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89-552: The Linux API includes the kernel–user space API, which allows code in user space to access system resources and services of the Linux kernel. It is composed of the system call interface of the Linux kernel and the subroutines in the C standard library . The focus of the development of the Linux API has been to provide the usable features of the specifications defined in POSIX in a way which

178-400: A word mark set at their high-order (lowest-addressed) position. When an operation such as addition is performed, the processor begins at the low-order positions at the high addresses of the two fields and works its way down to the high-order. Another important attribute of a byte being part of a "field" is its "significance". These attributes of the parts of a field play an important role in

267-474: A 32-bit base address of the segment stored in little-endian order, but in four nonconsecutive bytes, at relative positions 2, 3, 4 and 7 of the descriptor start. Hardware description languages (HDLs) used to express digital logic often support arbitrary endianness, with arbitrary granularity. For example, in SystemVerilog , a word can be defined as little-endian or big-endian. The recognition of endianness

356-448: A C library in shared library form, but the header files (and compiler toolchain) may be absent from an installation so C development may not be possible. The C library is considered part of the operating system on Unix-like systems; in addition to functions specified by the C standard, it includes other functions that are part of the operating system API, such as functions specified in the POSIX standard. The C library functions, including

445-485: A C library, or linked to a dynamic version of the library that is shipped with these applications, rather than relied upon to be present on the targeted systems. Functions in a compiler's C library are not regarded as interfaces to Microsoft Windows. Many C library implementations exist, provided with both various operating systems and C compilers. Some of the popular implementations are the following: Some compilers (for example, GCC ) provide built-in versions of many of

534-585: A consecutive sequence of bytes and represents a "simple data value" which – at least potentially – can be manipulated by one single hardware instruction . On most systems, the address of a multi-byte simple data value is the address of its first byte (the byte with the lowest address). There are exceptions to this rule – for example, the Add instruction of the IBM 1401 addresses variable-length fields at their low-order (highest-addressed) position with their lengths being defined by

623-486: A kernel. In the Linux kernel, various subsystems, such as the Direct Rendering Manager (DRM), define their own system calls, all of which are part of the system call interface. Various issues with the organization of the Linux kernel system calls are being publicly discussed. Issues have been pointed out by Andy Lutomirski, Michael Kerrisk and others. A C standard library for Linux includes wrappers around

712-472: A little-endian should start with FF FE 00 00 . Application binary data formats, such as MATLAB .mat files, or the .bil data format, used in topography, are usually endianness-independent. This is achieved by storing the data always in one fixed endianness or carrying with the data a switch to indicate the endianness. An example of the former is the binary XLS file format that is portable between Windows and Mac systems and always little-endian, requiring

801-547: A number of hardware architectures where floating-point numbers are represented in big-endian form while integers are represented in little-endian form. There are ARM processors that have mixed-endian floating-point representation for double-precision numbers: each of the two 32-bit words is stored as little-endian, but the most significant word is stored first. VAX floating point stores little-endian 16-bit words in big-endian order. Because there have been many floating-point formats with no network standard representation for them,

890-437: A number of routines that should be available over and above those in the basic C standard library. The POSIX specification includes header files for, among other uses, multi-threading , networking , and regular expressions . These are often implemented alongside the C standard library functionality, with varying degrees of closeness. For example, glibc implements functions such as fork within libc.so , but before NPTL

979-421: A number system, the value of a digit which it contributes to the whole number is determined not only by its value as a single digit, but also by the position it holds in the complete number, called its significance. These positions can be mapped to memory mainly in two ways: In these expressions, the term "end" is meant as the extremity where the big resp. little significance is written first , namely where

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1068-483: A processor treats data accesses. Instruction accesses (fetches of instruction words) on a given processor may still assume a fixed endianness, even if data accesses are fully bi-endian, though this is not always the case, such as on Intel's IA-64 -based Itanium CPU, which allows both. Some nominally bi-endian CPUs require motherboard help to fully switch endianness. For instance, the 32-bit desktop-oriented PowerPC processors in little-endian mode act as little-endian from

