[489740f] | 1 | .. comment SPDX-License-Identifier: CC-BY-SA-4.0 |
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| 2 | |
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[2abceef] | 3 | .. COMMENT: COPYRIGHT (c) 1988-2008, 2018. |
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[8ef6ea8] | 4 | .. COMMENT: On-Line Applications Research Corporation (OAR). |
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| 5 | .. COMMENT: All rights reserved. |
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| 6 | |
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[fd6dc8c8] | 7 | Barrier Manager |
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[4da4a15] | 8 | *************** |
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[fd6dc8c8] | 9 | |
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| 10 | .. index:: barrier |
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| 11 | |
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| 12 | Introduction |
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| 13 | ============ |
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| 14 | |
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[8ef6ea8] | 15 | The barrier manager provides a unique synchronization capability which can be |
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| 16 | used to have a set of tasks block and be unblocked as a set. The directives |
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| 17 | provided by the barrier manager are: |
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[fd6dc8c8] | 18 | |
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[8ef6ea8] | 19 | - rtems_barrier_create_ - Create a barrier |
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[fd6dc8c8] | 20 | |
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[8ef6ea8] | 21 | - rtems_barrier_ident_ - Get ID of a barrier |
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[fd6dc8c8] | 22 | |
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[8ef6ea8] | 23 | - rtems_barrier_delete_ - Delete a barrier |
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[fd6dc8c8] | 24 | |
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[8ef6ea8] | 25 | - rtems_barrier_wait_ - Wait at a barrier |
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[fd6dc8c8] | 26 | |
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[8ef6ea8] | 27 | - rtems_barrier_release_ - Release a barrier |
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[fd6dc8c8] | 28 | |
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| 29 | Background |
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| 30 | ========== |
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| 31 | |
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[8ef6ea8] | 32 | A barrier can be viewed as a gate at which tasks wait until the gate is opened. |
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| 33 | This has many analogies in the real world. Horses and other farm animals may |
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| 34 | approach a closed gate and gather in front of it, waiting for someone to open |
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| 35 | the gate so they may proceed. Similarly, cticket holders gather at the gates |
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| 36 | of arenas before concerts or sporting events waiting for the arena personnel to |
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| 37 | open the gates so they may enter. |
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[fd6dc8c8] | 38 | |
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[8ef6ea8] | 39 | Barriers are useful during application initialization. Each application task |
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| 40 | can perform its local initialization before waiting for the application as a |
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| 41 | whole to be initialized. Once all tasks have completed their independent |
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| 42 | initializations, the "application ready" barrier can be released. |
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[fd6dc8c8] | 43 | |
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| 44 | Automatic Versus Manual Barriers |
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| 45 | -------------------------------- |
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| 46 | |
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[8ef6ea8] | 47 | Just as with a real-world gate, barriers may be configured to be manually |
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| 48 | opened or automatically opened. All tasks calling the ``rtems_barrier_wait`` |
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| 49 | directive will block until a controlling task invokes |
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| 50 | the ``rtems_barrier_release`` directive. |
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[fd6dc8c8] | 51 | |
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[8ef6ea8] | 52 | Automatic barriers are created with a limit to the number of tasks which may |
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| 53 | simultaneously block at the barrier. Once this limit is reached, all of the |
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| 54 | tasks are released. For example, if the automatic limit is ten tasks, then the |
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| 55 | first nine tasks calling the ``rtems_barrier_wait`` directive will block. When |
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[f02e872] | 56 | the tenth task calls the ``rtems_barrier_wait`` directive, the nine blocked |
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[8ef6ea8] | 57 | tasks will be released and the tenth task returns to the caller without |
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| 58 | blocking. |
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[fd6dc8c8] | 59 | |
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| 60 | Building a Barrier Attribute Set |
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| 61 | -------------------------------- |
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| 62 | |
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[8ef6ea8] | 63 | In general, an attribute set is built by a bitwise OR of the desired attribute |
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| 64 | components. The following table lists the set of valid barrier attributes: |
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[fd6dc8c8] | 65 | |
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[8ef6ea8] | 66 | ``RTEMS_BARRIER_AUTOMATIC_RELEASE`` |
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| 67 | automatically release the barrier when the configured number of tasks are |
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| 68 | blocked |
