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External Variable
In the C programming language, and its predecessor B, an external variable is a variable defined outside any function block. On the other hand, a local (automatic) variable is a variable defined inside a function block. Definition, declaration and the extern keyword extrn In B, all variables must be declared, as one of auto, extrn, or implicitly as function arguments. An external variable is defined outside functions, has a lifetime of the whole program execution, and introduced to function by the mean of extrn declaration. Using the following code in the tutorial as an example: main( ) a 'hell'; b 'o, w'; c 'orld'; a, b, c are all external variables of the program defined outside functions, and introduced to the main function by extrn declarations. extern To understand how external variables relate to the extern keyword, it is necessary to know the difference between defining and declaring a variable. When a variable is ''defined'', the compiler allocates memory for ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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C Programming Language
C (''pronounced'' '' – like the letter c'') is a general-purpose programming language. It was created in the 1970s by Dennis Ritchie and remains very widely used and influential. By design, C's features cleanly reflect the capabilities of the targeted Central processing unit, CPUs. It has found lasting use in operating systems code (especially in Kernel (operating system), kernels), device drivers, and protocol stacks, but its use in application software has been decreasing. C is commonly used on computer architectures that range from the largest supercomputers to the smallest microcontrollers and embedded systems. A successor to the programming language B (programming language), B, C was originally developed at Bell Labs by Ritchie between 1972 and 1973 to construct utilities running on Unix. It was applied to re-implementing the kernel of the Unix operating system. During the 1980s, C gradually gained popularity. It has become one of the most widely used programming langu ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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B Programming Language
B is a programming language developed at Bell Labs circa 1969 by Ken Thompson and Dennis Ritchie. B was derived from BCPL, and its name may possibly be a contraction of BCPL. Thompson's coworker Dennis Ritchie speculated that the name might be based on Bon, an earlier, but unrelated, programming language that Thompson designed for use on Multics. B was designed for recursive, non-numeric, machine-independent applications, such as system and language software. It was a typeless language, with the only data type being the underlying machine's natural memory word format, whatever that might be. Depending on the context, the word was treated either as an integer or a memory address. As machines with ASCII processing became common, notably the DEC PDP-11 that arrived at Bell Labs, support for character data stuffed in memory words became important. The typeless nature of the language was seen as a disadvantage, which led Thompson and Ritchie to develop an expanded version of the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Pascal (programming Language)
Pascal is an imperative and procedural programming language, designed by Niklaus Wirth as a small, efficient language intended to encourage good programming practices using structured programming and data structuring. It is named after French mathematician, philosopher and physicist Blaise Pascal. Pascal was developed on the pattern of the ALGOL 60 language. Wirth was involved in the process to improve the language as part of the ALGOL X efforts and proposed a version named ALGOL W. This was not accepted, and the ALGOL X process bogged down. In 1968, Wirth decided to abandon the ALGOL X process and further improve ALGOL W, releasing this as Pascal in 1970. On top of ALGOL's scalars and arrays, Pascal enables defining complex datatypes and building dynamic and recursive data structures such as lists, trees and graphs. Pascal has strong typing on all objects, which means that one type of data cannot be converted to or interpreted as another without explicit conversions ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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The C Programming Language
''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the C programming language, as well as co-designed the Unix operating system with which development of the language was closely intertwined. The book was central to the development and popularization of C and is still widely read and used today. Because the book was co-authored by the original language designer, and because the first edition of the book served for many years as the ''de facto'' standard for the language, the book was regarded by many to be the authoritative reference on C. History C was created by Dennis Ritchie at Bell Labs in the early 1970s as an augmented version of Ken Thompson's B (programming language), B. Another Bell Labs employee, Brian Kernighan, had written the first C tutorial, and he persuaded Ritchie to coauthor a book on the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Header Files