1157-624: A revision to the C Standard published in 1999, and five more files ( stdalign.h , stdatomic.h , stdnoreturn.h , threads.h , and uchar.h ) with C11 in 2011. In total, there are now 29 header files: Three of the header files ( complex.h , stdatomic.h , and threads.h ) are conditional features that implementations are not required to support. The POSIX standard added several nonstandard C headers for Unix-specific functionality. Many have found their way to other architectures. Examples include fcntl.h and unistd.h . A number of other groups are using other nonstandard headers –

1246-436: A setting which allows for switchable endianness in data fetches and stores, instruction fetches, or both; those instruction set architectures are referred to as bi-endian . Architectures that support switchable endianness include PowerPC / Power ISA , SPARC V9, ARM versions 3 and above, DEC Alpha , MIPS , Intel i860 , PA-RISC , SuperH SH-4 , IA-64 , C-Sky , and RISC-V . This feature can improve performance or simplify

1335-475: A similar approach, placing C compatibility functions/routines under a common namespace; these include D , Perl , and Ruby . CPython includes wrappers for some of the C library functions in its own common library, and it also grants more direct access to C functions and variables via its ctypes package. More generally, Python   2. x specifies the built-in file objects as being “implemented using C's stdio package ,” and frequent reference

1424-406: A single byte, so the complexity of the hardware is not affected by the byte ordering. Addition, subtraction, and multiplication start at the least significant digit position and propagate the carry to the subsequent more significant position. On most systems, the address of a multi-byte value is the address of its first byte (the byte with the lowest address). The implementation of these operations

1513-442: A single libc library and providing an empty libm. According to the C standard the macro __STDC_HOSTED__ shall be defined to 1 if the implementation is hosted. A hosted implementation has all the headers specified by the C standard. An implementation can also be freestanding which means that these headers will not be present. If an implementation is freestanding , it shall define __STDC_HOSTED__ to 0 . Some functions in

1602-491: A word in a register to the opposite endianness, that is, they swap the order of the bytes in a 16-, 32- or 64-bit word. Recent Intel x86 and x86-64 architecture CPUs have a MOVBE instruction ( Intel Core since generation 4, after Atom ), which fetches a big-endian format word from memory or writes a word into memory in big-endian format. These processors are otherwise thoroughly little-endian. There are also devices which use different formats in different places. For instance,

1691-504: Is a feature supported by numerous computer architectures that feature switchable endianness in data fetches and stores or for instruction fetches. Other orderings are generically called middle-endian or mixed-endian . Big-endianness is the dominant ordering in networking protocols, such as in the Internet protocol suite , where it is referred to as network order , transmitting the most significant byte first. Conversely, little-endianness

1780-421: Is accessed first for division and comparison . See § Calculation order . When character (text) strings are to be compared with one another, e.g. in order to support some mechanism like sorting , this is very frequently done lexicographically where a single positional element (character) also has a positional value. Lexicographical comparison means almost everywhere: first character ranks highest – as in

1869-430: Is called a byte. Larger groups comprise two or more bytes, for example, a 32-bit word contains four bytes. There are two possible ways a computer could number the individual bytes in a larger group, starting at either end. Both types of endianness are in widespread use in digital electronic engineering. The initial choice of endianness of a new design is often arbitrary, but later technology revisions and updates perpetuate

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1958-696: Is criticized for being thread unsafe and otherwise vulnerable to race conditions . The error handling of the functions in the C standard library is not consistent and sometimes confusing. According to the Linux manual page math_error , "The current (version 2.8) situation under glibc is messy. Most (but not all) functions raise exceptions on errors. Some also set errno . A few functions set errno , but do not raise an exception. A very few functions do neither." The original C language provided no built-in functions such as I/O operations, unlike traditional languages such as COBOL and Fortran . Over time, user communities of C shared ideas and implementations of what

2047-405: Is identified and accessed in hardware and software by its memory address . If the total number of bytes in memory is n , then addresses are enumerated from 0 to n  − 1. Computer programs often use data structures or fields that may consist of more data than can be stored in one byte. In the context of this article where its type cannot be arbitrarily complicated, a "field" consists of

2136-406: Is important when reading a file or filesystem created on a computer with different endianness. Fortran sequential unformatted files created with one endianness usually cannot be read on a system using the other endianness because Fortran usually implements a record (defined as the data written by a single Fortran statement) as data preceded and succeeded by count fields, which are integers equal to

