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Installable File Systems (1.2)
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Installable File Systems
This topic describes how MS OS/2 enables applications to use installable
file systems. A file system is the combination of software and hardware that
supports storing information on a storage device. An installable file system
is a file system whose software can be installed when the operating system
starts. MS OS/2 supports installable file systems and permits users to have
multiple file systems active at the same time.
This section also describes some of the MS OS/2 functions that enable
applications to create, read, and write data files in installable file
systems. Because installable file systems are not available with releases of
MS OS/2 prior to version 1.2 or with MS-DOS versions 2.0 through 3.3,
applications that use the family application programming interface (family
API) cannot use functions that are specific to installable file systems.
In MS OS/2 version 1.2, users install a file system by specifying the
file-system components in the config.sys file. The file-system software
consists of device drivers that access storage devices and dynamic-link
libraries that control the format of information on a device and manage the
flow of data to and from the device. The user must use the device command to
specify the device driver and the ifs command to specify the dynamic-link
library. MS OS/2 loads the device driver and dynamic-link library and
initializes a specific device for use as a file system.
MS OS/2 version 1.2 has two file systems: the file allocation table (FAT)
file system and the high-performance file system (HPFS). These file systems
define how information is organized on the storage devices. Both file
systems create data files supported by one or more tables that specify the
location and size of the data files on the storage device.
The file allocation table (FAT) file system is the default file system for
MS OS/2; it does not need to be installed. The FAT file system, used in
previous releases of MS OS/2 and also in MS-DOS, controls storage of data
files for fixed and floppy disks. The FAT file system is hierarchical,
allowing multiple directories on the disk. Each directory can contain one or
more files. The distinguishing feature of the FAT file system is its 8.3
filename convention. Under this convention, the filename consists of a
filename (up to eight characters), a separating period (.), and a filename
extension (up to three characters).
The high-performance file system (HPFS) is an installable file system for MS
OS/2. It is an hierarchical file system and allows for multiple directories.
HPFS controls storage of data for fixed disks. Filenames under HPFS can be
any practical length and can contain characters that are not valid for the
FAT file system, for example, spaces and underscores (__). In many cases,
accessing files under HPFS is faster than accessing similar files under the
FAT file system.
A user can choose either or both file systems. Applications must be able to
work with any file system. Fortunately, MS OS/2 provides a common set of
file-system functions that are not dependent upon a particular file system;
it also gives guidelines for working with file systems, such as specific
filename conventions.
File-System Functions
MS OS/2 provides a standard set of file-system functions. This means that
applications can create, open, read, write, copy, and delete files and
directories by using the same functions, regardless of which file system is
used. When an application calls a file-system function, MS OS/2 passes the
request to the dynamic-link library that supports the file system. Most
file-system processing, such as validating filenames, is carried out by the
dynamic-link library. If an error occurs, the file system returns the error
to MS OS/2, which then passes it back to the calling application.
Occasionally, a file system may extend the standard set of file-system
functions by providing file-system control functions. The control functions
are specific to the given file system. An application can call a control
function by using the DosFSCtl function, which directs MS OS/2 to pass the
control-function information to the dynamic-link library for the particular
file system.
Volumes
MS OS/2 allows more than one file system on a single storage device. If the
device can have more than one logical partition (or volume), each partition
can be initialized as an MS OS/2 partition and given a valid MS OS/2 file
system. For each volume, MS OS/2 determines the type of file system the
first time the volume is accessed by a function or when the medium in the
drive changes. After that, MS OS/2 manages all input and output to that
volume by using the corresponding dynamic-link library for the file system.
MS OS/2 uses the volume label and serial number to ensure that the medium in
the drive does not change while there are outstanding requests for input and
output. Each volume has a volume label and a 32-bit volume serial number,
stored in a reserved location in logical sector zero at the time of
formatting. If the volume label and serial number do not match, MS OS/2
signals the critical-error handler to prompt the user to insert the volume
that has the specified serial number and label. MS OS/2 maintains the
connection between the medium and the volume label and serial number until
all open files on the volume are closed and all search references and
cache-buffer references are removed. The system redetermines the type of the
file system and the volume label and serial number for the volume only when
the medium changes.
