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96-490: The GNU Coding Standards specify exactly how to format most C programming language constructs. Here is a characteristic example: The consistent treatment of blocks as statements (for the purpose of indentation) is a very distinctive feature of the GNU C code formatting style; as is the mandatory space before parentheses. All code formatted in the GNU style has the property that each closing brace, bracket or parenthesis appears to

192-540: A static type system . It was designed to be compiled to provide low-level access to memory and language constructs that map efficiently to machine instructions , all with minimal runtime support . Despite its low-level capabilities, the language was designed to encourage cross-platform programming. A standards -compliant C program written with portability in mind can be compiled for a wide variety of computer platforms and operating systems with few changes to its source code. Since 2000, C has consistently ranked among

288-428: A system call to perform a block I/O write operation, then the system call might execute the following instructions: While the writing takes place, the operating system will context switch to other processes as normal. When the device finishes writing, the device will interrupt the currently running process by asserting an interrupt request . The device will also place an integer onto the data bus. Upon accepting

384-452: A certain platform or with a particular compiler, due, for example, to the use of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the exact size of data types and byte endianness . In cases where code must be compilable by either standard-conforming or K&R C-based compilers, the __STDC__ macro can be used to split the code into Standard and K&R sections to prevent

480-399: A code sample which uses it with a "Coding Horror" icon, symbolizing especially dangerous code, and states that it impedes readability by requiring an extra level of indentation for braces. C (programming language) This is an accepted version of this page C ( pronounced / ˈ s iː / – like the letter c ) is a general-purpose programming language . It was created in

576-645: A computer even if they are not compatible with the base operating system. A library operating system (libOS) is one in which the services that a typical operating system provides, such as networking, are provided in the form of libraries and composed with a single application and configuration code to construct a unikernel : a specialized (only the absolute necessary pieces of code are extracted from libraries and bound together ), single address space , machine image that can be deployed to cloud or embedded environments. The operating system code and application code are not executed in separated protection domains (there

672-585: A development of MULTICS for a single user. Because UNIX's source code was available, it became the basis of other, incompatible operating systems, of which the most successful were AT&T 's System V and the University of California 's Berkeley Software Distribution (BSD). To increase compatibility, the IEEE released the POSIX standard for operating system application programming interfaces (APIs), which

768-484: A large legal settlement was paid. In the twenty-first century, Windows continues to be popular on personal computers but has less market share of servers. UNIX operating systems, especially Linux, are the most popular on enterprise systems and servers but are also used on mobile devices and many other computer systems. On mobile devices, Symbian OS was dominant at first, being usurped by BlackBerry OS (introduced 2002) and iOS for iPhones (from 2007). Later on,

864-442: A library with no protection between applications, such as eCos . A hypervisor is an operating system that runs a virtual machine . The virtual machine is unaware that it is an application and operates as if it had its own hardware. Virtual machines can be paused, saved, and resumed, making them useful for operating systems research, development, and debugging. They also enhance portability by enabling applications to be run on

960-447: A malformed machine instruction . However, the most common error conditions are division by zero and accessing an invalid memory address . Users can send messages to the kernel to modify the behavior of a currently running process. For example, in the command-line environment , pressing the interrupt character (usually Control-C ) might terminate the currently running process. To generate software interrupts for x86 CPUs,

1056-455: A particular application's memory is stored, or even whether or not it has been allocated yet. In modern operating systems, memory which is accessed less frequently can be temporarily stored on a disk or other media to make that space available for use by other programs. This is called swapping , as an area of memory can be used by multiple programs, and what that memory area contains can be swapped or exchanged on demand. Virtual memory provides

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1152-503: A program does not interfere with memory already in use by another program. Since programs time share, each program must have independent access to memory. Cooperative memory management, used by many early operating systems, assumes that all programs make voluntary use of the kernel 's memory manager, and do not exceed their allocated memory. This system of memory management is almost never seen any more, since programs often contain bugs which can cause them to exceed their allocated memory. If