2225-476: Is in principle a 16-bit little-endian system. The instructions to convert between floating-point and integer values in the optional floating-point processor of the PDP-11/45, PDP-11/70, and in some later processors, stored 32-bit "double precision integer long" values with the 16-bit halves swapped from the expected little-endian order. The UNIX C compiler used the same format for 32-bit long integers. This ordering

2314-534: Is known as PDP-endian . UNIX was one of the first systems to allow the same code to be compiled for platforms with different internal representations. One of the first programs converted was supposed to print out Unix , but on the Series/1 it printed nUxi instead. A way to interpret this endianness is that it stores a 32-bit integer as two little-endian 16-bit words, with a big-endian word ordering: Segment descriptors of IA-32 and compatible processors keep

2403-482: Is made to C standard library behaviors; the available operations ( open , read , write , etc.) are expected to have the same behavior as the corresponding C functions ( fopen , fread , fwrite , etc.). Python 3 ’s specification relies considerably less on C specifics than Python 2 , however. Rust offers crate libc , which allows various C standard (and other) library functions and type definitions to be used. The C standard library

2492-434: Is marginally simpler using little-endian machines where this first byte contains the least significant digit. Comparison and division start at the most significant digit and propagate a possible carry to the subsequent less significant digits. For fixed-length numerical values (typically of length 1,2,4,8,16), the implementation of these operations is marginally simpler on big-endian machines. Some big-endian processors (e.g.

2581-412: Is no guarantee for stability. A kernel-internal API can be changed when such a need is indicated by new research or insights; all necessary modifications and testing have to be done by the author. The Linux kernel is a monolithic kernel, hence device drivers are kernel components. To ease the burden of companies maintaining their (proprietary) device drivers outside of the main kernel tree, stable APIs for

2670-412: Is now called C standard libraries. Many of these ideas were incorporated eventually into the definition of the standardized C language. Both Unix and C were created at AT&T's Bell Laboratories in the late 1960s and early 1970s. During the 1970s the C language became increasingly popular. Many universities and organizations began creating their own variants of the language for their own projects. By

2759-427: Is primarily expressed as big-endian (BE) or little-endian (LE), terms introduced by Danny Cohen into computer science for data ordering in an Internet Experiment Note published in 1980. The adjective endian has its origin in the writings of 18th century Anglo-Irish writer Jonathan Swift . In the 1726 novel Gulliver's Travels , he portrays the conflict between sects of Lilliputians divided into those breaking

Linux kernel interfaces - Misplaced Pages Continue

2848-535: Is reasonably compatible, robust and performant, and to provide additional useful features not defined in POSIX, just as the kernel–user space APIs of other systems implementing the POSIX API also provide additional features not defined in POSIX. The Linux API, by choice, has been kept stable over the decades through a policy of not introducing breaking changes; this stability guarantees the portability of source code . At

2937-442: Is redirected to the corresponding address and unaligned access is not allowed. ARMv6 introduces BE-8 or byte-invariant mode, where access to a single byte works as in little-endian mode, but accessing a 16-bit, 32-bit or (starting with ARMv8) 64-bit word results in a byte swap of the data. This simplifies unaligned memory access as well as memory-mapped access to registers other than 32-bit. Many processors have instructions to convert

3026-513: Is small compared to the standard libraries of some other languages. The C library provides a basic set of mathematical functions, string manipulation, type conversions , and file and console-based I/O. It does not include a standard set of " container types " like the C++ Standard Template Library , let alone the complete graphical user interface (GUI) toolkits, networking tools, and profusion of other functionality that Java and

3115-420: Is subtly incompatible with both TR 24731-1 and Annex K, so it’s common for portable projects to disable or ignore these warnings. They can be disabled directly by issuing before/around the call site[s] in question, or indirectly by issuing before including any headers. Command-line option /D_CRT_NO_SECURE_WARNINGS=1 should have the same effect as this #define .) The strerror () routine

3204-475: Is the dominant ordering for processor architectures ( x86 , most ARM implementations, base RISC-V implementations) and their associated memory. File formats can use either ordering; some formats use a mixture of both or contain an indicator of which ordering is used throughout the file. Computer memory consists of a sequence of storage cells (smallest addressable units); in machines that support byte addressing , those units are called bytes . Each byte

