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rtems/doc/user/task.t
Joel Sherrill 61389eac9f first cut at Ada bindings manual
1997-05-29 21:53:58 +00:00

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@c
@c COPYRIGHT (c) 1996.
@c On-Line Applications Research Corporation (OAR).
@c All rights reserved.
@c
@c
@c COPYRIGHT (c) 1996.
@c On-Line Applications Research Corporation (OAR).
@c All rights reserved.
@c
@ifinfo
@node Task Manager, Task Manager Introduction, SHUTDOWN_EXECUTIVE - Shutdown RTEMS, Top
@end ifinfo
@chapter Task Manager
@ifinfo
@menu
* Task Manager Introduction::
* Task Manager Background::
* Task Manager Operations::
* Task Manager Directives::
@end menu
@end ifinfo
@ifinfo
@node Task Manager Introduction, Task Manager Background, Task Manager, Task Manager
@end ifinfo
@section Introduction
The task manager provides a comprehensive set of directives to
create, delete, and administer tasks. The directives provided
by the task manager are:
@itemize @bullet
@item @code{task_create} - Create a task
@item @code{task_ident} - Get ID of a task
@item @code{task_start} - Start a task
@item @code{task_restart} - Restart a task
@item @code{task_delete} - Delete a task
@item @code{task_suspend} - Suspend a task
@item @code{task_resume} - Resume a task
@item @code{task_set_priority} - Set task priority
@item @code{task_mode} - Change current task's mode
@item @code{task_get_note} - Get task notepad entry
@item @code{task_set_note} - Set task notepad entry
@item @code{task_wake_after} - Wake up after interval
@item @code{task_wake_when} - Wake up when specified
@end itemize
@ifinfo
@node Task Manager Background, Task Definition, Task Manager Introduction, Task Manager
@end ifinfo
@section Background
@ifinfo
@menu
* Task Definition::
* Task Control Block::
* Task States::
* Task Priority::
* Task Mode::
* Accessing Task Arguments::
* Floating Point Considerations::
* Building a Task's Attribute Set::
* Building a Mode and Mask::
@end menu
@end ifinfo
@ifinfo
@node Task Definition, Task Control Block, Task Manager Background, Task Manager Background
@end ifinfo
@subsection Task Definition
Many definitions of a task have been proposed in computer
literature. Unfortunately, none of these definitions
encompasses all facets of the concept in a manner which is
operating system independent. Several of the more common
definitions are provided to enable each user to select a
definition which best matches their own experience and
understanding of the task concept:
@itemize @bullet
@item a "dispatchable" unit.
@item an entity to which the processor is allocated.
@item an atomic unit of a real-time, multiprocessor system.
@item single threads of execution which concurrently compete for resources.
@item a sequence of closely related computations which can execute
concurrently with other computational sequences.
@end itemize
From RTEMS' perspective, a task is the smallest thread of
execution which can compete on its own for system resources. A
task is manifested by the existence of a task control block
(TCB).
@ifinfo
@node Task Control Block, Task States, Task Definition, Task Manager Background
@end ifinfo
@subsection Task Control Block
The Task Control Block (TCB) is an RTEMS defined data structure
which contains all the information that is pertinent to the
execution of a task. During system initialization, RTEMS
reserves a TCB for each task configured. A TCB is allocated
upon creation of the task and is returned to the TCB free list
upon deletion of the task.
The TCB's elements are modified as a result of system calls made
by the application in response to external and internal stimuli.
TCBs are the only RTEMS internal data structure that can be
accessed by an application via user extension routines. The TCB
contains a task's name, ID, current priority, current and
starting states, execution mode, set of notepad locations, TCB
user extension pointer, scheduling control structures, as well
as data required by a blocked task.
A task's context is stored in the TCB when a task switch occurs.
When the task regains control of the processor, its context is
restored from the TCB. When a task is restarted, the initial
state of the task is restored from the starting context area in
the task's TCB.
@ifinfo
@node Task States, Task Priority, Task Control Block, Task Manager Background
@end ifinfo
@subsection Task States
A task may exist in one of the following five states:
@itemize @bullet
@item @code{executing} - Currently scheduled to the CPU
@item @code{ready} - May be scheduled to the CPU
@item @code{blocked} - Unable to be scheduled to the CPU
@item @code{dormant} - Created task that is not started
@item @code{non-existent} - Uncreated or deleted task
@end itemize
An active task may occupy the executing, ready, blocked or
dormant state, otherwise the task is considered non-existent.
One or more tasks may be active in the system simultaneously.
Multiple tasks communicate, synchronize, and compete for system
resources with each other via system calls. The multiple tasks
appear to execute in parallel, but actually each is dispatched
to the CPU for periods of time determined by the RTEMS
scheduling algorithm. The scheduling of a task is based on its
current state and priority.
@ifinfo
@node Task Priority, Task Mode, Task States, Task Manager Background
@end ifinfo
@subsection Task Priority
A task's priority determines its importance in relation to the
other tasks executing on the same processor. RTEMS supports 255
levels of priority ranging from 1 to 255. Tasks of numerically
smaller priority values are more important tasks than tasks of
numerically larger priority values. For example, a task at
priority level 5 is of higher privilege than a task at priority
level 10. There is no limit to the number of tasks assigned to
the same priority.
Each task has a priority associated with it at all times. The
initial value of this priority is assigned at task creation
time. The priority of a task may be changed at any subsequent
time.
Priorities are used by the scheduler to determine which ready
task will be allowed to execute. In general, the higher the
logical priority of a task, the more likely it is to receive
processor execution time.
@ifinfo
@node Task Mode, Accessing Task Arguments, Task Priority, Task Manager Background
@end ifinfo
@subsection Task Mode
A task's mode is a combination of the following four components:
@itemize @bullet
@item preemption
@item ASR processing
@item timeslicing
@item interrupt level
@end itemize
It is used to modify RTEMS' scheduling process and to alter the
execution environment of the task.
The preemption component allows a task to determine when control
of the processor is relinquished. If preemption is disabled
(NO_PREEMPT), the task will retain control of the processor as
long as it is in the executing state -- even if a higher
priority task is made ready. If preemption is enabled (PREEMPT)
and a higher priority task is made ready, then the processor
will be taken away from the current task immediately and given
to the higher priority task.