1248-408: A program fails, it may cause memory used by one or more other programs to be affected or overwritten. Malicious programs or viruses may purposefully alter another program's memory, or may affect the operation of the operating system itself. With cooperative memory management, it takes only one misbehaved program to crash the system. Memory protection enables the kernel to limit a process' access to

1344-440: A program tries to access memory that is not accessible memory, but nonetheless has been allocated to it, the kernel is interrupted (see § Memory management ) . This kind of interrupt is typically a page fault . When the kernel detects a page fault it generally adjusts the virtual memory range of the program which triggered it, granting it access to the memory requested. This gives the kernel discretionary power over where

1440-590: A semicolon; as a side effect of the evaluation, functions may be called and variables assigned new values. To modify the normal sequential execution of statements, C provides several control-flow statements identified by reserved keywords. Structured programming is supported by if ... [ else ] conditional execution and by do ... while , while , and for iterative execution (looping). The for statement has separate initialization, testing, and reinitialization expressions, any or all of which can be omitted. break and continue can be used within

1536-470: A significant amount of CPU time. Direct memory access (DMA) is an architecture feature to allow devices to bypass the CPU and access main memory directly. (Separate from the architecture, a device may perform direct memory access to and from main memory either directly or via a bus.) When a computer user types a key on the keyboard, typically the character appears immediately on the screen. Likewise, when

1632-402: A specific moment in time. Hard real-time systems require exact timing and are common in manufacturing , avionics , military, and other similar uses. With soft real-time systems, the occasional missed event is acceptable; this category often includes audio or multimedia systems, as well as smartphones. In order for hard real-time systems be sufficiently exact in their timing, often they are just

1728-417: A user moves a mouse , the cursor immediately moves across the screen. Each keystroke and mouse movement generates an interrupt called Interrupt-driven I/O . An interrupt-driven I/O occurs when a process causes an interrupt for every character or word transmitted. Devices such as hard disk drives , solid-state drives , and magnetic tape drives can transfer data at a rate high enough that interrupting

1824-453: A variation of the classic reader/writer problem . The writer receives a pipe from the shell for its output to be sent to the reader's input stream. The command-line syntax is alpha | bravo . alpha will write to the pipe when its computation is ready and then sleep in the wait queue. bravo will then be moved to the ready queue and soon will read from its input stream. The kernel will generate software interrupts to coordinate

1920-423: A warning message if a local function was called with the wrong number of arguments, or if different calls to an external function used different numbers or types of arguments. Separate tools such as Unix's lint utility were developed that (among other things) could check for consistency of function use across multiple source files. In the years following the publication of K&R C, several features were added to

2016-612: A wide variety of mainframe computers , minicomputers , and microcomputers , including the IBM PC , as its popularity began to increase significantly. In 1983 the American National Standards Institute (ANSI) formed a committee, X3J11, to establish a standard specification of C. X3J11 based the C standard on the Unix implementation; however, the non-portable portion of the Unix C library was handed off to

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2112-562: Is remote direct memory access , which enables each CPU to access memory belonging to other CPUs. Multicomputer operating systems often support remote procedure calls where a CPU can call a procedure on another CPU, or distributed shared memory , in which the operating system uses virtualization to generate shared memory that does not physically exist. A distributed system is a group of distinct, networked computers—each of which might have their own operating system and file system. Unlike multicomputers, they may be dispersed anywhere in

2208-428: Is system software that manages computer hardware and software resources, and provides common services for computer programs . Time-sharing operating systems schedule tasks for efficient use of the system and may also include accounting software for cost allocation of processor time , mass storage , peripherals, and other resources. For hardware functions such as input and output and memory allocation ,

2304-484: Is a change away from the currently running process. Similarly, both hardware and software interrupts execute an interrupt service routine . Software interrupts may be normally occurring events. It is expected that a time slice will occur, so the kernel will have to perform a context switch . A computer program may set a timer to go off after a few seconds in case too much data causes an algorithm to take too long. Software interrupts may be error conditions, such as

