7. Task Manager

7.1. Introduction

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

7.2. Background

7.2.1. 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).

7.2.2. 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.

7.2.3. Task Memory

The system uses two separate memory areas to manage a task. One memory area is the Task Control Block. The other memory area is allocated from the stack space or provided by the user and contains

  • the task stack,

  • the thread-local storage (TLS), and

  • an optional architecture-specific floating-point context.

The size of the thread-local storage is determined at link time. A user-provided task stack must take the size of the thread-local storage into account.

On architectures with a dedicated floating-point context, the application configuration assumes that every task is a floating-point task, but whether or not a task is actually floating-point is determined at runtime during task creation (see Floating Point Considerations). In highly memory constrained systems this potential overestimate of the task stack space can be mitigated through the CONFIGURE_MINIMUM_TASK_STACK_SIZE configuration option and aligned task stack sizes for the tasks. A user-provided task stack must take the potential floating-point context into account.

7.2.4. Task Name

By default, the task name is defined by the task object name given to rtems_task_create(). The task name can be obtained with the pthread_getname_np() function. Optionally, a new task name may be set with the pthread_setname_np() function. The maximum size of a task name is defined by the application configuration option CONFIGURE_MAXIMUM_THREAD_NAME_SIZE.

7.2.5. 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.

7.2.6. 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.

7.2.7. 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.

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.

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.

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.

7.2.8. 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:

rtems_task user_task(
    rtems_task_argument argument
);

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

7.2.9. Floating Point Considerations

Please consult the RTEMS CPU Architecture Supplement if this section is relevant on your architecture. On some architectures the floating-point context is contained in the normal task context and this section does not apply.

Creating a task with the RTEMS_FLOATING_POINT attribute flag results in additional memory being allocated for the task 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. In uniprocessor configurations, 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 uniprocessor 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. Some architectures with a dedicated floating-point context raise a processor exception if a task with RTEMS_NO_FLOATING_POINT issues a floating-point instruction, so this approach may not work at all.

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.

7.2.10. Building a Task 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:

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 | 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 | RTEMS_FLOATING_POINT.

7.2.11. 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:

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) | RTEMS_NO_PREEMPT to indicate the desired preemption mode and interrupt level, while the mask parameter should be set to RTEMS_INTERRUPT_MASK | RTEMS_NO_PREEMPT_MASK to indicate that the calling task’s interrupt level and preemption mode are being altered.

7.3. Operations

7.3.1. 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.

7.3.2. 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.

7.3.3. 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.

7.3.4. 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.

7.3.5. 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.

7.3.6. 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.

7.3.7. 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.

7.3.8. 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. A specialization of rtems_task_delete is rtems_task_exit which deletes the calling task.

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.

7.3.9. Setting Affinity to a Single Processor

On some embedded applications targeting SMP systems, it may be beneficial to lock individual tasks to specific processors. In this way, one can designate a processor for I/O tasks, another for computation, etc.. The following illustrates the code sequence necessary to assign a task an affinity for processor with index processor_index.

#include <rtems.h>
#include <assert.h>

void pin_to_processor(rtems_id task_id, int processor_index)
{
    rtems_status_code sc;
    cpu_set_t         cpuset;
    CPU_ZERO(&cpuset);
    CPU_SET(processor_index, &cpuset);
    sc = rtems_task_set_affinity(task_id, sizeof(cpuset), &cpuset);
    assert(sc == RTEMS_SUCCESSFUL);
}

It is important to note that the cpuset is not validated until the rtems_task_set_affinity call is made. At that point, it is validated against the current system configuration.

7.3.10. Transition Advice for Removed Notepads

Task notepads and the associated directives TASK_GET_NOTE - Get task notepad entry and TASK_SET_NOTE - Set task notepad entry were removed in RTEMS 5.1. 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 (TLS) is an option for some use cases.

7.3.11. Transition Advice for Removed Task Variables

Task notepads and the associated directives TASK_VARIABLE_ADD - Associate per task variable, TASK_VARIABLE_GET - Obtain value of a per task variable and TASK_VARIABLE_DELETE - Remove per task variable were removed in RTEMS 5.1. Task variables must be replaced by POSIX Keys or thread-local storage (TLS). POSIX Keys are available in all configurations and support value destructors. For the TLS support consult the RTEMS CPU Architecture Supplement.

7.4. 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.

