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                         Using Extended Attributes
 
Applications can examine, add, and replace extended attributes at any time.
The DosOpen2 function adds extended attributes to new or existing files; the
DosMkDir2 function adds extended attributes to new directories. Any
application can read the extended attributes by using the DosQFileInfo or
DosQPathInfo function. Applications can also search for files that have
specific extended attributes by using the DosFindFirst and DosFindNext
functions.
 
A file can have any number of extended attributes. Each extended attribute
can be up to 64K long. For MS OS/2 version 1.2, the sum of all extended
attributes for a file must not exceed 64K.
 
Naming Conventions
 
Although an application can choose any name for the extended attributes it
creates, other applications cannot read the extended attributes unless they
also recognize the corresponding format. Because many applications use
extended attributes consisting of text, bitmaps, and other similar data, a
set of names has been adopted to help identify these formats when used in
extended attributes. An application need not be limited to this set of
standard extended attributes but should use it as a way for many
applications to access a common set of information.
 
The names for all standard extended attributes use a dot (.) as a prefix.
The leading dot is considered reserved, so no application should define
extended attributes that start with a dot. Also, extended attributes that
start with the characters $, @, &, and + are reserved for system use. To
ensure that its extended attributes are unique, an application should use
the vendor and application name as a prefix for application-specific
extended attributes. For example, Microsoft Excel would use MS
EXCEL.MYSTUFF, MS EXCEL.MORESTUFF, and so forth.
 
Data-Type Conventions
 
Extended attributes can contain any type of data. To identify the type of
information, the first word of extended-attribute data should specify one of
the following data types:
 
Value         Meaning
────────────────────────────────────────────────────────────────────────────
EAT_BINARY    Binary data; the first word specifies length.
 
EAT_ASCII     ASCII text; the first word specifies length.
 
EAT_BITMAP    Bitmap data; the first word specifies length.
 
EAT_METAFILE  Metafile data; the first word specifies length.
 
EAT_ICON      Icon data; the first word specifies length.
 
EAT_EA        ASCII name of associated data; the first word specifies
              length.
 
EAT_MVMT      Two or more consecutive extended-attribute values; each value
              has an explicitly specified type.
 
EAT_MVST      Two or more consecutive extended-attribute values; all values
              have the same type.
 
EAT_ASN1      ASN.1 field data.
 
In all cases, the length specifies the number of bytes of data. Other values
for data types, in the range 0x0000 through 0x7FFF, can be used for
user-defined extended attributes. User-defined data should also specify the
length.
 
For example, here is how to represent the string "Hello":
 
EAT_ASCII               0005            Hello
 
Standard Extended Attributes
 
The standard extended attributes are listed in the following sections. The
field format follows the data-type conventions given previously. A field can
be a multivalue or single-value field.
 
.TYPE
 
The .TYPE extended attribute indicates the type of file. It is similar to
the earlier use of filename extensions. The following file types are
predefined:
 
Plain Text
OS/2 Command File
DOS Command File
Executable
Metafile
Bitmap
Icon
Binary Data
Dynamic-Link Library
C Code
Pascal Code
BASIC Code
COBOL Code
FORTRAN Code
Assembler Code
Library
Resource File
 
Applications can use their own type names, such as Microsoft Excel Chart.
The first words in the type name should be the name of the vendor and the
application. For example, Microsoft Excel Chart, Microsoft Excel Worksheet,
Lotus 1-2-3 Spreadsheet.
 
Entries should be ASCII. Case is important.
 
The performance of extended attributes is dependent on the file system.
Because some file systems store extended attributes in first-in/first out
(FIFO) order, it is important to write the .TYPE entry first so that File
Manager can access that information quickly.
 
.KEYPHRASES
 
The .KEYPHRASES extended attribute specifies text key phrases for the file.
Such phrases can be used for a database-style search or to help the user
understand the nature of the file.
 
If there is more than one key phrase, each should be stored in a separate
entry in a multivalue field. Each entry should be ASCII.
 
.SUBJECT
 
The .SUBJECT extended attribute contains a brief summary of the file's
content and/or purpose. This attribute should be less than 40 characters
long.
 
This field should be a single-value ASCII entry.
 
.COMMENTS
 
The .COMMENTS extended attribute contains miscellaneous notes about the
file. It can be a multivalue field and be of any type. This field is
intended as a reminder note. For example, it could contain some notes about
the intent of a file or a picture.
 