2400-456: Is an informal name for the current major C language standard revision. It was informally known as "C2X" through most of its development. C23 was published in October 2024 as ISO/IEC 9899:2024. The standard macro __STDC_VERSION__ is defined as 202311L to indicate that C23 support is available. C2Y is an informal name for the next major C language standard revision, after C23 (C2X), that

2496-504: Is defined as 201112L to indicate that C11 support is available. C17 is an informal name for ISO/IEC 9899:2018, a standard for the C programming language published in June 2018. It introduces no new language features, only technical corrections, and clarifications to defects in C11. The standard macro __STDC_VERSION__ is defined as 201710L to indicate that C17 support is available. C23

2592-422: Is difficult to define, but has been called "the layer of software that manages a computer's resources for its users and their applications ". Operating systems include the software that is always running, called a kernel —but can include other software as well. The two other types of programs that can run on a computer are system programs —which are associated with the operating system, but may not be part of

2688-524: Is for the most part backward compatible with C90, but is stricter in some ways; in particular, a declaration that lacks a type specifier no longer has int implicitly assumed. A standard macro __STDC_VERSION__ is defined with value 199901L to indicate that C99 support is available. GCC , Solaris Studio , and other C compilers now support many or all of the new features of C99. The C compiler in Microsoft Visual C++ , however, implements

2784-469: Is hoped to be released later in the 2020s decade, hence the '2' in "C2Y". An early working draft of C2Y was released in February 2024 as N3220 by the working group ISO/IEC JTC1/SC22 /WG14. Historically, embedded C programming requires non-standard extensions to the C language to support exotic features such as fixed-point arithmetic , multiple distinct memory banks , and basic I/O operations. In 2008,

2880-497: Is made for programs whose job it is to update system configuration files in /etc . Another exception is made for storing files in a directory when the user has explicitly asked to modify a file in the same directory. The GNU Coding Standards define the issue of portability in this way: portability in the Unix world means 'between Unixes'; in a GNU program this kind of portability is desirable, but not vitally important. According to

2976-622: Is now also referred to as C78 . The second edition of the book covers the later ANSI C standard, described below. K&R introduced several language features: Even after the publication of the 1989 ANSI standard, for many years K&R C was still considered the " lowest common denominator " to which C programmers restricted themselves when maximum portability was desired, since many older compilers were still in use, and because carefully written K&R C code can be legal Standard C as well. In early versions of C, only functions that return types other than int must be declared if used before

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3072-443: Is only a single application running, at least conceptually, so there is no need to prevent interference between applications) and OS services are accessed via simple library calls (potentially inlining them based on compiler thresholds), without the usual overhead of context switches , in a way similarly to embedded and real-time OSes. Note that this overhead is not negligible: to the direct cost of mode switching it's necessary to add

3168-420: Is sometimes called C90. Therefore, the terms "C89" and "C90" refer to the same programming language. ANSI, like other national standards bodies, no longer develops the C standard independently, but defers to the international C standard, maintained by the working group ISO/IEC JTC1/SC22 /WG14. National adoption of an update to the international standard typically occurs within a year of ISO publication. One of

3264-499: Is supported by most UNIX systems. MINIX was a stripped-down version of UNIX, developed in 1987 for educational uses, that inspired the commercially available, free software Linux . Since 2008, MINIX is used in controllers of most Intel microchips , while Linux is widespread in data centers and Android smartphones. The invention of large scale integration enabled the production of personal computers (initially called microcomputers ) from around 1980. For around five years,

3360-473: Is that they do not load user-installed software. Consequently, they do not need protection between different applications, enabling simpler designs. Very small operating systems might run in less than 10 kilobytes , and the smallest are for smart cards . Examples include Embedded Linux , QNX , VxWorks , and the extra-small systems RIOT and TinyOS . A real-time operating system is an operating system that guarantees to process events or data by or at

3456-435: Is the part of the operating system that provides protection between different applications and users. This protection is key to improving reliability by keeping errors isolated to one program, as well as security by limiting the power of malicious software and protecting private data, and ensuring that one program cannot monopolize the computer's resources. Most operating systems have two modes of operation: in user mode ,