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[fd6dc8c8] | 69 | |
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[8ef6ea8] | 70 | ``RTEMS_BARRIER_MANUAL_RELEASE`` |
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| 71 | only release the barrier when the application invokes the |
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| 72 | ``rtems_barrier_release`` directive. (default) |
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[fd6dc8c8] | 73 | |
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[8ef6ea8] | 74 | .. note:: |
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[fd6dc8c8] | 75 | |
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[8ef6ea8] | 76 | Barriers only support FIFO blocking order because all waiting tasks are |
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| 77 | released as a set. Thus the released tasks will all become ready to execute |
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| 78 | at the same time and compete for the processor based upon their priority. |
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| 79 | |
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| 80 | Attribute values are specifically designed to be mutually exclusive, therefore |
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| 81 | bitwise OR and addition operations are equivalent as long as each attribute |
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| 82 | appears exactly once in the component list. An attribute listed as a default |
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| 83 | is not required to appear in the attribute list, although it is a good |
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| 84 | programming practice to specify default attributes. If all defaults are |
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| 85 | desired, the attribute ``RTEMS_DEFAULT_ATTRIBUTES`` should be specified on this |
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| 86 | call. |
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[fd6dc8c8] | 87 | |
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| 88 | This example demonstrates the attribute_set parameter needed to create a |
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[8ef6ea8] | 89 | barrier with the automatic release policy. The ``attribute_set`` parameter |
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| 90 | passed to the ``rtems_barrier_create`` directive will be |
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| 91 | ``RTEMS_BARRIER_AUTOMATIC_RELEASE``. In this case, the user must also specify |
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| 92 | the ``maximum_waiters`` parameter. |
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[fd6dc8c8] | 93 | |
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| 94 | Operations |
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| 95 | ========== |
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| 96 | |
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| 97 | Creating a Barrier |
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| 98 | ------------------ |
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| 99 | |
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[8ef6ea8] | 100 | The ``rtems_barrier_create`` directive creates a barrier with a user-specified |
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| 101 | name and the desired attributes. RTEMS allocates a Barrier Control Block (BCB) |
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| 102 | from the BCB free list. This data structure is used by RTEMS to manage the |
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| 103 | newly created barrier. Also, a unique barrier ID is generated and returned to |
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[fd6dc8c8] | 104 | the calling task. |
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| 105 | |
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| 106 | Obtaining Barrier IDs |
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| 107 | --------------------- |
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| 108 | |
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[8ef6ea8] | 109 | When a barrier is created, RTEMS generates a unique barrier ID and assigns it |
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| 110 | to the created barrier until it is deleted. The barrier ID may be obtained by |
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[f02e872] | 111 | either of two methods. First, as the result of an invocation of the |
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| 112 | ``rtems_barrier_create`` directive, the barrier ID is stored in a user provided |
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| 113 | location. Second, the barrier ID may be obtained later using the |
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[8ef6ea8] | 114 | ``rtems_barrier_ident`` directive. The barrier ID is used by other barrier |
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| 115 | manager directives to access this barrier. |
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[fd6dc8c8] | 116 | |
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| 117 | Waiting at a Barrier |
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| 118 | -------------------- |
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| 119 | |
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| 120 | The ``rtems_barrier_wait`` directive is used to wait at |
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| 121 | the specified barrier. Since a barrier is, by definition, never immediately, |
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| 122 | the task may wait forever for the barrier to be released or it may |
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| 123 | specify a timeout. Specifying a timeout limits the interval the task will |
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| 124 | wait before returning with an error status code. |
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| 125 | |
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[8ef6ea8] | 126 | If the barrier is configured as automatic and there are already one less then |
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| 127 | the maximum number of waiters, then the call will unblock all tasks waiting at |
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| 128 | the barrier and the caller will return immediately. |
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[fd6dc8c8] | 129 | |
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[8ef6ea8] | 130 | When the task does wait to acquire the barrier, then it is placed in the |
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| 131 | barrier's task wait queue in FIFO order. All tasks waiting on a barrier are |
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| 132 | returned an error code when the barrier is deleted. |
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[fd6dc8c8] | 133 | |
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| 134 | Releasing a Barrier |
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| 135 | ------------------- |
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| 136 | |
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[8ef6ea8] | 137 | The ``rtems_barrier_release`` directive is used to release the specified |
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| 138 | barrier. When the ``rtems_barrier_release`` is invoked, all tasks waiting at |
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| 139 | the barrier are immediately made ready to execute and begin to compete for the |
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| 140 | processor to execute. |
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[fd6dc8c8] | 141 | |
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| 142 | Deleting a Barrier |
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| 143 | ------------------ |
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| 144 | |
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[8ef6ea8] | 145 | The ``rtems_barrier_delete`` directive removes a barrier from the system and |
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| 146 | frees its control block. A barrier can be deleted by any local task that knows |
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| 147 | the barrier's ID. As a result of this directive, all tasks blocked waiting for |
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| 148 | the barrier to be released, will be readied and returned a status code which |
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| 149 | indicates that the barrier was deleted. Any subsequent references to the |
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| 150 | barrier's name and ID are invalid. |
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[fd6dc8c8] | 151 | |
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| 152 | Directives |
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| 153 | ========== |
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| 154 | |
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[8ef6ea8] | 155 | This section details the barrier manager's directives. A subsection is |
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| 156 | dedicated to each of this manager's directives and describes the calling |
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| 157 | sequence, related constants, usage, and status codes. |
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| 158 | |
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[53bb72e] | 159 | .. raw:: latex |
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| 160 | |
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| 161 | \clearpage |
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| 162 | |
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[8ef6ea8] | 163 | .. _rtems_barrier_create: |
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[fd6dc8c8] | 164 | |
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| 165 | BARRIER_CREATE - Create a barrier |
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| 166 | --------------------------------- |
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| 167 | .. index:: create a barrier |
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| 168 | .. index:: rtems_barrier_create |
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| 169 | |
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[53bb72e] | 170 | CALLING SEQUENCE: |
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| 171 | .. code-block:: c |
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| 172 | |
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| 173 | rtems_status_code rtems_barrier_create( |
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| 174 | rtems_name name, |
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| 175 | rtems_attribute attribute_set, |
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| 176 | uint32_t maximum_waiters, |
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| 177 | rtems_id *id |
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| 178 | ); |
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| 179 | |
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| 180 | DIRECTIVE STATUS CODES: |
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| 181 | .. list-table:: |
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| 182 | :class: rtems-table |
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| 183 | |
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| 184 | * - ``RTEMS_SUCCESSFUL`` |
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| 185 | - barrier created successfully |
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| 186 | * - ``RTEMS_INVALID_NAME`` |
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| 187 | - invalid barrier name |
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| 188 | * - ``RTEMS_INVALID_ADDRESS`` |
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| 189 | - ``id`` is NULL |
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| 190 | * - ``RTEMS_TOO_MANY`` |
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| 191 | - too many barriers created |
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| 192 | |
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| 193 | DESCRIPTION: |
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| 194 | This directive creates a barrier which resides on the local node. The |
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| 195 | created barrier has the user-defined name specified in ``name`` and the |
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| 196 | initial count specified in ``count``. For control and maintenance of the |
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| 197 | barrier, RTEMS allocates and initializes a BCB. The RTEMS-assigned barrier |
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| 198 | id is returned in ``id``. This barrier id is used with other barrier |
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| 199 | related directives to access the barrier. |
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| 200 | |
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| 201 | .. list-table:: |
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| 202 | :class: rtems-table |
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| 203 | |
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| 204 | * - ``RTEMS_BARRIER_MANUAL_RELEASE`` |
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| 205 | - only release |
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| 206 | |
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| 207 | Specifying ``RTEMS_BARRIER_AUTOMATIC_RELEASE`` in ``attribute_set`` causes |
