23. User Extensions Manager¶

23.1. Introduction¶

The RTEMS User Extensions Manager allows the application developer to augment the executive by allowing them to supply extension routines which are invoked at critical system events. The directives provided by the user extensions manager are:

rtems_extension_create - Create an extension set
rtems_extension_ident - Get ID of an extension set
rtems_extension_delete - Delete an extension set

23.2. Background¶

User extension routines are invoked when the following system events occur:

Task creation
Task initiation
Task reinitiation
Task deletion
Task context switch
Post task context switch
Task begin
Task exits
Fatal error detection

These extensions are invoked as a function with arguments that are appropriate to the system event.

23.2.1. Extension Sets¶

An extension set is defined as a set of routines which are invoked at each of the critical system events at which user extension routines are invoked. Together a set of these routines typically perform a specific functionality such as performance monitoring or debugger support. RTEMS is informed of the entry points which constitute an extension set via the following structure:.. index:: rtems_extensions_table

typedef struct {
    rtems_task_create_extension      thread_create;
    rtems_task_start_extension       thread_start;
    rtems_task_restart_extension     thread_restart;
    rtems_task_delete_extension      thread_delete;
    rtems_task_switch_extension      thread_switch;
    rtems_task_begin_extension       thread_begin;
    rtems_task_exitted_extension     thread_exitted;
    rtems_fatal_extension            fatal;
} rtems_extensions_table;

RTEMS allows the user to have multiple extension sets active at the same time. First, a single static extension set may be defined as the application’s User Extension Table which is included as part of the Configuration Table. This extension set is active for the entire life of the system and may not be deleted. This extension set is especially important because it is the only way the application can provided a FATAL error extension which is invoked if RTEMS fails during the initialize_executive directive. The static extension set is optional and may be configured as NULL if no static extension set is required.

Second, the user can install dynamic extensions using the rtems_extension_create directive. These extensions are RTEMS objects in that they have a name, an ID, and can be dynamically created and deleted. In contrast to the static extension set, these extensions can only be created and installed after the initialize_executive directive successfully completes execution. Dynamic extensions are useful for encapsulating the functionality of an extension set. For example, the application could use extensions to manage a special coprocessor, do performance monitoring, and to do stack bounds checking. Each of these extension sets could be written and installed independently of the others.

All user extensions are optional and RTEMS places no naming restrictions on the user. The user extension entry points are copied into an internal RTEMS structure. This means the user does not need to keep the table after creating it, and changing the handler entry points dynamically in a table once created has no effect. Creating a table local to a function can save space in space limited applications.

Extension switches do not effect the context switch overhead if no switch handler is installed.

23.2.2. TCB Extension Area¶

RTEMS provides for a pointer to a user-defined data area for each extension set to be linked to each task’s control block. This set of pointers is an extension of the TCB and can be used to store additional data required by the user’s extension functions.

The TCB extension is an array of pointers in the TCB. The index into the table can be obtained from the extension id returned when the extension is created:

index = rtems_object_id_get_index(extension_id);

The number of pointers in the area is the same as the number of user extension sets configured. This allows an application to augment the TCB with user-defined information. For example, an application could implement task profiling by storing timing statistics in the TCB’s extended memory area. When a task context switch is being executed, the TASK_SWITCH extension could read a real-time clock to calculate how long the task being swapped out has run as well as timestamp the starting time for the task being swapped in.

If used, the extended memory area for the TCB should be allocated and the TCB extension pointer should be set at the time the task is created or started by either the TASK_CREATE or TASK_START extension. The application is responsible for managing this extended memory area for the TCBs. The memory may be reinitialized by the TASK_RESTART extension and should be deallocated by the TASK_DELETE extension when the task is deleted. Since the TCB extension buffers would most likely be of a fixed size, the RTEMS partition manager could be used to manage the application’s extended memory area. The application could create a partition of fixed size TCB extension buffers and use the partition manager’s allocation and deallocation directives to obtain and release the extension buffers.

23.2.3. Extensions¶

The sections that follow will contain a description of each extension. Each section will contain a prototype of a function with the appropriate calling sequence for the corresponding extension. The names given for the C function and its arguments are all defined by the user. The names used in the examples were arbitrarily chosen and impose no naming conventions on the user.

23.2.3.1. TASK_CREATE Extension¶

The TASK_CREATE extension directly corresponds to the rtems_task_create directive. If this extension is defined in any static or dynamic extension set and a task is being created, then the extension routine will automatically be invoked by RTEMS. The extension should have a prototype similar to the following:

bool user_task_create(
   rtems_tcb *current_task,
   rtems_tcb *new_task
);

where current_task can be used to access the TCB for the currently executing task, and new_task can be used to access the TCB for the new task being created. This extension is invoked from the rtems_task_create directive after new_task has been completely initialized, but before it is placed on a ready TCB chain.