7.4.1. TASK_CREATE - Create a task

CALLING SEQUENCE:
rtems_status_code rtems_task_create(
    rtems_name           name,
    rtems_task_priority  initial_priority,
    size_t               stack_size,
    rtems_mode           initial_modes,
    rtems_attribute      attribute_set,
    rtems_id            *id
);
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_TOO_MANY

too many tasks created

RTEMS_UNSATISFIED

not enough memory for stack/FP context

RTEMS_UNSATISFIED

non-preemption mode not supported on SMP system

RTEMS_UNSATISFIED

interrupt level mode not supported on SMP system

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 may cause the calling task to be preempted.

The scheduler of the new task is the scheduler of the executing task at some point during the task creation. The specified task priority must be valid for the selected scheduler.

The task processor affinity is initialized to the set of online processors.

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

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 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.

7.4.2. TASK_IDENT - Get ID of a task

CALLING SEQUENCE:
rtems_status_code rtems_task_ident(
    rtems_name  name,
    uint32_t    node,
    rtems_id   *id
);
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.

7.4.3. TASK_SELF - Obtain ID of caller

CALLING SEQUENCE:
rtems_id rtems_task_self(void);
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.

7.4.4. TASK_START - Start a task

CALLING SEQUENCE:
rtems_status_code rtems_task_start(
    rtems_id            id,
    rtems_task_entry    entry_point,
    rtems_task_argument argument
);
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.

7.4.5. TASK_RESTART - Restart a task

CALLING SEQUENCE:
rtems_status_code rtems_task_restart(
   rtems_id            id,
   rtems_task_argument argument
);
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.

7.4.6. TASK_DELETE - Delete a task

CALLING SEQUENCE:
rtems_status_code rtems_task_delete(
    rtems_id id
);
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.

7.4.7. TASK_EXIT - Delete the calling task

CALLING SEQUENCE:
void rtems_task_exit( void ) RTEMS_NO_RETURN;
DIRECTIVE STATUS CODES:

NONE - This function will not return to the caller.

DESCRIPTION:

This directive deletes the calling task.

NOTES:

This directive must be called from a regular task context with enabled interrupts, otherwise one of the fatal errors

will occur.

The rtems_task_exit() call is equivalent to the following code sequence:

pthread_detach(pthread_self());
pthread_exit(NULL);

See also rtems_task_delete().

7.4.8. TASK_SUSPEND - Suspend a task

CALLING SEQUENCE:
rtems_status_code rtems_task_suspend(
    rtems_id id
);
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.

7.4.9. TASK_RESUME - Resume a task

CALLING SEQUENCE:
rtems_status_code rtems_task_resume(
    rtems_id id
);
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.

7.4.10. TASK_IS_SUSPENDED - Determine if a task is Suspended

CALLING SEQUENCE:
rtems_status_code rtems_task_is_suspended(
    rtems_id id
);
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.

7.4.11. TASK_SET_PRIORITY - Set task priority

CALLING SEQUENCE:
rtems_status_code rtems_task_set_priority(
    rtems_id             id,
    rtems_task_priority  new_priority,
    rtems_task_priority *old_priority
);
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.

7.4.12. TASK_GET_PRIORITY - Get task priority

CALLING SEQUENCE:
rtems_status_code rtems_task_get_priority(
    rtems_id             task_id,
    rtems_id             scheduler_id,
    rtems_task_priority *priority
);
DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL

Successful operation.

RTEMS_ILLEGAL_ON_REMOTE_OBJECT

Directive is illegal on remote tasks.

RTEMS_INVALID_ADDRESS

The priority parameter is NULL.

RTEMS_INVALID_ID

Invalid task or scheduler identifier.

RTEMS_NOT_DEFINED

The task has no priority within the specified scheduler instance. This error is only possible in SMP configurations.
DESCRIPTION:

This directive returns the current priority of the task specified by task_id with respect to the scheduler instance specified by scheduler_id. A task id of RTEMS_SELF is used to indicate the calling task.

NOTES:

The current priority reflects temporary priority adjustments due to locking protocols, the rate-monotonic period objects on some schedulers and other mechanisms.

7.4.13. TASK_MODE - Change the current task mode

CALLING SEQUENCE:
rtems_status_code rtems_task_mode(
    rtems_mode  mode_set,
    rtems_mode  mask,
    rtems_mode *previous_mode_set
);
DIRECTIVE STATUS CODES:
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

7.4.14. TASK_WAKE_AFTER - Wake up after interval

CALLING SEQUENCE:
rtems_status_code rtems_task_wake_after(
    rtems_interval ticks
);
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 clock tick directives 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.

