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Sizeof
sizeof is a unary operator in the programming languages C and C++. It generates the storage size of an expression or a data type, measured in the number of ''char''-sized units. Consequently, the construct ''sizeof (char)'' is guaranteed to be ''1''. The actual number of bits of type char is specified by the preprocessor macro , defined in the standard include file limits.h. On most modern computing platforms this is eight bits. The result of ''sizeof'' has an unsigned integer type that is usually denoted by size_t. The operator has a single operand, which is either an expression or the cast of a data type, which is a data type enclosed in parentheses. Data types may not only be primitive types, such as integer and floating-point types, but also pointer types, and compound datatypes ( unions, structs, and C++ classes). Purpose Many programs must know the storage size of a particular datatype. Though for any given implementation of C or C++ the size of a particular datatype i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Data Pointer
In computer science, a pointer is an object in many programming languages that stores a memory address. This can be that of another value located in computer memory, or in some cases, that of memory-mapped computer hardware. A pointer ''references'' a location in memory, and obtaining the value stored at that location is known as ''dereferencing'' the pointer. As an analogy, a page number in a book's index could be considered a pointer to the corresponding page; dereferencing such a pointer would be done by flipping to the page with the given page number and reading the text found on that page. The actual format and content of a pointer variable is dependent on the underlying computer architecture. Using pointers significantly improves performance for repetitive operations, like traversing iterable data structures (e.g. strings, lookup tables, control tables and tree structures). In particular, it is often much cheaper in time and space to copy and dereference pointers than ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Unary Operator
In mathematics, an unary operation is an operation with only one operand, i.e. a single input. This is in contrast to binary operations, which use two operands. An example is any function , where is a set. The function is a unary operation on . Common notations are prefix notation (e.g. ¬, −), postfix notation (e.g. factorial ), functional notation (e.g. or ), and superscripts (e.g. transpose ). Other notations exist as well, for example, in the case of the square root, a horizontal bar extending the square root sign over the argument can indicate the extent of the argument. Examples Unary negative and positive As unary operations have only one operand they are evaluated before other operations containing them. Here is an example using negation: :3 − −2 Here, the first '−' represents the binary subtraction operation, while the second '−' represents the unary negation of the 2 (or '−2' could be taken to mean the integer −2). Therefore, the expression is ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Magic Number (programming)
In computer programming, a magic number is any of the following: * A unique value with unexplained meaning or multiple occurrences which could (preferably) be replaced with a named constant * A constant numerical or text value used to identify a file format or protocol; for files, see List of file signatures * A distinctive unique value that is unlikely to be mistaken for other meanings (e.g., Globally Unique Identifiers) Unnamed numerical constants The term ''magic number'' or ''magic constant'' refers to the anti-pattern of using numbers directly in source code. This has been referred to as breaking one of the oldest rules of programming, dating back to the COBOL, FORTRAN and PL/1 manuals of the 1960s. The use of unnamed magic numbers in code obscures the developers' intent in choosing that number, increases opportunities for subtle errors (e.g. is every digit correct in 3.14159265358979323846 and is this equal to 3.14159?) and makes it more difficult for the program to be a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Computer Architecture
In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, the description may include the instruction set architecture design, microarchitecture design, logic design, and implementation. History The first documented computer architecture was in the correspondence between Charles Babbage and Ada Lovelace, describing the analytical engine. When building the computer Z1 in 1936, Konrad Zuse described in two patent applications for his future projects that machine instructions could be stored in the same storage used for data, i.e., the stored-program concept. Two other early and important examples are: * John von Neumann's 1945 paper, First Draft of a Report on the EDVAC, which described an organization of logical elements; and *Alan Turing's more detailed ''Proposed Electronic Calculator'' ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Data Structure Alignment
Data structure alignment is the way data is arranged and accessed in computer memory. It consists of three separate but related issues: data alignment, data structure padding, and packing. The CPU in modern computer hardware performs reads and writes to memory most efficiently when the data is ''naturally aligned'', which generally means that the data's memory address is a multiple of the data size. For instance, in a 32-bit architecture, the data may be aligned if the data is stored in four consecutive bytes and the first byte lies on a 4-byte boundary. ''Data alignment'' is the aligning of elements according to their natural alignment. To ensure natural alignment, it may be necessary to insert some ''padding'' between structure elements or after the last element of a structure. For example, on a 32-bit machine, a data structure containing a 16-bit value followed by a 32-bit value could have 16 bits of ''padding'' between the 16-bit value and the 32-bit value to align the 32-b ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Application Binary Interface
In computer software, an application binary interface (ABI) is an interface between two binary program modules. Often, one of these modules is a library or operating system facility, and the other is a program that is being run by a user. An ''ABI'' defines how data structures or computational routines are accessed in machine code, which is a low-level, hardware-dependent format. In contrast, an ''API'' defines this access in source code, which is a relatively high-level, hardware-independent, often human-readable format. A common aspect of an ABI is the calling convention, which determines how data is provided as input to, or read as output from, computational routines. Examples of this are the x86 calling conventions. Adhering to an ABI (which may or may not be officially standardized) is usually the job of a compiler, operating system, or library author. However, an application programmer may have to deal with an ABI directly when writing a program in a mix of programming l ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Variable-length Array