3552-542: The CP/M (Control Program for Microcomputers) was the most popular operating system for microcomputers. Later, IBM bought the DOS (Disk Operating System) from Microsoft . After modifications requested by IBM, the resulting system was called MS-DOS (MicroSoft Disk Operating System) and was widely used on IBM microcomputers. Later versions increased their sophistication, in part by borrowing features from UNIX. Apple 's Macintosh

3648-547: The IEEE working group 1003 to become the basis for the 1988 POSIX standard. In 1989, the C standard was ratified as ANSI X3.159-1989 "Programming Language C". This version of the language is often referred to as ANSI C , Standard C, or sometimes C89. In 1990 the ANSI C standard (with formatting changes) was adopted by the International Organization for Standardization (ISO) as ISO/IEC 9899:1990, which

3744-504: The INT assembly language instruction is available. The syntax is INT X , where X is the offset number (in hexadecimal format) to the interrupt vector table . To generate software interrupts in Unix-like operating systems, the kill(pid,signum) system call will send a signal to another process. pid is the process identifier of the receiving process. signum is

3840-498: The personal computer market, as of September 2024 , Microsoft Windows holds a dominant market share of around 73%. macOS by Apple Inc. is in second place (15%), Linux is in third place (5%), and ChromeOS is in fourth place (2%). In the mobile sector (including smartphones and tablets ), as of September 2023 , Android's share is 68.92%, followed by Apple's iOS and iPadOS with 30.42%, and other operating systems with .66%. Linux distributions are dominant in

3936-420: The transistor in the mid-1950s, mainframes began to be built. These still needed professional operators who manually do what a modern operating system would do, such as scheduling programs to run, but mainframes still had rudimentary operating systems such as Fortran Monitor System (FMS) and IBSYS . In the 1960s, IBM introduced the first series of intercompatible computers ( System/360 ). All of them ran

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4032-429: The 1970s by Dennis Ritchie and remains very widely used and influential. By design, C's features cleanly reflect the capabilities of the targeted CPUs. It has found lasting use in operating systems code (especially in 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

4128-483: The C Standards Committee published a technical report extending the C language to address these issues by providing a common standard for all implementations to adhere to. It includes a number of features not available in normal C, such as fixed-point arithmetic, named address spaces, and basic I/O hardware addressing. C has a formal grammar specified by the C standard. Line endings are generally not significant in C; however, line boundaries do have significance during

4224-597: The C89 standard and those parts of C99 that are required for compatibility with C++11 . In addition, the C99 standard requires support for identifiers using Unicode in the form of escaped characters (e.g. \u0040 or \U0001f431 ) and suggests support for raw Unicode names. Work began in 2007 on another revision of the C standard, informally called "C1X" until its official publication of ISO/IEC 9899:2011 on December 8, 2011. The C standards committee adopted guidelines to limit

4320-410: The CPU for every byte or word transferred, and having the CPU transfer the byte or word between the device and memory, would require too much CPU time. Data is, instead, transferred between the device and memory independently of the CPU by hardware such as a channel or a direct memory access controller; an interrupt is delivered only when all the data is transferred. If a computer program executes

4416-474: The CPU to re-enter supervisor mode , placing the kernel in charge. This is called a segmentation violation or Seg-V for short, and since it is both difficult to assign a meaningful result to such an operation, and because it is usually a sign of a misbehaving program, the kernel generally resorts to terminating the offending program, and reports the error. Windows versions 3.1 through ME had some level of memory protection, but programs could easily circumvent

4512-531: The address of the first item in the array. Pass-by-reference is simulated in C by explicitly passing pointers to the thing being referenced. C program source text is free-form code. Semicolons terminate statements , while curly braces are used to group statements into blocks . The C language also exhibits the following characteristics: While C does not include certain features found in other languages (such as object orientation and garbage collection ), these can be implemented or emulated, often through