3293-543: The .NET Framework provide as standard. The main advantage of the small standard library is that providing a working ISO C environment is much easier than it is with other languages, and consequently porting C to a new platform is comparatively easy. Endianness In computing , endianness is the order in which bytes within a word of digital data are transmitted over a data communication medium or addressed (by rising addresses) in computer memory , counting only byte significance compared to earliness. Endianness

3382-524: The 6809 and the 68000 series of processors use the big-endian format. Solely big-endian architectures include the IBM z/Architecture and OpenRISC . The PDP-11 minicomputer, however, uses little-endian byte order, as does its VAX successor. The Datapoint 2200 used simple bit-serial logic with little-endian to facilitate carry propagation . When Intel developed the 8008 microprocessor for Datapoint, they used little-endian for compatibility. However, as Intel

3471-652: The Altera Nios II , the Atmel AVR , the Andes Technology NDS32, the Qualcomm Hexagon , and many other processors and processor families are also little-endian. The Intel 8051 , unlike other Intel processors, expects 16-bit addresses for LJMP and LCALL in big-endian format; however, xCALL instructions store the return address onto the stack in little-endian format. Some instruction set architectures feature

3560-645: The Cray T3E ). IBM AIX and IBM i run in big-endian mode on bi-endian Power ISA; Linux originally ran in big-endian mode, but by 2019, IBM had transitioned to little-endian mode for Linux to ease the porting of Linux software from x86 to Power. SPARC has no relevant little-endian deployment, as both Oracle Solaris and Linux run in big-endian mode on bi-endian SPARC systems, and can be considered big-endian in practice. ARM, C-Sky, and RISC-V have no relevant big-endian deployments, and can be considered little-endian in practice. The term bi-endian refers primarily to how

3649-560: The GNU C Library has alloca.h , and OpenVMS has the va_count() function. On Unix-like systems, the authoritative documentation of the API is provided in the form of man pages . On most systems, man pages on standard library functions are in section 3; section 7 may contain some more generic pages on underlying concepts (e.g. man 7 math_error in Linux ). Unix-like systems typically have

Linux kernel interfaces - Misplaced Pages Continue

3738-456: The Intel Fortran compiler supports the non-standard CONVERT specifier when opening a file, e.g.: OPEN ( unit , CONVERT = 'BIG_ENDIAN' ,...) . Other compilers have options for generating code that globally enables the conversion for all file IO operations. This permits the reuse of code on a system with the opposite endianness without code modification. On most systems,

3827-451: The XDR standard uses big-endian IEEE 754 as its representation. It may therefore appear strange that the widespread IEEE 754 floating-point standard does not specify endianness. Theoretically, this means that even standard IEEE floating-point data written by one machine might not be readable by another. However, on modern standard computers (i.e., implementing IEEE 754), one may safely assume that

3916-411: The control-flow characteristics of the built-in variants), but may cause confusion when debugging (for example, the built-in versions cannot be replaced with instrumented variants). However, the built-in functions must behave like ordinary functions in accordance with ISO C. The main implication is that the program must be able to create a pointer to these functions by taking their address, and invoke

4005-481: The endianness , if both are supported. It should be able to compile the software with different compilers against the definitions specified in the ABI and achieve full binary compatibility. Compilers that are free and open-source software are e.g. GNU Compiler Collection , LLVM / Clang . Many kernel-internal APIs exist, allowing kernel subsystems to interface with one another. These are being kept fairly stable, but there

4094-400: The 2D drivers would be available in the X.Org Server . DRM was developed for Linux, and since has been ported to other operating systems as well. The term Linux ABI refers to a kernel–user space ABI. The application binary interface refers to the compiled binaries, in machine code . Any such ABI is therefore bound to the instruction set . Defining a useful ABI and keeping it stable is less

4183-659: The BQ27421 Texas Instruments battery gauge uses the little-endian format for its registers and the big-endian format for its random-access memory . SPARC historically used big-endian until version 9, which is bi-endian. Similarly early IBM POWER processors were big-endian, but the PowerPC and Power ISA descendants are now bi-endian. The ARM architecture was little-endian before version 3 when it became bi-endian. Although many processors use little-endian storage for all types of data (integer, floating point), there are

