C Standard
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| C Standard | |
|---|---|
| Name | C Standard |
| Family | ALGOL |
| Designed by | Dennis Ritchie, Ken Thompson |
| First appeared | 1978 |
| Latest release | ISO/IEC 9899:2018 |
| Influenced by | B (programming language), ALGOL 68 |
| Influenced | C++, Objective-C, C#, Java |
C Standard The C Standard defines the syntax, semantics, and library of the C programming language as maintained by international and national bodies such as International Organization for Standardization, International Electrotechnical Commission, and American National Standards Institute. It provides a common baseline for compiler vendors like GCC, Clang and Microsoft Visual C++ while interacting with operating systems such as Unix, Linux kernel, and Windows NT. The Standard balances efficiency for systems programming, exemplified by projects like Bell Labs's UNIX, with portability across architectures like x86 architecture, ARM architecture, and RISC-V.
Overview
The Standard codifies lexical grammar, translation phases, diagnostics, and the abstract machine used by implementations, aligning with organizations including ISO/IEC JTC 1/SC 22 and national committees like ANSI. It specifies core language constructs—declarations, expressions, statements—and the normative and informative contents of the normative document ISO/IEC 9899. The Standard also enumerates constraints, implementation-defined behaviors, and undefined behaviors important for compilers used by vendors such as Intel Corporation and ARM Holdings.
History and Standardization Process
Standardization began with efforts at Bell Labs and publication of K&R C in 1978, followed by ANSI work culminating in ANSI X3.159-1989 (C89). Subsequent international adoption led to ISO/IEC 9899:1990 (C90). Major revisions emerged as ISO/IEC 9899:1999 (C99), ISO/IEC 9899:2011 (C11), ISO/IEC 9899:2018 (C17/C18), each produced through ballots and working groups within ISO and IEC subcommittees. Prominent contributors and reviewers include researchers from University of Cambridge, engineers from Bell Labs, and implementers from companies such as Silicon Graphics and Sun Microsystems. The process involves committee drafts, enquiry drafts, and final text approvals, influenced by public comment and technical corrigenda.
Language Features and Changes by Revision
C89 standardized core syntax and the preprocessor, adding prototypes and linkage rules impacting systems like POSIX. C99 introduced features including inline functions, variable-length arrays, new integer types from ISO/IEC TR 18037, interoperation with IEEE 754 floating-point models, and complex types used in signal processing implementations. C11 added _Atomic, threading support aligning with POSIX threads, and bounds-checking Annexes influenced by safety research in organizations like CERT Coordination Center. C17 focused on defect corrections and editorial fixes, while C23 proposals include pattern matching, modules discussed in venues such as WG14 meetings, and interactions with proposals from compiler projects like LLVM.
Standard Library
The Standard Library specifies headers and functions for input/output, memory management, and string manipulation used by projects like glibc, musl, and Newlib. It defines facilities in headers such as
Compiler Conformance and Implementation Issues
Conformance testing involves test suites and validation activities such as those produced by Open Group and community-driven test suites used by GCC and Clang. Implementers must reconcile undefined behavior with optimization passes in backends like LLVM and code generators targeting microarchitectures from Intel Corporation and ARM Holdings. Tooling ecosystems including static analyzers from Coverity and sanitizers integrated into Clang address issues like buffer overflows and integer overflow. Platform-specific concerns arise with calling conventions implemented by System V AMD64 ABI or Microsoft x64 calling convention.
Portability and Undefined Behavior
The Standard distinguishes implementation-defined, unspecified, and undefined behaviors; understanding these distinctions is essential for projects like Linux kernel and embedded systems firmware from vendors such as TI and Microchip Technology. Undefined behavior permits aggressive optimizations exploited by compilers, affecting portability across instruction set architectures like PowerPC and MIPS. Portability guidance often references standards and profiles used by industries represented in consortia such as POSIX and certification programs run by organizations like NIST.
Future Developments and Proposals
Ongoing work in committees like ISO/IEC JTC 1/SC 22/WG 14 addresses proposals for improved safety features, module systems, extended diagnostics, and interactions with modern hardware features from companies such as NVIDIA and ARM Holdings. Discussions at standards meetings and conferences like ACM SIGPLAN and IO-driven collaboration with open source communities including LLVM and GCC shape proposals for upcoming revisions. Emerging topics include stronger memory-safety interfaces, better interoperability with C++ and managed runtimes, and formal semantics driven by research groups at institutions like MIT and ETH Zurich.