In computer programming, a variable-length array (VLA), also called variable-sized or runtime-sized, is an array data structure whose length is determined at run time (instead of at compile time). In C, the VLA is said to have a variably modified type that depends on a value (see Dependent type). The main purpose of VLAs is to simplify programming of numerical algorithms. Programming languages that support VLAs include Ada (programming language), Ada, Algol 68 (for non-flexible rows), APL (programming language), APL, C99 (although subsequently relegated in C11 (C standard revision), C11 to a conditional feature, which implementations are not required to support; on some platforms, VLAs could be implemented previously with alloca() or similar functions) and C Sharp (programming language), C# (as unsafe-mode stack-allocated arrays), COBOL, Fortran, Fortran 90, J (programming language), J, and Object Pascal (the language used in Borland Delphi and Lazarus (software), Lazarus, that uses ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ellipsis
The ellipsis (, also known informally as dot dot dot) is a series of dots that indicates an intentional omission of a word, sentence, or whole section from a text without altering its original meaning. The plural is ellipses. The term originates from the grc, ἔλλειψις, meaning 'leave out'. Opinions differ as to how to render ellipses in printed material. According to ''The Chicago Manual of Style'', it should consist of three periods, each separated from its neighbor by a non-breaking space: . According to the ''AP Stylebook'', the periods should be rendered with no space between them: . A third option is to use the Unicode character U+2026 . Background The ellipsis is also called a suspension point, points of ellipsis, periods of ellipsis, or (colloquially) "dot-dot-dot".. According to Toner it is difficult to establish when the "dot dot dot" phrase was first used. There is an early instance, which is perhaps the first in a piece of fiction, in Virginia Woolf's ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Variadic Template
In computer programming, variadic templates are templates that take a variable number of arguments. Variadic templates are supported by C++ (since the C++11 standard), and the D programming language. C++ The variadic template feature of C++ was designed by Douglas Gregor and Jaakko Järvi and was later standardized in C++11. Prior to C++11, templates (classes and functions) could only take a fixed number of arguments, which had to be specified when a template was first declared. C++11 allows template definitions to take an arbitrary number of arguments of any type. template class tuple; // takes zero or more arguments The above template class will take any number of typenames as its template parameters. Here, an instance of the above template class is instantiated with three type arguments: tuple some_instance_name; The number of arguments can be zero, so will also work. If the variadic template should only allow a positive number of arguments, then this d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Declaration (computer Science)
In computer programming, a declaration is a language construct specifying identifier properties: it declares a word's (identifier's) meaning."A declaration specifies the interpretation and attributes of a set of identifiers. A ''definition'' of an identifier is a declaration for that identifier that: * for an object ariable or constant causes storage to be reserved for that object; * for a function, includes the function body; * for an enumeration constant, is the (only) declaration of the identifier; * for a typedef name, is the first (or only) declaration of the identifier." C11 specification, 6.7: Declarations, paragraph 5. Declarations are most commonly used for functions, variables, constants, and classes, but can also be used for other entities such as enumerations and type definitions. Beyond the name (the identifier itself) and the kind of entity (function, variable, etc.), declarations typically specify the data type (for variables and constants), or the type signatur ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Offsetof
C's macro is an ANSI C library feature found in . It evaluates to the offset (in bytes) of a given member within a struct or union type, an expression of type . The offsetof() macro takes two parameters, the first being a structure name, and the second being the name of a member within the structure. It cannot be described as a C prototype. Implementation The "traditional" implementation of the macro relied on the compiler obtaining the offset of a member by specifying a hypothetical structure that begins at address zero: #define offsetof(st, m) \ ((size_t)&(((st *)0)->m)) This can be understood as taking a null pointer of type structure , and then obtaining the address of member within said structure. While this implementation works correctly in many compilers, it has generated some debate regarding whether this is undefined behavior according to the C standard, since it appears to involve a dereference of a null pointer (although, according to the standard, section 6.6 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Buffer Overflow
In information security and programming, a buffer overflow, or buffer overrun, is an anomaly whereby a program, while writing data to a buffer, overruns the buffer's boundary and overwrites adjacent memory locations. Buffers are areas of memory set aside to hold data, often while moving it from one section of a program to another, or between programs. Buffer overflows can often be triggered by malformed inputs; if one assumes all inputs will be smaller than a certain size and the buffer is created to be that size, then an anomalous transaction that produces more data could cause it to write past the end of the buffer. If this overwrites adjacent data or executable code, this may result in erratic program behavior, including memory access errors, incorrect results, and crashes. Exploiting the behavior of a buffer overflow is a well-known security exploit. On many systems, the memory layout of a program, or the system as a whole, is well defined. By sending in data designed to ca ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
