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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML Strict//EN"><HTML><HEAD><TITLE>SP - System identifiers</TITLE></HEAD><BODY><H1>System identifiers</H1><P>There are two kinds of system identifier: formal system identifiersand simple system identifiers. A system identifier that does notstart with <SAMP><</SAMP> will always be interpreted as a simplesystem identifier. A simple system identifier will always beinterpreted either as a filename or as a URL.<H2>Formal system identifiers</H2><P>Formal system identifiers are based on theSystem Identifier facility defined in ISO/IEC 10744 (HyTime) TechnicalCorrigendum 1, Annex D.A system identifier that is a formal systemidentifier consists of a sequence of one or more storage objectspecifications. The objects specified by the storage objectspecifications are concatenated to form the entity. A storage objectspecification consists of an SGML start-tag in the reference concretesyntax followed by character data content. The generic identifier ofthe start-tag is the name of a storage manager. The content is astorage object identifier which identifies the storage object in amanner dependent on the storage manager. The start-tag can alsospecify attributes giving additional information about the storageobject. Numeric character references are recognized in storage objectidentifiers and attribute value literals in the start-tag. Recordends are ignored in the storage object identifier as with SGML. Asystem identifier will be interpreted as a formal system identifier ifit starts with a <SAMP><</SAMP> followed by a storage manager name,followed by either <SAMP>></SAMP> or white-space; otherwise it will beinterpreted as a simple system identifier. A storage objectidentifier extends until the end of the system identifier or until thefirst occurrence of <SAMP><</SAMP> followed by a storage managername, followed by either <SAMP>></SAMP> or white-space.<P>The following storage managers are available:<DL><DT><A NAME="osfile"><SAMP>osfile</SAMP></A><DD>The storage object identifier is a filename. If the filename isrelative it is resolved using a base filename. Normally the basefilename is the name of the file in which the storage objectidentifier was specified, but this can be changed using the<SAMP>base</SAMP> attribute. The filename will be searched for firstin the directory of the base filename. If it is not found there, thenit will be searched for in directories specified with the<SAMP>-D</SAMP> option in the order in which they were specified onthe command line, and then in the list of directories specified by theenvironment variable <SAMP>SGML_SEARCH_PATH</SAMP>. The listis separated by colons under Unix and by semi-colons under MSDOS.<DT><SAMP>osfd</SAMP><DD>The storage object identifier is an integer specifying a filedescriptor. Thus a system identifier of <SAMP><osfd>0</SAMP> willrefer to the standard input.<DT><SAMP>url</SAMP><DD>The storage object identifier is a URL. Only the <SAMP>http</SAMP>scheme is currently supported and not on all systems.<DT><SAMP>neutral</SAMP><DD>The storage manager is the storage manager of storage object in whichthe system identifier was specified (the <I>underlying storagemanager</I>). However if the underlying storage manager does notsupport named storage objects (ie it is <SAMP>osfd</SAMP>), then thestorage manager will be <SAMP>osfile</SAMP>. The storage objectidentifier is treated as a relative, hierarchical name separated byslashes (<SAMP>/</SAMP>) and will be transformed as appropriate forthe underlying storage manager.<DT><SAMP>literal</SAMP><DD>The bit combinations of the storage object identifier arethe contents of the storage object.</DL><P>The following attributes are supported:<DL><DT><SAMP>records</SAMP><DD>This describes how records are delimited in the storage object:<DL><DT><SAMP>cr</SAMP><DD>Records are terminated by a carriage return.<DT><SAMP>lf</SAMP><DD>Records are terminated by a line feed.<DT><SAMP>crlf</SAMP><DD>Records are terminated by a carriage return followed by a line feed.<DT><SAMP>find</SAMP><DD>Records are terminated by whichever of<SAMP>cr</SAMP>,<SAMP>lf</SAMP>or<SAMP>crlf</SAMP>is first encountered in the storage object.<DT><SAMP>asis</SAMP><DD>No recognition of records is performed.</DL><P>The default is <SAMP>find</SAMP> except for NDATA entities for whichthe default is <SAMP>asis</SAMP>. This attribute is not applicable tothe <SAMP>literal</SAMP> storage manager.<P>When records are recognized in a storage object, a record start isinserted at the beginning of each record, and a record end at the endof each record. If there is a partial record (a record that doesn'tend with the record terminator) at the end of the entity, then arecord start will be inserted before it but no record end will beinserted after it.