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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta name="generator" content="HTML Tidy, see www.w3.org"><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 --><!-- Copyright (c) 2001-2003 The Open Group, All Rights Reserved --><title>Rationale</title></head><body><basefont size="3"> <center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2003 Edition<br>Copyright © 2001-2003 The IEEE and The Open Group</font></center><hr size="2" noshade><h3><a name="tag_01_02"></a>Conformance</h3><p>The terms "profile" and "profiling" are used throughout this section.</p><p>A profile of a standard or standards is a codified set of option selections, such that by being conformant to a profile,particular classes of users are specifically supported.</p><p>These conformance definitions are descended from those in the ISO POSIX-1:1996 standard, but with changes for thefollowing:</p><ul><li><p>The addition of profiling options, allowing larger profiles of options such as the XSI extension used by the Single UNIXSpecification. In effect, it has profiled itself (that is, created a self-profile).</p></li><li><p>The addition of a definition of subprofiling considerations, to allow smaller profiles of options.</p></li><li><p>The addition of a hierarchy of super-options for XSI; these were formerly known as "Feature Groups" in the System Interfacesand Headers, Issue 5 specification.</p></li><li><p>Options from the ISO POSIX-2:1993 standard are also now included, as IEEE Std 1003.1-2001 merges thefunctionality from it.</p></li></ul><h4><a name="tag_01_02_01"></a>Implementation Conformance</h4><p>These definitions allow application developers to know what to depend on in an implementation.</p><p>There is no definition of a "strictly conforming implementation''; that would be an implementation that provides <i>only</i>those facilities specified by POSIX.1 with no extensions whatsoever. This is because no actual operating system implementation canexist without system administration and initialization facilities that are beyond the scope of POSIX.1.</p><h5><a name="tag_01_02_01_01"></a>Requirements</h5><p>The word "support" is used in certain instances, rather than "provide", in order to allow an implementation that has noresident software development facilities, but that supports the execution of a <i>Strictly Conforming POSIX.1 Application</i>, tobe a <i>conforming implementation</i>.</p><h5><a name="tag_01_02_01_02"></a>Documentation</h5><p>The conformance documentation is required to use the same numbering scheme as POSIX.1 for purposes of cross-referencing. Alloptions that an implementation chooses are reflected in <a href="../basedefs/limits.h.html"><i><limits.h></i></a> and <ahref="../basedefs/unistd.h.html"><i><unistd.h></i></a>.</p><p>Note that the use of "may" in terms of where conformance documents record where implementations may vary, implies that it isnot required to describe those features identified as undefined or unspecified.</p><p>Other aspects of systems must be evaluated by purchasers for suitability. Many systems incorporate buffering facilities,maintaining updated data in volatile storage and transferring such updates to non-volatile storage asynchronously. Variousexception conditions, such as a power failure or a system crash, can cause this data to be lost. The data may be associated with afile that is still open, with one that has been closed, with a directory, or with any other internal system data structuresassociated with permanent storage. This data can be lost, in whole or part, so that only careful inspection of file contents coulddetermine that an update did not occur.</p><p>Also, interrelated file activities, where multiple files and/or directories are updated, or where space is allocated or releasedin the file system structures, can leave inconsistencies in the relationship between data in the various files and directories, orin the file system itself. Such inconsistencies can break applications that expect updates to occur in a specific sequence, so thatupdates in one place correspond with related updates in another place.</p><p>For example, if a user creates a file, places information in the file, and then records this action in another file, a system orpower failure at this point followed by restart may result in a state in which the record of the action is permanently recorded,but the file created (or some of its information) has been lost. The consequences of this to the user may be undesirable. For auser on such a system, the only safe action may be to require the system administrator to have a policy that requires, after anysystem or power failure, that the entire file system must be restored from the most recent backup copy (causing all interveningwork to be lost).</p><p>The characteristics of each implementation will vary in this respect and may or may not meet the requirements of a givenapplication or user. Enforcement of such requirements is beyond the scope of POSIX.1. It is up to the purchaser to determine whatfacilities are provided in an implementation that affect the exposure to possible data or sequence loss, and also what underlyingimplementation techniques and/or facilities are provided that reduce or limit such loss or its consequences.</p><h5><a name="tag_01_02_01_03"></a>POSIX Conformance</h5><p>This really means conformance to the base standard; however, since this revision includes the core material of the Single UNIXSpecification, the standard developers decided that it was appropriate to segment the conformance requirements into two, the formerfor the base standard, and the latter for the Single UNIX Specification.</p><p>Within POSIX.1 there are some symbolic constants that, if defined, indicate that a certain option is enabled. Other symbolicconstants exist in POSIX.1 for other reasons.</p><p>As part of the revision some alignment has occurred of the options with the FIPS 151-2 profile on the POSIX.1-1990 standard. Thefollowing options from the POSIX.1-1990 standard are now mandatory:</p><ul><li><p>_POSIX_JOB_CONTROL</p></li><li><p>_POSIX_SAVED_IDS</p></li><li><p>_POSIX_VDISABLE</p></li></ul><p>A POSIX-conformant system may support the XSI extensions of the Single UNIX Specification. This was intentional since thestandard developers intend them to be upwards-compatible, so that a system conforming to the Single UNIX Specification can alsoconform to the base standard at the same time.</p><h5><a name="tag_01_02_01_04"></a>XSI Conformance</h5><p>This section is added since the revision merges in the base volumes of the Single UNIX Specification.</p><p>XSI conformance can be thought of as a profile, selecting certain options from IEEE Std 1003.1-2001.</p><h5><a name="tag_01_02_01_05"></a>Option Groups</h5><p>The concept of "Option Groups" is introduced to IEEE Std 1003.1-2001 to allow collections of related functions oroptions to be grouped together. This has been used as follows: the "XSI Option Groups" have been created to allow super-options,collections of underlying options and related functions, to be collectively supported by XSI-conforming systems. These reflect the"Feature Groups" from the System Interfaces and Headers, Issue 5 specification.</p><p>The standard developers considered the matter of subprofiling and decided it was better to include an enabling mechanism ratherthan detailed normative requirements. A set of subprofiling options was developed and included later in this volume ofIEEE Std 1003.1-2001 as an informative illustration.</p><h5><a name="tag_01_02_01_06"></a>Subprofiling Considerations</h5><p>The goal of not simultaneously fixing maximums and minimums was to allow implementations of the base standard or standards tosupport multiple profiles without conflict.</p><p>The following summarizes the rules for the limit types:</p><center><table border="1" cellpadding="3" align="center"><tr valign="top"><th align="center"><p class="tent"><b>Limit</b></p></th><th align="center"><p class="tent"><b>Fixed</b></p></th><th align="center"><p class="tent"><b>Minimum Acceptable</b></p></th><th align="center"><p class="tent"><b>Maximum Acceptable</b></p></th></tr><tr valign="top"><th align="center"><p class="tent"><b>Type</b></p></th><th align="center"><p class="tent"><b>Value</b></p></th>⌨️ 快捷键说明
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