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Task Manager

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.. index:: tasks

Introduction

The task manager provides a comprehensive set of directives to create, delete, and administer tasks. The directives provided by the task manager are:

  • rtems.task_create - Create a task
  • rtems.task_ident - Get ID of a task
  • rtems.task_self - Obtain ID of caller
  • rtems.task_start - Start a task
  • rtems.task_restart - Restart a task
  • rtems.task_delete - Delete a task
  • rtems.task_suspend - Suspend a task
  • rtems.task_resume - Resume a task
  • rtems.task_is_suspended - Determine if a task is suspended
  • rtems.task_set_priority - Set task priority
  • rtems.task_mode - Change current task’s mode
  • rtems.task_wake_after - Wake up after interval
  • rtems.task_wake_when - Wake up when specified
  • rtems.iterate_over_all_threads - Iterate Over Tasks
  • rtems.task_variable_add - Associate per task variable
  • rtems.task_variable_get - Obtain value of a a per task variable
  • rtems.task_variable_delete - Remove per task variable

Background

Task Definition

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.. index:: 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:

  • a "dispatchable" unit.
  • an entity to which the processor is allocated.
  • an atomic unit of a real-time, multiprocessor system.
  • single threads of execution which concurrently compete for resources.
  • a sequence of closely related computations which can execute concurrently with other computational sequences.

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).

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, 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.

Task States

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.. index:: task states

A task may exist in one of the following five states:

  • executing - Currently scheduled to the CPU
  • ready - May be scheduled to the CPU
  • blocked - Unable to be scheduled to the CPU
  • dormant - Created task that is not started
  • non-existent - Uncreated or deleted task

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.

Task Priority

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.. index:: task priority
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.. index:: priority, task
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.. index:: rtems_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. The data type``rtems.task_priority`` is used to store task priorities.

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.

Task Mode

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.. index:: task mode
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.. index:: rtems_task_mode

A task’s execution mode is a combination of the following four components:

  • preemption
  • ASR processing
  • timeslicing
  • interrupt level

It is used to modify RTEMS’ scheduling process and to alter the execution environment of the task. The data type``rtems.task_mode`` is used to manage the task execution mode... index:: preemption

The preemption component allows a task to determine when control of the processor is relinquished. If preemption is disabled (RTEMS.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 (RTEMS.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... index:: timeslicing

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 (RTEMS.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 (RTEMS.NO_TIMESLICE), then the task will be allowed to execute until a task of higher priority is made ready. If``RTEMS.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 (RTEMS.ASR), then signals sent to the task will be processed the next time the task executes. If signal processing is disabled (RTEMS.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... index:: interrupt level, task

The interrupt level component is used to determine which interrupts will be enabled when the task is executing.``RTEMS.INTERRUPT_LEVEL(n)`` specifies that the task will execute at interrupt level n.

  • RTEMS.PREEMPT - enable preemption (default)
  • RTEMS.NO_PREEMPT - disable preemption
  • RTEMS.NO_TIMESLICE - disable timeslicing (default)
  • RTEMS.TIMESLICE - enable timeslicing
  • RTEMS.ASR - enable ASR processing (default)
  • RTEMS.NO_ASR - disable ASR processing
  • RTEMS.INTERRUPT_LEVEL(0) - enable all interrupts (default)
  • RTEMS.INTERRUPT_LEVEL(n) - execute at interrupt level n

The set of default modes may be selected by specifying the``RTEMS.DEFAULT_MODES`` constant.

Accessing Task Arguments

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.. index:: task arguments
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.. index:: task prototype

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:

procedure User_Task (
Argument : in    RTEMS.Task_Argument_Ptr
);

Application tasks requiring more information may view this single argument as an index into an array of parameter blocks.

Floating Point Considerations

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.. index:: floating point

Creating a task with the RTEMS.FLOATING_POINT attribute 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 RTEMS.NO_FLOATING_POINT tasks. Saving and restoring the context of a RTEMS.FLOATING_POINT task takes longer than that of a RTEMS.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``RTEMS.FLOATING_POINT`` task is dispatched and that task was not the last task to utilize the coprocessor. In a system with only one``RTEMS.FLOATING_POINT`` task, the state of the numeric coprocessor will never be saved or restored.

Although the overhead imposed by RTEMS.FLOATING_POINT tasks is minimal, some applications may wish to completely avoid the overhead associated with RTEMS.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``RTEMS.NO_FLOATING_POINT`` task can utilize the numeric coprocessor without incurring the overhead of a``RTEMS.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 RTEMS.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``RTEMS.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 RTEMS.NO_FLOATING_POINT whether created as RTEMS.FLOATING_POINT or``RTEMS.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 RTEMS.FLOATING_POINT attribute. The consequence of a RTEMS.NO_FLOATING_POINT task attempting to access the floating point unit is CPU dependent but will generally result in an exception condition.

Per Task Variables

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.. index:: per task variables

Per task variables are deprecated, see the warning below.

Per task variables are used to support global variables whose value may be unique to a task. After indicating that a variable should be treated as private (i.e. per-task) the task can access and modify the variable, but the modifications will not appear to other tasks, and other tasks’ modifications to that variable will not affect the value seen by the task. This is accomplished by saving and restoring the variable’s value each time a task switch occurs to or from the calling task.

The value seen by other tasks, including those which have not added the variable to their set and are thus accessing the variable as a common location shared among tasks, cannot be affected by a task once it has added a variable to its local set. Changes made to the variable by other tasks will not affect the value seen by a task which has added the variable to its private set.

This feature can be used when a routine is to be spawned repeatedly as several independent tasks. Although each task will have its own stack, and thus separate stack variables, they will all share the same static and global variables. To make a variable not shareable (i.e. a "global" variable that is specific to a single task), the tasks can call``rtems_task_variable_add`` to make a separate copy of the variable for each task, but all at the same physical address.

Task variables increase the context switch time to and from the tasks that own them so it is desirable to minimize the number of task variables. One efficient method is to have a single task variable that is a pointer to a dynamically allocated structure containing the task’s private "global" data.

A critical point with per-task variables is that each task must separately request that the same global variable is per-task private.

WARNING: Per-Task variables are inherently broken on SMP systems. They only work correctly when there is one task executing in the system and that task is the logical owner of the value in the per-task variable’s location. There is no way for a single memory image to contain the correct value for each task executing on each core. Consequently, per-task variables are disabled in SMP configurations of RTEMS. Instead the application developer should consider the use of POSIX Keys or Thread Local Storage (TLS). POSIX Keys are not enabled in all RTEMS configurations.

Building a Task Attribute Set

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.. index:: task attributes, building

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:

  • RTEMS.NO_FLOATING_POINT - does not use coprocessor (default)
  • RTEMS.FLOATING_POINT - uses numeric coprocessor
  • RTEMS.LOCAL - local task (default)
  • RTEMS.GLOBAL - global task

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 RTEMS.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 RTEMS.FLOATING_POINT or``RTEMS.LOCAL or RTEMS.FLOATING_POINT``. The attribute_set parameter can be set to``RTEMS.FLOATING_POINT`` because RTEMS.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``RTEMS.GLOBAL or RTEMS.FLOATING_POINT``.

Building a Mode and Mask

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.. index:: task mode, building

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:

  • RTEMS.PREEMPT is masked by``RTEMS.PREEMPT_MASK`` and enables preemption
  • RTEMS.NO_PREEMPT is masked by``RTEMS.PREEMPT_MASK`` and disables preemption
  • RTEMS.NO_TIMESLICE is masked by``RTEMS.TIMESLICE_MASK`` and disables timeslicing
  • RTEMS.TIMESLICE is masked by``RTEMS.TIMESLICE_MASK`` and enables timeslicing
  • RTEMS.ASR is masked by``RTEMS.ASR_MASK`` and enables ASR processing
  • RTEMS.NO_ASR is masked by``RTEMS.ASR_MASK`` and disables ASR processing
  • RTEMS.INTERRUPT_LEVEL(0) is masked by``RTEMS.INTERRUPT_MASK`` and enables all interrupts
  • RTEMS.INTERRUPT_LEVEL(n) is masked by``RTEMS.INTERRUPT_MASK`` and sets interrupts level n

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 RTEMS.DEFAULT_MODES and the mask RTEMS.ALL_MODE_MASKS should be used.

The following example demonstrates the mode and mask parameters used with the rtems.task_mode directive to place a task at interrupt level 3 and make it non-preemptible. The mode should be set to``RTEMS.INTERRUPT_LEVEL(3) or RTEMS.NO_PREEMPT`` to indicate the desired preemption mode and interrupt level, while the mask parameter should be set to``RTEMS.INTERRUPT_MASK or RTEMS.NO_PREEMPT_MASK`` to indicate that the calling task’s interrupt level and preemption mode are being altered.

Operations

Creating Tasks

The rtems.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.

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 rtems.task_create directive, the task ID is stored in a user provided location. Second, the task ID may be obtained later using the rtems.task_ident directive. The task ID is used by other directives to manipulate this task.

Starting and Restarting Tasks

The rtems.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 rtems.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 rtems.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.

Suspending and Resuming Tasks

The rtems.task_suspend directive is used to place either the caller or another task into a suspended state. The task remains suspended until a rtems.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 rtems.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.

Suspending a task which is already suspended or resuming a task which is not suspended is considered an error. The rtems.task_is_suspended can be used to determine if a task is currently suspended.

Delaying the Currently Executing Task

The rtems.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 rtems.task_wake_after directive with a delay interval of RTEMS.YIELD_PROCESSOR ticks will yield the processor to any other ready task of equal or greater priority and remain ready to execute.

The rtems.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.

Changing Task Priority

The rtems.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``RTEMS.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 rtems.task_restart directive resets the priority of a task to its original value.

Changing Task Mode

The rtems.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 rtems.task_restart directive resets the mode of a task to its original value.

Task Deletion

RTEMS provides the rtems.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``rtems.task_wake_after`` and``rtems.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.

Transition Advice for Obsolete Directives

Notepads

Task notepads and the associated directives``rtems.task_get_note`` and``rtems.task_set_note`` were removed after the 4.11 Release Series. These were never thread-safe to access and subject to conflicting use of the notepad index by libraries which were designed independently.

It is recommended that applications be modified to use services which are thread safe and not subject to issues with multiple applications conflicting over the key (e.g. notepad index) selection. For most applications, POSIX Keys should be used. These are available in all RTEMS build configurations. It is also possible that Thread Local Storage is an option for some use cases.

Directives

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.

TASK_CREATE - Create a task

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.. index:: create a task

CALLING SEQUENCE:

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
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - task created successfully RTEMS.INVALID_ADDRESS - id is NULL RTEMS.INVALID_NAME - invalid task name RTEMS.INVALID_PRIORITY - invalid task priority RTEMS.MP_NOT_CONFIGURED - multiprocessing not configured RTEMS.TOO_MANY - too many tasks created RTEMS.UNSATISFIED - not enough memory for stack/FP context RTEMS.TOO_MANY - too many global objects

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``RTEMS.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 RTEMS.NO_FLOATING_POINT attribute. If the RTEMS.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 rtems.task_start.

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.

If the requested stack size is less than the configured minimum stack size, then RTEMS will use the configured minimum as the stack size for this task. In addition to being able to specify the task stack size as a integer, there are two constants which may be specified:

  • RTEMS.MINIMUM_STACK_SIZE is the minimum stack size RECOMMENDED for use on this processor. This value is selected by the RTEMS developers conservatively to minimize the risk of blown stacks for most user applications. Using this constant when specifying the task stack size, indicates that the stack size will be at least``RTEMS.MINIMUM_STACK_SIZE`` bytes in size. If the user configured minimum stack size is larger than the recommended minimum, then it will be used.
  • RTEMS.CONFIGURED_MINIMUM_STACK_SIZE indicates that this task is to be created with a stack size of the minimum stack size that was configured by the application. If not explicitly configured by the application, the default configured minimum stack size is the processor dependent value``RTEMS.MINIMUM_STACK_SIZE``. Since this uses the configured minimum stack size value, you may get a stack size that is smaller or larger than the recommended minimum. This can be used to provide large stacks for all tasks on complex applications or small stacks on applications that are trying to conserve memory.

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:

  • RTEMS.NO_FLOATING_POINT - does not use coprocessor (default)
  • RTEMS.FLOATING_POINT - uses numeric coprocessor
  • RTEMS.LOCAL - local task (default)
  • RTEMS.GLOBAL - global task

The following task mode constants are defined by RTEMS:

  • RTEMS.PREEMPT - enable preemption (default)
  • RTEMS.NO_PREEMPT - disable preemption
  • RTEMS.NO_TIMESLICE - disable timeslicing (default)
  • RTEMS.TIMESLICE - enable timeslicing
  • RTEMS.ASR - enable ASR processing (default)
  • RTEMS.NO_ASR - disable ASR processing
  • RTEMS.INTERRUPT_LEVEL(0) - enable all interrupts (default)
  • RTEMS.INTERRUPT_LEVEL(n) - execute at interrupt level n

The interrupt level portion of the task execution mode supports a maximum of 256 interrupt levels. These levels are mapped onto the interrupt levels actually supported by the target processor in a processor dependent fashion.

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.

TASK_IDENT - Get ID of a task

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.. index:: get ID of a task

CALLING SEQUENCE:

procedure Task_Ident (
Name   : in     RTEMS.Name;
Node   : in     RTEMS.Node;
ID     :    out RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - task identified successfully RTEMS.INVALID_ADDRESS - id is NULL RTEMS.INVALID_NAME - invalid task name RTEMS.INVALID_NODE - invalid node id

DESCRIPTION:

This directive obtains the task id associated with the task name specified in name. A task may obtain its own id by specifying``RTEMS.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.

NOTES:

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

If node is RTEMS.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.

TASK_SELF - Obtain ID of caller

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.. index:: obtain ID of caller

CALLING SEQUENCE:

function Task_Self return RTEMS.ID;

DIRECTIVE STATUS CODES:

Returns the object Id of the calling task.

DESCRIPTION:

This directive returns the Id of the calling task.

NOTES:

If called from an interrupt service routine, this directive will return the Id of the interrupted task.

TASK_START - Start a task

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.. index:: starting a task

CALLING SEQUENCE:

procedure Task_Start (
ID          : in     RTEMS.ID;
Entry_Point : in     RTEMS.Task_Entry;
Argument    : in     RTEMS.Task_Argument;
Result      :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - ask started successfully RTEMS.INVALID_ADDRESS - invalid task entry point RTEMS.INVALID_ID - invalid task id RTEMS.INCORRECT_STATE - task not in the dormant state RTEMS.ILLEGAL_ON_REMOTE_OBJECT - cannot start remote task

DESCRIPTION:

This directive readies the task, specified by id, 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. The type of this numeric argument is an unsigned integer type with the property that any valid pointer to void can be converted to this type and then converted back to a pointer to void. The result will compare equal to the original pointer.

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 rtems.task_start directive.

TASK_RESTART - Restart a task

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.. index:: restarting a task

CALLING SEQUENCE:

procedure Task_Restart (
ID       : in     RTEMS.ID;
Argument : in     RTEMS.Task_Argument;
Result   :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - task restarted successfully RTEMS.INVALID_ID - task id invalid RTEMS.INCORRECT_STATE - task never started RTEMS.ILLEGAL_ON_REMOTE_OBJECT - cannot restart remote task

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. The type of this numeric argument is an unsigned integer type with the property that any valid pointer to void can be converted to this type and then converted back to a pointer to void. The result will compare equal to the original pointer. This new argument may be used to distinguish between the initial rtems.task_start of the task and any ensuing calls to rtems.task_restart of the task. This can be beneficial in deleting a task. Instead of deleting a task using the rtems.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.

NOTES:

If id is RTEMS.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 RTEMS.GLOBAL option.

TASK_DELETE - Delete a task

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.. index:: deleting a task

CALLING SEQUENCE:

procedure Task_Delete (
ID     : in     RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - task deleted successfully RTEMS.INVALID_ID - task id invalid RTEMS.ILLEGAL_ON_REMOTE_OBJECT - cannot restart remote task

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 RTEMS.FLOATING_POINT, its floating point context area. RTEMS does not reclaim the following resources: region segments, partition buffers, semaphores, timers, or rate monotonic periods.

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 (RTEMS.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 RTEMS.GLOBAL option.

TASK_SUSPEND - Suspend a task

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.. index:: suspending a task

CALLING SEQUENCE:

procedure Task_Suspend (
ID     : in     RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - task suspended successfully RTEMS.INVALID_ID - task id invalid RTEMS.ALREADY_SUSPENDED - task already suspended

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 rtems.task_resume directive for this task and any blocked state has been removed.

NOTES:

The requesting task can suspend itself by specifying RTEMS.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.

If the task specified by id is already suspended, then the``RTEMS.ALREADY_SUSPENDED`` status code is returned.

TASK_RESUME - Resume a task

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.. index:: resuming a task

CALLING SEQUENCE:

procedure Task_Resume (
ID     : in     RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - task resumed successfully RTEMS.INVALID_ID - task id invalid RTEMS.INCORRECT_STATE - task not suspended

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.

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.

If the task specified by id is not suspended, then the``RTEMS.INCORRECT_STATE`` status code is returned.