Local and Remote File Systems
Installable file systems work with a variety of storage devices. A file
system on a local device such as a disk drive or virtual disk is called a
local file system. A file system on a remote device such as a disk drive on
another computer is called a remote file system. An application can
establish a connection to a local or a remote file system by using the
DosFSAttach function.
For a local file system, MS OS/2 uses a block device driver to handle input
and output to the device. MS OS/2 automatically connects most (if not all)
local file systems when it starts. However, an application can connect and
disconnect (sometimes called mount and dismount) additional file systems as
needed.
For a remote file system, the corresponding device driver typically accesses
a communications or network device instead of a block device driver used to
access disk hardware. Typically, the actual storage device is located on
another computer, and the two computers communicate requests and data
through a network connection. An application can connect a remote file
system to a drive letter by using the DosFSAttach function. Once the
connection is made, the application can access directories and files on the
remote device simply by using the assigned drive letter, treating the remote
device as if it were on the same computer.
Pseudo-Character Device
An application can attach a device name to a file system and use the file
system as a pseudo-character device (also called a single-file device).
Attaching a device name to a file system lets an application open the device
associated with the file system as if it were a character device (for
example, a serial port) and read from and write to the device by using the
DosRead and DosWrite functions. Unlike with a character device, an
application can use the DosChgFilePtr and DosFileLocks functions for working
with a pseudo-character device. An MS OS/2 pseudo-character device name is a
null-terminated string in the format of an MS OS/2 filename in a
subdirectory called \dev.
A file system that can be attached to a pseudo-character device is typically
associated with a single disk file or with a special storage device such as
a tape drive. The file system establishes a connection with the device and
transfers requests and data between MS OS/2 and the device. The following
example attaches the device associated with the file system cabinet1 to the
pseudo-character device named \dev\host:
BYTE bData[];
USHORT cbData;
DosFSAttach("\dev\host", "cabinet1", bData, cbData, 0);
If the application successfully attaches the file system, the application
can then open the file \dev\host by using the DosOpen function, read
host-created data by using the DosRead function, and write data and commands
to the host by using the DosWrite function. This example assumes that the
name cabinet1 corresponds to an installable file system and that the file
system can perform the necessary host communication and translation.
Filename Conventions
Filename conventions are the rules used to form names that uniquely identify
files in a given file system. Although each installable file system can have
specific rules about how individual components in a directory or filename
are formed, all file systems follow the same general conventions for
combining components. For example, the FAT file system requires that file
and directory names have the 8.3 filename format, HPFS allows names to be
any length, but both file systems use the backslash (\) character to
separate directory names and the filename when forming a path.
When creating names for directories and files or when processing names
supplied by the user, applications should follow these general rules:
1 Process a path as a null-terminated string. An application can determine
maximum length for a path by using the DosQSysInfo function.
2 Use any character in the current code page for a name, but do not use a
path separator, a character in the range 0 through 31, or any character
explicitly disallowed by the file system. Although a name can contain
characters above 127, an application must be able to switch code pages if
necessary to access the corresponding file.
3 Compare names using a case-insensitive comparison. Names such as ABC,
Abc, and abc are considered to be the same name.
4 Use the backslash (\) and/or the forward slash (/) to separate components
in a path. No other character is accepted as a path separator.
5 Use the dot (.) as a directory component in a path to represent the
current directory.
6 Use two dots (..) as a directory component in a path to represent the
parent of the current directory.
7 Use a period (.) to separate components in a directory name or filename.
Unless explicitly defined by a file system, there are no restrictions on
the number of components in a name.
Filenames in DOS-Compatibility Mode
For compatibility with existing DOS 3.x applications, all file systems
support the FAT file system's 8.3 filename format. This means that
applications running in DOS-compatibility mode can access files on non-FAT
file systems if the filenames have the 8.3 format. To guarantee this rule,
MS OS/2 automatically applies the 8.3 truncation rules to all filenames
given in file-system requests from DOS-compatibility mode.
