to 3, and face_value can hold values from 0 to15. Notice that these bit fields were declared as unsigned int; hadthey been signed integers, then their ranges would have been from-2 to 1, and from -8 to 7, respectively.
Bit fields can be specified without a name in order to control whichactual bits within the containing unit are used. However,the effect of this is not very portable and it is rarely useful.You can also specify a bit field of size 0, which indicates thatsubsequent bit fields not further bit fields should be packed into theunit containing the previous bit field. This is likewise notgenerally useful.
The size of a structure type is equal to the sum of the size of all of itsmembers, possibly including padding to cause the structure type to align toa particular byte boundary. The details vary depending on your computerplatform, but it would not be atypical to see structures padded to alignon four- or eight-byte boundaries. This is done in order to speed upmemory accesses of instances of the structure type.
As with initializing structures which contain structure members, theadditional inner grouping braces are optional. But, if you use theadditional braces, then you can partially initialize some of thestructures in the array, and fully initialize others:
There are two cases where this technique does not work. The first iswhere the array element has zero size (GCC supports zero-sizedstructures as a GNU extension). The second is where the array is infact a function parameter (see Function Parameters).
In each of those, all that happens is that each expression isevaluated. However, they are useless because they do not storea value anywhere, nor do they actually do anything, other thanthe evaluation itself. The compiler is free to ignore suchstatements.
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