TASK_IS_SUSPENDED - Determine if a task is Suspended

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.. index:: is task suspended

CALLING SEQUENCE:

procedure Task_Is_Suspended (
ID     : in     RTEMS.ID;
Result :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - task is NOT suspended RTEMS.ALREADY_SUSPENDED - task is currently suspended RTEMS.INVALID_ID - task id invalid RTEMS.ILLEGAL_ON_REMOTE_OBJECT - not supported on remote tasks

DESCRIPTION:

This directive returns a status code indicating whether or not the specified task is currently suspended.

NOTES:

This operation is not currently supported on remote tasks.

TASK_SET_PRIORITY - Set task priority

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.. index:: rtems_task_set_priority
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.. index:: current task priority
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.. index:: set task priority
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.. index:: get task priority
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.. index:: obtain task priority

CALLING SEQUENCE:

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
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - task priority set successfully RTEMS.INVALID_ID - invalid task id RTEMS.INVALID_ADDRESS - invalid return argument pointer RTEMS.INVALID_PRIORITY - invalid task priority

DESCRIPTION:

This directive manipulates the priority of the task specified by id. An id of RTEMS.SELF is used to indicate the calling task. When new_priority is not equal to``RTEMS.CURRENT_PRIORITY``, the specified task’s previous priority is returned in old_priority. When new_priority is RTEMS.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.

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.

In case the new priority equals the current priority of the task, then nothing happens.

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.

TASK_MODE - Change the current task mode

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.. index:: current task mode
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.. index:: set task mode
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.. index:: get task mode
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.. index:: set task preemption mode
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.. index:: get task preemption mode
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.. index:: obtain task mode

CALLING SEQUENCE:

procedure Task_Mode (
Mode_Set          : in     RTEMS.Mode;
Mask              : in     RTEMS.Mode;
Previous_Mode_Set : in     RTEMS.Mode;
Result            :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - task mode set successfully RTEMS.INVALID_ADDRESS - previous_mode_set is NULL

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.

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 disabled. For a task to be timesliced, that task must have both preemption and timeslicing enabled.

A task can obtain its current execution mode, without modifying it, by calling this directive with a mask value of``RTEMS.CURRENT_MODE``.

To temporarily disable the processing of a valid ASR, a task should call this directive with the RTEMS.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:

  • RTEMS.PREEMPT is masked by``RTEMS.PREEMPT_MASK`` and enables preemption
  • RTEMS.NO_PREEMPT is masked by``RTEMS.PREEMPT_MASK`` and disables preemption
  • RTEMS.NO_TIMESLICE is masked by``RTEMS.TIMESLICE_MASK`` and disables timeslicing
  • RTEMS.TIMESLICE is masked by``RTEMS.TIMESLICE_MASK`` and enables timeslicing
  • RTEMS.ASR is masked by``RTEMS.ASR_MASK`` and enables ASR processing
  • RTEMS.NO_ASR is masked by``RTEMS.ASR_MASK`` and disables ASR processing
  • RTEMS.INTERRUPT_LEVEL(0) is masked by``RTEMS.INTERRUPT_MASK`` and enables all interrupts
  • RTEMS.INTERRUPT_LEVEL(n) is masked by``RTEMS.INTERRUPT_MASK`` and sets interrupts level n

TASK_WAKE_AFTER - Wake up after interval

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.. index:: delay a task for an interval
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.. index:: wake up after an interval

CALLING SEQUENCE:

procedure Task_Wake_After (
Ticks  : in     RTEMS.Interval;
Result :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - always successful

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 rtems.clock_tick directive automatically updates the delay period.

NOTES:

Setting the system date and time with the``rtems.clock_set`` directive has no effect on a rtems.task_wake_after blocked task.

A task may give up the processor and remain in the ready state by specifying a value of RTEMS.YIELD_PROCESSOR in ticks.

The maximum timer interval that can be specified is the maximum value which can be represented by the uint32_t type.

A clock tick is required to support the functionality of this directive.

TASK_WAKE_WHEN - Wake up when specified

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.. index:: delay a task until a wall time
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.. index:: wake up at a wall time

CALLING SEQUENCE:

procedure Task_Wake_When (
Time_Buffer : in     RTEMS.Time_Of_Day;
Result      :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - awakened at date/time successfully RTEMS.INVALID_ADDRESS - time_buffer is NULL RTEMS.INVALID_TIME_OF_DAY - invalid time buffer RTEMS.NOT_DEFINED - system date and time is not set

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.

NOTES:

The ticks portion of time_buffer record is ignored. The timing granularity of this directive is a second.

A clock tick is required to support the functionality of this directive.