.HISTORY
 
The .HISTORY extended attribute lists the history of a file's modification.
It lists the author of the file and all subsequent changes. Each action
entry should be a separate field in a multivalue field. Each entry should be
ASCII.
 
The application can let the user decide when an entry is placed into the
history field, to avoid unnecessary file growth. For example, there are some
cases when it is important to note when a document is printed; however, it
is probably unnecessary to note every time the file was printed.
 
.VERSION
 
The .VERSION extended attribute is a version number of the file format (for
example, Excel Worksheet 1).
 
This attribute should be ASCII or binary. It should be modified only by the
application. This attribute can also be used to indicate an application or
dynamic-link library version.
 
.ICON
 
The .ICON extended attribute specifies the icon to be used for the file
representation, whether in File Manager or when minimized. File Manager can
use the .TYPE entry to determine the default application to run and to
determine the default icon for that type of file. If there is a .ICON entry,
however, it is used instead of the icon associated with a particular type.
 
If the data type is for an icon, the icon data follows. It is best to
provide as much icon information as possible. Ideally, an icon should be
64-by-64 bits in 8-color, device-independent format.
 
Executable files should simply store the binary icon data in this extended
attribute. They should use the .ASSOCTABLE extended attribute to install
icons for data files.
 
.ASSOCTABLE
 
The .ASSOCTABLE extended attribute contains association data for a file. It
is created by the Microsoft Operating System/2 Resource Compiler (rc), from
a table with the following form:
 
ASSOCTABLE assoctable -id
BEGIN
        "type name", "extension", [flags], [icon filename]
        .
        .
END
 
The .ASSOCTABLE extended attribute contains information that associates
icons with the data files an application creates. The file-association table
associates icons by data type.
 
The .ASSOCTABLE extended attribute allows an application to indicate the
type, extension, and icon for the data files it recognizes. It also contains
an ownership flag. This data can be installed automatically by File
Manager.
 
For example, the table for Microsoft Excel could be:
 
"MS Excel Worksheet", "XLS", AF_DEFAULTOWNER, excel.sheet.icon
"MS Excel Chart", "XLC", AF_DEFAULTOWNER, excel.chart.icon
 
The flag entry indicates if the application owns or merely recognizes the
type. The icon file contains an icon for that data type.
 
.HPFSNAME
 
The .HPFSNAME attribute is used when an application attempts to write a file
with a long name to a file system that does not support long names. The
application should generate a unique short name for the file and notify the
user of the new short name. It should then save the original (long) name in
the .HPFSNAME extended attribute.
 
When a file is copied from a system that uses short names to a system that
uses long names, the application should check the .HPFSNAME extended
attribute. If a value is present, the application should allow the file to
be renamed to a long name. The .HPFSNAME extended attribute should then be
removed.
 
Supporting Extended Attributes
 
To support extended attributes, applications should do the following:
 
1  Fill in the .ASSOCTABLE extended attribute for all major file types that
   the application recognizes or uses.
 
2  Fill in the .ICON extended attribute for executable files.
 
3  Set the .TYPE field for data files it creates.
 
4  Fill in and use the .HPFSNAME extended attribute as appropriate.
 
5  Support .HISTORY and .VERSION.
 
6  Support the other standard extended attributes as appropriate.
 
Multivalue Data-Type Fields
 
In many cases, extended attributes need to store more than a single piece of
information. For example, an extended attribute can store a list of names of
people to whom a mail document was sent. The multivalue formats specify how
individual pieces of data are stored.
 
In a multivalue field, the first entry in the list is assumed to be the
default. For example, suppose the .TYPE entry contains Text and C Code. Text
is the default type. If C Code is the first entry in the list (C Code and
Text), then C Code is the default type.
 
Multivalue, Multitype Attributes
 
The EAT_MVMT type allows a single extended attribute to contain several
pieces of information; each piece of information can be a different type.
 
Multivalue, Single-Type Attributes
 
The EAT_MVST type sets up a multivalue field in which each piece of
information is of the same type.
 
ASN.1
 
The EAT_ASN1 type is an ISO standard for describing multivalue data
streams.
 
Include-Extended-Attribute Type
 
The EAT_EA type indicates that the data is continued in another
extended-attribute entry associated with the file. Among other things, this
allows for extended attributes greater than 64K (but not exceeding the limit
per file).
 
 
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