ISO/IEC 9945-2:2003

Information technology - Portable Operating System Interface (POSIX) - Part 2: System Interfaces

ISO/IEC 9945 (parts 1 to 4):2003 defines a standard operating system interface and environment, including a command interpreter (or "shell"), and common utility programs to support applications portability at the source code level. ISO/IEC 9945-1:2003 is a minor revision and, together with ISO/IEC 9945-2:2003, ISO/IEC 9945-3:2003 and ISO/IEC 9945-4:2003, cancels and replaces ISO/IEC 9945-1:2002, ISO/IEC 9945-2:2002, ISO/IEC 9945-3:2002 and ISO/IEC 9945-4:2002. ISO/IEC 9945 (parts 1 to 4):2003 is the single common revision to ISO/IEC 9945-1:1996 (IEEE Std 1003.1-1996), ISO/IEC 9945-2:1993 (IEEE Std 1003.2-1992), and the Base Specifications of The Open Group Single UNIX® Specification, Version 2. ISO/IEC 9945 (parts 1 to 4):2003 is intended to be used by both applications developers and system implementors and comprises four major components (each in an associated volume). General terms, concepts, and interfaces common to all volumes of ISO/IEC 9945 (parts 1 to 4):2003, including utility conventions and C-language header definitions, are included in the Base Definitions volume (ISO/IEC 9945-1:2003). Definitions for system service functions and subroutines, language-specific system services for the C programming language, function issues, including portability, error handling, and error recovery, are included in the System Interfaces volume (ISO/IEC 9945-2:2003). Definitions for a standard source code-level interface to command interpretation services (a "shell") and common utility programs for application programs are included in the Shell and Utilities volume (ISO/IEC 9945-3:2003). Extended rationale that did not fit well into the rest of the document structure, containing historical information concerning the contents of ISO/IEC 9945 (parts 1 to 4):2003 and why features were included or discarded by the standard developers, is included in the Rationale (Informative) volume (ISO/IEC 9945-4:2003). The following areas are outside the scope of ISO/IEC 9945 (parts 1 to 4):2003: Graphics interfaces Database management system interfaces Record I/O considerations Object or binary code portability System configuration and resource availability ISO/IEC 9945 (parts 1 to 4):2003 describes the external characteristics and facilities that are of importance to applications developers, rather than the internal construction techniques employed to achieve these capabilities. Special emphasis is placed on those functions and facilities that are needed in a wide variety of commercial applications.

Technologies de l'information — Interface pour la portabilité des systèmes (POSIX) — Partie 2: Interfaces systèmes

General Information

• Status: Withdrawn
• Publication Date: 17-Aug-2003
• Withdrawal Date: 17-Aug-2003
• ICS: 35.060 - Languages used in information technology
• Technical Committee: ISO/IEC JTC 1/SC 22 - Programming languages, their environments and system software interfaces
• Drafting Committee: ISO/IEC JTC 1/SC 22 - Programming languages, their environments and system software interfaces
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 15-Sep-2009
• Completion Date: 30-Oct-2025

Relations

• Revised: ISO/IEC/IEEE 9945:2009 - Information technology - Portable Operating System Interface (POSIX®) Base Specifications, Issue 7
• Effective Date: 12-May-2008
• Revises: ISO/IEC 9945-2:2002 - Information technology - Portable Operating System Interface (POSIX) - Part 2: System Interfaces
• Effective Date: 15-Apr-2008

Standards Content (Sample)

INTERNATIONAL ISO/IEC
STANDARD 9945-2
System Interfaces,
IEEE Std 1003.1™, 2003 Edition
The Open Group Technical Standard
Includes IEEE Std 1003.1™-2001 and IEEE Std 1003.1™-2001/Cor 1-2002
Third edition
2003-08-15
Information technology — Portable ®
Operating System Interface (POSIX ) —

Part 2:
System Interfaces
Technologies de l'information — Interface pour la portabilité des systèmes ®
(POSIX ) —
Partie 2: Interfaces systèmes
Reference number
System Interfaces, IEEE Std 1003.1, 2003 Edition
The Open Group Technical Standard, Issue 6

System Interfaces, IEEE Std 1003.1, 2003 Edition
The Open Group Technical Standard, Issue 6
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Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail [e-mail]
IEEE Std 1003.1™, 2003 Edition

The Open Group Technical Standard
Base Specifications, Issue 6
Includes IEEE Std 1003.1™-2001 and IEEE Std 1003.1™-2001/Cor 1-2002