The timeslicing component is used by the RTEMS scheduler to
determine how the processor is allocated to tasks of equal
priority. If timeslicing is enabled (TIMESLICE), then RTEMS
will limit the amount of time the task can execute before the
processor is allocated to another ready task of equal priority.
The length of the timeslice is application dependent and
specified in the Configuration Table. If timeslicing is
disabled (NO_TIMESLICE), then the task will be allowed to
execute until a task of higher priority is made ready. If
NO_PREEMPT is selected, then the timeslicing component is
ignored by the scheduler.
The asynchronous signal processing component is used to
determine when received signals are to be processed by the task.
If signal processing is enabled (ASR), then signals sent to the
task will be processed the next time the task executes. If
signal processing is disabled (NO_ASR), then all signals
received by the task will remain posted until signal processing
is enabled. This component affects only tasks which have
established a routine to process asynchronous signals.
The interrupt level component is used to determine which
interrupts will be enabled when the task is executing.
INTERRUPT_LEVEL(n) specifies that the task will execute at
interrupt level n.
@itemize @bullet
@item @code{PREEMPT} - enable preemption (default)
@item @code{NO_PREEMPT} - disable preemption
@item @code{NO_TIMESLICE} - disable timeslicing (default)
@item @code{TIMESLICE} - enable timeslicing
@item @code{ASR} - enable ASR processing (default)
@item @code{NO_ASR} - disable ASR processing
@item @code{INTERRUPT_LEVEL(0)} - enable all interrupts (default)
@item @code{INTERRUPT_LEVEL(n)} - execute at interrupt level n
@end itemize
@ifinfo
@node Accessing Task Arguments, Floating Point Considerations, Task Mode, Task Manager Background
@end ifinfo
@subsection Accessing Task Arguments
All RTEMS tasks are invoked with a single argument which is
specified when they are started or restarted. The argument is
commonly used to communicate startup information to the task.
The simplest manner in which to define a task which accesses it
argument is:
@ifset is-C
@example
rtems_task user_task(
rtems_task_argument argument
);
@end example
@end ifset
@ifset is-Ada
@example
procedure User_Task (
Argument : in RTEMS.Task_Argument_Ptr
);
@end example
@end ifset
Application tasks requiring more information may view this
single argument as an index into an array of parameter blocks.
@ifinfo
@node Floating Point Considerations, Building a Task's Attribute Set, Accessing Task Arguments, Task Manager Background
@end ifinfo
@subsection Floating Point Considerations
Creating a task with the FLOATING_POINT flag results in
additional memory being allocated for the TCB to store the state
of the numeric coprocessor during task switches. This
additional memory is NOT allocated for NO_FLOATING_POINT tasks.
Saving and restoring the context of a FLOATING_POINT task takes
longer than that of a NO_FLOATING_POINT task because of the
relatively large amount of time required for the numeric
coprocessor to save or restore its computational state.
Since RTEMS was designed specifically for embedded military
applications which are floating point intensive, the executive
is optimized to avoid unnecessarily saving and restoring the
state of the numeric coprocessor. The state of the numeric
coprocessor is only saved when a FLOATING_POINT task is
dispatched and that task was not the last task to utilize the
coprocessor. In a system with only one FLOATING_POINT task, the
state of the numeric coprocessor will never be saved or
restored.
Although the overhead imposed by FLOATING_POINT tasks is
minimal, some applications may wish to completely avoid the
overhead associated with FLOATING_POINT tasks and still utilize
a numeric coprocessor. By preventing a task from being
preempted while performing a sequence of floating point
operations, a NO_FLOATING_POINT task can utilize the numeric
coprocessor without incurring the overhead of a FLOATING_POINT
context switch. This approach also avoids the allocation of a
floating point context area. However, if this approach is taken
by the application designer, NO tasks should be created as
FLOATING_POINT tasks. Otherwise, the floating point context
will not be correctly maintained because RTEMS assumes that the
state of the numeric coprocessor will not be altered by
NO_FLOATING_POINT tasks.
If the supported processor type does not have hardware floating
capabilities or a standard numeric coprocessor, RTEMS will not
provide built-in support for hardware floating point on that
processor. In this case, all tasks are considered
NO_FLOATING_POINT whether created as FLOATING_POINT or
NO_FLOATING_POINT tasks. A floating point emulation software
library must be utilized for floating point operations.
On some processors, it is possible to disable the floating point
unit dynamically. If this capability is supported by the target
processor, then RTEMS will utilize this capability to enable the
floating point unit only for tasks which are created with the
FLOATING_POINT attribute. The consequence of a
NO_FLOATING_POINT task attempting to access the floating point
unit is CPU dependent but will i general result in an exception
condition.
@ifinfo
@node Building a Task's Attribute Set, Building a Mode and Mask, Floating Point Considerations, Task Manager Background
@end ifinfo
@subsection Building a Task's Attribute Set
In general, an attribute set is built by a bitwise OR of the
desired components. The set of valid task attribute components
is listed below:
@itemize @bullet
@item @code{NO_FLOATING_POINT} - does not use coprocessor (default)
@item @code{FLOATING_POINT} - uses numeric coprocessor
@item @code{LOCAL} - local task (default)
@item @code{GLOBAL} - global task
@end itemize
Attribute values are specifically designed to be mutually
exclusive, therefore bitwise OR and addition operations are
equivalent as long as each attribute appears exactly once in the
component list. A component listed as a default is not required
to appear in the component list, although it is a good
programming practice to specify default components. If all
defaults are desired, then DEFAULT_ATTRIBUTES should be used.
This example demonstrates the attribute_set parameter needed to
create a local task which utilizes the numeric coprocessor. The
attribute_set parameter could be FLOATING_POINT or LOCAL |
FLOATING_POINT. The attribute_set parameter can be set to
FLOATING_POINT because LOCAL is the default for all created
tasks. If the task were global and used the numeric
coprocessor, then the attribute_set parameter would be GLOBAL |
FLOATING_POINT.
