POSIX

POSIX. The Portable Operating System Interface is a family of standards specified by the IEEE for maintaining compatibility between operating systems. It defines the application programming interface, along with command line shells and utility interfaces, for software written to run on variants of Unix and other compatible systems. These specifications are crucial for ensuring source code portability across different computing platforms, forming a foundational layer for software development in open systems.

Overview

The primary goal is to establish a standardized environment for application development across diverse systems, particularly those derived from or inspired by the original AT&T Bell Labs Unix system. By providing a consistent set of system calls, libraries, and utilities, it enables developers to write programs that can be compiled and run on multiple compliant platforms with minimal modification. This standardization is managed and maintained by the IEEE Computer Society under the auspices of the Austin Group, which consolidates work from several major standards bodies. The widespread adoption of its principles has been instrumental in the growth of open systems and the free software movement.

History and development

The need for a common standard became acute in the 1980s as the Unix market fragmented into incompatible proprietary versions from vendors like Sun Microsystems, IBM, and Hewlett-Packard. In response, the IEEE initiated the Standard 1003 committee in 1988, with Richard Stallman notably influencing the naming. The first ratified standard, IEEE 1003.1-1988, focused on core system interfaces for the C programming language. Subsequent development involved collaboration with organizations like The Open Group, which publishes the Single UNIX Specification, and the work was later aligned with the ISO/IEC JTC 1 international standards process. Major revisions include IEEE 1003.1-2001, which incorporated the Single UNIX Specification, and the ongoing work of the Austin Group.

Core components and standards

The family encompasses several interrelated specifications covering different aspects of an operating system. The foundational IEEE 1003.1 standard defines the core services for process management, file and directory operations, and I/O primitives. Other parts include IEEE 1003.2 for shell and utility standards, which formalizes the behavior of tools found in environments like the Bourne shell, and IEEE 1003.1b for real-time extensions. Key defined interfaces include those for thread management, synchronization primitives, memory-mapped files, and semaphore operations. These components collectively describe a comprehensive environment for developing portable applications, influencing the design of systems like Linux, FreeBSD, and macOS.

Compliance and certification

Formal verification that an operating system conforms to the specifications is a rigorous process. Historically, certification was offered through programs like the Open Brand from The Open Group, which tested against the Single UNIX Specification that incorporates the base standards. While many modern systems, such as those based on the Linux kernel or Berkeley Software Distribution derivatives, are designed to be largely compliant, they often do not pursue formal certification due to cost and complexity. Instead, compliance is frequently demonstrated through implementation of features and passing test suites like the POSIX Conformance Test Suite. The Linux Standard Base builds upon this foundation to ensure greater consistency across Linux distributions.

Influence and related standards

Its impact on the computing industry is profound, serving as the de facto model for interoperability in open systems. It directly shaped the development of the Single UNIX Specification and is a mandatory component for any system using the UNIX trademark. The standard heavily influenced the design of the Linux kernel and the GNU Core Utilities, which are central to the GNU/Linux operating system. Furthermore, it has affected other operating systems, including Microsoft Windows, which provides a partial compatibility layer through services like the Windows Subsystem for Linux. Related standards that extend or specialize its concepts include the Filesystem Hierarchy Standard and various ISO/IEC standards for programming languages and system interfaces.

Category:Computing standards Category:Unix