Filenames in User Input
Users often supply filenames as part of an application's command line or in
response to a prompt from the application. Traditionally, users have been
able to supply more than one filename on the command line or in a prompt by
separating the names with certain characters, such as a blank space. In some
file systems, however, traditional separators can be used as valid filename
characters. This means that some additional conventions are required to
ensure that an application processes all characters in a name.
When an application processes arguments (including filenames) from its
command line, the application should treat the double quotation mark (") and
the caret (^) as quotation characters. All characters between the starting
and closing double quotation marks should be processed as a single argument.
The character immediately following the caret should be processed as part of
the current argument. In both cases, the quotation characters are discarded
and not treated as part of the final argument.
When an application processes two or more filenames from a dialog box or
other prompt, it expects the user to enter each filename on a new line. For
example, a Presentation Manager application should use a multiple-line entry
field to prompt for multiple filenames. This makes the use of quotation
characters unnecessary.
When an application is started from File Manager, File Manager may construct
a command line for the application. If the command line includes filenames,
File Manager separates each argument with a space character and marks the
end of the argument list with two null characters. Programs that start other
applications by using the DosExecPgm function also can pass arguments using
this convention or by using quotation characters. In practice, most
applications receive a command line as a single, null-terminated string.
Therefore, applications that use the DosExecPgm function should prepare
command lines as a single string with any filenames in the string enclosed
in quotation marks.
Metacharacters in Filenames
To give the user a shortcut to entering long lists of names, applications
that accept more than one filename on their command line can allow
metacharacters in filenames. The metacharacters, the asterisk (*) and the
question mark (?), represent placeholders in a filename. Although a name
that contains metacharacters is not a complete filename, an application can
use functions, such as DosFindFirst and DosEditName, to expand the name
(replace the metacharacters) to create one or more valid filenames.
An application can expand a name with metacharacters to a list of filenames
by using the DosFindFirst function. The asterisk (*) matches one or more
characters, including blanks. The question mark (?) matches one character,
unless that character is a period (.). To match a period, the original name
must contain a period.
An application can create a new filename from an existing name by using the
DosEditName function. This function takes a template (a name with
metacharacters) and expands it, using characters from an existing name. An
asterisk (*) in the template directs the function to copy all characters in
the existing name until it locates a character that matches the character
following the asterisk. A question mark (?) directs the function to copy one
character unless that character is a period. The period (.) in the template
directs the function look for and move to the next period in the existing
name, skipping any characters between the current position and the period.
The metacharacters are illegal in all but the last component of a path.
File-System Errors
Some MS OS/2 file-system functions return the following errors:
Value Meaning
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ERROR_WRITE_PROTECT The disk in the drive is write-protected.
ERROR_BAD_UNIT There is a breakdown of internal consistency in
mapping between the logical drive and the device
driver. Internal error.
ERROR_NOT_READY The device is not ready.
ERROR_BAD_COMMAND There is a breakdown of internal consistency between
the expected capability of a device driver and its
true capability.
ERROR_CRC The device driver detected a cyclic redundancy check
(CRC) mismatch.
ERROR_BAD_LENGTH There is a breakdown of internal consistency between
the expected length of a request packet and the true
length. Internal error.
ERROR_SEEK The device driver detected an error during a seek
operation.
ERROR_NOT_DOS_DISK The disk is not recognized as being manageable by MS
OS/2.
ERROR_SECTOR_NOT_FOUND The device is unable to find the specific sector.
ERROR_OUT_OF_PAPER The printer is out of paper.
ERROR_WRITE_FAULT Other write-specific error.
ERROR_READ_FAULT Other read-specific error.
ERROR_GEN_FAILURE Other error.
There are also errors defined by and specific to the specific device driver.
These are indicated by either 0xFF or 0xFE in the high byte of the error
code.
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