7.4.15. TASK_WAKE_WHEN - Wake up when specified

CALLING SEQUENCE:
rtems_status_code rtems_task_wake_when(
    rtems_time_of_day *time_buffer
);
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 structure is ignored. The timing granularity of this directive is a second.

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

7.4.16. TASK_GET_SCHEDULER - Get scheduler of a task

CALLING SEQUENCE:
rtems_status_code rtems_task_get_scheduler(
    rtems_id  task_id,
    rtems_id *scheduler_id
);
DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL

successful operation

RTEMS_INVALID_ADDRESS

scheduler_id is NULL

RTEMS_INVALID_ID

invalid task id
DESCRIPTION:

Returns the scheduler identifier of a task identified by task_id in scheduler_id.

NOTES:

None.

7.4.17. TASK_SET_SCHEDULER - Set scheduler of a task

CALLING SEQUENCE:
rtems_status_code rtems_task_set_scheduler(
  rtems_id            task_id,
  rtems_id            scheduler_id,
  rtems_task_priority priority
);
DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL

successful operation

RTEMS_INVALID_ID

invalid task or scheduler id

RTEMS_INVALID_PRIORITY

invalid task priority

RTEMS_RESOURCE_IN_USE

the task is in the wrong state to perform a scheduler change

RTEMS_UNSATISFIED

the processor set of the scheduler is empty

RTEMS_ILLEGAL_ON_REMOTE_OBJECT

not supported on remote tasks
DESCRIPTION:

Sets the scheduler of a task identified by task_id to the scheduler identified by scheduler_id. The scheduler of a task is initialized to the scheduler of the task that created it. The priority of the task is set to priority.

NOTES:

It is recommended to set the scheduler of a task before it is started or in case it is guaranteed that the task owns no resources. Otherwise, sporadic RTEMS_RESOURCE_IN_USE errors may occur.

EXAMPLE:
 1 #include <rtems.h>
 2 #include <assert.h>
 3 
 4 rtems_task task( rtems_task_argument arg );
 5 
 6 void example( void )
 7 {
 8   rtems_status_code sc;
 9   rtems_id          task_id;
10   rtems_id          scheduler_id;
11   rtems_name        scheduler_name;
12 
13   scheduler_name = rtems_build_name( 'W', 'O', 'R', 'K' );
14 
15   sc = rtems_scheduler_ident( scheduler_name, &scheduler_id );
16   assert( sc == RTEMS_SUCCESSFUL );
17 
18   sc = rtems_task_create(
19     rtems_build_name( 'T', 'A', 'S', 'K' ),
20     1,
21     RTEMS_MINIMUM_STACK_SIZE,
22     RTEMS_DEFAULT_MODES,
23     RTEMS_DEFAULT_ATTRIBUTES,
24     &task_id
25   );
26   assert( sc == RTEMS_SUCCESSFUL );
27 
28   sc = rtems_task_set_scheduler( task_id, scheduler_id, 2 );
29   assert( sc == RTEMS_SUCCESSFUL );
30 
31   sc = rtems_task_start( task_id, task, 0 );
32   assert( sc == RTEMS_SUCCESSFUL );
33 }

7.4.18. TASK_GET_AFFINITY - Get task processor affinity

CALLING SEQUENCE:
rtems_status_code rtems_task_get_affinity(
    rtems_id   id,
    size_t     cpusetsize,
    cpu_set_t *cpuset
);
DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL

successful operation

RTEMS_INVALID_ADDRESS

cpuset is NULL

RTEMS_INVALID_ID

invalid task id

RTEMS_INVALID_NUMBER

the affinity set buffer is too small for the current processor affinity set of the task
DESCRIPTION:

Returns the current processor affinity set of the task in cpuset. A set bit in the affinity set means that the task can execute on this processor and a cleared bit means the opposite.

NOTES:

The task processor affinity is initialized to the set of online processors.

7.4.19. TASK_SET_AFFINITY - Set task processor affinity

CALLING SEQUENCE:
rtems_status_code rtems_task_set_affinity(
    rtems_id         id,
    size_t           cpusetsize,
    const cpu_set_t *cpuset
);
DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL

successful operation

RTEMS_INVALID_ADDRESS

cpuset is NULL

RTEMS_INVALID_ID

invalid task id

RTEMS_INVALID_NUMBER

invalid processor affinity set
DESCRIPTION:

Sets the processor affinity set for the task specified by cpuset. A set bit in the affinity set means that the task can execute on this processor and a cleared bit means the opposite.