4272-401: The C standard library have been notorious for having buffer overflow vulnerabilities and generally encouraging buggy programming ever since their adoption. The most criticized items are: Except the extreme case with gets() , all the security vulnerabilities can be avoided by introducing auxiliary code to perform memory management, bounds checking, input checking, etc. This is often done in

4361-496: The C standard library is declared in a number of header files . Each header file contains one or more function declarations, data type definitions, and macros. After a long period of stability, three new header files ( iso646.h , wchar.h , and wctype.h ) were added with Normative Addendum 1 (NA1), an addition to the C Standard ratified in 1995. Six more header files ( complex.h , fenv.h , inttypes.h , stdbool.h , stdint.h , and tgmath.h ) were added with C99 ,

4450-678: The C++98 bool , false , and true keywords in C. C++11 requires <stdbool.h> and <cstdbool> for compatibility, but all they need to define is __bool_true_false_are_defined . C23 obsoletes older _Bool keyword in favor of new, C++98-equivalent bool , false , and true keywords, so the C≥23 and C++≥11 <stdbool.h> / <cstdbool> headers are fully equivalent. (In particular, C23 doesn’t require any __STDC_VERSION_BOOL_H__ macro for <stdbool.h> .) Access to C library functions via namespace ::std and

4539-498: The C++≥98 header names is preferred where possible. To encourage adoption, C++98 obsoletes the C ( *.h ) header names, so it’s possible that use of C compatibility headers will cause an especially strict C++98– 20 preprocessor to raise a diagnostic of some sort. However, C++23 (unusually) de- obsoletes these headers, so newer C++ implementations/modes shouldn’t complain without being asked to specifically. Other languages take

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4628-492: The C11 standard and commonly used in code interacting with hardware. Some operations in positional number systems have a natural or preferred order in which the elementary steps are to be executed. This order may affect their performance on small-scale byte-addressable processors and microcontrollers . However, high-performance processors usually fetch multi-byte operands from memory in the same amount of time they would have fetched

4717-713: The IBM System/360 and its successors) contain hardware instructions for lexicographically comparing varying length character strings . The normal data transport by an assignment statement is in principle independent of the endianness of the processor. Many historical and extant processors use a big-endian memory representation, either exclusively or as a design option. The IBM System/360 uses big-endian byte order, as do its successors System/370 , ESA/390 , and z/Architecture . The PDP-10 uses big-endian addressing for byte-oriented instructions. The IBM Series/1 minicomputer uses big-endian byte order. The Motorola 6800 / 6801,

4806-458: The ISO C standard ones, are widely used by programs, and are regarded as if they were not only an implementation of something in the C language, but also de facto part of the operating system interface. Unix-like operating systems generally cannot function if the C library is erased. This is true for applications which are dynamically as opposed to statically linked. Further, the kernel itself (at least in

4895-445: The Linux API is considered too low-level, so APIs of higher abstraction must be used. Higher-level APIs must be implemeted on top of lower-level APIs. Examples: C standard library The C standard library , sometimes referred to as libc , is the standard library for the C programming language , as specified in the ISO C standard. Starting from the original ANSI C standard, it

4984-431: The Linux kernel's user-space API, describing that it contains multiple design errors by being non-extensible, unmaintainable, overly complex, of limited purpose, in violation of standards, and inconsistent. Most of those mistakes cannot be fixed because doing so would break the ABI that the kernel presents to the user space. The system call interface of a kernel is the set of all implemented and available system calls in

5073-920: The above two is: A using namespace :: std declaration above or within main can be issued to apply the ::std:: prefix automatically, although it’s generally considered poor practice to use it globally in headers because it pollutes the global namespace. A few of the C++≥98 versions of C’s headers are missing; e.g., C≥11 <stdnoreturn.h> and <threads.h> have no C++ counterparts. Others are reduced to placeholders, such as (until C++20 ) <ciso646> for C95 <iso646.h> , all of whose requisite macros are rendered as keywords in C++98. C-specific syntactic constructs aren’t generally supported, even if their header is. Several C headers exist primarily for C++ compatibility, and these tend to be near-empty in C++. For example, C99 – 17 <stdbool.h> require only in order to feign support for