4608-438: The adoption of new features that had not been tested by existing implementations. The C11 standard adds numerous new features to C and the library, including type generic macros, anonymous structures, improved Unicode support, atomic operations, multi-threading, and bounds-checked functions. It also makes some portions of the existing C99 library optional, and improves compatibility with C++. The standard macro __STDC_VERSION__

4704-428: The aims of the C standardization process was to produce a superset of K&R C, incorporating many of the subsequently introduced unofficial features. The standards committee also included several additional features such as function prototypes (borrowed from C++), void pointers, support for international character sets and locales , and preprocessor enhancements. Although the syntax for parameter declarations

4800-538: The application program, which then interacts with the user and with hardware devices. However, in some systems an application can request that the operating system execute another application within the same process, either as a subroutine or in a separate thread, e.g., the LINK and ATTACH facilities of OS/360 and successors . An interrupt (also known as an abort , exception , fault , signal , or trap ) provides an efficient way for most operating systems to react to

4896-416: The basis for several implementations of C on new platforms. In 1978 Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Language . Known as K&R from the initials of its authors, the book served for many years as an informal specification of the language. The version of C that it describes is commonly referred to as " K&R C ". As this was released in 1978, it

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4992-453: The computer's memory. Various methods of memory protection exist, including memory segmentation and paging . All methods require some level of hardware support (such as the 80286 MMU), which does not exist in all computers. In both segmentation and paging, certain protected mode registers specify to the CPU what memory address it should allow a running program to access. Attempts to access other addresses trigger an interrupt, which causes

5088-471: The details of how interrupt service routines behave vary from operating system to operating system. However, several interrupt functions are common. The architecture and operating system must: A software interrupt is a message to a process that an event has occurred. This contrasts with a hardware interrupt — which is a message to the central processing unit (CPU) that an event has occurred. Software interrupts are similar to hardware interrupts — there

5184-515: The different 32- and 64-bit systems is absolutely necessary. The GNU coding standards are primarily used by GNU projects, though its use is not limited to GNU projects alone. The Linux kernel strongly discourages this style for kernel code, and refers to the style pejoratively: "First off, I’d suggest printing out a copy of the GNU coding standards, and NOT read it. Burn them, it’s a great symbolic gesture.". Steve McConnell , in his book Code Complete , also advises against using this style; he marks

5280-493: The end of each expression statement, and the entry to and return from each function call. Sequence points also occur during evaluation of expressions containing certain operators ( && , || , ?: and the comma operator ). This permits a high degree of object code optimization by the compiler, but requires C programmers to take more care to obtain reliable results than is needed for other programming languages. Operating system An operating system ( OS )

5376-422: The environment. Interrupts cause the central processing unit (CPU) to have a control flow change away from the currently running program to an interrupt handler , also known as an interrupt service routine (ISR). An interrupt service routine may cause the central processing unit (CPU) to have a context switch . The details of how a computer processes an interrupt vary from architecture to architecture, and

5472-600: The features of the more-powerful PDP-11. A significant addition was a character data type. He called this New B (NB). Thompson started to use NB to write the Unix kernel, and his requirements shaped the direction of the language development. Through to 1972, richer types were added to the NB language: NB had arrays of int and char . Pointers, the ability to generate pointers to other types, arrays of all types, and types to be returned from functions were all also added. Arrays within expressions became pointers. A new compiler

5568-477: The function definition; functions used without prior declaration were presumed to return type int . For example: The int type specifiers which are commented out could be omitted in K&;R C, but are required in later standards. Since K&R function declarations did not include any information about function arguments, function parameter type checks were not performed, although some compilers would issue

5664-410: The hardware checks that the software is only executing legal instructions, whereas the kernel has unrestricted powers and is not subject to these checks. The kernel also manages memory for other processes and controls access to input/output devices. The operating system provides an interface between an application program and the computer hardware, so that an application program can interact with

5760-493: The hardware only by obeying rules and procedures programmed into the operating system. The operating system is also a set of services which simplify development and execution of application programs. Executing an application program typically involves the creation of a process by the operating system kernel , which assigns memory space and other resources, establishes a priority for the process in multi-tasking systems, loads program binary code into memory, and initiates execution of