@ifinfo
@node Building a Mode and Mask, Task Manager Operations, Building a Task's Attribute Set, Task Manager Background
@end ifinfo
@subsection Building a Mode and Mask
In general, a mode and its corresponding mask is built by a
bitwise OR of the desired components. The set of valid mode
constants and each mode's corresponding mask constant is
listed below:
@ifset use-ascii
@itemize @bullet
@item PREEMPT is masked by PREEMPT_MASK and enables preemption
@item NO_PREEMPT is masked by PREEMPT_MASK and disables preemption
@item NO_TIMESLICE is masked by TIMESLICE_MASK and disables timeslicing
@item TIMESLICE is masked by TIMESLICE_MASK and enables timeslicing
@item ASR is masked by ASR_MASK and enables ASR processing
@item NO_ASR is masked by ASR_MASK and disables ASR processing
@item INTERRUPT_LEVEL(0) is masked by INTERRUPT_MASK and enables all interrupts
@item INTERRUPT_LEVEL(n) is masked by INTERRUPT_MASK and sets interrupts level n
@end itemize
@end ifset
@ifset use-tex
@sp 1
@c this is temporary
@itemize @bullet
@item PREEMPT is masked by PREEMPT_MASK and enables preemption
@item NO_PREEMPT is masked by PREEMPT_MASK and disables preemption
@item NO_TIMESLICE is masked by TIMESLICE_MASK and disables timeslicing
@item TIMESLICE is masked by TIMESLICE_MASK and enables timeslicing
@item ASR is masked by ASR_MASK and enables ASR processing
@item NO_ASR is masked by ASR_MASK and disables ASR processing
@item INTERRUPT_LEVEL(0) is masked by INTERRUPT_MASK and enables all interrupts
@item INTERRUPT_LEVEL(n) is masked by INTERRUPT_MASK and sets interrupts level n
@end itemize
@tex
@end tex
@end ifset
@ifset use-html
@html
<CENTER>
<TABLE COLS=3 WIDTH="80%" BORDER=2>
<TR><TD ALIGN=center><STRONG>Mode Constant</STRONG></TD>
<TD ALIGN=center><STRONG>Mask Constant</STRONG></TD>
<TD ALIGN=center><STRONG>Description</STRONG></TD></TR>
<TR><TD ALIGN=center>PREEMPT</TD>
<TD ALIGN=center>PREEMPT_MASK</TD>
<TD ALIGN=center>enables preemption</TD></TR>
<TR><TD ALIGN=center>NO_PREEMPT</TD>
<TD ALIGN=center>PREEMPT_MASK</TD>
<TD ALIGN=center>disables preemption</TD></TR>
<TR><TD ALIGN=center>NO_TIMESLICE</TD>
<TD ALIGN=center>TIMESLICE_MASK</TD>
<TD ALIGN=center>disables timeslicing</TD></TR>
<TR><TD ALIGN=center>TIMESLICE</TD>
<TD ALIGN=center>TIMESLICE_MASK</TD>
<TD ALIGN=center>enables timeslicing</TD></TR>
<TR><TD ALIGN=center>ASR</TD>
<TD ALIGN=center>ASR_MASK</TD>
<TD ALIGN=center>enables ASR processing</TD></TR>
<TR><TD ALIGN=center>NO_ASR</TD>
<TD ALIGN=center>ASR_MASK</TD>
<TD ALIGN=center>disables ASR processing</TD></TR>
<TR><TD ALIGN=center>INTERRUPT_LEVEL(0)</TD>
<TD ALIGN=center>INTERRUPT_MASK</TD>
<TD ALIGN=center>enables all interrupts</TD></TR>
<TR><TD ALIGN=center>INTERRUPT_LEVEL(n)</TD>
<TD ALIGN=center>INTERRUPT_MASK</TD>
<TD ALIGN=center>sets interrupts level n</TD></TR>
</TABLE>
</CENTER>
@end html
@end ifset
Mode values are specifically designed to be mutually exclusive,
therefore bitwise OR and addition operations are equivalent as
long as each mode appears exactly once in the component list. A
mode component listed as a default is not required to appear in
the mode component list, although it is a good programming
practice to specify default components. If all defaults are
desired, the mode DEFAULT_MODES and the mask ALL_MODE_MASKS
should be used.
The following example demonstrates the mode and mask parameters
used with the task_mode directive to place a task at interrupt
level 3 and make it non-preemptible. The mode should be set to
INTERRUPT_LEVEL(3) | NO_PREEMPT to indicate the desired
preemption mode and interrupt level, while the mask parameter
should be set to INTERRUPT_MASK | NO_PREEMPT_MASK to indicate
that the calling task's interrupt level and preemption mode are
being altered.
@ifinfo
@node Task Manager Operations, Creating Tasks, Building a Mode and Mask, Task Manager
@end ifinfo
@section Operations
@ifinfo
@menu
* Creating Tasks::
* Obtaining Task IDs::
* Starting and Restarting Tasks::
* Suspending and Resuming Tasks::
* Delaying the Currently Executing Task::
* Changing Task Priority::
* Changing Task Mode::
* Notepad Locations::
* Task Deletion::
@end menu
@end ifinfo
@ifinfo
@node Creating Tasks, Obtaining Task IDs, Task Manager Operations, Task Manager Operations
@end ifinfo
@subsection Creating Tasks
The task_create directive creates a task by allocating a task
control block, assigning the task a user-specified name,
allocating it a stack and floating point context area, setting a
user-specified initial priority, setting a user-specified
initial mode, and assigning it a task ID. Newly created tasks
are initially placed in the dormant state. All RTEMS tasks
execute in the most privileged mode of the processor.
@ifinfo
@node Obtaining Task IDs, Starting and Restarting Tasks, Creating Tasks, Task Manager Operations
@end ifinfo
@subsection Obtaining Task IDs
When a task is created, RTEMS generates a unique task ID and
assigns it to the created task until it is deleted. The task ID
may be obtained by either of two methods. First, as the result
of an invocation of the task_create directive, the task ID is
stored in a user provided location. Second, the task ID may be
obtained later using the task_ident directive. The task ID is
used by other directives to manipulate this task.
@ifinfo
@node Starting and Restarting Tasks, Suspending and Resuming Tasks, Obtaining Task IDs, Task Manager Operations
@end ifinfo
@subsection Starting and Restarting Tasks
The task_start directive is used to place a dormant task in the
ready state. This enables the task to compete, based on its
current priority, for the processor and other system resources.
Any actions, such as suspension or change of priority, performed
on a task prior to starting it are nullified when the task is
started.
With the task_start directive the user specifies the task's
starting address and argument. The argument is used to
communicate some startup information to the task. As part of
this directive, RTEMS initializes the task's stack based upon
the task's initial execution mode and start address. The
starting argument is passed to the task in accordance with the
target processor's calling convention.