An include directive instructs a text file processor to replace the directive text with the content of a specified file. The act of including may be logical in nature. The processor may simply process the include file content at the location of the directive without creating a combined file. Different processors may use different syntax. The C preprocessor (used with C, C++ and in other contexts) defines an include directive as a line that starts #include and is followed by a file specification. COBOL defines an include directive indicated by copy in order to include a copybook. Generally, for C/C++ the include directive is used to include a header file, but can include any file. Although relatively uncommon, it is sometimes used to include a body file such as a .c file. The include directive can support encapsulation and reuse. Different parts of a system can be segregated into logical groupings yet rely on one another via file inclusion. C and C++ are designed to leverage ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Automatic Variable
In computer programming, an automatic variable is a local variable which is allocated and deallocated automatically when program flow enters and leaves the variable's scope. The scope is the lexical context, particularly the function or block in which a variable is defined. Local data is typically (in most languages) invisible outside the function or lexical context where it is defined. Local data is also invisible and inaccessible to a ''called'' function,unless it is a nested function, which itself is ''defined'' along that local data but is not deallocated, coming back in scope as the execution thread returns to the caller. Automatic local variables primarily applies to recursive lexically-scoped languages.although they exist in a somewhat similar, but not identical, form also in recursive languages with dynamic scoping, such as older variants of LISP Automatic local variables are normally allocated in the stack frame of the procedure in which they are declared.unless ot ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Global Variable
In computer programming, a global variable is a variable with global scope, meaning that it is visible (hence accessible) throughout the program, unless shadowed. The set of all global variables is known as the ''global environment'' or ''global state.'' In compiled languages, global variables are generally static variables, whose extent (lifetime) is the entire runtime of the program, though in interpreted languages (including command-line interpreters), global variables are generally dynamically allocated when declared, since they are not known ahead of time. In some languages, all variables are global, or global by default, while in most modern languages variables have limited scope, generally lexical scope, though global variables are often available by declaring a variable at the top level of the program. In other languages, however, global variables do not exist; these are generally modular programming languages that enforce a module structure, or class-based object-or ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Static (keyword)
static is a reserved word in many programming languages to modify a declaration. The effect of the keyword varies depending on the details of the specific programming language, most commonly used to modify the lifetime (as a static variable) and visibility (depending on '' linkage''), or to specify a class member instead of an instance member in classes. Origin In the predecessors of C, including BCPL and B, there was already a concept of static storage., which meant a storage which is always in existence. However, In B, there wasn't a static keyword, but there was an extrn keyword to specify external storage (external to all functions and must be defined outside a function), which is always static, in contrast with auto keyword, which declared an variable with ''auto storage'' - one appears whenever the function is invoked, and disappears when the function is left. All variables must be declared, as one of auto, extrn, or implicitly as function arguments. C was developed ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Function Pointer
A function pointer, also called a subroutine pointer or procedure pointer, is a pointer referencing executable code, rather than data. Dereferencing the function pointer yields the referenced function, which can be invoked and passed arguments just as in a normal function call. Such an invocation is also known as an "indirect" call, because the function is being invoked ''indirectly'' through a variable instead of ''directly'' through a fixed identifier or address. Function pointers allow different code to be executed at runtime. They can also be passed to a function to enable callbacks. Function pointers are supported by third-generation programming languages (such as PL/I, COBOL, Fortran, dBASE dBL, and C) and object-oriented programming languages (such as C++, C#, and D). Simple function pointers The simplest implementation of a function (or subroutine) pointer is as a variable containing the address of the function within executable memory. Older third-generati ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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Mebibyte
The byte is a units of information, unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character (computing), character of text in a computer and for this reason it is the smallest address space, addressable unit of Computer memory, memory in many computer architectures. To disambiguate arbitrarily sized bytes from the common 8-bit computing, 8-bit definition, Computer network, network protocol documents such as the Internet Protocol () refer to an 8-bit byte as an Octet (computing), octet. Those bits in an octet are usually counted with numbering from 0 to 7 or 7 to 0 depending on the bit numbering, bit endianness. The size of the byte has historically been Computer hardware, hardware-dependent and no definitive standards existed that mandated the size. Sizes from 1 to 48 bits have been used. The six-bit character code was an often-used implementation in early encoding systems, and compute ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |