source: rtems/doc/user/userext.t @ 9c8530f7

@c
@c  COPYRIGHT (c) 1988-2002.
@c  On-Line Applications Research Corporation (OAR).
@c  All rights reserved.
@c
@c  $Id$
@c

@chapter User Extensions Manager

@cindex user extensions

@section 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:

@itemize @bullet
@item @code{@value{DIRPREFIX}extension_create} - Create an extension set
@item @code{@value{DIRPREFIX}extension_ident} - Get ID of an extension set
@item @code{@value{DIRPREFIX}extension_delete} - Delete an extension set
@end itemize

@section Background

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

@itemize @bullet
@item Task creation
@item Task initiation
@item Task reinitiation
@item Task deletion
@item Task context switch
@item Post task context switch
@item Task begin
@item Task exits
@item Fatal error detection
@end itemize

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

@subsection Extension Sets

@cindex extension set

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 @value{STRUCTURE}:

@findex rtems_extensions_table
@ifset is-C
@example
@group
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;
@end group
@end example
@end ifset

@ifset is-Ada
@example
type Extensions_Table is
   record
      Task_Create      : RTEMS.Task_Create_Extension;
      Task_Start       : RTEMS.Task_Start_Extension;
      Task_Restart     : RTEMS.Task_Restart_Extension;
      Task_Delete      : RTEMS.Task_Delete_Extension;
      Task_Switch      : RTEMS.Task_Switch_Extension;
      Task_Post_Switch : RTEMS.Task_Post_Switch_Extension;
      Task_Begin       : RTEMS.Task_Begin_Extension;
      Task_Exitted     : RTEMS.Task_Exitted_Extension;
      Fatal            : RTEMS.Fatal_Error_Extension;
   end record;
@end example
@end ifset


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 @code{@value{DIRPREFIX}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.

@subsection TCB Extension Area

@cindex 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.  It is also possible for a user extension
to utilize the notepad locations associated with each task
although this may conflict with application usage of those
particular notepads.

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:

@findex rtems extensions table index
@ifset is-C
@example
@group
index = rtems_get_index(extension_id);
@end group
@end example
@end ifset

@ifset is-Ada
@example
There is currently no example for Ada.
@end example
@end ifset

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.

@subsection 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 @value{LANGUAGE} 
@value{ROUTINE} 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.

@subsubsection TASK_CREATE Extension

The TASK_CREATE extension directly corresponds to the
@code{@value{DIRPREFIX}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:

@findex rtems_task_create_extension
@findex rtems_extension
@ifset is-C
@example
boolean user_task_create(
  rtems_tcb *current_task,
  rtems_tcb *new_task
);
@end example
@end ifset

@ifset is-Ada
@example
function User_Task_Create (
   Current_Task : in     RTEMS.TCB_Pointer;
   New_Task     : in     RTEMS.TCB_Pointer
) returns Boolean;
@end example
@end ifset

where @code{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 @code{@value{DIRPREFIX}task_create}
directive after @code{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 @code{TRUE} if it successfully executed and
@code{FALSE} otherwise.  A task create user extension
will frequently attempt to allocate resources.  If this
allocation fails, then the extension should return
@code{FALSE} and the entire task create operation
will fail.

@subsubsection 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:

@findex rtems_task_start_extension
@ifset is-C
@example
rtems_extension user_task_start(
  rtems_tcb *current_task,
  rtems_tcb *started_task
);
@end example
@end ifset

@ifset is-Ada
@example
procedure User_Task_Start (
   Current_Task : in     RTEMS.TCB_Pointer;
   Started_Task : in     RTEMS.TCB_Pointer
);
@end example
@end ifset

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.

@subsubsection 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:

@findex rtems_task_restart_extension
@ifset is-C
@example
rtems_extension user_task_restart(
  rtems_tcb *current_task,
  rtems_tcb *restarted_task
);
@end example
@end ifset

@ifset is-Ada
@example
procedure User_Task_Restart (
   Current_Task   : in     RTEMS.TCB_Pointer;
   Restarted_Task : in     RTEMS.TCB_Pointer
);
@end example
@end ifset

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.

@subsubsection 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:

@findex rtems_task_delete_extension
@ifset is-C
@example
rtems_extension user_task_delete(
  rtems_tcb *current_task,
  rtems_tcb *deleted_task
);
@end example
@end ifset

@ifset is-Ada
@example
procedure User_Task_Delete (
   Current_Task : in     RTEMS.TCB_Pointer;
   Deleted_Task : in     RTEMS.TCB_Pointer
);
@end example
@end ifset

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).

@subsubsection 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:

@findex rtems_task_switch_extension
@ifset is-C
@example
rtems_extension user_task_switch(
  rtems_tcb *current_task,
  rtems_tcb *heir_task
);
@end example
@end ifset

@ifset is-Ada
@example
procedure User_Task_Switch (
   Current_Task : in     RTEMS.TCB_Pointer;
   Heir_Task    : in     RTEMS.TCB_Pointer
);
@end example
@end ifset

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.