5162-421: The address of a multi-byte value is the address of its first byte (the byte with the lowest address); little-endian systems of that type have the property that, for sufficiently low data values, the same value can be read from memory at different lengths without using different addresses (even when alignment restrictions are imposed). For example, a 32-bit memory location with content 4A 00 00 00 can be read at

5251-586: The beginning of the 1980s compatibility problems between the various C implementations became apparent. In 1983 the American National Standards Institute (ANSI) formed a committee to establish a standard specification of C known as " ANSI C ". This work culminated in the creation of the so-called C89 standard in 1989. Part of the resulting standard was a set of software libraries called the ANSI C standard library. POSIX , as well as SUS , specify

5340-591: The case of Linux) operates independently of any libraries. On Microsoft Windows, the core system dynamic libraries ( DLLs ) provide an implementation of the C standard library for the Microsoft Visual ;C++ compiler v6.0; the C standard library for newer versions of the Microsoft Visual ;C++ compiler is provided by each compiler individually, as well as redistributable packages. Compiled applications written in C are either statically linked with

5429-518: The device drivers have been repeatedly requested. The Linux kernel developers have repeatedly denied guaranteeing stable in-kernel APIs for device drivers. Guaranteeing such would have faltered the development of the Linux kernel in the past and would still in the future and, due to the nature of free and open-source software, are not necessary. Ergo, by choice, the Linux kernel has no stable in-kernel API. Since there are no stable in-kernel APIs, there cannot be stable in-kernel ABIs. For many use cases,

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5518-446: The endianness is the same for floating-point numbers as for integers, making the conversion straightforward regardless of data type. Small embedded systems using special floating-point formats may be another matter, however. Most instructions considered so far contain the size (lengths) of their operands within the operation code . Frequently available operand lengths are 1, 2, 4, 8, or 16 bytes. But there are also architectures where

5607-510: The existing endianness to maintain backward compatibility . A big-endian system stores the most significant byte of a word at the smallest memory address and the least significant byte at the largest. A little-endian system, in contrast, stores the least-significant byte at the smallest address. Of the two, big-endian is thus closer to the way the digits of numbers are written left-to-right in English, comparing digits to bytes. Bi-endianness

5696-457: The field starts . The integer data that are directly supported by the computer hardware have a fixed width of a low power of 2, e.g. 8 bits ≙ 1 byte, 16 bits ≙ 2 bytes, 32 bits ≙ 4 bytes, 64 bits ≙ 8 bytes, 128 bits ≙ 16 bytes. The low-level access sequence to the bytes of such a field depends on the operation to be performed. The least-significant byte is accessed first for addition , subtraction and multiplication . The most-significant byte

5785-546: The form of wrappers that make standard library functions safer and easier to use. This dates back to as early as The Practice of Programming book by B. Kernighan and R. Pike where the authors commonly use wrappers that print error messages and quit the program if an error occurs. The ISO C committee published Technical reports TR 24731-1 and is working on TR 24731-2 to propose adoption of some functions with bounds checking and automatic buffer allocation, correspondingly. The former has met severe criticism with some praise, and

5874-399: The full Linux API, rather than just the POSIX API, may provide advantages in cases where those additional features are useful. Well-known current examples are udev , systemd and Weston . People such as Lennart Poettering openly advocate to prefer the Linux API over the POSIX API, where this offers advantages. At FOSDEM 2016, Michael Kerrisk explained some of the perceived issues with

5963-400: The function by means of that pointer. If two pointers to the same function are derived in two different translation units in the program, these two pointers must compare equal; that is, the address comes by resolving the name of the function, which has external (program-wide) linkage. Under FreeBSD and glibc, some functions such as sin() are not linked in by default and are instead bundled in

6052-406: The functions in the C standard library; that is, the implementations of the functions are written into the compiled object file , and the program calls the built-in versions instead of the functions in the C library shared object file. This reduces function-call overhead, especially if function calls are replaced with inline variants, and allows other forms of optimization (as the compiler knows

6141-406: The late 1990s (SPARC v9 compliant processors) allow data endianness to be chosen with each individual instruction that loads from or stores to memory. The ARM architecture supports two big-endian modes, called BE-8 and BE-32 . CPUs up to ARMv5 only support BE-32 or word-invariant mode. Here any naturally aligned 32-bit access works like in little-endian mode, but access to a byte or 16-bit word