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| 208 | tasks calling the ``rtems_barrier_wait`` directive to block until there are |
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| 209 | ``maximum_waiters - 1`` tasks waiting at the barrier. When the |
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| 210 | ``maximum_waiters`` task invokes the ``rtems_barrier_wait`` directive, the |
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| 211 | previous ``maximum_waiters - 1`` tasks are automatically released and the |
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| 212 | caller returns. |
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| 213 | |
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| 214 | In contrast, when the ``RTEMS_BARRIER_MANUAL_RELEASE`` attribute is |
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| 215 | specified, there is no limit on the number of tasks that will block at the |
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| 216 | barrier. Only when the ``rtems_barrier_release`` directive is invoked, are |
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| 217 | the tasks waiting at the barrier unblocked. |
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| 218 | |
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| 219 | NOTES: |
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| 220 | This directive will not cause the calling task to be preempted. |
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| 221 | |
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| 222 | The following barrier attribute constants are defined by RTEMS: |
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| 223 | |
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| 224 | .. list-table:: |
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| 225 | :class: rtems-table |
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| 226 | |
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| 227 | * - ``RTEMS_BARRIER_AUTOMATIC_RELEASE`` |
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| 228 | - automatically release the barrier when the configured number of tasks are |
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| 229 | blocked |
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| 230 | * - ``RTEMS_BARRIER_MANUAL_RELEASE`` |
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| 231 | - only release the barrier when the application invokes |
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| 232 | the ``rtems_barrier_release`` directive. (default) |
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| 233 | |
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| 234 | .. raw:: latex |
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| 235 | |
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| 236 | \clearpage |
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[fd6dc8c8] | 237 | |
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[8ef6ea8] | 238 | .. _rtems_barrier_ident: |
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[fd6dc8c8] | 239 | |
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| 240 | BARRIER_IDENT - Get ID of a barrier |
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| 241 | ----------------------------------- |
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| 242 | .. index:: get ID of a barrier |
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| 243 | .. index:: obtain ID of a barrier |
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| 244 | .. index:: rtems_barrier_ident |
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| 245 | |
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[53bb72e] | 246 | CALLING SEQUENCE: |
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| 247 | .. code-block:: c |
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[fd6dc8c8] | 248 | |
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[53bb72e] | 249 | rtems_status_code rtems_barrier_ident( |
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| 250 | rtems_name name, |
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| 251 | rtems_id *id |
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| 252 | ); |
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[fd6dc8c8] | 253 | |
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[53bb72e] | 254 | DIRECTIVE STATUS CODES: |
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| 255 | .. list-table:: |
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| 256 | :class: rtems-table |
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[fd6dc8c8] | 257 | |
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[53bb72e] | 258 | * - ``RTEMS_SUCCESSFUL`` |
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| 259 | - barrier identified successfully |
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| 260 | * - ``RTEMS_INVALID_NAME`` |
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| 261 | - barrier name not found |
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| 262 | * - ``RTEMS_INVALID_NODE`` |
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| 263 | - invalid node id |
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[8ef6ea8] | 264 | |
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[53bb72e] | 265 | DESCRIPTION: |
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| 266 | This directive obtains the barrier id associated with the barrier name. If |
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| 267 | the barrier name is not unique, then the barrier id will match one of the |
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| 268 | barriers with that name. However, this barrier id is not guaranteed to |
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| 269 | correspond to the desired barrier. The barrier id is used by other barrier |
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| 270 | related directives to access the barrier. |
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[fd6dc8c8] | 271 | |
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[53bb72e] | 272 | NOTES: |
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| 273 | This directive will not cause the running task to be preempted. |
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[fd6dc8c8] | 274 | |
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[53bb72e] | 275 | .. raw:: latex |
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[fd6dc8c8] | 276 | |
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[53bb72e] | 277 | \clearpage |
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[8ef6ea8] | 278 | |
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| 279 | .. _rtems_barrier_delete: |
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[fd6dc8c8] | 280 | |
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| 281 | BARRIER_DELETE - Delete a barrier |