The user extension is expected to return the boolean value true if it successfully executed and false otherwise. A task create user extension will frequently attempt to allocate resources. If this allocation fails, then the extension should return false and the entire task create operation will fail.

23.2.3.2. TASK_START Extension¶

The TASK_START extension directly corresponds to the task_start directive. If this extension is defined in any static or dynamic extension set and a task is being started, then the extension routine will automatically be invoked by RTEMS. The extension should have a prototype similar to the following:

void user_task_start(
    rtems_tcb *current_task,
    rtems_tcb *started_task
);

where current_task can be used to access the TCB for the currently executing task, and started_task can be used to access the TCB for the dormant task being started. This extension is invoked from the task_start directive after started_task has been made ready to start execution, but before it is placed on a ready TCB chain.

23.2.3.3. TASK_RESTART Extension¶

The TASK_RESTART extension directly corresponds to the task_restart directive. If this extension is defined in any static or dynamic extension set and a task is being restarted, then the extension should have a prototype similar to the following:

void user_task_restart(
    rtems_tcb *current_task,
    rtems_tcb *restarted_task
);

where current_task can be used to access the TCB for the currently executing task, and restarted_task can be used to access the TCB for the task being restarted. This extension is invoked from the task_restart directive after restarted_task has been made ready to start execution, but before it is placed on a ready TCB chain.

23.2.3.4. TASK_DELETE Extension¶

The TASK_DELETE extension is associated with the task_delete directive. If this extension is defined in any static or dynamic extension set and a task is being deleted, then the extension routine will automatically be invoked by RTEMS. The extension should have a prototype similar to the following:

void user_task_delete(
    rtems_tcb *current_task,
    rtems_tcb *deleted_task
);

where current_task can be used to access the TCB for the currently executing task, and deleted_task can be used to access the TCB for the task being deleted. This extension is invoked from the task_delete directive after the TCB has been removed from a ready TCB chain, but before all its resources including the TCB have been returned to their respective free pools. This extension should not call any RTEMS directives if a task is deleting itself (current_task is equal to deleted_task).

23.2.3.5. TASK_SWITCH Extension¶

The TASK_SWITCH extension corresponds to a task context switch. If this extension is defined in any static or dynamic extension set and a task context switch is in progress, then the extension routine will automatically be invoked by RTEMS. The extension should have a prototype similar to the following:

void user_task_switch(
    rtems_tcb *current_task,
    rtems_tcb *heir_task
);

where current_task can be used to access the TCB for the task that is being swapped out, and heir_task can be used to access the TCB for the task being swapped in. This extension is invoked from RTEMS’ dispatcher routine after the current_task context has been saved, but before the heir_task context has been restored. This extension should not call any RTEMS directives.

23.2.3.6. TASK_BEGIN Extension¶

The TASK_BEGIN extension is invoked when a task begins execution. It is invoked immediately before the body of the starting procedure and executes in the context in the task. This user extension have a prototype similar to the following:

void user_task_begin(
    rtems_tcb *current_task
);

where current_task can be used to access the TCB for the currently executing task which has begun. The distinction between the TASK_BEGIN and TASK_START extension is that the TASK_BEGIN extension is executed in the context of the actual task while the TASK_START extension is executed in the context of the task performing the task_start directive. For most extensions, this is not a critical distinction.

23.2.3.7. TASK_EXITTED Extension¶

The TASK_EXITTED extension is invoked when a task exits the body of the starting procedure by either an implicit or explicit return statement. This user extension have a prototype similar to the following:

void user_task_exitted(
    rtems_tcb *current_task
);

where current_task can be used to access the TCB for the currently executing task which has just exitted.

Although exiting of task is often considered to be a fatal error, this extension allows recovery by either restarting or deleting the exiting task. If the user does not wish to recover, then a fatal error may be reported. If the user does not provide a TASK_EXITTED extension or the provided handler returns control to RTEMS, then the RTEMS default handler will be used. This default handler invokes the directive fatal_error_occurred with the RTEMS_TASK_EXITTED directive status.

23.2.3.8. FATAL Error Extension¶

The FATAL error extension is associated with the fatal_error_occurred directive. If this extension is defined in any static or dynamic extension set and the fatal_error_occurred directive has been invoked, then this extension will be called. This extension should have a prototype similar to the following:

void user_fatal_error(
    Internal_errors_Source  the_source,
    bool                    is_internal,
    uint32_t                the_error
);

where the_error is the error code passed to the fatal_error_occurred directive. This extension is invoked from the fatal_error_occurred directive.

