1 | .. SPDX-License-Identifier: CC-BY-SA-4.0 |
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2 | |
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3 | .. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR) |
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4 | |
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5 | .. index:: semaphores |
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6 | .. index:: binary semaphores |
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7 | .. index:: counting semaphores |
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8 | .. index:: mutual exclusion |
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9 | |
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10 | Semaphore Manager |
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11 | ***************** |
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12 | |
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13 | Introduction |
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14 | ============ |
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15 | |
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16 | The semaphore manager utilizes standard Dijkstra |
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17 | counting semaphores to provide synchronization and mutual |
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18 | exclusion capabilities. The directives provided by the |
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19 | semaphore manager are: |
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20 | |
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21 | - rtems_semaphore_create_ - Create a semaphore |
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22 | |
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23 | - rtems_semaphore_ident_ - Get ID of a semaphore |
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24 | |
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25 | - rtems_semaphore_delete_ - Delete a semaphore |
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26 | |
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27 | - rtems_semaphore_obtain_ - Acquire a semaphore |
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28 | |
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29 | - rtems_semaphore_release_ - Release a semaphore |
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30 | |
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31 | - rtems_semaphore_flush_ - Unblock all tasks waiting on a semaphore |
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32 | |
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33 | - rtems_semaphore_set_priority_ - Set priority by scheduler for a semaphore |
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34 | |
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35 | Background |
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36 | ========== |
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37 | |
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38 | A semaphore can be viewed as a protected variable whose value can be modified |
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39 | only with the ``rtems_semaphore_create``, ``rtems_semaphore_obtain``, and |
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40 | ``rtems_semaphore_release`` directives. RTEMS supports both binary and |
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41 | counting semaphores. A binary semaphore is restricted to values of zero or one, |
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42 | while a counting semaphore can assume any non-negative integer value. |
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43 | |
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44 | A binary semaphore (not a simple binary semaphore) can be used to control |
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45 | access to a single resource. In particular, it can be used to enforce mutual |
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46 | exclusion for a critical section in user code (mutex). In this instance, the |
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47 | semaphore would be created with an initial count of one to indicate that no |
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48 | task is executing the critical section of code. Upon entry to the critical |
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49 | section, a task must issue the ``rtems_semaphore_obtain`` directive to prevent |
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50 | other tasks from entering the critical section. Upon exit from the critical |
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51 | section, the task that obtained the binary semaphore must issue the |
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52 | ``rtems_semaphore_release`` directive to allow another task to execute the |
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53 | critical section. A binary semaphore must be released by the task that |
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54 | obtained it. |
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55 | |
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56 | A counting semaphore can be used to control access to a pool of two or more |
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57 | resources. For example, access to three printers could be administered by a |
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58 | semaphore created with an initial count of three. When a task requires access |
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59 | to one of the printers, it issues the ``rtems_semaphore_obtain`` directive to |
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60 | obtain access to a printer. If a printer is not currently available, the task |
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61 | can wait for a printer to become available or return immediately. When the |
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62 | task has completed printing, it should issue the ``rtems_semaphore_release`` |
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63 | directive to allow other tasks access to the printer. |
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64 | |
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65 | Task synchronization may be achieved by creating a semaphore with an initial |
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66 | count of zero. One task waits for the arrival of another task by issuing a |
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67 | ``rtems_semaphore_obtain`` directive when it reaches a synchronization point. |
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68 | The other task performs a corresponding ``rtems_semaphore_release`` operation |
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69 | when it reaches its synchronization point, thus unblocking the pending task. |
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70 | |
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71 | .. _Nested Resource Access: |
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72 | |
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73 | Nested Resource Access |
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74 | ---------------------- |
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75 | |
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76 | Deadlock occurs when a task owning a binary semaphore attempts to acquire that |
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77 | same semaphore and blocks as result. Since the semaphore is allocated to a |
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78 | task, it cannot be deleted. Therefore, the task that currently holds the |
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79 | semaphore and is also blocked waiting for that semaphore will never execute |
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80 | again. |
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81 | |
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82 | RTEMS addresses this problem by allowing the task holding the binary semaphore |
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83 | to obtain the same binary semaphore multiple times in a nested manner. Each |
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84 | ``rtems_semaphore_obtain`` must be accompanied with a |
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85 | ``rtems_semaphore_release``. The semaphore will only be made available for |
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86 | acquisition by other tasks when the outermost ``rtems_semaphore_obtain`` is |
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87 | matched with a ``rtems_semaphore_release``. |
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88 | |
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89 | Simple binary semaphores do not allow nested access and so can be used for task |
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90 | synchronization. |
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91 | |
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92 | .. _Priority Inheritance: |
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93 | |
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94 | Priority Inheritance |
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95 | -------------------- |
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96 | |
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97 | RTEMS supports :ref:`priority inheritance <PriorityInheritance>` for local, |
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98 | binary semaphores that use the priority task wait queue blocking discipline. |
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99 | In SMP configurations, the :ref:`OMIP` is used instead. |
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100 | |
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101 | .. _Priority Ceiling: |
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102 | |
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103 | Priority Ceiling |
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104 | ---------------- |
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105 | |
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106 | RTEMS supports :ref:`priority ceiling <PriorityCeiling>` for local, binary |
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107 | semaphores that use the priority task wait queue blocking discipline. |
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108 | |
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109 | .. _Multiprocessor Resource Sharing Protocol: |
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110 | |
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111 | Multiprocessor Resource Sharing Protocol |
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112 | ---------------------------------------- |
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113 | |
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114 | RTEMS supports the :ref:`MrsP` for local, binary semaphores that use the |
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115 | priority task wait queue blocking discipline. In uniprocessor configurations, |
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116 | the :ref:`PriorityCeiling` is used instead. |
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117 | |
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118 | .. _Building a Semaphore Attribute Set: |
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119 | |
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120 | Building a Semaphore Attribute Set |
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121 | ---------------------------------- |
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122 | |
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123 | In general, an attribute set is built by a bitwise OR of the desired attribute |
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124 | components. The following table lists the set of valid semaphore attributes: |
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125 | |
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126 | .. list-table:: |
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127 | :class: rtems-table |
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128 | |
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129 | * - ``RTEMS_FIFO`` |
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130 | - tasks wait by FIFO (default) |
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131 | * - ``RTEMS_PRIORITY`` |
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132 | - tasks wait by priority |
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133 | * - ``RTEMS_BINARY_SEMAPHORE`` |
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134 | - restrict values to 0 and 1 |
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135 | * - ``RTEMS_COUNTING_SEMAPHORE`` |
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136 | - no restriction on values (default) |
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137 | * - ``RTEMS_SIMPLE_BINARY_SEMAPHORE`` |
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138 | - restrict values to 0 and 1, do not allow nested access, allow deletion of |
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139 | locked semaphore. |
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140 | * - ``RTEMS_NO_INHERIT_PRIORITY`` |
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141 | - do not use priority inheritance (default) |
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142 | * - ``RTEMS_INHERIT_PRIORITY`` |
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143 | - use priority inheritance |
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144 | * - ``RTEMS_NO_PRIORITY_CEILING`` |
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145 | - do not use priority ceiling (default) |
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146 | * - ``RTEMS_PRIORITY_CEILING`` |
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147 | - use priority ceiling |
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148 | * - ``RTEMS_NO_MULTIPROCESSOR_RESOURCE_SHARING`` |
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149 | - do not use Multiprocessor Resource Sharing Protocol (default) |
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150 | * - ``RTEMS_MULTIPROCESSOR_RESOURCE_SHARING`` |
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151 | - use Multiprocessor Resource Sharing Protocol |
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152 | * - ``RTEMS_LOCAL`` |
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153 | - local semaphore (default) |
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154 | * - ``RTEMS_GLOBAL`` |
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155 | - global semaphore |
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156 | |
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157 | Attribute values are specifically designed to be mutually exclusive, therefore |
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158 | bitwise OR and addition operations are equivalent as long as each attribute |
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159 | appears exactly once in the component list. An attribute listed as a default |
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160 | is not required to appear in the attribute list, although it is a good |
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161 | programming practice to specify default attributes. If all defaults are |
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162 | desired, the attribute ``RTEMS_DEFAULT_ATTRIBUTES`` should be specified on this |
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163 | call. |
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164 | |
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165 | This example demonstrates the attribute_set parameter needed to create a local |
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166 | semaphore with the task priority waiting queue discipline. The attribute_set |
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167 | parameter passed to the ``rtems_semaphore_create`` directive could be either |
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168 | ``RTEMS_PRIORITY`` or ``RTEMS_LOCAL | RTEMS_PRIORITY``. The attribute_set |
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169 | parameter can be set to ``RTEMS_PRIORITY`` because ``RTEMS_LOCAL`` is the |
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170 | default for all created tasks. If a similar semaphore were to be known |
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171 | globally, then the attribute_set parameter would be ``RTEMS_GLOBAL | |
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172 | RTEMS_PRIORITY``. |
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173 | |
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174 | Some combinatinos of these attributes are invalid. For example, priority |
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175 | ordered blocking discipline must be applied to a binary semaphore in order to |
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176 | use either the priority inheritance or priority ceiling functionality. The |
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177 | following tree figure illustrates the valid combinations. |
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178 | |
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179 | .. figure:: ../images/c_user/semaphore_attributes.png |
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180 | :width: 90% |
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181 | :align: center |
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182 | :alt: Semaphore Attributes |
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183 | |
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184 | .. _Building a SEMAPHORE_OBTAIN Option Set: |
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185 | |
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186 | Building a SEMAPHORE_OBTAIN Option Set |
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187 | -------------------------------------- |
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188 | |
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189 | In general, an option is built by a bitwise OR of the desired option |
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190 | components. The set of valid options for the ``rtems_semaphore_obtain`` |
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191 | directive are listed in the following table: |
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192 | |
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193 | .. list-table:: |
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194 | :class: rtems-table |
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195 | |
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196 | * - ``RTEMS_WAIT`` |
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197 | - task will wait for semaphore (default) |
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198 | * - ``RTEMS_NO_WAIT`` |
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199 | - task should not wait |
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200 | |
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201 | Option values are specifically designed to be mutually exclusive, therefore |
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202 | bitwise OR and addition operations are equivalent as long as each attribute |
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203 | appears exactly once in the component list. An option listed as a default is |
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204 | not required to appear in the list, although it is a good programming practice |
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205 | to specify default options. If all defaults are desired, the option |
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206 | ``RTEMS_DEFAULT_OPTIONS`` should be specified on this call. |
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207 | |
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208 | This example demonstrates the option parameter needed to poll for a semaphore. |
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209 | The option parameter passed to the ``rtems_semaphore_obtain`` directive should |
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210 | be ``RTEMS_NO_WAIT``. |
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211 | |
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212 | Operations |
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213 | ========== |
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214 | |
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215 | .. _Creating a Semaphore: |
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216 | |
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217 | Creating a Semaphore |
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218 | -------------------- |
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219 | |
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220 | The ``rtems_semaphore_create`` directive creates a binary or counting semaphore |
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221 | with a user-specified name as well as an initial count. If a binary semaphore |
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222 | is created with a count of zero (0) to indicate that it has been allocated, |
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223 | then the task creating the semaphore is considered the current holder of the |
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224 | semaphore. At create time the method for ordering waiting tasks in the |
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225 | semaphore's task wait queue (by FIFO or task priority) is specified. |
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226 | Additionally, the priority inheritance or priority ceiling algorithm may be |
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227 | selected for local, binary semaphores that use the priority task wait queue |
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228 | blocking discipline. If the priority ceiling algorithm is selected, then the |
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229 | highest priority of any task which will attempt to obtain this semaphore must |
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230 | be specified. RTEMS allocates a Semaphore Control Block (SMCB) from the SMCB |
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231 | free list. This data structure is used by RTEMS to manage the newly created |
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232 | semaphore. Also, a unique semaphore ID is generated and returned to the |
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233 | calling task. |
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234 | |
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235 | .. _Obtaining Semaphore IDs: |
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236 | |
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237 | Obtaining Semaphore IDs |
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238 | ----------------------- |
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239 | |
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240 | When a semaphore is created, RTEMS generates a unique semaphore ID and assigns |
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241 | it to the created semaphore until it is deleted. The semaphore ID may be |