The task_restart directive restarts a task at its initial
starting address with its original priority and execution mode,
but with a possibly different argument. The new argument may be
used to distinguish between the original invocation of the task
and subsequent invocations. The task's stack and control block
are modified to reflect their original creation values.
Although references to resources that have been requested are
cleared, resources allocated by the task are NOT automatically
returned to RTEMS. A task cannot be restarted unless it has
previously been started (i.e. dormant tasks cannot be
restarted). All restarted tasks are placed in the ready state.
@ifinfo
@node Suspending and Resuming Tasks, Delaying the Currently Executing Task, Starting and Restarting Tasks, Task Manager Operations
@end ifinfo
@subsection Suspending and Resuming Tasks
The task_suspend directive is used to place either the caller or
another task into a suspended state. The task remains suspended
until a task_resume directive is issued. This implies that a
task may be suspended as well as blocked waiting either to
acquire a resource or for the expiration of a timer.
The task_resume directive is used to remove another task from
the suspended state. If the task is not also blocked, resuming
it will place it in the ready state, allowing it to once again
compete for the processor and resources. If the task was
blocked as well as suspended, this directive clears the
suspension and leaves the task in the blocked state.
@ifinfo
@node Delaying the Currently Executing Task, Changing Task Priority, Suspending and Resuming Tasks, Task Manager Operations
@end ifinfo
@subsection Delaying the Currently Executing Task
The task_wake_after directive creates a sleep timer which allows
a task to go to sleep for a specified interval. The task is
blocked until the delay interval has elapsed, at which time the
task is unblocked. A task calling the task_wake_after directive
with a delay interval of YIELD_PROCESSOR ticks will yield the
processor to any other ready task of equal or greater priority
and remain ready to execute.
The task_wake_when directive creates a sleep timer which allows
a task to go to sleep until a specified date and time. The
calling task is blocked until the specified date and time has
occurred, at which time the task is unblocked.
@ifinfo
@node Changing Task Priority, Changing Task Mode, Delaying the Currently Executing Task, Task Manager Operations
@end ifinfo
@subsection Changing Task Priority
The task_set_priority directive is used to obtain or change the
current priority of either the calling task or another task. If
the new priority requested is CURRENT_PRIORITY or the task's
actual priority, then the current priority will be returned and
the task's priority will remain unchanged. If the task's
priority is altered, then the task will be scheduled according
to its new priority.
The task_restart directive resets the priority of a task to its
original value.
@ifinfo
@node Changing Task Mode, Notepad Locations, Changing Task Priority, Task Manager Operations
@end ifinfo
@subsection Changing Task Mode
The task_mode directive is used to obtain or change the current
execution mode of the calling task. A task's execution mode is
used to enable preemption, timeslicing, ASR processing, and to
set the task's interrupt level.
The task_restart directive resets the mode of a task to its
original value.
@ifinfo
@node Notepad Locations, Task Deletion, Changing Task Mode, Task Manager Operations
@end ifinfo
@subsection Notepad Locations
RTEMS provides sixteen notepad locations for each task. Each
notepad location may contain a note consisting of four bytes of
information. RTEMS provides two directives, task_set_note and
task_get_note, that enable a user to access and change the
notepad locations. The task_set_note directive enables the user
to set a task's notepad entry to a specified note. The
task_get_note directive allows the user to obtain the note
contained in any one of the sixteen notepads of a specified task.
@ifinfo
@node Task Deletion, Task Manager Directives, Notepad Locations, Task Manager Operations
@end ifinfo
@subsection Task Deletion
RTEMS provides the task_delete directive to allow a task to
delete itself or any other task. This directive removes all
RTEMS references to the task, frees the task's control block,
removes it from resource wait queues, and deallocates its stack
as well as the optional floating point context. The task's name
and ID become inactive at this time, and any subsequent
references to either of them is invalid. In fact, RTEMS may
reuse the task ID for another task which is created later in the
application.
Unexpired delay timers (i.e. those used by task_wake_after and
task_wake_when) and timeout timers associated with the task are
automatically deleted, however, other resources dynamically
allocated by the task are NOT automatically returned to RTEMS.
Therefore, before a task is deleted, all of its dynamically
allocated resources should be deallocated by the user. This may
be accomplished by instructing the task to delete itself rather
than directly deleting the task. Other tasks may instruct a
task to delete itself by sending a "delete self" message, event,
or signal, or by restarting the task with special arguments
which instruct the task to delete itself.
@ifinfo
@node Task Manager Directives, TASK_CREATE - Create a task, Task Deletion, Task Manager
@end ifinfo
@section Directives
@ifinfo
@menu
* TASK_CREATE - Create a task::
* TASK_IDENT - Get ID of a task::
* TASK_START - Start a task::
* TASK_RESTART - Restart a task::
* TASK_DELETE - Delete a task::
* TASK_SUSPEND - Suspend a task::
* TASK_RESUME - Resume a task::
* TASK_SET_PRIORITY - Set task priority::
* TASK_MODE - Change current task's mode::
* TASK_GET_NOTE - Get task notepad entry::
* TASK_SET_NOTE - Set task notepad entry::
* TASK_WAKE_AFTER - Wake up after interval::
* TASK_WAKE_WHEN - Wake up when specified::
@end menu
@end ifinfo
This section details the task 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.
@page
@ifinfo
@node TASK_CREATE - Create a task, TASK_IDENT - Get ID of a task, Task Manager Directives, Task Manager Directives
@end ifinfo
@subsection TASK_CREATE - Create a task
@subheading CALLING SEQUENCE:
@ifset is-C
@example
rtems_status_code rtems_task_create(
rtems_name name,
rtems_task_priority initial_priority,
rtems_unsigned32 stack_size,
rtems_mode initial_modes,
rtems_attribute attribute_set,
rtems_id *id
);
@end example
@end ifset
@ifset is-Ada
@example
procedure Task_Create (
Name : in RTEMS.Name;
Initial_Priority : in RTEMS.Task_Priority;
Stack_Size : in RTEMS.Unsigned32;
Initial_Modes : in RTEMS.Mode;
Attribute_Set : in RTEMS.Attribute;
ID : out RTEMS.ID;
Result : out RTEMS.Status_Codes
);
@end example
@end ifset
@subheading DIRECTIVE STATUS CODES:
@code{SUCCESSFUL} - task created successfully@*
@code{INVALID_NAME} - invalid task name@*
@code{INVALID_SIZE} - stack too small@*
@code{INVALID_PRIORITY} - invalid task priority@*
@code{MP_NOT_CONFIGURED} - multiprocessing not configured@*
@code{TOO_MANY} - too many tasks created@*
@code{UNSATISFIED} - not enough memory for stack/FP context@*
@code{TOO_MANY} - too many global objects
@subheading DESCRIPTION:
This directive creates a task which resides on the local node.
It allocates and initializes a TCB, a stack, and an optional
floating point context area. The mode parameter contains values
which sets the task's initial execution mode. The
FLOATING_POINT attribute should be specified if the created task
is to use a numeric coprocessor. For performance reasons, it is
recommended that tasks not using the numeric coprocessor should
specify the NO_FLOATING_POINT attribute. If the GLOBAL
attribute is specified, the task can be accessed from remote
nodes. The task id, returned in id, is used in other task
related directives to access the task. When created, a task is
placed in the dormant state and can only be made ready to
execute using the directive task_start.
@subheading NOTES:
This directive will not cause the calling task to be preempted.
Valid task priorities range from a high of 1 to a low of 255.
RTEMS supports a maximum of 256 interrupt levels which are
mapped onto the interrupt levels actually supported by the
target processor.
The requested stack size should be at least MINIMUM_STACK_SIZE
bytes. The value of MINIMUM_STACK_SIZE is processor dependent.
Application developers should consider the stack usage of the
device drivers when calculating the stack size required for
tasks which utilize the driver.
The following task attribute constants are defined by RTEMS:
@itemize @bullet
@item @code{NO_FLOATING_POINT} - does not use coprocessor (default)
@item @code{FLOATING_POINT} - uses numeric coprocessor
@item @code{LOCAL} - local task (default)
@item @code{GLOBAL} - global task
@end itemize
The following task mode constants are defined by RTEMS:
@itemize @bullet
@item @code{PREEMPT} - enable preemption (default)
@item @code{NO_PREEMPT} - disable preemption
@item @code{NO_TIMESLICE} - disable timeslicing (default)
@item @code{TIMESLICE} - enable timeslicing
@item @code{ASR} - enable ASR processing (default)
@item @code{NO_ASR} - disable ASR processing
@item @code{INTERRUPT_LEVEL(0)} - enable all interrupts (default)
@item @code{INTERRUPT_LEVEL(n)} - execute at interrupt level n
@end itemize
Tasks should not be made global unless remote tasks must
interact with them. This avoids the system overhead incurred by
the creation of a global task. When a global task is created,
the task's name and id must be transmitted to every node in the
system for insertion in the local copy of the global object
table.
The total number of global objects, including tasks, is limited
by the maximum_global_objects field in the Configuration Table.
@page
@ifinfo
@node TASK_IDENT - Get ID of a task, TASK_START - Start a task, TASK_CREATE - Create a task, Task Manager Directives
@end ifinfo
@subsection TASK_IDENT - Get ID of a task
@subheading CALLING SEQUENCE:
@ifset is-C
@example
rtems_status_code rtems_task_ident(
rtems_name name,
rtems_unsigned32 node,
rtems_id *id
);
@end example
@end ifset
@ifset is-Ada
@example
procedure Task_Ident (
Name : in RTEMS.Name;
Node : in RTEMS.Node;
ID : out RTEMS.ID;
Result : out RTEMS.Status_Codes
);
@end example
@end ifset
@subheading DIRECTIVE STATUS CODES:
@code{SUCCESSFUL} - task identified successfully@*
@code{INVALID_NAME} - invalid task name@*
@code{INVALID_NODE} - invalid node id
@subheading DESCRIPTION:
This directive obtains the task id associated with the task name
specified in name. A task may obtain its own id by specifying
SELF or its own task name in name. If the task name is not
unique, then the task id returned will match one of the tasks
with that name. However, this task id is not guaranteed to
correspond to the desired task. The task id, returned in id, is
used in other task related directives to access the task.
@subheading NOTES:
This directive will not cause the running task to be preempted.
If node is SEARCH_ALL_NODES, all nodes are searched with the
local node being searched first. All other nodes are searched
with the lowest numbered node searched first.
If node is a valid node number which does not represent the
local node, then only the tasks exported by the designated node
are searched.
This directive does not generate activity on remote nodes. It
accesses only the local copy of the global object table.
@page
@ifinfo
@node TASK_START - Start a task, TASK_RESTART - Restart a task, TASK_IDENT - Get ID of a task, Task Manager Directives
@end ifinfo
@subsection TASK_START - Start a task
@subheading CALLING SEQUENCE:
@ifset is-C
@example
rtems_status_code rtems_task_start(
rtems_id id,
rtems_task_entry entry_point,
rtems_task_argument argument
);
@end example
@end ifset
@ifset is-Ada
@example
procedure Task_Start (
ID : in RTEMS.ID;
Entry_Point : in System.Address;
Argument : in RTEMS.Task_Argument_PTR;
Result : out RTEMS.Status_Codes
);
@end example
@end ifset
@subheading DIRECTIVE STATUS CODES:
@code{SUCCESSFUL} - ask started successfully@*
@code{INVALID_ADDRESS} - invalid task entry point@*
@code{INVALID_ID} - invalid task id@*
@code{INCORRECT_STATE} - task not in the dormant state@*
@code{ILLEGAL_ON_REMOTE_OBJECT} - cannot start remote task
@subheading DESCRIPTION:
This directive readies the task, specified by tid, for execution
based on the priority and execution mode specified when the task
was created. The starting address of the task is given in
entry_point. The task's starting argument is contained in
argument. This argument can be a single value or used as an
index into an array of parameter blocks.
@subheading NOTES:
The calling task will be preempted if its preemption mode is
enabled and the task being started has a higher priority.
Any actions performed on a dormant task such as suspension or
change of priority are nullified when the task is initiated via
the task_start directive.