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| 282 | --------------------------------- |
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| 283 | .. index:: delete a barrier |
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| 284 | .. index:: rtems_barrier_delete |
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| 285 | |
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[53bb72e] | 286 | CALLING SEQUENCE: |
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| 287 | .. code-block:: c |
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[fd6dc8c8] | 288 | |
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[53bb72e] | 289 | rtems_status_code rtems_barrier_delete( |
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| 290 | rtems_id id |
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| 291 | ); |
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[fd6dc8c8] | 292 | |
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[53bb72e] | 293 | DIRECTIVE STATUS CODES: |
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| 294 | .. list-table:: |
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| 295 | :class: rtems-table |
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[8ef6ea8] | 296 | |
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[53bb72e] | 297 | * - ``RTEMS_SUCCESSFUL`` |
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| 298 | - barrier deleted successfully |
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| 299 | * - ``RTEMS_INVALID_ID`` |
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| 300 | - invalid barrier id |
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[fd6dc8c8] | 301 | |
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[53bb72e] | 302 | DESCRIPTION: |
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| 303 | This directive deletes the barrier specified by ``id``. All tasks blocked |
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| 304 | waiting for the barrier to be released will be readied and returned a |
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| 305 | status code which indicates that the barrier was deleted. The BCB for this |
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| 306 | barrier is reclaimed by RTEMS. |
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[fd6dc8c8] | 307 | |
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[53bb72e] | 308 | NOTES: |
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| 309 | The calling task will be preempted if it is enabled by the task's execution |
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| 310 | mode and a higher priority local task is waiting on the deleted barrier. |
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| 311 | The calling task will NOT be preempted if all of the tasks that are waiting |
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| 312 | on the barrier are remote tasks. |
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[fd6dc8c8] | 313 | |
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[53bb72e] | 314 | The calling task does not have to be the task that created the barrier. |
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| 315 | Any local task that knows the barrier id can delete the barrier. |
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[fd6dc8c8] | 316 | |
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[53bb72e] | 317 | .. raw:: latex |
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[fd6dc8c8] | 318 | |
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[53bb72e] | 319 | \clearpage |
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[fd6dc8c8] | 320 | |
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[8ef6ea8] | 321 | .. _rtems_barrier_wait: |
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[fd6dc8c8] | 322 | |
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[2abceef] | 323 | BARRIER_OBTAIN - Wait at a barrier |
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[fd6dc8c8] | 324 | ---------------------------------- |
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| 325 | .. index:: obtain a barrier |
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| 326 | .. index:: lock a barrier |
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| 327 | .. index:: rtems_barrier_wait |
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| 328 | |
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[53bb72e] | 329 | CALLING SEQUENCE: |
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| 330 | .. code-block:: c |
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[fd6dc8c8] | 331 | |
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[53bb72e] | 332 | rtems_status_code rtems_barrier_wait( |
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| 333 | rtems_id id, |
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| 334 | rtems_interval timeout |
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| 335 | ); |
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[fd6dc8c8] | 336 | |
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[53bb72e] | 337 | DIRECTIVE STATUS CODES: |
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| 338 | .. list-table:: |
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| 339 | :class: rtems-table |
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[fd6dc8c8] | 340 | |
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[53bb72e] | 341 | * - ``RTEMS_SUCCESSFUL`` |
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| 342 | - barrier released and task unblocked |
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| 343 | * - ``RTEMS_UNSATISFIED`` |
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| 344 | - barrier not available |
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| 345 | * - ``RTEMS_TIMEOUT`` |
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| 346 | - timed out waiting for barrier |
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| 347 | * - ``RTEMS_OBJECT_WAS_DELETED`` |
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| 348 | - barrier deleted while waiting |
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| 349 | * - ``RTEMS_INVALID_ID`` |
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| 350 | - invalid barrier id |
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[8ef6ea8] | 351 | |
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[53bb72e] | 352 | DESCRIPTION: |
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[fd6dc8c8] | 353 | |
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[53bb72e] | 354 | This directive acquires the barrier specified by ``id``. The |
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| 355 | ``RTEMS_WAIT`` and ``RTEMS_NO_WAIT`` components of the options parameter |
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| 356 | indicate whether the calling task wants to wait for the barrier to become |