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242 | obtained by either of two methods. First, as the result of an invocation of |
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243 | the ``rtems_semaphore_create`` directive, the semaphore ID is stored in a user |
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244 | provided location. Second, the semaphore ID may be obtained later using the |
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245 | ``rtems_semaphore_ident`` directive. The semaphore ID is used by other |
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246 | semaphore manager directives to access this semaphore. |
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247 | |
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248 | .. _Acquiring a Semaphore: |
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249 | |
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250 | Acquiring a Semaphore |
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251 | --------------------- |
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252 | |
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253 | The ``rtems_semaphore_obtain`` directive is used to acquire the |
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254 | specified semaphore. A simplified version of the ``rtems_semaphore_obtain`` |
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255 | directive can be described as follows: |
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256 | |
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257 | If the semaphore's count is greater than zero then decrement the |
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258 | semaphore's count else wait for release of semaphore then return |
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259 | SUCCESSFUL. |
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260 | |
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261 | When the semaphore cannot be immediately acquired, one of the following |
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262 | situations applies: |
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263 | |
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264 | - By default, the calling task will wait forever to acquire the semaphore. |
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265 | |
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266 | - Specifying ``RTEMS_NO_WAIT`` forces an immediate return with an error status |
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267 | code. |
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268 | |
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269 | - Specifying a timeout limits the interval the task will wait before returning |
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270 | with an error status code. |
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271 | |
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272 | If the task waits to acquire the semaphore, then it is placed in the |
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273 | semaphore's task wait queue in either FIFO or task priority order. If the task |
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274 | blocked waiting for a binary semaphore using priority inheritance and the |
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275 | task's priority is greater than that of the task currently holding the |
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276 | semaphore, then the holding task will inherit the priority of the blocking |
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277 | task. All tasks waiting on a semaphore are returned an error code when the |
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278 | semaphore is deleted. |
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279 | |
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280 | When a task successfully obtains a semaphore using priority ceiling and the |
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281 | priority ceiling for this semaphore is greater than that of the holder, then |
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282 | the holder's priority will be elevated. |
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283 | |
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284 | .. _Releasing a Semaphore: |
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285 | |
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286 | Releasing a Semaphore |
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287 | --------------------- |
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288 | |
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289 | The ``rtems_semaphore_release`` directive is used to release the specified |
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290 | semaphore. A simplified version of the ``rtems_semaphore_release`` directive |
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291 | can be described as follows: |
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292 | |
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293 | If there are no tasks are waiting on this semaphore then increment the |
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294 | semaphore's count else assign semaphore to a waiting task and return |
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295 | SUCCESSFUL. |
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296 | |
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297 | If this is the outermost release of a binary semaphore that uses priority |
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298 | inheritance or priority ceiling and the task does not currently hold any other |
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299 | binary semaphores, then the task performing the ``rtems_semaphore_release`` |
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300 | will have its priority restored to its normal value. |
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301 | |
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302 | .. _Deleting a Semaphore: |
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303 | |
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304 | Deleting a Semaphore |
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305 | -------------------- |
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306 | |
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307 | The ``rtems_semaphore_delete`` directive removes a semaphore from the system |
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308 | and frees its control block. A semaphore can be deleted by any local task that |
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309 | knows the semaphore's ID. As a result of this directive, all tasks blocked |
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310 | waiting to acquire the semaphore will be readied and returned a status code |
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311 | which indicates that the semaphore was deleted. Any subsequent references to |
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312 | the semaphore's name and ID are invalid. |
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313 | |
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314 | Directives |
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315 | ========== |
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316 | |
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317 | This section details the semaphore manager's directives. A subsection is |
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318 | dedicated to each of this manager's directives and describes the calling |
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319 | sequence, related constants, usage, and status codes. |
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320 | |
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321 | .. raw:: latex |