6230-624: The latter saw mixed response. Despite concerns, TR 24731-1 was integrated into the C standards track in ISO/IEC ;9899:2011 (C11), Annex K ( Bounds-checking interfaces ), and implemented approximately in Microsoft’s C/++ runtime (CRT) library for the Win32 and Win64 platforms. (By default, Microsoft Visual Studio’s C and C++ compilers issue warnings when using older, "insecure" functions. However, Microsoft’s implementation of TR 24731-1

6319-588: The length of an operand may be held in a separate field of the instruction or with the operand itself, e.g. by means of a word mark . Such an approach allows operand lengths up to 256 bytes or larger. The data types of such operands are character strings or BCD . Machines able to manipulate such data with one instruction (e.g. compare, add) include the IBM 1401 , 1410 , 1620 , System/360 , System/370 , ESA/390 , and z/Architecture , all of them of type big-endian. Numerous other orderings, generically called middle-endian or mixed-endian , are possible. The PDP-11

6408-497: The logic of networking devices and software. The word bi-endian , when said of hardware, denotes the capability of the machine to compute or pass data in either endian format. Many of these architectures can be switched via software to default to a specific endian format (usually done when the computer starts up); however, on some systems, the default endianness is selected by hardware on the motherboard and cannot be changed via software (e.g. Alpha, which runs only in big-endian mode on

6497-986: The majority of the C standard library’s constructs into its own, excluding C-specific machinery. C standard library functions are exported from the C++ standard library in two ways. For backwards-/cross-compatibility to C and pre-Standard C++, functions can be accessed in the global namespace ( :: ), after #include ing the C standard header name as in C. Thus, the C++98 program should exhibit (apparently-)identical behavior to C95 program From C++98 on, C functions are also made available in namespace ::std (e.g., C  printf as C++  ::std::printf , atoi as ::std::atoi , feof as ::std::feof ), by including header <c hdrname > instead of corresponding C header < hdrname .h> . E.g., <cstdio> substitutes for <stdio.h> and <cmath> for <math.h> ; note lack of .h extension on C++ header names. Thus, an equivalent (generally preferable) C++≥98 program to

6586-421: The mathematical library libm . If any of them are used, the linker must be given the directive -lm . POSIX requires that the c99 compiler supports -lm , and that the functions declared in the headers math.h , complex.h , and fenv.h are available for linking if -lm is specified, but does not specify if the functions are linked by default. musl satisfies this requirement by putting everything into

6675-403: The number of bytes in the data. An attempt to read such a file using Fortran on a system of the other endianness results in a run-time error, because the count fields are incorrect. Unicode text can optionally start with a byte order mark (BOM) to signal the endianness of the file or stream. Its code point is U+FEFF. In UTF-32 for example, a big-endian file should start with 00 00 FE FF ;

6764-444: The original standard, many of which first appeared in 1994's 4.4BSD release (the first to be largely developed after the first standard was issued in 1989). Some of the extensions of BSD libc are: Some languages include the functionality of the standard C library in their own libraries. The library may be adapted to better suit the language's structure, but the operational semantics are kept similar. The C++ language incorporates

6853-560: The point of view of the executing programs, but they require the motherboard to perform a 64-bit swap across all 8 byte lanes to ensure that the little-endian view of things will apply to I/O devices. In the absence of this unusual motherboard hardware, device driver software must write to different addresses to undo the incomplete transformation and also must perform a normal byte swap. Some CPUs, such as many PowerPC processors intended for embedded use and almost all SPARC processors, allow per-page choice of endianness. SPARC processors since

6942-521: The responsibility of the Linux kernel developers or of the developers of the GNU C Library, and more the task for Linux distributions and independent software vendors (ISVs) who wish to sell and provide support for their proprietary software as binaries only for such a single Linux ABI, as opposed to supporting multiple Linux ABIs. An ABI has to be defined for every instruction set, such as x86 , x86-64 , MIPS , ARMv7-A (32-Bit), ARMv8-A (64-Bit), etc. with