Information Technology — ®
Portable Operating System Interface (POSIX )

Part 2: System Interfaces
Sponsor
Portable Applications Standards Committee
of the
IEEE Computer Society
and
The Open Group
Adopted as an International Standard by the
International Organization for Standardization
and by the
International Electrotechnical Commission

System Interfaces, IEEE Std 1003.1, 2003 Edition
The Open Group Technical Standard, Issue 6

International Standard ISO/IEC 9945-2:2003(E)

ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission)
form the specialized system for worldwide standardization. National bodies that are members of ISO or IEC
participate in the development of International Standards through technical committees established by the
respective organization to deal with particular fields of technical activity. ISO and IEC technical committees
collaborate in fields of mutual interest. Other international organizations, governmental and non-governmental, in
liaison with ISO and IEC, also take part in the work. In the field of information technology, ISO and IEC have
established a joint technical committee, ISO/IEC JTC 1.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of the joint technical committee is to prepare International Standards. Draft International
Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as an
International Standard requires approval by at least 75% of the national bodies casting a vote.

Attention is drawn to the possibility that some of the elements of this part of ISO/IEC 9945 may be the subject of
patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.

ISO/IEC 9945-2 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 22, Programming languages, their environments and system software interfaces.

This third edition of ISO/IEC 9945-2 is a minor revision and, together with ISO/IEC 9945-1, ISO/IEC 9945-3, and
ISO/IEC 9945-4, cancels and replaces ISO/IEC 9945-1:2002 and ISO/IEC 9945-2:2002, ISO/IEC 9945-3:2002
and ISO/IEC 9945-4:2002.
ISO/IEC 9945 consists of the following parts, under the general title Information technology — Portable ®
Operating System Interface (POSIX ):
• Part 1: Base Definitions
• Part 2: System Interfaces
• Part 3: Shell and Utilities
• Part 4: Rationale
International Organization for Standardization/International Electrotechnical Commission
Case postale 56 • CH-1211 Genève 20 • Switzerland

Abstract
This standard is simultaneously ISO/IEC 9945: 2003, IEEE Std 1003.1-2001, and forms the core of the Single UNIX Specification,
Version 3.
The IEEE Std 1003.1, 2003 Edition includes IEEE Std 1003.1-2001/Cor 1-2002 incorporated into IEEE Std 1003.1-2001 (base
document). The Corrigendum addresses problems discovered since the approval of IEEE Std 1003.1-2001. These changes are mainly
due to resolving integration issues raised by the merger of the base documents that were incorporated into IEEE Std 1003.1-2001,
TM TM
which is the single common revision to IEEE Std 1003.1 -1996, IEEE Std 1003.2 -1992, ISO/IEC 9945-1: 1996, ISO/IEC 9945-2: 1993,

and the Base Specifications of The Open Group Single UNIX Specification, Version 2.
This standard defines a standard operating system interface and environment, including a command interpreter (or ‘‘shell’’), and
common utility programs to support applications portability at the source code level. This standard is intended to be used by both
applications developers and system implementors and comprises four major components (each in an associated volume):
• General terms, concepts, and interfaces common to all volumes of this standard, including utility conventions and C-language
header definitions, are included in the Base Definitions volume.
• Definitions for system service functions and subroutines, language-specific system services for the C programming language,
function issues, including portability, error handling, and error recovery, are included in the System Interfaces volume.
• Definitions for a standard source code-level interface to command interpretation services (a ‘‘shell’’) and common utility
programs for application programs are included in the Shell and Utilities volume.
• Extended rationale that did not fit well into the rest of the document structure, which contains historical information concerning
the contents of this standard and why features were included or discarded by the standard developers, is included in the
Rationale (Informative) volume.
The following areas are outside the scope of this standard:
• Graphics interfaces
• Database management system interfaces
• Record I/O considerations
• Object or binary code portability
• System configuration and resource availability
This standard describes the external characteristics and facilities that are of importance to applications developers, rather than the
internal construction techniques employed to achieve these capabilities. Special emphasis is placed on those functions and facilities
that are needed in a wide variety of commercial applications.
Keywords
application program interface (API), argument, asynchronous, basic regular expression (BRE), batch job, batch system, built-in
utility, byte, child, command language interpreter, CPU, extended regular expression (ERE), FIFO, file access control mechanism,

input/output (I/O), job control, network, portable operating system interface (POSIX ), parent, shell, stream, string, synchronous,
system, thread, X/Open System Interface (XSI)
System Interfaces, Issue 6 — Copyright  2001-2003, IEEE and The Open Group. All rights reserved. i