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322 | |
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323 | \clearpage |
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324 | |
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325 | .. index:: create a semaphore |
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326 | .. index:: rtems_semaphore_create |
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327 | |
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328 | .. _rtems_semaphore_create: |
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329 | |
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330 | SEMAPHORE_CREATE - Create a semaphore |
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331 | ------------------------------------- |
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332 | |
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333 | |
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334 | CALLING SEQUENCE: |
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335 | .. code-block:: c |
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336 | |
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337 | rtems_status_code rtems_semaphore_create( |
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338 | rtems_name name, |
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339 | uint32_t count, |
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340 | rtems_attribute attribute_set, |
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341 | rtems_task_priority priority_ceiling, |
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342 | rtems_id *id |
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343 | ); |
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344 | |
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345 | DIRECTIVE STATUS CODES: |
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346 | .. list-table:: |
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347 | :class: rtems-table |
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348 | |
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349 | * - ``RTEMS_SUCCESSFUL`` |
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350 | - semaphore created successfully |
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351 | * - ``RTEMS_INVALID_NAME`` |
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352 | - invalid semaphore name |
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353 | * - ``RTEMS_INVALID_ADDRESS`` |
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354 | - ``id`` is NULL |
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355 | * - ``RTEMS_TOO_MANY`` |
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356 | - too many semaphores created |
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357 | * - ``RTEMS_NOT_DEFINED`` |
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358 | - invalid attribute set |
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359 | * - ``RTEMS_INVALID_NUMBER`` |
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360 | - invalid starting count for binary semaphore |
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361 | * - ``RTEMS_MP_NOT_CONFIGURED`` |
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362 | - multiprocessing not configured |
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363 | * - ``RTEMS_TOO_MANY`` |
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364 | - too many global objects |
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365 | |
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366 | DESCRIPTION: |
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367 | This directive creates a semaphore which resides on the local node. The |
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368 | created semaphore has the user-defined name specified in name and the |
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369 | initial count specified in count. For control and maintenance of the |
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370 | semaphore, RTEMS allocates and initializes a SMCB. The RTEMS-assigned |
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371 | semaphore id is returned in id. This semaphore id is used with other |
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372 | semaphore related directives to access the semaphore. |
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373 | |
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374 | Specifying PRIORITY in attribute_set causes tasks waiting for a semaphore |
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375 | to be serviced according to task priority. When FIFO is selected, tasks |
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376 | are serviced in First In-First Out order. |
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377 | |
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378 | NOTES: |
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379 | This directive will not cause the calling task to be preempted. |
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380 | |
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381 | The priority inheritance and priority ceiling algorithms are only supported |
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382 | for local, binary semaphores that use the priority task wait queue blocking |
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383 | discipline. |
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384 | |
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385 | The following semaphore attribute constants are defined by RTEMS: |
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386 | |
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387 | .. list-table:: |
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388 | :class: rtems-table |
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389 | |
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390 | * - ``RTEMS_FIFO`` |
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391 | - tasks wait by FIFO (default) |
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392 | * - ``RTEMS_PRIORITY`` |
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393 | - tasks wait by priority |
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394 | * - ``RTEMS_BINARY_SEMAPHORE`` |
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395 | - restrict values to 0 and 1 |
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396 | * - ``RTEMS_COUNTING_SEMAPHORE`` |
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397 | - no restriction on values (default) |
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398 | * - ``RTEMS_SIMPLE_BINARY_SEMAPHORE`` |
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399 | - restrict values to 0 and 1, block on nested access, allow deletion of locked semaphore. |
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400 | * - ``RTEMS_NO_INHERIT_PRIORITY`` |
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401 | - do not use priority inheritance (default) |
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402 | * - ``RTEMS_INHERIT_PRIORITY`` |
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403 | - use priority inheritance |
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404 | * - ``RTEMS_NO_PRIORITY_CEILING`` |
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405 | - do not use priority ceiling (default) |
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406 | * - ``RTEMS_PRIORITY_CEILING`` |
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407 | - use priority ceiling |
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408 | * - ``RTEMS_NO_MULTIPROCESSOR_RESOURCE_SHARING`` |
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409 | - do not use Multiprocessor Resource Sharing Protocol (default) |
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410 | * - ``RTEMS_MULTIPROCESSOR_RESOURCE_SHARING`` |