5856-644: The importance of English-language comments : Please write the comments in a GNU program in English, because English is the one language that nearly all programmers in all countries can read. If you do not write English well, please write comments in English as well as you can, then ask other people to help rewrite them. If you can't write comments in English, please find someone to work with you and translate your comments into English. Comments should consist of complete, capitalized sentences, each followed by two spaces (so that Emacs can tell where one sentence ends and

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5952-418: The indirect pollution of important processor structures (like CPU caches , the instruction pipeline , and so on) which affects both user-mode and kernel-mode performance. The first computers in the late 1940s and 1950s were directly programmed either with plugboards or with machine code inputted on media such as punch cards , without programming languages or operating systems. After the introduction of

6048-404: The interrupt request, the operating system will: When the writing process has its time slice expired, the operating system will: With the program counter now reset, the interrupted process will resume its time slice. Among other things, a multiprogramming operating system kernel must be responsible for managing all system memory which is currently in use by the programs. This ensures that

6144-431: The kernel—and applications—all other software. There are three main purposes that an operating system fulfills: With multiprocessors multiple CPUs share memory. A multicomputer or cluster computer has multiple CPUs, each of which has its own memory . Multicomputers were developed because large multiprocessors are difficult to engineer and prohibitively expensive; they are universal in cloud computing because of

6240-438: The language, supported by compilers from AT&T (in particular PCC ) and some other vendors. These included: The large number of extensions and lack of agreement on a standard library , together with the language popularity and the fact that not even the Unix compilers precisely implemented the K&R specification, led to the necessity of standardization. During the late 1970s and 1980s, versions of C were implemented for

6336-413: The loop. Break is used to leave the innermost enclosing loop statement and continue is used to skip to its reinitialisation. There is also a non-structured goto statement which branches directly to the designated label within the function. switch selects a case to be executed based on the value of an integer expression. Different from many other languages, control-flow will fall through to

6432-400: The memory allocated to a different one. Around the same time, teleprinters began to be used as terminals so multiple users could access the computer simultaneously. The operating system MULTICS was intended to allow hundreds of users to access a large computer. Despite its limited adoption, it can be considered the precursor to cloud computing . The UNIX operating system originated as

6528-408: The need to use it. A general protection fault would be produced, indicating a segmentation violation had occurred; however, the system would often crash anyway. The use of virtual memory addressing (such as paging or segmentation) means that the kernel can choose what memory each program may use at any given time, allowing the operating system to use the same memory locations for multiple tasks. If

6624-411: The next case unless terminated by a break . Expressions can use a variety of built-in operators and may contain function calls. The order in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even be interleaved. However, all side effects (including storage to variables) will occur before the next " sequence point "; sequence points include

6720-491: The next begins). For long or complex preprocessor conditionals, every #else and #endif should have a comment explaining the condition for the code below (for #else ) or above (for #endif ). The standards require that all programs be able to operate when /usr and /etc are mounted read-only. Therefore, files that are modified for internal purposes (log files, lock files, temporary files, etc.) should not be stored in either /usr or /etc . An exception

6816-408: The open-source Android operating system (introduced 2008), with a Linux kernel and a C library ( Bionic ) partially based on BSD code, became most popular. The components of an operating system are designed to ensure that various parts of a computer function cohesively. With the de facto obsoletion of DOS , all user software must interact with the operating system to access hardware. The kernel

6912-420: The operating system acts as an intermediary between programs and the computer hardware, although the application code is usually executed directly by the hardware and frequently makes system calls to an OS function or is interrupted by it. Operating systems are found on many devices that contain a computer – from cellular phones and video game consoles to web servers and supercomputers . In

7008-419: The operating system to a PDP-11 . The original PDP-11 version of Unix was also developed in assembly language. Thompson wanted a programming language for developing utilities for the new platform. He first tried writing a Fortran compiler, but he soon gave up the idea and instead created a cut-down version of the recently developed systems programming language called BCPL . The official description of BCPL