Copyright 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc. and The Open Group. All rights reserved. This
printing is by the International Organization for Standardization with special permission of the Institute of Electrical and Electronics
Engineers, Inc. and The Open Group. Published in Switzerland.
System Interfaces, Issue 6
Published 31 March 2003 by the Institute of Electrical and Electronics Engineers, Inc.
3 Park Avenue, New York, NY 10016-5997, U.S.A.
ISBN: 0-7381-3436-8 PDF 0-7381-3564-X/SS95078 CD-ROM 0-7381-3563-1/SE95078
Printed in the United States of America by the IEEE.
Published 31 March 2003 by The Open Group
Apex Plaza, Forbury Road, Reading, Berkshire RG1 1AX, U.K.
Document Number: C032
ISBN: 1-931624-24-0
Printed in the U.K. by The Open Group.
All rights reserved. No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise,
without prior written permission from both the IEEE and The Open Group.
Portions of this standard are derived with permission from copyrighted material owned by Hewlett-Packard Company,
International Business Machines Corporation, Novell Inc., The Open Software Foundation, and Sun Microsystems, Inc.
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ii System Interfaces, Issue 6 — Copyright  2001-2003, IEEE and The Open Group. All rights reserved.

IEEE
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System Interfaces, Issue 6 — Copyright  2001-2003, IEEE and The Open Group. All rights reserved. iii

The Open Group
The Open Group, a vendor and technology-neutral consortium, is committed to delivering greater business efficiency by bringing
together buyers and suppliers of information technology to lower the time, cost, and risks associated with integrating new
technology across the enterprise.
The Open Group’s mission is to offer all organizations concerned with open information infrastructures a forum to share knowledge,
integrate open initiatives, and certify approved products and processes in a manner in which they continue to trust our impartiality.
In the global eCommerce world of today, no single economic entity can achieve independence while still ensuring interoperability.
The assurance that products will interoperate with each other across differing systems and platforms is essential to the success of
eCommerce and business workflow. The Open Group, with its proven testing and certification program, is the international
guarantor of interoperability in the new century.
The Open Group provides opportunities to exchange information and shape the future of IT. The Open Group’s members include
some of the largest and most influential organizations in the world. The flexible structure of The Open Groups membership allows
for almost any organization, no matter what their size, to join and have a voice in shaping the future of the IT world.
More information is available on The Open Group web site at http://www.opengroup.org.
The Open Group has over 15 years’ experience in developing and operating certification programs and has extensive experience
developing and facilitating industry adoption of test suites used to validate conformance to an open standard or specification. The
Open Group portfolio of test suites includes the Westwood family of tests for this standard and the associated certification program
for Version 3 of the Single UNIX Specification, as well tests for CDE, CORBA, Motif, Linux, LDAP, POSIX.1, POSIX.2, POSIX
Realtime, Sockets, UNIX, XPG4, XNFS, XTI, and X11. The Open Group test tools are essential for proper development and
maintenance of standards-based products, ensuring conformance of products to industry-standard APIs, applications portability,
and interoperability. In-depth testing identifies defects at the earliest possible point in the development cycle, saving costs in
development and quality assurance.
More information is available at http://www.opengroup.org/testing.
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iv System Interfaces, Issue 6 — Copyright  2001-2003, IEEE and The Open Group. All rights reserved.