@page
@ifinfo
@node TASK_RESTART - Restart a task, TASK_DELETE - Delete a task, TASK_START - Start a task, Task Manager Directives
@end ifinfo
@subsection TASK_RESTART - Restart a task
@subheading CALLING SEQUENCE:
@ifset is-C
@example
rtems_status_code rtems_task_restart(
rtems_id id,
rtems_task_argument argument
);
@end example
@end ifset
@ifset is-Ada
@example
procedure Task_Restart (
ID : in RTEMS.ID;
Argument : in RTEMS.Task_Argument_PTR;
Result : out RTEMS.Status_Codes
);
@end example
@end ifset
@subheading DIRECTIVE STATUS CODES:
@code{SUCCESSFUL} - task restarted successfully@*
@code{INVALID_ID} - task id invalid@*
@code{INCORRECT_STATE} - task never started@*
@code{ILLEGAL_ON_REMOTE_OBJECT} - cannot restart remote task
@subheading DESCRIPTION:
This directive resets the task specified by id to begin
execution at its original starting address. The task's priority
and execution mode are set to the original creation values. If
the task is currently blocked, RTEMS automatically makes the
task ready. A task can be restarted from any state, except the
dormant state.
The task's starting argument is contained in argument. This
argument can be a single value or an index into an array of
parameter blocks. This new argument may be used to distinguish
between the initial task_start of the task and any ensuing calls
to task_restart of the task. This can be beneficial in deleting
a task. Instead of deleting a task using the task_delete
directive, a task can delete another task by restarting that
task, and allowing that task to release resources back to RTEMS
and then delete itself.
@subheading NOTES:
If id is SELF, the calling task will be restarted and will not
return from this directive.
The calling task will be preempted if its preemption mode is
enabled and the task being restarted has a higher priority.
The task must reside on the local node, even if the task was
created with the GLOBAL option.
@page
@ifinfo
@node TASK_DELETE - Delete a task, TASK_SUSPEND - Suspend a task, TASK_RESTART - Restart a task, Task Manager Directives
@end ifinfo
@subsection TASK_DELETE - Delete a task
@subheading CALLING SEQUENCE:
@ifset is-C
@example
rtems_status_code rtems_task_delete(
rtems_id id
);
@end example
@end ifset
@ifset is-Ada
@example
procedure Task_Delete (
ID : in RTEMS.ID;
Result : out RTEMS.Status_Codes
);
@end example
@end ifset
@subheading DIRECTIVE STATUS CODES:
@code{SUCCESSFUL} - task restarted successfully@*
@code{INVALID_ID} - task id invalid@*
@code{ILLEGAL_ON_REMOTE_OBJECT} - cannot restart remote task
@subheading DESCRIPTION:
This directive deletes a task, either the calling task or
another task, as specified by id. RTEMS stops the execution of
the task and reclaims the stack memory, any allocated delay or
timeout timers, the TCB, and, if the task is FLOATING_POINT, its
floating point context area. RTEMS does not reclaim the
following resources: region segments, partition buffers,
semaphores, timers, or rate monotonic periods.
@subheading NOTES:
A task is responsible for releasing its resources back to RTEMS
before deletion. To insure proper deallocation of resources, a
task should not be deleted unless it is unable to execute or
does not hold any RTEMS resources. If a task holds RTEMS
resources, the task should be allowed to deallocate its
resources before deletion. A task can be directed to release
its resources and delete itself by restarting it with a special
argument or by sending it a message, an event, or a signal.
Deletion of the current task (SELF) will force RTEMS to select
another task to execute.
When a global task is deleted, the task id must be transmitted
to every node in the system for deletion from the local copy of
the global object table.
The task must reside on the local node, even if the task was
created with the GLOBAL option.
@page
@ifinfo
@node TASK_SUSPEND - Suspend a task, TASK_RESUME - Resume a task, TASK_DELETE - Delete a task, Task Manager Directives
@end ifinfo
@subsection TASK_SUSPEND - Suspend a task
@subheading CALLING SEQUENCE:
@ifset is-C
@example
rtems_status_code rtems_task_suspend(
rtems_id id
);
@end example
@end ifset
@ifset is-Ada
@example
procedure Task_Suspend (
ID : in RTEMS.ID;
Result : out RTEMS.Status_Codes
);
@end example
@end ifset
@subheading DIRECTIVE STATUS CODES:
@code{SUCCESSFUL} - task restarted successfully@*
@code{INVALID_ID} - task id invalid@*
@code{ALREADY_SUSPENDED} - task already suspended
@subheading DESCRIPTION:
This directive suspends the task specified by id from further
execution by placing it in the suspended state. This state is
additive to any other blocked state that the task may already be
in. The task will not execute again until another task issues
the task_resume directive for this task and any blocked state
has been removed.
@subheading NOTES:
The requesting task can suspend itself by specifying SELF as id.
In this case, the task will be suspended and a successful
return code will be returned when the task is resumed.
Suspending a global task which does not reside on the local node
will generate a request to the remote node to suspend the
specified task.
@page
@ifinfo
@node TASK_RESUME - Resume a task, TASK_SET_PRIORITY - Set task priority, TASK_SUSPEND - Suspend a task, Task Manager Directives
@end ifinfo
@subsection TASK_RESUME - Resume a task
@subheading CALLING SEQUENCE:
@ifset is-C
@example
rtems_status_code rtems_task_resume(
rtems_id id
);
@end example
@end ifset
@ifset is-Ada
@example
procedure Task_Resume (
ID : in RTEMS.ID;
Result : out RTEMS.Status_Codes
);
@end example
@end ifset
@subheading DIRECTIVE STATUS CODES:
@code{SUCCESSFUL} - task restarted successfully@*
@code{INVALID_ID} - task id invalid@*
@code{INCORRECT_STATE} - task not suspended
@subheading DESCRIPTION:
This directive removes the task specified by id from the
suspended state. If the task is in the ready state after the
suspension is removed, then it will be scheduled to run. If the
task is still in a blocked state after the suspension is
removed, then it will remain in that blocked state.
@subheading NOTES:
The running task may be preempted if its preemption mode is
enabled and the local task being resumed has a higher priority.
Resuming a global task which does not reside on the local node
will generate a request to the remote node to resume the
specified task.