7104-567: The original language designer, served for many years as the de facto standard for the language. C has been standardized since 1989 by the American National Standards Institute (ANSI) and, subsequently, jointly by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). C is an imperative procedural language, supporting structured programming , lexical variable scope , and recursion , with

7200-421: The piping. Signals may be classified into 7 categories. The categories are: Input/output (I/O) devices are slower than the CPU. Therefore, it would slow down the computer if the CPU had to wait for each I/O to finish. Instead, a computer may implement interrupts for I/O completion, avoiding the need for polling or busy waiting. Some computers require an interrupt for each character or word, costing

7296-652: The preprocessing phase. Comments may appear either between the delimiters /* and */ , or (since C99) following // until the end of the line. Comments delimited by /* and */ do not nest, and these sequences of characters are not interpreted as comment delimiters if they appear inside string or character literals. C source files contain declarations and function definitions. Function definitions, in turn, contain declarations and statements . Declarations either define new types using keywords such as struct , union , and enum , or assign types to and perhaps reserve storage for new variables, usually by writing

7392-408: The recognizable expression and statement syntax of C with underlying type systems, data models, and semantics that can be radically different. The origin of C is closely tied to the development of the Unix operating system, originally implemented in assembly language on a PDP-7 by Dennis Ritchie and Ken Thompson , incorporating several ideas from colleagues. Eventually, they decided to port

7488-596: The right of its corresponding opening delimiter, or in the same column. Being tightly integrated with the GNU system, GNU Emacs provides automatic formatting of C code to match the GNU coding standards. Rather than manually modifying code formatting in a way that strays from the GNU coding standards, the formatted layout of the code can be tweaked by writing it in a more Emacs-friendly form—for example, by inserting additional parentheses. "When you split an expression into multiple lines, split it before an operator, not after one." For example: The standards greatly emphasise

7584-418: The same operating system— OS/360 —which consisted of millions of lines of assembly language that had thousands of bugs . The OS/360 also was the first popular operating system to support multiprogramming , such that the CPU could be put to use on one job while another was waiting on input/output (I/O). Holding multiple jobs in memory necessitated memory partitioning and safeguards against one job accessing

7680-619: The server and supercomputing sectors. Other specialized classes of operating systems (special-purpose operating systems), such as embedded and real-time systems, exist for many applications. Security-focused operating systems also exist. Some operating systems have low system requirements (e.g. light-weight Linux distribution ). Others may have higher system requirements. Some operating systems require installation or may come pre-installed with purchased computers ( OEM -installation), whereas others may run directly from media (i.e. live CD ) or flash memory (i.e. USB stick). An operating system

7776-400: The signal number (in mnemonic format) to be sent. (The abrasive name of kill was chosen because early implementations only terminated the process.) In Unix-like operating systems, signals inform processes of the occurrence of asynchronous events. To communicate asynchronously, interrupts are required. One reason a process needs to asynchronously communicate to another process solves

7872-400: The size of the machine needed. The different CPUs often need to send and receive messages to each other; to ensure good performance, the operating systems for these machines need to minimize this copying of packets . Newer systems are often multiqueue —separating groups of users into separate queues —to reduce the need for packet copying and support more concurrent users. Another technique

7968-571: The smallest microcontrollers and embedded systems . A successor to the 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 languages, with C compilers available for practically all modern computer architectures and operating systems. The book The C Programming Language , co-authored by

8064-480: The standard, portability problems are very limited as GNU programs are designed to be compiled with one compiler, the GNU C Compiler , and only run on one system, which is the GNU system. There is one form of portability problem though, and that is the fact that the standard makes it clear that a program should run on different CPU types. The standard says that GNU doesn't and won't support 16-bit systems, but handling all

8160-537: The top four languages in the TIOBE index , a measure of the popularity of programming languages. C is an imperative , procedural language in the ALGOL tradition. It has a static type system . In C, all executable code is contained within subroutines (also called "functions", though not in the sense of functional programming ). Function parameters are passed by value, although arrays are passed as pointers , i.e.