Contents
Chapter 1 Introduction. 1
1.1  Scope. 1
1.2  Conformance . 1
1.3  Normative References . 1
1.4  Change History . 1
1.5  Terminology. 1
1.6  Definitions . 3
1.7  Relationship to Other Formal Standards . 3
1.8  Portability . 3
1.8.1   Codes. 3
1.9  Format of Entries. 11
Chapter 2 General Information . 13
2.1  Use and Implementation of Functions . 13
2.2  The Compilation Environment. 13
2.2.1   POSIX.1 Symbols . 13
2.2.1.1     The _POSIX_C_SOURCE Feature Test Macro. 14
2.2.1.2     The _XOPEN_SOURCE Feature Test Macro. 14
2.2.2   The Name Space. 14
2.3  Error Numbers. 21
2.3.1   Additional Error Numbers. 28
2.4  Signal Concepts. 28
2.4.1   Signal Generation and Delivery. 28
2.4.2   Realtime Signal Generation and Delivery . 29
2.4.3   Signal Actions. 30
2.4.4   Signal Effects on Other Functions . 34
2.5  Standard I/O Streams. 34
2.5.1   Interaction of File Descriptors and Standard I/O Streams. 35
2.5.2   Stream Orientation and Encoding Rules . 36
2.6  STREAMS . 38
2.6.1   Accessing STREAMS. 39
2.7  XSI Interprocess Communication . 39
2.7.1   IPC General Description. 40
2.8  Realtime . 41
2.8.1   Realtime Signals. 41
2.8.2   Asynchronous I/O . 41
2.8.3   Memory Management . 43
2.8.3.1     Memory Locking. 43
2.8.3.2     Memory Mapped Files. 43
2.8.3.3     Memory Protection. 43
2.8.3.4     Typed Memory Objects . 44
2.8.4   Process Scheduling . 44
System Interfaces, Issue 6 — Copyright  2001-2003, IEEE and The Open Group. All rights reserved. v

Contents
2.8.5   Clocks and Timers . 48
2.9  Threads. 50
2.9.1   Thread-Safety. 50
2.9.2   Thread IDs. 51
2.9.3   Thread Mutexes. 51
2.9.4   Thread Scheduling. 52
2.9.5   Thread Cancellation . 54
2.9.5.1     Cancelability States . 54
2.9.5.2     Cancellation Points. 55
2.9.5.3     Thread Cancellation Cleanup Handlers . 57
2.9.5.4     Async-Cancel Safety. 57
2.9.6   Thread Read-Write Locks. 58
2.9.7   Thread Interactions with Regular File Operations . 58
2.10  Sockets. 58
2.10.1   Address Families. 58
2.10.2   Addressing . 59
2.10.3   Protocols . 59
2.10.4   Routing. 59
2.10.5   Interfaces. 59
2.10.6   Socket Types. 59
2.10.7   Socket I/O Mode. 60
2.10.8   Socket Owner. 60
2.10.9   Socket Queue Limits . 60
2.10.10   Pending Error. 60
2.10.11   Socket Receive Queue. 61
2.10.12   Socket Out-of-Band Data State . 61
2.10.13   Connection Indication Queue . 62
2.10.14   Signals . 62
2.10.15   Asynchronous Errors . 62
2.10.16   Use of Options. 63
2.10.17   Use of Sockets for Local UNIX Connections. 66
2.10.17.1     Headers . 66
2.10.18   Use of Sockets over Internet Protocols. 66
2.10.19   Use of Sockets over Internet Protocols Based on IPv4. 67
2.10.19.1     Headers . 67
2.10.20   Use of Sockets over Internet Protocols Based on IPv6. 67
2.10.20.1     Addressing . 67
2.10.20.2     Compatibility with IPv4. 68
2.10.20.3     Interface Identification. 68
2.10.20.4     Options. 69
2.10.20.5     Headers . 70
2.11  Tracing. 70
2.11.1   Tracing Data Definitions. 71
2.11.1.1     Structures. 71
2.11.1.2     Trace Stream Attributes. 75
2.11.2   Trace Event Type Definitions . 76
2.11.2.1     System Trace Event Type Definitions . 76
2.11.2.2     User Trace Event Type Definitions. 79
vi System Interfaces, Issue 6 — Copyright  2001-2003, IEEE and The Open Group. All rights reserved.

Contents
2.11.3   Trace Functions. 79
2.12  Data Types. 80
Chapter 3 System Interfaces . 83
Index. 1669
List of Tables
2-1 Value of Level for Socket Options. 63
2-2 Socket-Level Options . 64
2-3 Trace Option: System Trace Events. 77
2-4 Trace and Trace Event Filter Options: System Trace Events. 78
2-5 Trace and Trace Log Options: System Trace Events. 78
2-6 Trace, Trace Log, and Trace Event Filter Options: System
Trace Events . 79
2-7 Trace Option: User Trace Event . 79
System Interfaces, Issue 6 — Copyright  2001-2003, IEEE and The Open Group. All rights reserved. vii

Contents
viii System Interfaces, Issue 6 — Copyright  2001-2003, IEEE and The Open Group. All rights reserved.

