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4. The Entity Manager

SGML can refer to an external file (really entity) with an external identifier, this is a public identifier or a system identifier, or both.

A typical public identifier looks like

 
PUBLIC "ISO 8879:1986//ENTITIES Added Latin 1//EN"

where "ISO 8879:1986" is the owner, "ENTITIES" is the text class and "Added Latin 1" is the text description (and "EN" is language).

A system identifier looks like

 
SYSTEM "htmlplus.dtd"

where "htmlplus.dtd" is a system-specific identifier.

To map external identifiers to file names, PSGML first searches entity catalog files and then search the list of file name templates in the variable sgml-public-map.

The catalog format is according to SGML/Opens resolution on entity management. The catalog consists of a series of entries and comments. A comment is delimited by `--' like in a markup declaration. The entry types recognized are described in the following table.

`public pubid file'
The file will be used for the entity text of an entity with the public identifier pubid.

`entity name file'
The file will be used for the entity text of an entity with the name name. If the name starts with a `%' the rest of the name will be matched against parameter entities.

`doctype name file'
The file will be used for the entity text of an entity used as external subset of a document declaration with name as document type name.

`sgmldecl file'
Used to specify a default SGML declaration. Recognized but not used by PSGML other than to pass to an external validation command (sgml-validate-command).

When PSGML is looking for the file containing an external entity, the following things will be tried in order:

  1. Try the system identifier, as a file name, if there is a system identifier and the variable sgml-system-identifiers-are-preferred is non-nil and there is no elements containing `%s' in sgml-public-map. If the system identifier is a relative file name it will be relative to the directory containing the defining entity.

  2. Look thru each catalog in sgml-local-catalogs and sgml-catalog-files in order. For each catalog look first for entries matching the public identifier, if any. Then look for other matching entries in the order they appear in the catalog.

    Currently an entry will be ignored if it is matching but its file is non-existent or unreadable. (This is under reconsideration, perhaps it should signal error instead).

  3. Try the system identifier, if any, as a file name. If sgml-system-identifiers-are-preferred is nil and there is no elements containing `%s' in sgml-public-map.

  4. Try the entries in sgml-public-map. Using the catalogs are preferred. The sgml-public-map may disappear in a future version of PSGML (not soon though).

The sgml-public-map variable can contain a list of file name templates where `%P' will be substituted with the whole public identifier, owner is substituted for `%O', public text class for `%C', and public text description for `%D'. The text class will be converted to lower case and the owner and description will be transliterated according to the variable sgml-public-transliterations. The templates in the list is tried in order until an existing file is found. The sgml-public-map is modeled after `sgmls' environment variable SGML_PATH and psgml understand the following substitution characters: %%, %N, %P, %S, %Y, %C, %L, %O, %T, and %V. The default value of sgml-public-map is taken from the environment variable SGML_PATH.

Given the public identifier above and the file name template `/usr/local/lib/sgml/%o/%c/%d', the resulting file name is

 
/usr/local/lib/sgml/ISO_8879:1986/entities/Added_Latin_1

Note: blanks are transliterated to `_' (and also `/' to `%') and the text class is down cased.

User Option: sgml-catalog-files
This is a list of catalog entry files. The files are in the format defined in the SGML Open Draft Technical Resolution on Entity Management. The Emacs variable is initialized from the environment variable SGML_CATALOG_FILES or if this variable is undefined the default is

 
("CATALOG" "/usr/local/lib/sgml/CATALOG")

User Option: sgml-local-catalogs
A list of SGML entity catalogs to be searched first when parsing the buffer. This is used in addition to sgml-catalog-files, and sgml-public-map. This variable is automatically local to the buffer.

User Option: sgml-system-identifiers-are-preferred
If nil, PSGML will look up external entities by searching the catalogs in sgml-local-catalogs and sgml-catalog-files and only if the entity is not found in the catalogs will a given system identifier be used. If the variable is non-nil and a system identifier is given, the system identifier will be used for the entity. If no system identifier is given the catalogs will searched.

User Option: sgml-public-map
This should be a list of file name templates. This variable is initialized from the environment variable SGML_PATH. This is the same environment variable that `sgmls' uses. If the environment variable is undefined the default is

 
("%S" "/usr/local/lib/sgml/%o/%c/%d")

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