8256-468: The type followed by the variable name. Keywords such as char and int specify built-in types. Sections of code are enclosed in braces ( { and } , sometimes called "curly brackets") to limit the scope of declarations and to act as a single statement for control structures. As an imperative language, C uses statements to specify actions. The most common statement is an expression statement , consisting of an expression to be evaluated, followed by

8352-476: The urging of Alan Snyder and also in recognition of the usefulness of the file-inclusion mechanisms available in BCPL and PL/I . Its original version provided only included files and simple string replacements: #include and #define of parameterless macros. Soon after that, it was extended, mostly by Mike Lesk and then by John Reiser, to incorporate macros with arguments and conditional compilation . Unix

8448-715: The use of external libraries (e.g., the GLib Object System or the Boehm garbage collector ). Many later languages have borrowed directly or indirectly from C, including C++ , C# , Unix's C shell , D , Go , Java , JavaScript (including transpilers ), Julia , Limbo , LPC , Objective-C , Perl , PHP , Python , Ruby , Rust , Swift , Verilog and SystemVerilog (hardware description languages). These languages have drawn many of their control structures and other basic features from C. Most of them also express highly similar syntax to C, and they tend to combine

8544-541: The use on a K&R C-based compiler of features available only in Standard C. After the ANSI/ISO standardization process, the C language specification remained relatively static for several years. In 1995, Normative Amendment 1 to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally as C95) was published, to correct some details and to add more extensive support for international character sets. The C standard

8640-473: The world. Middleware , an additional software layer between the operating system and applications, is often used to improve consistency. Although it functions similarly to an operating system, it is not a true operating system. Embedded operating systems are designed to be used in embedded computer systems , whether they are internet of things objects or not connected to a network. Embedded systems include many household appliances. The distinguishing factor

8736-513: Was augmented to include the style used in C++, the K&R interface continued to be permitted, for compatibility with existing source code. C89 is supported by current C compilers, and most modern C code is based on it. Any program written only in Standard C and without any hardware-dependent assumptions will run correctly on any platform with a conforming C implementation, within its resource limits. Without such precautions, programs may compile only on

8832-706: Was further revised in the late 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999, which is commonly referred to as " C99 ". It has since been amended three times by Technical Corrigenda. C99 introduced several new features, including inline functions , several new data types (including long long int and a complex type to represent complex numbers ), variable-length arrays and flexible array members , improved support for IEEE 754 floating point, support for variadic macros (macros of variable arity ), and support for one-line comments beginning with // , as in BCPL or C++. Many of these had already been implemented as extensions in several C compilers. C99

8928-511: Was not available at the time, and Thompson modified the syntax to be less 'wordy' and similar to a simplified ALGOL known as SMALGOL. He called the result B , describing it as "BCPL semantics with a lot of SMALGOL syntax". Like BCPL, B had a bootstrapping compiler to facilitate porting to new machines. Ultimately, few utilities were written in B because it was too slow and could not take advantage of PDP-11 features such as byte addressability. In 1971 Ritchie started to improve B, to use

9024-628: Was one of the first operating system kernels implemented in a language other than assembly . Earlier instances include the Multics system (which was written in PL/I ) and Master Control Program (MCP) for the Burroughs B5000 (which was written in ALGOL ) in 1961. In around 1977, Ritchie and Stephen C. Johnson made further changes to the language to facilitate portability of the Unix operating system. Johnson's Portable C Compiler served as

9120-406: Was the first popular computer to use a graphical user interface (GUI). The GUI proved much more user friendly than the text-only command-line interface earlier operating systems had used. Following the success of Macintosh, MS-DOS was updated with a GUI overlay called Windows . Windows later was rewritten as a stand-alone operating system, borrowing so many features from another ( VAX VMS ) that

9216-530: Was written, and the language was renamed C. The C compiler and some utilities made with it were included in Version 2 Unix , which is also known as Research Unix . At Version 4 Unix , released in November 1973, the Unix kernel was extensively re-implemented in C. By this time, the C language had acquired some powerful features such as struct types. The preprocessor was introduced around 1973 at

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