Foreword
Structure of the Standard
This standard was originally developed by the Austin Group, a joint working group of members
of the IEEE, members of The Open Group, and members of ISO/IEC Joint Technical
Committee 1, as one of the four volumes of IEEE Std 1003.1-2001. The standard was approved by
ISO and IEC and published in four parts, correlating to the original volumes.
A mapping of the parts to the volumes is shown below:
__________________________________________________________________________________
LLLLISO/IEC 9945 IEEE Std 1003.1
LLLL
Part Volume Description
__________________________________________________________________________________
LLLL
9945-1 Base Definitions Includes general terms, concepts, and interfaces
LLLL
common to all parts of ISO/IEC 9945, including utilityLLLL
LLLLconventions and C-language header definitions.
__________________________________________________________________________________
LLLL
9945-2 System Interfaces Includes definitions for system service functions andLLLL
LLLsubroutines, language-specific system services for the CL
LLLLprogramming language, function issues, including
LLLL
portability, error handling, and error recovery.
_L_________________________________________________________________________________LLL
L9945-3LShell and UtilitiesLIncludes definitions for a standard source code-levelL
LLLLinterface to command interpretation services (a ‘‘shell’’)
LLLL
and common utility programs for application programs.
__________________________________________________________________________________
LLLL
9945-4 Rationale Includes extended rationale that did not fit well into theLLLL
LLLLrest of the document structure, containing historical
LLLL
information concerning the contents of ISO/IEC 9945
LLLL
and why features were included or discarded by the
LLLL
standard developers.
L_LL_________________________________________________________________________________LLLLL
All four parts comprise the entire standard, and are intended to be used together to
accommodate significant internal referencing among them. POSIX-conforming systems are
required to support all four parts.
System Interfaces, Issue 6 — Copyright  2001-2003, IEEE and The Open Group. All rights reserved. ix

Introduction
Note: This introduction is not part of IEEE Std 1003.1-2001, Standard for Information Technology —
Portable Operating System Interface (POSIX).
This standard has been jointly developed by the IEEE and The Open Group. It is simultaneously
an IEEE Standard, an ISO/IEC Standard, and an Open Group Technical Standard.
The Austin Group
This standard was developed, and is maintained, by a joint working group of members of the
IEEE Portable Applications Standards Committee, members of The Open Group, and members
of ISO/IEC Joint Technical Committee 1. This joint working group is known as the Austin
Group. The Austin Group arose out of discussions amongst the parties which started in early
1998, leading to an initial meeting and formation of the group in September 1998. The purpose of
the Austin Group has been to revise, combine, and update the following standards: ISO/IEC
9945-1, ISO/IEC 9945-2, IEEE Std 1003.1, IEEE Std 1003.2, and the Base Specifications of The
Open Group Single UNIX Specification.
After two initial meetings, an agreement was signed in July 1999 between The Open Group and
the Institute of Electrical and Electronics Engineers (IEEE), Inc., to formalize the project with the
first draft of the revised specifications being made available at the same time. Under this
agreement, The Open Group and IEEE agreed to share joint copyright of the resulting work. The
Open Group has provided the chair and secretariat for the Austin Group.
The base document for the revision was The Open Group’s Base volumes of its Single UNIX
Specification, Version 2. These were selected since they were a superset of the existing POSIX.1
and POSIX.2 specifications and had some organizational aspects that would benefit the audience
for the new revision.
The approach to specification development has been one of ‘‘write once, adopt everywhere’’,
with the deliverables being a set of specifications that carry the IEEE POSIX designation, The
Open Group’s Technical Standard designation, and an ISO/IEC designation. This set of
specifications forms the core of the Single UNIX Specification, Version 3.
This unique development has combined both the industry-led efforts and the formal
standardization activities into a single initiative, and included a wide spectrum of participants.
The Austin Group continues as the maintenance body for this document.
Anyone wishing to participate in the Austin Group should contact the chair with their request.
There are no fees for participation or membership. You may participate as an observer or as a
contributor. You do not have to attend face-to-face meetings to participate; electronic
participation is most welcome. For more information on the Austin Group and how to
participate, see http://www.opengroup.org/austin.
__________________
3. The Austin Group is named after the location of the inaugural meeting held at the IBM facility in Austin, Texas in September
1998.
x System Interfaces, Issue 6 — Copyright  2001-2003, IEEE and The Open Group. All rights reserved.