@page
@ifinfo
@node TASK_SET_PRIORITY - Set task priority, TASK_MODE - Change current task's mode, TASK_RESUME - Resume a task, Task Manager Directives
@end ifinfo
@subsection TASK_SET_PRIORITY - Set task priority
@subheading CALLING SEQUENCE:
@ifset is-C
@example
rtems_status_code rtems_task_set_priority(
rtems_id id,
rtems_task_priority new_priority,
rtems_task_priority *old_priority
);
@end example
@end ifset
@ifset is-Ada
@example
procedure Task_Set_Priority (
ID : in RTEMS.ID;
New_Priority : in RTEMS.Task_Priority;
Old_Priority : out RTEMS.Task_Priority;
Result : out RTEMS.Status_Codes
);
@end example
@end ifset
@subheading DIRECTIVE STATUS CODES:
@code{SUCCESSFUL} - task priority set successfully@*
@code{INVALID_ID} - invalid task id@*
@code{INVALID_PRIORITY} - invalid task priority
@subheading DESCRIPTION:
This directive manipulates the priority of the task specified by
id. An id of SELF is used to indicate the calling task. When
new_priority is not equal to CURRENT_PRIORITY, the specified
task's previous priority is returned in old_priority. When
new_priority is CURRENT_PRIORITY, the specified task's current
priority is returned in old_priority. Valid priorities range
from a high of 1 to a low of 255.
@subheading NOTES:
The calling task may be preempted if its preemption mode is
enabled and it lowers its own priority or raises another task's
priority.
Setting the priority of a global task which does not reside on
the local node will generate a request to the remote node to
change the priority of the specified task.
If the task specified by id is currently holding any binary
semaphores which use the priority inheritance algorithm, then
the task's priority cannot be lowered immediately. If the
task's priority were lowered immediately, then priority
inversion results. The requested lowering of the task's
priority will occur when the task has released all priority
inheritance binary semaphores. The task's priority can be
increased regardless of the task's use of priority inheritance
binary semaphores.
@page
@ifinfo
@node TASK_MODE - Change current task's mode, TASK_GET_NOTE - Get task notepad entry, TASK_SET_PRIORITY - Set task priority, Task Manager Directives
@end ifinfo
@subsection TASK_MODE - Change current task's mode
@subheading CALLING SEQUENCE:
@ifset is-C
@example
rtems_status_code rtems_task_mode(
rtems_mode mode_set,
rtems_mode mask,
rtems_mode *previous_mode_set
);
@end example
@end ifset
@ifset is-Ada
@example
procedure Task_Delete (
Mode_Set : in RTEMS.Mode;
Mask : in RTEMS.Mode;
Previous_Mode_Set : in RTEMS.Mode;
Result : out RTEMS.Status_Codes
);
@end example
@end ifset
@subheading DIRECTIVE STATUS CODES:
@code{SUCCESSFUL} - task mode set successfully
@subheading DESCRIPTION:
This directive manipulates the execution mode of the calling
task. A task's execution mode enables and disables preemption,
timeslicing, asynchronous signal processing, as well as
specifying the current interrupt level. To modify an execution
mode, the mode class(es) to be changed must be specified in the
mask parameter and the desired mode(s) must be specified in the
mode parameter.
@subheading NOTES:
The calling task will be preempted if it enables preemption and
a higher priority task is ready to run.
Enabling timeslicing has no effect if preemption is enabled.
A task can obtain its current execution mode, without modifying
it, by calling this directive with a mask value of CURRENT_MODE.
To temporarily disable the processing of a valid ASR, a task
should call this directive with the NO_ASR indicator specified
in mode.
The set of task mode constants and each mode's corresponding
mask constant is provided in the following table:
@ifset use-ascii
@itemize @bullet
@item PREEMPT is masked by PREEMPT_MASK and enables preemption
@item NO_PREEMPT is masked by PREEMPT_MASK and disables preemption
@item NO_TIMESLICE is masked by TIMESLICE_MASK and disables timeslicing
@item TIMESLICE is masked by TIMESLICE_MASK and enables timeslicing
@item ASR is masked by ASR_MASK and enables ASR processing
@item NO_ASR is masked by ASR_MASK and disables ASR processing
@item INTERRUPT_LEVEL(0) is masked by INTERRUPT_MASK and enables all interrupts
@item INTERRUPT_LEVEL(n) is masked by INTERRUPT_MASK and sets interrupts level n
@end itemize
@end ifset
@ifset use-tex
@sp 1
@c this is temporary
@itemize @bullet
@item PREEMPT is masked by PREEMPT_MASK and enables preemption
@item NO_PREEMPT is masked by PREEMPT_MASK and disables preemption
@item NO_TIMESLICE is masked by TIMESLICE_MASK and disables timeslicing
@item TIMESLICE is masked by TIMESLICE_MASK and enables timeslicing
@item ASR is masked by ASR_MASK and enables ASR processing
@item NO_ASR is masked by ASR_MASK and disables ASR processing
@item INTERRUPT_LEVEL(0) is masked by INTERRUPT_MASK and enables all interrupts
@item INTERRUPT_LEVEL(n) is masked by INTERRUPT_MASK and sets interrupts level n
@end itemize
@tex
@end tex
@end ifset
@ifset use-html
@html
<CENTER>
<TABLE COLS=3 WIDTH="80%" BORDER=2>
<TR><TD ALIGN=center><STRONG>Mode Constant</STRONG></TD>
<TD ALIGN=center><STRONG>Mask Constant</STRONG></TD>
<TD ALIGN=center><STRONG>Description</STRONG></TD></TR>
<TR><TD ALIGN=center>PREEMPT</TD>
<TD ALIGN=center>PREEMPT_MASK</TD>
<TD ALIGN=center>enables preemption</TD></TR>
<TR><TD ALIGN=center>NO_PREEMPT</TD>
<TD ALIGN=center>PREEMPT_MASK</TD>
<TD ALIGN=center>disables preemption</TD></TR>
<TR><TD ALIGN=center>NO_TIMESLICE</TD>
<TD ALIGN=center>TIMESLICE_MASK</TD>
<TD ALIGN=center>disables timeslicing</TD></TR>
<TR><TD ALIGN=center>TIMESLICE</TD>
<TD ALIGN=center>TIMESLICE_MASK</TD>
<TD ALIGN=center>enables timeslicing</TD></TR>
<TR><TD ALIGN=center>ASR</TD>
<TD ALIGN=center>ASR_MASK</TD>
<TD ALIGN=center>enables ASR processing</TD></TR>
<TR><TD ALIGN=center>NO_ASR</TD>
<TD ALIGN=center>ASR_MASK</TD>
<TD ALIGN=center>disables ASR processing</TD></TR>
<TR><TD ALIGN=center>INTERRUPT_LEVEL(0)</TD>
<TD ALIGN=center>INTERRUPT_MASK</TD>
<TD ALIGN=center>enables all interrupts</TD></TR>
<TR><TD ALIGN=center>INTERRUPT_LEVEL(n)</TD>
<TD ALIGN=center>INTERRUPT_MASK</TD>
<TD ALIGN=center>sets interrupts level n</TD></TR>
</TABLE>
</CENTER>
@end html
@end ifset
@page
@ifinfo
@node TASK_GET_NOTE - Get task notepad entry, TASK_SET_NOTE - Set task notepad entry, TASK_MODE - Change current task's mode, Task Manager Directives
@end ifinfo
@subsection TASK_GET_NOTE - Get task notepad entry
@subheading CALLING SEQUENCE:
@ifset is-C
@example
rtems_status_code rtems_task_get_note(
rtems_id id,
rtems_unsigned32 notepad,
rtems_unsigned32 *note
);
@end example
@end ifset
@ifset is-Ada
@example
procedure Task_Get_Note (
ID : in RTEMS.ID;
Notepad : in RTEMS.Notepad_Index;
Note : out RTEMS.Unsigned32;
Result : out RTEMS.Status_Codes
);
@end example
@end ifset
@subheading DIRECTIVE STATUS CODES:
@code{SUCCESSFUL} - note obtained successfully@*
@code{INVALID_ID} - invalid task id@*
@code{INVALID_NUMBER} - invalid notepad location
@subheading DESCRIPTION:
This directive returns the note contained in the notepad
location of the task specified by id.