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411 | - use Multiprocessor Resource Sharing Protocol |
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412 | * - ``RTEMS_LOCAL`` |
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413 | - local semaphore (default) |
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414 | * - ``RTEMS_GLOBAL`` |
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415 | - global semaphore |
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416 | |
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417 | Semaphores should not be made global unless remote tasks must interact with |
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418 | the created semaphore. This is to avoid the system overhead incurred by |
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419 | the creation of a global semaphore. When a global semaphore is created, |
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420 | the semaphore's name and id must be transmitted to every node in the system |
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421 | for insertion in the local copy of the global object table. |
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422 | |
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423 | *Note*, some combinations of attributes are not valid. See the earlier |
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424 | discussion on this. |
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425 | |
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426 | The total number of global objects, including semaphores, is limited by the |
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427 | maximum_global_objects field in the Configuration Table. |
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428 | |
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429 | It is not allowed to create an initially locked MrsP semaphore and the |
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430 | ``RTEMS_INVALID_NUMBER`` status code will be returned in SMP configurations |
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431 | in this case. This prevents lock order reversal problems with the |
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432 | allocator mutex. |
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433 | |
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434 | .. raw:: latex |
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435 | |
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436 | \clearpage |
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437 | |
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438 | .. index:: get ID of a semaphore |
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439 | .. index:: obtain ID of a semaphore |
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440 | .. index:: rtems_semaphore_ident |
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441 | |
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442 | .. _rtems_semaphore_ident: |
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443 | |
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444 | SEMAPHORE_IDENT - Get ID of a semaphore |
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445 | --------------------------------------- |
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446 | |
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447 | CALLING SEQUENCE: |
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448 | .. code-block:: c |
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449 | |
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450 | rtems_status_code rtems_semaphore_ident( |
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451 | rtems_name name, |
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452 | uint32_t node, |
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453 | rtems_id *id |
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454 | ); |
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455 | |
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456 | DIRECTIVE STATUS CODES: |
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457 | .. list-table:: |
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458 | :class: rtems-table |
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459 | |
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460 | * - ``RTEMS_SUCCESSFUL`` |
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461 | - semaphore identified successfully |
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462 | * - ``RTEMS_INVALID_NAME`` |
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463 | - semaphore name not found |
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464 | * - ``RTEMS_INVALID_NODE`` |
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465 | - invalid node id |
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466 | |
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467 | DESCRIPTION: |
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468 | This directive obtains the semaphore id associated with the semaphore name. |
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469 | If the semaphore name is not unique, then the semaphore id will match one |
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470 | of the semaphores with that name. However, this semaphore id is not |
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471 | guaranteed to correspond to the desired semaphore. The semaphore id is |
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472 | used by other semaphore related directives to access the semaphore. |
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473 | |
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474 | NOTES: |
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475 | This directive will not cause the running task to be preempted. |
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476 | |
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477 | If node is ``RTEMS_SEARCH_ALL_NODES``, all nodes are searched with the |
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478 | local node being searched first. All other nodes are searched with the |
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479 | lowest numbered node searched first. |
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480 | |
---|
481 | If node is a valid node number which does not represent the local node, |
---|
482 | then only the semaphores exported by the designated node are searched. |
---|
483 | |
---|
484 | This directive does not generate activity on remote nodes. It accesses |
---|
485 | only the local copy of the global object table. |
---|
486 | |
---|
487 | .. raw:: latex |
---|
488 | |
---|
489 | \clearpage |
---|
490 | |
---|
491 | .. index:: delete a semaphore |
---|
492 | .. index:: rtems_semaphore_delete |
---|
493 | |
---|
494 | .. _rtems_semaphore_delete: |
---|
495 | |
---|
496 | SEMAPHORE_DELETE - Delete a semaphore |
---|
497 | ------------------------------------- |
---|
498 | |
---|
499 | CALLING SEQUENCE: |
---|
500 | .. code-block:: c |
---|
501 | |
---|
502 | rtems_status_code rtems_semaphore_delete( |
---|
503 | rtems_id id |
---|
504 | ); |
---|
505 | |
---|
506 | DIRECTIVE STATUS CODES: |
---|
507 | .. list-table:: |
---|
508 | :class: rtems-table |
---|
509 | |
---|
510 | * - ``RTEMS_SUCCESSFUL`` |
---|
511 | - semaphore deleted successfully |
---|
512 | * - ``RTEMS_INVALID_ID`` |
---|
513 | - invalid semaphore id |
---|
514 | * - ``RTEMS_RESOURCE_IN_USE`` |
---|
515 | - binary semaphore is in use |
---|
516 | * - ``RTEMS_ILLEGAL_ON_REMOTE_OBJECT`` |
---|
517 | - cannot delete remote semaphore |
---|
518 | |
---|
519 | DESCRIPTION: |
---|
520 | This directive deletes the semaphore specified by ``id``. All tasks |
---|
521 | blocked waiting to acquire the semaphore will be readied and returned a |
---|
522 | status code which indicates that the semaphore was deleted. The SMCB for |
---|