Introduction
Background
The developers of this standard represent a cross section of hardware manufacturers, vendors of
operating systems and other software development tools, software designers, consultants,
academics, authors, applications programmers, and others.
Conceptually, this standard describes a set of fundamental services needed for the efficient
construction of application programs. Access to these services has been provided by defining an
interface, using the C programming language, a command interpreter, and common utility
programs that establish standard semantics and syntax. Since this interface enables application
writers to write portable applications—it was developed with that goal in mind—it has been
designated POSIX, an acronym for Portable Operating System Interface.
Although originated to refer to the original IEEE Std 1003.1-1988, the name POSIX more correctly
refers to a family of related standards: IEEE Std 1003.n and the parts of ISO/IEC 9945. In earlier
editions of the IEEE standard, the term POSIX was used as a synonym for IEEE Std 1003.1-1988.
A preferred term, POSIX.1, emerged. This maintained the advantages of readability of the
symbol ‘‘POSIX’’ without being ambiguous with the POSIX family of standards.
Audience
The intended audience for this standard is all persons concerned with an industry-wide standard
operating system based on the UNIX system. This includes at least four groups of people:
1. Persons buying hardware and software systems
2. Persons managing companies that are deciding on future corporate computing directions
3. Persons implementing operating systems, and especially
4. Persons developing applications where portability is an objective
Purpose
Several principles guided the development of this standard:
• Application-Oriented
The basic goal was to promote portability of application programs across UNIX system
environments by developing a clear, consistent, and unambiguous standard for the interface
specification of a portable operating system based on the UNIX system documentation. This
standard codifies the common, existing definition of the UNIX system.
• Interface, Not Implementation
This standard defines an interface, not an implementation. No distinction is made between
library functions and system calls; both are referred to as functions. No details of the
implementation of any function are given (although historical practice is sometimes
indicated in the RATIONALE section). Symbolic names are given for constants (such as
signals and error numbers) rather than numbers.
__________________
4. The name POSIX was suggested by Richard Stallman. It is expected to be pronounced pahz-icks,as in positive , not poh-six,or
other variations. The pronunciation has been published in an attempt to promulgate a standardized way of referring to a
standard operating system interface.
System Interfaces, Issue 6 — Copyright  2001-2003, IEEE and The Open Group. All rights reserved. xi

Introduction
• Source, Not Object, Portability
This standard has been written so that a program written and translated for execution on one
conforming implementation may also be translated for execution on another conforming
implementation. This standard does not guarantee that executable (object or binary) code
will execute under a different conforming implementation than that for which it was
translated, even if the underlying hardware is identical.
• The C Language
The system interfaces and header definitions are written in terms of the standard C language
as specified in the ISO C standard.
• No Superuser, No System Administration
There was no intention to specify all aspects of an operating system. System administration
facilities and functions are excluded from this standard, and functions usable only by the
superuser have not been included. Still, an implementation of the standard interface may also
implement features not in this standard. This standard is also not concerned with hardware
constraints or system maintenance.
• Minimal Interface, Minimally Defined
In keeping with the historical design principles of the UNIX system, the mandatory core
facilities of this standard have been kept as minimal as possible. Additional capabilities have
been added as optional extensions.
• Broadly Implementable
The developers of this standard endeavored to make all specified functions implementable
across a wide range of existing and potential systems, including:
1. All of the current major systems that are ultimately derived from the original UNIX
system code (Version 7 or later)
2. Compatible systems that are not derived from the original UNIX system code
3. Emulations hosted on entirely different operating systems
4. Networked systems
5. Distributed systems
6. Systems running on a broad range of hardware
No direct references to this goal appear in this standard, but some results of it are mentioned
in the Rationale (Informative) volume.
• Minimal Changes to Historical Implementations
When the original version of IEEE Std 1003.1 was published, there were no known historical
implementations that did not have to change. However, there was a broad consensus on a set
of functions, types, definitions, and concepts that formed an interface that was common to
most historical implementations.
The adoption of the 1988 and 1990 IEEE system interface standards, the 1992 IEEE shell and
utilities standard, the various Open Group (formerly X/Open) specifications, and the
subsequent revisions and addenda to all of them have consolidated this consensus, and this
revision reflects the significantly increased level of consensus arrived at since the original
versions. The earlier standards and their modifications specified a number of areas where
consensus had not been reached before, and these are now reflected in this revision. The
authors of the original versions tried, as much as possible, to follow the principles below
xii System Interfaces, Issue 6 — Copyright  2001-2003, IEEE and The Open Group. All rights reserved.

Introduction
when creating new specifications:
1. By standardizing an interface like one in an historical implementation; for example,
directories
2. By specifying an interface that is readily implementable in terms of,
...