ITERATE_OVER_ALL_THREADS - Iterate Over Tasks

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.. index:: iterate over all threads

CALLING SEQUENCE:

NOT SUPPORTED FROM Ada BINDING

DIRECTIVE STATUS CODES: NONE

DESCRIPTION:

This directive iterates over all of the existant threads in the system and invokes routine on each of them. The user should be careful in accessing the contents of the_thread.

This routine is intended for use in diagnostic utilities and is not intented for routine use in an operational system.

NOTES:

There is NO protection while this routine is called. Thus it is possible that the_thread could be deleted while this is operating. By not having protection, the user is free to invoke support routines from the C Library which require semaphores for data structures.

TASK_VARIABLE_ADD - Associate per task variable

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.. index:: per-task variable
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.. index:: task private variable
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.. index:: task private data

CALLING SEQUENCE:

type Task_Variable_Dtor is access procedure (
Argument : in     RTEMS.Address;
);
procedure Task_Variable_Add (
ID            : in     RTEMS.ID;
Task_Variable : in     RTEMS.Address;
Dtor          : in     RTEMS.Task_Variable_Dtor;
Result        :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - per task variable added successfully RTEMS.INVALID_ADDRESS - task_variable is NULL RTEMS.INVALID_ID - invalid task id RTEMS.NO_MEMORY - invalid task id RTEMS.ILLEGAL_ON_REMOTE_OBJECT - not supported on remote tasks

DESCRIPTION:

This directive adds the memory location specified by the ptr argument to the context of the given task. The variable will then be private to the task. The task can access and modify the variable, but the modifications will not appear to other tasks, and other tasks’ modifications to that variable will not affect the value seen by the task. This is accomplished by saving and restoring the variable’s value each time a task switch occurs to or from the calling task. If the dtor argument is non-NULL it specifies the address of a ‘destructor’ function which will be called when the task is deleted. The argument passed to the destructor function is the task’s value of the variable.

NOTES:

This directive is deprecated and task variables will be removed.

Task variables increase the context switch time to and from the tasks that own them so it is desirable to minimize the number of task variables. One efficient method is to have a single task variable that is a pointer to a dynamically allocated structure containing the task’s private ‘global’ data. In this case the destructor function could be ‘free’.

Per-task variables are disabled in SMP configurations and this service is not available.

TASK_VARIABLE_GET - Obtain value of a per task variable

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.. index:: get per-task variable
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.. index:: obtain per-task variable

CALLING SEQUENCE:

procedure Task_Variable_Get (
ID                  : in     RTEMS.ID;
Task_Variable       :    out RTEMS.Address;
Task_Variable_Value :    out RTEMS.Address;
Result              :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - per task variable obtained successfully RTEMS.INVALID_ADDRESS - task_variable is NULL RTEMS.INVALID_ADDRESS - task_variable_value is NULL RTEMS.INVALID_ADDRESS - task_variable is not found RTEMS.NO_MEMORY - invalid task id RTEMS.ILLEGAL_ON_REMOTE_OBJECT - not supported on remote tasks

DESCRIPTION:

This directive looks up the private value of a task variable for a specified task and stores that value in the location pointed to by the result argument. The specified task is usually not the calling task, which can get its private value by directly accessing the variable.

NOTES:

This directive is deprecated and task variables will be removed.

If you change memory which task_variable_value points to, remember to declare that memory as volatile, so that the compiler will optimize it correctly. In this case both the pointer``task_variable_value`` and data referenced by task_variable_value should be considered volatile.

Per-task variables are disabled in SMP configurations and this service is not available.

TASK_VARIABLE_DELETE - Remove per task variable

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.. index:: per-task variable
?
.. index:: task private variable
?
.. index:: task private data

CALLING SEQUENCE:

procedure Task_Variable_Delete (
ID                  : in     RTEMS.ID;
Task_Variable       :    out RTEMS.Address;
Result              :    out RTEMS.Status_Codes
);

DIRECTIVE STATUS CODES:

RTEMS.SUCCESSFUL - per task variable deleted successfully RTEMS.INVALID_ID - invalid task id RTEMS.NO_MEMORY - invalid task id RTEMS.INVALID_ADDRESS - task_variable is NULL RTEMS.ILLEGAL_ON_REMOTE_OBJECT - not supported on remote tasks

DESCRIPTION:

This directive removes the given location from a task’s context.

NOTES:

This directive is deprecated and task variables will be removed.

Per-task variables are disabled in SMP configurations and this service is not available.

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