@subsubsection 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:

@findex rtems_task_begin_extension
@ifset is-C
@example
rtems_extension user_task_begin(
  rtems_tcb *current_task
);
@end example
@end ifset

@ifset is-Ada
@example
procedure User_Task_Begin (
   Current_Task : in     RTEMS.TCB_Pointer
);
@end example
@end ifset

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.

@subsubsection 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:

@findex rtems_task_exitted_extension
@ifset is-C
@example
rtems_extension user_task_exitted(
  rtems_tcb *current_task
);
@end example
@end ifset

@ifset is-Ada
@example
procedure User_Task_Exitted (
   Current_Task : in     RTEMS.TCB_Pointer
);
@end example
@end ifset

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 @code{@value{RPREFIX}TASK_EXITTED} directive status.

@subsubsection 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:

@findex rtems_fatal_extension
@ifset is-C
@example
rtems_extension user_fatal_error(
  Internal_errors_Source  the_source,
  rtems_boolean           is_internal,
  rtems_unsigned32        the_error
);
@end example
@end ifset

@ifset is-Ada
@example
procedure User_Fatal_Error (
   Error : in     RTEMS.Unsigned32
);
@end example
@end ifset

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.

@subsection 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:

@itemize @bullet
@item Task creation
@item Task initiation
@item Task reinitiation
@item Task deletion
@item Task context switch
@item Post task context switch
@item Task begins to execute
@end itemize


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

@itemize @bullet
@item Task deletion
@item Fatal error detection
@end itemize

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.

@section Operations

@subsection Creating an Extension Set

The @code{@value{DIRPREFIX}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.

@subsection 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 @code{@value{DIRPREFIX}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 @code{@value{DIRPREFIX}extension_ident}
directive.  The extension set ID is used by other directives
to manipulate this extension set.

@subsection Deleting an Extension Set

The @code{@value{DIRPREFIX}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.

@section 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.

@c
@c
@c
@page
@subsection EXTENSION_CREATE - Create a extension set

@cindex create an extension set

@subheading CALLING SEQUENCE:

@ifset is-C
@findex rtems_extension_create
@example
rtems_status_code rtems_extension_create(
  rtems_name              name,
  rtems_extensions_table *table,
  rtems_id               *id
);
@end example
@end ifset

@ifset is-Ada
@example
procedure Extension_Create (
   Name   : in     RTEMS.Name;
   Table  : in     RTEMS.Extensions_Table_Pointer;
   ID     :    out RTEMS.ID;
   Result :    out RTEMS.Status_Codes
);
@end example
@end ifset

@subheading DIRECTIVE STATUS CODES:
@code{@value{RPREFIX}SUCCESSFUL} -  extension set created successfully@*
@code{@value{RPREFIX}INVALID_NAME} - invalid extension set name@*
@code{@value{RPREFIX}TOO_MANY} - too many extension sets created

@subheading 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.

@subheading NOTES:

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

@c
@c
@c
@page
@subsection EXTENSION_IDENT - Get ID of a extension set

@cindex get ID of an extension set
@cindex obtain ID of an extension set

@subheading CALLING SEQUENCE:

@ifset is-C
@findex rtems_extension_ident
@example
rtems_status_code rtems_extension_ident(
  rtems_name  name,
  rtems_id   *id
);
@end example
@end ifset

@ifset is-Ada
@example
procedure Extension_Ident (
   Name   : in     RTEMS.Name;
   ID     :    out RTEMS.ID;
   Result :    out RTEMS.Status_Codes
);
@end example
@end ifset

@subheading DIRECTIVE STATUS CODES:
@code{@value{RPREFIX}SUCCESSFUL} -  extension set identified successfully@*
@code{@value{RPREFIX}INVALID_NAME} - extension set name not found

@subheading 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.

@subheading NOTES:

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

@c
@c
@c
@page
@subsection EXTENSION_DELETE - Delete a extension set

@cindex delete an extension set

@subheading CALLING SEQUENCE:

@ifset is-C
@findex rtems_extension_delete
@example
rtems_status_code rtems_extension_delete(
  rtems_id id
);
@end example
@end ifset

@ifset is-Ada
@example
procedure Extension_Delete (
   ID     : in     RTEMS.ID;
   Result :    out RTEMS.Status_Codes
);
@end example
@end ifset

@subheading DIRECTIVE STATUS CODES:
@code{@value{RPREFIX}SUCCESSFUL} -  extension set deleted successfully@*
@code{@value{RPREFIX}INVALID_ID} - invalid extension set id

@subheading 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.

@subheading 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.

@subheading NOTES:

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