@subheading NOTES:
This directive will not cause the running task to be preempted.
If id is set to SELF, the calling task accesses its own notepad.
@c This version of the paragraph avoids the overfull hbox error.
@c The constants NOTEPAD_0 through NOTEPAD_15 can be used to access the
@c sixteen notepad locations.
The sixteen notepad locations can be accessed using the constants
NOTEPAD_0 through NOTEPAD_15.
Getting a note of a global task which does not reside on the
local node will generate a request to the remote node to obtain
the notepad entry of the specified task.
@page
@ifinfo
@node TASK_SET_NOTE - Set task notepad entry, TASK_WAKE_AFTER - Wake up after interval, TASK_GET_NOTE - Get task notepad entry, Task Manager Directives
@end ifinfo
@subsection TASK_SET_NOTE - Set task notepad entry
@subheading CALLING SEQUENCE:
@ifset is-C
@example
rtems_status_code rtems_task_set_note(
rtems_id id,
rtems_unsigned32 notepad,
rtems_unsigned32 note
);
@end example
@end ifset
@ifset is-Ada
@example
procedure Task_Set_Note (
ID : in RTEMS.ID;
Notepad : in RTEMS.Notepad_Index;
Note : in RTEMS.Unsigned32;
Result : out RTEMS.Status_Codes
);
@end example
@end ifset
@subheading DIRECTIVE STATUS CODES:
@code{SUCCESSFUL} - task's note set successfully@*
@code{INVALID_ID} - invalid task id@*
@code{INVALID_NUMBER} - invalid notepad location
@subheading DESCRIPTION:
This directive sets the notepad entry for the task specified by
id to the value note.
@subheading NOTES:
If id is set to SELF, the calling task accesses its own notepad
locations.
This directive will not cause the running task to be preempted.
@c This version of the paragraph avoids the overfull hbox error.
@c The constants NOTEPAD_0 through NOTEPAD_15 can be used to access the
@c sixteen notepad locations.
The sixteen notepad locations can be accessed using the constants
NOTEPAD_0 through NOTEPAD_15.
Setting a notepad location of a global task which does not
reside on the local node will generate a request to the remote
node to set the specified notepad entry.
@page
@ifinfo
@node TASK_WAKE_AFTER - Wake up after interval, TASK_WAKE_WHEN - Wake up when specified, TASK_SET_NOTE - Set task notepad entry, Task Manager Directives
@end ifinfo
@subsection TASK_WAKE_AFTER - Wake up after interval
@subheading CALLING SEQUENCE:
@ifset is-C
@example
rtems_status_code rtems_task_wake_after(
rtems_interval ticks
);
@end example
@end ifset
@ifset is-Ada
@example
procedure Task_Wake_After (
Ticks : in RTEMS.Interval;
Result : out RTEMS.Status_Codes
);
@end example
@end ifset
@subheading DIRECTIVE STATUS CODES:
@code{SUCCESSFUL} - always successful
@subheading DESCRIPTION:
This directive blocks the calling task for the specified number
of system clock ticks. When the requested interval has elapsed,
the task is made ready. The clock_tick directive automatically
updates the delay period.
@subheading NOTES:
Setting the system date and time with the clock_set directive
has no effect on a task_wake_after blocked task.
A task may give up the processor and remain in the ready state
by specifying a value of YIELD_PROCESSOR in ticks.
The maximum timer interval that can be specified is the maximum
value which can be represented by the rtems_unsigned32 type.
@page
@ifinfo
@node TASK_WAKE_WHEN - Wake up when specified, Interrupt Manager, TASK_WAKE_AFTER - Wake up after interval, Task Manager Directives
@end ifinfo
@subsection TASK_WAKE_WHEN - Wake up when specified
@subheading CALLING SEQUENCE:
@ifset is-C
@example
rtems_status_code rtems_task_wake_when(
rtems_time_of_day *time_buffer
);
@end example
@end ifset
@ifset is-Ada
@example
procedure Task_Wake_When (
Time_Buffer : in RTEMS.Time_Of_Day;
Result : out RTEMS.Status_Codes
);
@end example
@end ifset
@subheading DIRECTIVE STATUS CODES:
@code{SUCCESSFUL} - awakened at date/time successfully@*
@code{INVALID_TIME_OF_DAY} - invalid time buffer@*
@code{NOT_DEFINED} - system date and time is not set
@subheading DESCRIPTION:
This directive blocks a task until the date and time specified
in time_buffer. At the requested date and time, the calling
task will be unblocked and made ready to execute.
@subheading NOTES:
The ticks portion of time_buffer structure is ignored. The
timing granularity of this directive is a second.
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