2. Pathname Evaluation

This chapter describes the pathname evaluation process for the RTEMS Filesystem Infrastructure.

XXX Include graphic of the path evaluation process

2.1. Pathname Evaluation Handlers

There are two pathname evaluation routines. The handler patheval() is called to find, verify privlages on and return information on a node that exists. The handler evalformake() is called to find, verify permissions, and return information on a node that is to become a parent. Additionally, evalformake() returns a pointer to the start of the name of the new node to be created.

Pathname evaluation is specific to a filesystem. Each filesystem is required to provide both a patheval() and an evalformake() routine. Both of these routines gets a name to evaluate and a node indicating where to start the evaluation.

2.2. Crossing a Mount Point During Path Evaluation

If the filesystem supports the mount command, the evaluate routines must handle crossing the mountpoint. The evaluate routine should evaluate the name upto the first directory node where the new filesystem is mounted. The filesystem may process terminator characters prior to calling the evaluate routine for the new filesystem. A pointer to the portion of the name which has not been evaluated along with the root node of the new file system (gotten from the mount table entry) is passed to the correct mounted filesystem evaluate routine.

2.3. The rtems_filesystem_location_info_t Structure

The rtems_filesystem_location_info_t structure contains all information necessary for identification of a node.

The generic rtems filesystem code defines two global rtems_filesystem_location_info_t structures, the``rtems_filesystem_root`` and the rtems_filesystem_current. Both are initially defined to be the root node of the base filesystem. Once the chdir command is correctly used the rtems_filesystem_current is set to the location specified by the command.

The filesystem generic code peeks at the first character in the name to be evaluated. If this character is a valid seperator, the``rtems_filesystem_root`` is used as the node to start the evaluation with. Otherwise, the rtems_filesystem_current node is used as the node to start evaluating with. Therefore, a valid rtems_filesystem_location_info_t is given to the evaluate routine to start evaluation with. The evaluate routines are then responsible for making any changes necessary to this structure to correspond to the name being parsed.

struct rtems_filesystem_location_info_tt {
    void                                     *node_access;
    rtems_filesystem_file_handlers_r         *handlers;
    rtems_filesystem_operations_table        *ops;
    rtems_filesystem_mount_table_entry_t     *mt_entry;
};

node_access

This element is filesystem specific. A filesystem can define and store any information necessary to identify a node at this location. This element is normally filled in by the filesystem’s evaluate routine. For the filesystem’s root node, the filesystem’s initilization routine should fill this in, and it should remain valid until the instance of the filesystem is unmounted.

handlers

This element is defined as a set of routines that may change within a given filesystem based upon node type. For example a directory and a memory file may have to completely different read routines. This element is set to an initialization state defined by the mount table, and may be set to the desired state by the evaluation routines.

ops

This element is defined as a set of routines that remain static for the filesystem. This element identifies entry points into the filesystem to the generic code.

mt_entry

This element identifies the mount table entry for this instance of the filesystem.