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prefix

Examine or change the pathname prefix tree (QNX)

Syntax:

prefix [-A prefix=string] [-D prefix] 
       [-n node_id] [-R prefix=string]
       [-s]

Options:

-A prefix=string

Add prefix with alias string.

-D prefix

Delete the specified prefix. If the prefix is directly owned by a resource manager (is not an alias prefix), unmount the device. (See umount.)

-n node_id

Print the prefix string from the indicated node.

-R prefix=string

Replace the specified prefix with new alias string. This is the equivalent of deleting the prefix (-D prefix) then adding the prefix (-A prefix=string).

-s

When adding/changing an alias prefix, skip checks for real mount points.

Description:

The pathname space is administered through the use of two forms of prefixes:

1. prefixes registered by I/O managers
2. aliases

The prefix utility allows you to examine and modify the pathname prefix tree.

The prefix tree is essentially an in-memory pathname redirector that is maintained on each computer. When a file is opened, its pathname is matched against the prefix strings in order to direct the request to open the file to the appropriate I/O manager.

For example, the character device manager Dev registers the /dev prefix. Therefore, when a process calls open() with /dev/xxx, a prefix match of /dev will occur and the open() will be directed to Dev (the prefix's owner).

The form of an I/O manager prefix is as follows:

    prefix=pid,unit

where pid is the process ID of the I/O resource manager and the unit number is a single character assigned by the I/O manager. For example, if Fsys is process 3 and Dev is process 5, then the prefix tree might appear as:

    /=3,b
    /dev=5,a
    /dev/hd0=3,a
    /dev/hd0t77=3,d

Note that Fsys has registered three prefixes:

• the root (/)
• a block special file representing the entire hard disk (/dev/hd0)
• a block special file representing a QNX 4 partition (/dev/hd0t77)

In cases where prefixes overlap, the longest match always wins. Therefore, in the example above, references to /dev/hd0 and /dev/hd0t77 are directed to Fsys rather than to Dev, while all other pathnames starting with /dev are still forwarded to Dev.

Aliases

In addition to this absolute redirection to an I/O manager, another form of prefix, called an alias, lets you replace a matched prefix with a new prefix, which is then re-applied to the prefix tree. Unlike the I/O manager form of prefix, aliases can be modified using the prefix utility. An alias is of the form:

    prefix=replacement_string

Aliases are often used to map remote resources into the local pathname space. For example, a diskless workstation might typically create an alias for its root (/) to a filesystem on another node:

    prefix -A /=//10/

Aliases can also be used to create special device names. For example, the following command:

    prefix -A /dev/console=//20/dev/con1

would alias /dev/console to the primary console on node 20.

The -A option adds a new prefix to the prefix tree. The prefix being added must not already be in the tree.

The -R option deletes, then adds, a prefix to the prefix tree.

The -D option deletes an existing prefix from the prefix tree.

Examples:

List the prefix tree currently in effect:

    prefix

List the prefix tree in effect on node 20:

    prefix -n 20

Create prefix aliases for /home1=//10/home1 and /home2=//14/home2:

    prefix -A /home1=//10/home1 -A /home2=//14/home2

Create a prefix alias for /dev/printer pointing to //12/dev/spool/text:

    prefix -A /dev/printer=//12/dev/spool/text

Remove the prefix for /dev/printer:

    prefix -D /dev/printer

Caveats:

Since redefining the prefix aliases affects all the processes on the computer, you should do this with caution, especially when dealing with definitions for root (/).

See also:

ln

System Architecture

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