<P>The attribute name and <SAMP>=</SAMP> can be omitted for this attribute.<DT><SAMP>zapeof</SAMP><DD>This specifies whether a Control-Z character that occurs as the final bytein the storage object should be stripped.The following values are allowed:<DL><DT><SAMP>zapeof</SAMP><DD>A final Control-Z should be stripped.<DT><SAMP>nozapeof</SAMP><DD>A final Control-Z should not be stripped.</DL><P>The default is <SAMP>zapeof</SAMP> except for NDATA entities, entitiesdeclared in storage objects with <SAMP>zapeof=nozapeof</SAMP> andstorage objects with <SAMP>records=asis</SAMP>. This attribute is notapplicable to the <SAMP>literal</SAMP> storage manager.<P>The attribute name and <SAMP>=</SAMP> can be omitted for thisattribute.<DT><A NAME="encoding"><SAMP>encoding</SAMP></A><DD>The encoding attribute specifies the encoding of the storage object.This attribute is used when the encoding is independent of thedocument character set.The value must be the <A HREF="charset.htm#encodings">name of an encoding</A>.This attribute is not applicable to the<SAMP>literal</SAMP> storage manager.<DT><A NAME="bctf"><SAMP>bctf</SAMP></A><DD>The BCTF attribute specifies that the encoding of the storageobject.This attribute is used when the encoding isdocument character set dependent.The value must be the <A HREF="charset.htm#bctfs">name of a BCTF</A>.This attribute is not applicable to the<SAMP>literal</SAMP> storage manager.<DT><SAMP>tracking</SAMP><DD>This specifies whether line boundaries should be tracked for thisobject: a value of <SAMP>track</SAMP> specifies that they should; avalue of <SAMP>notrack</SAMP> specifies that they should not. Thedefault value is <SAMP>track</SAMP>. Keeping track of where lineboundaries occur in a storage object requires approximately one byteof storage per line and it may be desirable to disable this for verylarge storage objects.<P>The attribute name and<SAMP>=</SAMP>can be omitted for this attribute.<DT><SAMP>base</SAMP><DD>When the storage object identifier specified in the content of thestorage object specification is relative, this specifies the basestorage object identifier relative to which that storage objectidentifier should be resolved.When not specified a storage object identifier is interpretedrelative to the storage object in which it is specified,provided that this has the same storage manager.This applies both to system identifiers specified in SGMLdocuments and to system identifiers specified in the catalog entryfiles.<DT><SAMP>smcrd</SAMP><DD>The value is a single character that will be recognized in storageobject identifiers (both in the content of storage objectspecifications and in the value of <SAMP>base</SAMP> attributes) as astorage manager character reference delimiter when followed by adigit. A storage manager character reference is like an SGML numericcharacter reference except that the number is interpreted as acharacter number in the inherent character set of the storage managerrather than the document character set. The default is for nocharacter to be recognized as a storage manager character referencedelimiter. Numeric character references cannot be used to preventrecognition of storage manager character reference delimiters.<DT><SAMP>fold</SAMP><DD>This applies only to the <SAMP>neutral</SAMP> storage manager. Itspecifies whether the storage object identifier should be folded tothe customary case of the underlying storage manager if storage objectidentifiers for the underlying storage manager are case sensitive.The following values are allowed:<DL><DT><SAMP>fold</SAMP><DD>The storage object identifier will be folded.<DT><SAMP>nofold</SAMP><DD>The storage object identifier will not be folded.</DL><P>The default value is <SAMP>fold</SAMP>. The attribute name and<SAMP>=</SAMP> can be omitted for this attribute.<P>For example, on Unix filenames are case-sensitive and the customarycase is lower-case. So if the underlying storage manager were<SAMP>osfile</SAMP> and the system was a Unix system, then<SAMP><neutral>FOO.SGM</SAMP> would be equivalent to<SAMP><osfile>foo.sgm</SAMP>.</DL><H2>Simple system identfiers</H2><P>A simple system identifier is interpreted as a storage objectidentifier with a storage manager that depends on where the systemidentifier was specified: if it was specified in a storage objectwhose storage manager was <SAMP>url</SAMP> or if the system identifierlooks like an absolute URL in a supported scheme, the storage managerwill be <SAMP>url</SAMP>; otherwise the storage manager will be<SAMP>osfile</SAMP>. The storage manager attributes are defaulted asfor a formal system identifier. Numeric character references are notrecognized in simple system identifiers.<P><ADDRESS>James Clark<BR>jjc@jclark.com</ADDRESS></BODY></HTML>⌨️ 快捷键说明
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