User Extensions

The 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. More...

Modules
	User Extensions Implementation

Files
file	extension.h
	User Extensions API.
file	extension.c
	Extension Manager Information with Zero Objects.
file	extensioncreate.c
	User Extensions Implementation.
file	extensiondelete.c
	User Extensions Implementation.
file	extensionident.c
	User Extensions Implementation.

Typedefs
typedef User_extensions_thread_create_extension	rtems_task_create_extension
typedef User_extensions_thread_delete_extension	rtems_task_delete_extension
typedef User_extensions_thread_start_extension	rtems_task_start_extension
typedef User_extensions_thread_restart_extension	rtems_task_restart_extension
typedef User_extensions_thread_switch_extension	rtems_task_switch_extension
typedef User_extensions_thread_begin_extension	rtems_task_begin_extension
typedef User_extensions_thread_exitted_extension	rtems_task_exitted_extension
typedef User_extensions_fatal_extension	rtems_fatal_extension
typedef User_extensions_thread_terminate_extension	rtems_task_terminate_extension
typedef User_extensions_Table	rtems_extensions_table
typedef Internal_errors_Source	rtems_fatal_source
typedef Internal_errors_t	rtems_fatal_code

Functions
rtems_status_code	rtems_extension_create (rtems_name name, const rtems_extensions_table *extension_table, rtems_id *id)
	Creates an extension set object. More...
rtems_status_code	rtems_extension_ident (rtems_name name, rtems_id *id)
	Identifies an extension set object by a name. More...
rtems_status_code	rtems_extension_delete (rtems_id id)
	Deletes an extension set object specified by the identifier id. More...

Detailed Description

The 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.

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 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 rtems_initialize_data_structures() 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 rtems_initialize_data_structures() 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.

Task Control Block 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 (TCB). This area is only available for the dynamic extensions. 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 identifier returned when the extension is created:

rtems_tcb *task = some_task;
size_t index = rtems_object_id_get_index(extension_id);
void *extension_data = task->extensions [index];

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.

Order of Invokation

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 start
• Task restart
• Task context switch
• Post task context switch
• Task begins to execute

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

• Task exit
• 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.

Function Documentation

rtems_extension_create()

rtems_status_code rtems_extension_create	(	rtems_name	name,
		const rtems_extensions_table *	extension_table,
		rtems_id *	id
	)

Creates an extension set object.

This directive creates a extension set object from the extension table extension_table. The assigned extension set identifier is returned in id. The identifier is used to access this extension set in other extension set related directives. The name name will be assigned to the extension set object.

Newly created extension sets are immediately installed and are invoked upon the next system event supporting an extension.

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

Return values

RTEMS_SUCCESSFUL	Extension set created successfully.
RTEMS_INVALID_ADDRESS	Identifier pointer is NULL.
RTEMS_INVALID_NAME	Invalid extension set name.
RTEMS_TOO_MANY	Too many extension sets created.

rtems_extension_delete()

rtems_status_code rtems_extension_delete

(

rtems_id

id

)

Deletes an extension set object specified by the identifier id.

Any subsequent references to the extension's name and identifier are invalid.

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

Return values

RTEMS_SUCCESSFUL	Extension set deleted successfully.
RTEMS_INVALID_ID	Invalid extension set identifier.

rtems_extension_ident()

rtems_status_code rtems_extension_ident	(	rtems_name	name,
		rtems_id *	id
	)

Identifies an extension set object by a name.

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

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

Return values

RTEMS_SUCCESSFUL	Extension set identified successfully.
RTEMS_INVALID_ADDRESS	Identifier pointer is NULL.
RTEMS_INVALID_NAME	Extension set name not found or invalid name.