523 | this semaphore is reclaimed by RTEMS. |
---|
524 | |
---|
525 | NOTES: |
---|
526 | The calling task will be preempted if it is enabled by the task's execution |
---|
527 | mode and a higher priority local task is waiting on the deleted semaphore. |
---|
528 | The calling task will NOT be preempted if all of the tasks that are waiting |
---|
529 | on the semaphore are remote tasks. |
---|
530 | |
---|
531 | The calling task does not have to be the task that created the semaphore. Any |
---|
532 | local task that knows the semaphore id can delete the semaphore. |
---|
533 | |
---|
534 | When a global semaphore is deleted, the semaphore id must be transmitted to |
---|
535 | every node in the system for deletion from the local copy of the global |
---|
536 | object table. |
---|
537 | |
---|
538 | The semaphore must reside on the local node, even if the semaphore was |
---|
539 | created with the ``RTEMS_GLOBAL`` option. |
---|
540 | |
---|
541 | Proxies, used to represent remote tasks, are reclaimed when the semaphore |
---|
542 | is deleted. |
---|
543 | |
---|
544 | .. raw:: latex |
---|
545 | |
---|
546 | \clearpage |
---|
547 | |
---|
548 | .. index:: obtain a semaphore |
---|
549 | .. index:: lock a semaphore |
---|
550 | .. index:: rtems_semaphore_obtain |
---|
551 | |
---|
552 | .. _rtems_semaphore_obtain: |
---|
553 | |
---|
554 | SEMAPHORE_OBTAIN - Acquire a semaphore |
---|
555 | -------------------------------------- |
---|
556 | |
---|
557 | CALLING SEQUENCE: |
---|
558 | .. code-block:: c |
---|
559 | |
---|
560 | rtems_status_code rtems_semaphore_obtain( |
---|
561 | rtems_id id, |
---|
562 | rtems_option option_set, |
---|
563 | rtems_interval timeout |
---|
564 | ); |
---|
565 | |
---|
566 | DIRECTIVE STATUS CODES: |
---|
567 | .. list-table:: |
---|
568 | :class: rtems-table |
---|
569 | |
---|
570 | * - ``RTEMS_SUCCESSFUL`` |
---|
571 | - semaphore obtained successfully |
---|
572 | * - ``RTEMS_UNSATISFIED`` |
---|
573 | - semaphore not available |
---|
574 | * - ``RTEMS_TIMEOUT`` |
---|
575 | - timed out waiting for semaphore |
---|
576 | * - ``RTEMS_OBJECT_WAS_DELETED`` |
---|
577 | - semaphore deleted while waiting |
---|
578 | * - ``RTEMS_INVALID_ID`` |
---|
579 | - invalid semaphore id |
---|
580 | |
---|
581 | DESCRIPTION: |
---|
582 | This directive acquires the semaphore specified by id. The ``RTEMS_WAIT`` |
---|
583 | and ``RTEMS_NO_WAIT`` components of the options parameter indicate whether |
---|
584 | the calling task wants to wait for the semaphore to become available or |
---|
585 | return immediately if the semaphore is not currently available. With |
---|
586 | either ``RTEMS_WAIT`` or ``RTEMS_NO_WAIT``, if the current semaphore count |
---|
587 | is positive, then it is decremented by one and the semaphore is |
---|
588 | successfully acquired by returning immediately with a successful return |
---|
589 | code. |
---|
590 | |
---|
591 | If the calling task chooses to return immediately and the current semaphore |
---|
592 | count is zero or negative, then a status code is returned indicating that |
---|
593 | the semaphore is not available. If the calling task chooses to wait for a |
---|
594 | semaphore and the current semaphore count is zero or negative, then it is |
---|
595 | decremented by one and the calling task is placed on the semaphore's wait |
---|
596 | queue and blocked. If the semaphore was created with the |
---|
597 | ``RTEMS_PRIORITY`` attribute, then the calling task is inserted into the |
---|
598 | queue according to its priority. However, if the semaphore was created |
---|
599 | with the ``RTEMS_FIFO`` attribute, then the calling task is placed at the |
---|
600 | rear of the wait queue. If the binary semaphore was created with the |
---|
601 | ``RTEMS_INHERIT_PRIORITY`` attribute, then the priority of the task |
---|
602 | currently holding the binary semaphore is guaranteed to be greater than or |
---|
603 | equal to that of the blocking task. If the binary semaphore was created |
---|
604 | with the ``RTEMS_PRIORITY_CEILING`` attribute, a task successfully obtains |
---|
605 | the semaphore, and the priority of that task is greater than the ceiling |
---|
606 | priority for this semaphore, then the priority of the task obtaining the |
---|
607 | semaphore is elevated to that of the ceiling. |
---|
608 | |
---|
609 | The timeout parameter specifies the maximum interval the calling task is |
---|
610 | willing to be blocked waiting for the semaphore. If it is set to |
---|
611 | ``RTEMS_NO_TIMEOUT``, then the calling task will wait forever. If the |
---|
612 | semaphore is available or the ``RTEMS_NO_WAIT`` option component is set, |
---|
613 | then timeout is ignored. |
---|
614 | |
---|
615 | In case a semaphore is not available, then ``RTEMS_UNSATISFIED`` will be |
---|
616 | returned. This happens immediately in case ``RTEMS_NO_WAIT`` is specified, |
---|
617 | or as a result of another task invoking the ``rtems_semaphore_flush`` |
---|
618 | directive in case ``RTEMS_WAIT`` is specified. |
---|
619 | |
---|
620 | Deadlock situations are detected for MrsP semaphores and the |
---|
621 | ``RTEMS_UNSATISFIED`` status code will be returned in SMP configurations in |
---|
622 | this case. |
---|
623 | |
---|
624 | NOTES: |
---|
625 | The following semaphore acquisition option constants are defined by RTEMS: |
---|
626 | |
---|
627 | .. list-table:: |
---|
628 | :class: rtems-table |
---|
629 | |
---|
630 | * - ``RTEMS_WAIT`` |
---|
631 | - task will wait for semaphore (default) |
---|
632 | * - ``RTEMS_NO_WAIT`` |
---|
633 | - task should not wait |
---|
634 | |
---|
635 | Attempting to obtain a global semaphore which does not reside on the local |
---|
636 | node will generate a request to the remote node to access the semaphore. |
---|
637 | If the semaphore is not available and ``RTEMS_NO_WAIT`` was not specified, |
---|
638 | then the task must be blocked until the semaphore is released. A proxy is |
---|
639 | allocated on the remote node to represent the task until the semaphore is |
---|
640 | released. |
---|
641 | |
---|
642 | A clock tick is required to support the timeout functionality of this |
---|
643 | directive. |
---|
644 | |
---|
645 | It is not allowed to obtain a MrsP semaphore more than once by one task at |
---|
646 | a time (nested access) and the ``RTEMS_UNSATISFIED`` status code will be |
---|
647 | returned in SMP configurations in this case. |
---|
648 | |
---|
649 | .. raw:: latex |
---|
650 | |
---|
651 | \clearpage |
---|
652 | |
---|
653 | .. index:: release a semaphore |
---|
654 | .. index:: unlock a semaphore |
---|
655 | .. index:: rtems_semaphore_release |
---|
656 | |
---|
657 | .. _rtems_semaphore_release: |
---|
658 | |
---|
659 | SEMAPHORE_RELEASE - Release a semaphore |
---|
660 | --------------------------------------- |
---|
661 | |
---|
662 | CALLING SEQUENCE: |
---|
663 | .. code-block:: c |
---|
664 | |
---|
665 | rtems_status_code rtems_semaphore_release( |
---|
666 | rtems_id id |
---|
667 | ); |
---|
668 | |
---|
669 | DIRECTIVE STATUS CODES: |
---|
670 | .. list-table:: |
---|
671 | :class: rtems-table |
---|
672 | |
---|
673 | * - ``RTEMS_SUCCESSFUL`` |
---|
674 | - semaphore released successfully |
---|
675 | * - ``RTEMS_INVALID_ID`` |
---|
676 | - invalid semaphore id |
---|
677 | * - ``RTEMS_NOT_OWNER_OF_RESOURCE`` |
---|
678 | - calling task does not own semaphore |
---|
679 | * - ``RTEMS_INCORRECT_STATE`` |
---|
680 | - invalid unlock order |
---|
681 | |
---|
682 | DESCRIPTION: |
---|
683 | This directive releases the semaphore specified by id. The semaphore count |
---|
684 | is incremented by one. If the count is zero or negative, then the first |
---|
685 | task on this semaphore's wait queue is removed and unblocked. The |
---|
686 | unblocked task may preempt the running task if the running task's |
---|
687 | preemption mode is enabled and the unblocked task has a higher priority |
---|
688 | than the running task. |
---|
689 | |
---|
690 | NOTES: |
---|