NOTES:

This function will not change the scheduler of the task. The intersection of the processor affinity set and the set of processors owned by the scheduler of the task must be non-empty. It is not an error if the processor affinity set contains processors that are not part of the set of processors owned by the scheduler instance of the task. A task will simply not run under normal circumstances on these processors since the scheduler ignores them. Some locking protocols may temporarily use processors that are not included in the processor affinity set of the task. It is also not an error if the processor affinity set contains processors that are not part of the system.

In case a scheduler without support for task affinites is used for the task, then the task processor affinity set must contain all online processors of the system. This prevents odd corner cases if processors are added/removed at run-time to/from scheduler instances.

7.4.20. TASK_ITERATE - Iterate Over Tasks

CALLING SEQUENCE:
typedef bool ( *rtems_task_visitor )( rtems_tcb *tcb, void *arg );

void rtems_task_iterate(
    rtems_task_visitor  visitor,
    void               *arg
);
DIRECTIVE STATUS CODES:

NONE

DESCRIPTION:

Iterates over all tasks in the system. This operation covers all tasks of all APIs. The user should be careful in accessing the contents of the thread control block tcb. The visitor argument arg is passed to all invocations of visitor in addition to the thread control block. The iteration stops immediately in case the visitor function returns true.

NOTES:

Must be called from task context. This operation obtains and releases the objects allocator lock. The task visitor is called while owning the objects allocator lock. It is possible to perform blocking operations in the task visitor, however, take care that no deadlocks via the object allocator lock can occur.

7.5. Deprecated Directives

7.5.1. ITERATE_OVER_ALL_THREADS - Iterate Over Tasks

Warning

This directive is deprecated. Its use is unsafe. Use TASK_ITERATE - Iterate Over Tasks instead.

CALLING SEQUENCE:
typedef void (*rtems_per_thread_routine)(Thread_Control *the_thread);
void rtems_iterate_over_all_threads(
    rtems_per_thread_routine routine
);
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. The thread control block may be in an inconsistent state or may change due to interrupts or activity on other processors.

7.6. Removed Directives

7.6.1. TASK_GET_NOTE - Get task notepad entry

Warning

This directive was removed in RTEMS 5.1.

CALLING SEQUENCE:
rtems_status_code rtems_task_get_note(
  rtems_id  id,
  uint32_t  notepad,
  uint32_t *note
);
DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL

note value obtained successfully

RTEMS_INVALID_ADDRESS

note parameter is NULL

RTEMS_INVALID_ID

invalid task id

RTEMS_INVALID_NUMBER

invalid notepad location
DESCRIPTION:

This directive returns the note contained in the notepad location of the task specified by id.

NOTES:

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

If id is set to RTEMS_SELF, the calling task accesses its own notepad.

The sixteen notepad locations can be accessed using the constants RTEMS_NOTEPAD_0 through RTEMS_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.

7.6.2. TASK_SET_NOTE - Set task notepad entry

Warning

This directive was removed in RTEMS 5.1.

CALLING SEQUENCE:
rtems_status_code rtems_task_set_note(
  rtems_id  id,
  uint32_t  notepad,
  uint32_t  note
);
DIRECTIVE STATUS CODES:

RTEMS_SUCCESSFUL

note set successfully

RTEMS_INVALID_ID

invalid task id

RTEMS_INVALID_NUMBER

invalid notepad location
DESCRIPTION:

This directive sets the notepad entry for the task specified by id to the value note.

NOTES:

If id is set to RTEMS_SELF, the calling task accesses its own notepad.

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

The sixteen notepad locations can be accessed using the constants RTEMS_NOTEPAD_0 through RTEMS_NOTEPAD_15.

Setting a note of a global task which does not reside on the local node will generate a request to the remote node to set the notepad entry of the specified task.

7.6.3. TASK_VARIABLE_ADD - Associate per task variable

Warning

This directive was removed in RTEMS 5.1.

CALLING SEQUENCE:
rtems_status_code rtems_task_variable_add(
    rtems_id  tid,
    void    **task_variable,
    void    (*dtor)(void *)
);
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:

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.

7.6.4. TASK_VARIABLE_GET - Obtain value of a per task variable

Warning

This directive was removed in RTEMS 5.1.

CALLING SEQUENCE:
rtems_status_code rtems_task_variable_get(
    rtems_id  tid,
    void    **task_variable,
    void    **task_variable_value
);
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:

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.

7.6.5. TASK_VARIABLE_DELETE - Remove per task variable

Warning

This directive was removed in RTEMS 5.1.

CALLING SEQUENCE:
rtems_status_code rtems_task_variable_delete(
    rtems_id  id,
    void    **task_variable
);
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:

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