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| 357 | available or return immediately if the barrier is not currently available. |
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| 358 | With either ``RTEMS_WAIT`` or ``RTEMS_NO_WAIT``, if the current barrier |
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| 359 | count is positive, then it is decremented by one and the barrier is |
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| 360 | successfully acquired by returning immediately with a successful return |
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| 361 | code. |
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[fd6dc8c8] | 362 | |
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[53bb72e] | 363 | Conceptually, the calling task should always be thought of as blocking when |
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| 364 | it makes this call and being unblocked when the barrier is released. If |
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| 365 | the barrier is configured for manual release, this rule of thumb will |
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| 366 | always be valid. If the barrier is configured for automatic release, all |
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| 367 | callers will block except for the one which is the Nth task which trips the |
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| 368 | automatic release condition. |
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[fd6dc8c8] | 369 | |
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[53bb72e] | 370 | The timeout parameter specifies the maximum interval the calling task is |
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| 371 | willing to be blocked waiting for the barrier. If it is set to |
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| 372 | ``RTEMS_NO_TIMEOUT``, then the calling task will wait forever. If the |
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| 373 | barrier is available or the ``RTEMS_NO_WAIT`` option component is set, then |
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| 374 | timeout is ignored. |
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[fd6dc8c8] | 375 | |
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[53bb72e] | 376 | NOTES: |
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[fd6dc8c8] | 377 | |
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[53bb72e] | 378 | The following barrier acquisition option constants are defined by RTEMS: |
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[fd6dc8c8] | 379 | |
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[53bb72e] | 380 | .. list-table:: |
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| 381 | :class: rtems-table |
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[fd6dc8c8] | 382 | |
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[53bb72e] | 383 | * - ``RTEMS_WAIT`` |
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| 384 | - task will wait for barrier (default) |
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| 385 | * - ``RTEMS_NO_WAIT`` |
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| 386 | - task should not wait |
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[fd6dc8c8] | 387 | |
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[53bb72e] | 388 | A clock tick is required to support the timeout functionality of this |
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| 389 | directive. |
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[fd6dc8c8] | 390 | |
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[53bb72e] | 391 | .. raw:: latex |
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| 392 | |
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| 393 | \clearpage |
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[fd6dc8c8] | 394 | |
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[8ef6ea8] | 395 | .. _rtems_barrier_release: |
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[fd6dc8c8] | 396 | |
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| 397 | BARRIER_RELEASE - Release a barrier |
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| 398 | ----------------------------------- |
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| 399 | .. index:: wait at a barrier |
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| 400 | .. index:: release a barrier |
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| 401 | .. index:: rtems_barrier_release |
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| 402 | |
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[53bb72e] | 403 | CALLING SEQUENCE: |
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| 404 | .. code-block:: c |
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[8ef6ea8] | 405 | |
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[53bb72e] | 406 | rtems_status_code rtems_barrier_release( |
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| 407 | rtems_id id, |
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| 408 | uint32_t *released |
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| 409 | ); |
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[fd6dc8c8] | 410 | |
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[53bb72e] | 411 | DIRECTIVE STATUS CODES: |
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| 412 | .. list-table:: |
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| 413 | :class: rtems-table |
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[fd6dc8c8] | 414 | |
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[53bb72e] | 415 | * - ``RTEMS_SUCCESSFUL`` |
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| 416 | - barrier released successfully |
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| 417 | * - ``RTEMS_INVALID_ID`` |
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| 418 | - invalid barrier id |
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[fd6dc8c8] | 419 | |
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[53bb72e] | 420 | DESCRIPTION: |
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| 421 | This directive releases the barrier specified by id. All tasks waiting at |
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| 422 | the barrier will be unblocked. If the running task's preemption mode is |
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| 423 | enabled and one of the unblocked tasks has a higher priority than the |
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| 424 | running task. |
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[fd6dc8c8] | 425 | |
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[53bb72e] | 426 | NOTES: |
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| 427 | The calling task may be preempted if it causes a higher priority task to be |
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| 428 | made ready for execution. |
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