7031-427: The same address as either 8-bit (value = 4A), 16-bit (004A), 24-bit (00004A), or 32-bit (0000004A), all of which retain the same numeric value. Although this little-endian property is rarely used directly by high-level programmers, it is occasionally employed by code optimizers as well as by assembly language programmers. While not allowed by C++, such type punning code is allowed as "implementation-defined" by

7120-442: The same time, Linux kernel developers have historically been conservative and meticulous about introducing new system calls. Much available free and open-source software is written for the POSIX API. Since so much more development flows into the Linux kernel as compared to the other POSIX-compliant combinations of kernel and C standard library, the Linux kernel and its API have been augmented with additional features. Programming for

7209-444: The sequence the bytes are accessed by the computer hardware, more precisely: by the low-level algorithms contributing to the results of a computer instruction. Positional number systems (mostly base 2, or less often base 10) are the predominant way of representing and particularly of manipulating integer data by computers. In pure form this is valid for moderate sized non-negative integers, e.g. of C data type unsigned . In such

7298-406: The shell of a boiled egg from the big end or from the little end. By analogy, a CPU may read a digital word big end first, or little end first. Computers store information in various-sized groups of binary bits. Each group is assigned a number, called its address , that the computer uses to access that data. On most modern computers, the smallest data group with an address is eight bits long and

7387-457: The significance increasing from right to left. In other words, it appears backwards when visualized, which can be counter-intuitive. This behavior arises, for example, in FourCC or similar techniques that involve packing characters into an integer, so that it becomes a sequence of specific characters in memory. For example, take the string "JOHN", stored in hexadecimal ASCII . On big-endian machines,

7476-539: The system calls of the Linux kernel; the combination of the Linux kernel system call interface and a C standard library is what builds the Linux API. Some popular implementations of the C standard library are As in other Unix-like systems, additional capabilities of the Linux kernel exist that are not part of POSIX: DRM has been paramount for the development and implementations of well-defined and performant free and open-source graphics device drivers without which no rendering acceleration would be available at all, only

7565-423: The telephone book. Almost all machines which can do this using a single instruction are big-endian or at least mixed-endian. Integer numbers written as text are always represented most significant digit first in memory, which is similar to big-endian, independently of text direction . When memory bytes are printed sequentially from left to right (e.g. in a hex dump ), little-endian representation of integers has

7654-993: The value appears left-to-right, coinciding with the correct string order for reading the result ("J O H N"). But on a little-endian machine, one would see "N H O J". Middle-endian machines complicate this even further; for example, on the PDP-11 , the 32-bit value is stored as two 16-bit words "JO" "HN" in big-endian, with the characters in the 16-bit words being stored in little-endian, resulting in "O J N H". Byte-swapping consists of rearranging bytes to change endianness. Many compilers provide built-ins that are likely to be compiled into native processor instructions ( bswap / movbe ), such as __builtin_bswap32 . Software interfaces for swapping include: Some CPU instruction sets provide native support for endian byte swapping, such as bswap ( x86 — 486 and later, i960 — i960Jx and later ), and rev ( ARMv6 and later). Some compilers have built-in facilities for byte swapping. For example,

7743-573: Was developed at the same time as the C library POSIX specification , which is a superset of it. Since ANSI C was adopted by the International Organization for Standardization , the C standard library is also called the ISO C library . The C standard library provides macros , type definitions and functions for tasks such as string manipulation, mathematical computation, input/output processing, memory management , and input/output . The application programming interface (API) of

7832-474: Was merged into glibc it constituted a separate library with its own linker flag argument. Often, this POSIX-specified functionality will be regarded as part of the library; the basic C library may be identified as the ANSI or ISO C library. BSD libc is a superset of the POSIX standard library supported by the C libraries included with BSD operating systems such as FreeBSD , NetBSD , OpenBSD and macOS . BSD libc has some extensions that are not defined in

7921-504: Was unable to deliver the 8008 in time, Datapoint used a medium-scale integration equivalent, but the little-endianness was retained in most Intel designs, including the MCS-48 and the 8086 and its x86 successors, including IA-32 and x86-64 processors. The MOS Technology 6502 family (including Western Design Center 65802 and 65C816 ), the Zilog Z80 (including Z180 and eZ80 ),

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