If defined, the user’s FATAL error extension is invoked before RTEMS’ default fatal error routine is invoked and the processor is stopped. For example, this extension could be used to pass control to a debugger when a fatal error occurs. This extension should not call any RTEMS directives.

23.2.4. Order of Invocation¶

When one of the critical system events occur, the user extensions are invoked in either “forward” or “reverse” order. Forward order indicates that the static extension set is invoked followed by the dynamic extension sets in the order in which they were created. Reverse order means that the dynamic extension sets are invoked in the opposite of the order in which they were created followed by the static extension set. By invoking the extension sets in this order, extensions can be built upon one another. At the following system events, the extensions are invoked in forward order:

Task creation
Task initiation
Task reinitiation
Task deletion
Task context switch
Post task context switch
Task begins to execute

At the following system events, the extensions are invoked in reverse order:

Task deletion
Fatal error detection

At these system events, the extensions are invoked in reverse order to insure that if an extension set is built upon another, the more complicated extension is invoked before the extension set it is built upon. For example, by invoking the static extension set last it is known that the “system” fatal error extension will be the last fatal error extension executed. Another example is use of the task delete extension by the Standard C Library. Extension sets which are installed after the Standard C Library will operate correctly even if they utilize the C Library because the C Library’s TASK_DELETE extension is invoked after that of the other extensions.

23.3. Operations¶

23.3.1. Creating an Extension Set¶

The rtems_extension_create directive creates and installs an extension set by allocating a Extension Set Control Block (ESCB), assigning the extension set a user-specified name, and assigning it an extension set ID. Newly created extension sets are immediately installed and are invoked upon the next system even supporting an extension.

23.3.2. Obtaining Extension Set IDs¶

When an extension set is created, RTEMS generates a unique extension set ID and assigns it to the created extension set until it is deleted. The extension ID may be obtained by either of two methods. First, as the result of an invocation of the rtems_extension_create directive, the extension set ID is stored in a user provided location. Second, the extension set ID may be obtained later using the rtems_extension_ident directive. The extension set ID is used by other directives to manipulate this extension set.

23.3.3. Deleting an Extension Set¶

The rtems_extension_delete directive is used to delete an extension set. The extension set’s control block is returned to the ESCB free list when it is deleted. An extension set can be deleted by a task other than the task which created the extension set. Any subsequent references to the extension’s name and ID are invalid.

23.4. Directives¶

This section details the user extension manager’s directives. A subsection is dedicated to each of this manager’s directives and describes the calling sequence, related constants, usage, and status codes.

23.4.1. EXTENSION_CREATE - Create a extension set¶

CALLING SEQUENCE:

rtems_status_code rtems_extension_create(
    rtems_name              name,
    rtems_extensions_table *table,
    rtems_id               *id
);

DIRECTIVE STATUS CODES:

`RTEMS_SUCCESSFUL`	extension set created successfully
`RTEMS_INVALID_NAME`	invalid extension set name
`RTEMS_TOO_MANY`	too many extension sets created

DESCRIPTION:

This directive creates a extension set. The assigned extension set id is returned in id. This id is used to access the extension set with other user extension manager directives. For control and maintenance of the extension set, RTEMS allocates an ESCB from the local ESCB free pool and initializes it.

NOTES:

This directive will not cause the calling task to be preempted.

23.4.2. EXTENSION_IDENT - Get ID of a extension set¶

CALLING SEQUENCE:

rtems_status_code rtems_extension_ident(
    rtems_name  name,
    rtems_id   *id
);

DIRECTIVE STATUS CODES:

`RTEMS_SUCCESSFUL`	extension set identified successfully
`RTEMS_INVALID_NAME`	extension set name not found

DESCRIPTION:

This directive obtains the extension set id associated with the extension set name to be acquired. If the extension set name is not unique, then the extension set id will match one of the extension sets with that name. However, this extension set id is not guaranteed to correspond to the desired extension set. The extension set id is used to access this extension set in other extension set related directives.

NOTES:

This directive will not cause the running task to be preempted.

23.4.3. EXTENSION_DELETE - Delete a extension set¶

CALLING SEQUENCE:

rtems_status_code rtems_extension_delete(
    rtems_id id
);

DIRECTIVE STATUS CODES:

`RTEMS_SUCCESSFUL`	extension set deleted successfully
`RTEMS_INVALID_ID`	invalid extension set id

DESCRIPTION:

This directive deletes the extension set specified by id. If the extension set is running, it is automatically canceled. The ESCB for the deleted extension set is reclaimed by RTEMS.

NOTES:

This directive will not cause the running task to be preempted.

A extension set can be deleted by a task other than the task which created the extension set.

NOTES:

This directive will not cause the running task to be preempted.