691 | The calling task may be preempted if it causes a higher priority task to be |
---|
692 | made ready for execution. |
---|
693 | |
---|
694 | Releasing a global semaphore which does not reside on the local node will |
---|
695 | generate a request telling the remote node to release the semaphore. |
---|
696 | |
---|
697 | If the task to be unblocked resides on a different node from the semaphore, |
---|
698 | then the semaphore allocation is forwarded to the appropriate node, the |
---|
699 | waiting task is unblocked, and the proxy used to represent the task is |
---|
700 | reclaimed. |
---|
701 | |
---|
702 | The outermost release of a local, binary, priority inheritance or priority |
---|
703 | ceiling semaphore may result in the calling task having its priority |
---|
704 | lowered. This will occur if the calling task holds no other binary |
---|
705 | semaphores and it has inherited a higher priority. |
---|
706 | |
---|
707 | The MrsP semaphores must be released in the reversed obtain order, |
---|
708 | otherwise the ``RTEMS_INCORRECT_STATE`` status code will be returned in SMP |
---|
709 | configurations in this case. |
---|
710 | |
---|
711 | .. raw:: latex |
---|
712 | |
---|
713 | \clearpage |
---|
714 | |
---|
715 | .. index:: flush a semaphore |
---|
716 | .. index:: unblock all tasks waiting on a semaphore |
---|
717 | .. index:: rtems_semaphore_flush |
---|
718 | |
---|
719 | .. _rtems_semaphore_flush: |
---|
720 | |
---|
721 | SEMAPHORE_FLUSH - Unblock all tasks waiting on a semaphore |
---|
722 | ---------------------------------------------------------- |
---|
723 | |
---|
724 | CALLING SEQUENCE: |
---|
725 | .. code-block:: c |
---|
726 | |
---|
727 | rtems_status_code rtems_semaphore_flush( |
---|
728 | rtems_id id |
---|
729 | ); |
---|
730 | |
---|
731 | DIRECTIVE STATUS CODES: |
---|
732 | .. list-table:: |
---|
733 | :class: rtems-table |
---|
734 | |
---|
735 | * - ``RTEMS_SUCCESSFUL`` |
---|
736 | - semaphore released successfully |
---|
737 | * - ``RTEMS_INVALID_ID`` |
---|
738 | - invalid semaphore id |
---|
739 | * - ``RTEMS_NOT_DEFINED`` |
---|
740 | - operation not defined for the protocol of the semaphore |
---|
741 | * - ``RTEMS_ILLEGAL_ON_REMOTE_OBJECT`` |
---|
742 | - not supported for remote semaphores |
---|
743 | |
---|
744 | DESCRIPTION: |
---|
745 | This directive unblocks all tasks waiting on the semaphore specified by id. |
---|
746 | Since there are tasks blocked on the semaphore, the semaphore's count is |
---|
747 | not changed by this directive and thus is zero before and after this |
---|
748 | directive is executed. Tasks which are unblocked as the result of this |
---|
749 | directive will return from the ``rtems_semaphore_obtain`` directive with a |
---|
750 | status code of ``RTEMS_UNSATISFIED`` to indicate that the semaphore was not |
---|
751 | obtained. |
---|
752 | |
---|
753 | This directive may unblock any number of tasks. Any of the unblocked tasks |
---|
754 | may preempt the running task if the running task's preemption mode is |
---|
755 | enabled and an unblocked task has a higher priority than the running task. |
---|
756 | |
---|
757 | NOTES: |
---|
758 | The calling task may be preempted if it causes a higher priority task to be |
---|
759 | made ready for execution. |
---|
760 | |
---|
761 | If the task to be unblocked resides on a different node from the semaphore, |
---|
762 | then the waiting task is unblocked, and the proxy used to represent the |
---|
763 | task is reclaimed. |
---|
764 | |
---|
765 | It is not allowed to flush a MrsP semaphore and the ``RTEMS_NOT_DEFINED`` |
---|
766 | status code will be returned in SMP configurations in this case. |
---|
767 | |
---|
768 | Using the ``rtems_semaphore_flush`` directive for condition synchronization |
---|
769 | in concert with another semaphore may be subject to the lost wake-up |
---|
770 | problem. The following attempt to implement a condition variable is |
---|
771 | broken. |
---|
772 | |
---|
773 | .. code-block:: c |
---|
774 | |
---|
775 | #include <rtems.h> |
---|
776 | #include <assert.h> |
---|
777 | |
---|
778 | void cnd_wait( rtems_id cnd, rtems_id mtx ) |
---|
779 | { |
---|
780 | rtems_status_code sc; |
---|
781 | |
---|
782 | sc = rtems_semaphore_release( mtx ); |
---|
783 | assert( sc == RTEMS_SUCCESSFUL ); |
---|
784 | |
---|
785 | /* |
---|
786 | * Here, a higher priority task may run and satisfy the condition. We |
---|
787 | * may never wake up from the next semaphore obtain. |
---|
788 | */ |
---|
789 | |
---|
790 | sc = rtems_semaphore_obtain( cnd, RTEMS_WAIT, RTEMS_NO_TIMEOUT ); |
---|
791 | assert( sc == RTEMS_UNSATISFIED ); |
---|
792 | |
---|
793 | sc = rtems_semaphore_obtain( mtx, RTEMS_WAIT, RTEMS_NO_TIMEOUT ); |
---|
794 | assert( sc == RTEMS_SUCCESSFUL ); |
---|
795 | } |
---|
796 | |
---|
797 | void cnd_broadcast( rtems_id cnd ) |
---|
798 | { |
---|
799 | rtems_status_code sc; |
---|
800 | |
---|
801 | sc = rtems_semaphore_flush( cnd ); |
---|
802 | assert( sc == RTEMS_SUCCESSFUL ); |
---|
803 | } |
---|
804 | |
---|
805 | For barrier synchronization, the :ref:`barrier_manager` offers a cleaner |
---|
806 | alternative to using the `rtems_semaphore_flush` directive. Unlike POSIX |
---|
807 | barriers, they have a manual release option. |
---|
808 | |
---|
809 | .. raw:: latex |
---|
810 | |
---|
811 | \clearpage |
---|
812 | |
---|
813 | .. index:: set priority by scheduler for a semaphore |
---|
814 | .. index:: rtems_semaphore_set_priority |
---|
815 | |
---|
816 | .. _rtems_semaphore_set_priority: |
---|
817 | |
---|
818 | SEMAPHORE_SET_PRIORITY - Set priority by scheduler for a semaphore |
---|
819 | ------------------------------------------------------------------ |
---|
820 | |
---|
821 | CALLING SEQUENCE: |
---|
822 | .. code-block:: c |
---|
823 | |
---|
824 | rtems_status_code rtems_semaphore_set_priority( |
---|
825 | rtems_id semaphore_id, |
---|
826 | rtems_id scheduler_id, |
---|
827 | rtems_task_priority new_priority, |
---|
828 | rtems_task_priority *old_priority |
---|
829 | ); |
---|
830 | |
---|
831 | DIRECTIVE STATUS CODES: |
---|
832 | .. list-table:: |
---|
833 | :class: rtems-table |
---|
834 | |
---|
835 | * - ``RTEMS_SUCCESSFUL`` |
---|
836 | - successful operation |
---|
837 | * - ``RTEMS_INVALID_ID`` |
---|
838 | - invalid semaphore or scheduler id |
---|
839 | * - ``RTEMS_INVALID_ADDRESS`` |
---|
840 | - ``old_priority`` is NULL |
---|
841 | * - ``RTEMS_INVALID_PRIORITY`` |
---|
842 | - invalid new priority value |
---|
843 | * - ``RTEMS_NOT_DEFINED`` |
---|
844 | - operation not defined for the protocol ofthe semaphore |
---|
845 | * - ``RTEMS_ILLEGAL_ON_REMOTE_OBJECT`` |
---|
846 | - not supported for remote semaphores |
---|
847 | |
---|
848 | DESCRIPTION: |
---|
849 | This directive sets the priority value with respect to the specified |
---|
850 | scheduler of a semaphore. |
---|
851 | |
---|
852 | The special priority value ``RTEMS_CURRENT_PRIORITY`` can be used to get |
---|
853 | the current priority value without changing it. |
---|
854 | |
---|
855 | The interpretation of the priority value depends on the protocol of the |
---|
856 | semaphore object. |
---|
857 | |
---|
858 | - The Multiprocessor Resource Sharing Protocol needs a ceiling priority per |
---|
859 | scheduler instance. This operation can be used to specify these priority |
---|
860 | values. |
---|
861 | |
---|
862 | - For the Priority Ceiling Protocol the ceiling priority is used with this |
---|
863 | operation. |
---|
864 | |
---|
865 | - For other protocols this operation is not defined. |
---|
866 | |
---|
867 | EXAMPLE: |
---|
868 | .. code-block:: c |
---|
869 | :linenos: |
---|
870 | |
---|
871 | #include <assert.h> |
---|
872 | #include <stdlib.h> |
---|
873 | #include <rtems.h> |
---|
874 | |
---|
875 | #define SCHED_A rtems_build_name(' ', ' ', ' ', 'A') |
---|
876 | #define SCHED_B rtems_build_name(' ', ' ', ' ', 'B') |
---|
877 | |
---|
878 | static void Init(rtems_task_argument arg) |
---|
879 | { |
---|
880 | rtems_status_code sc; |
---|
881 | rtems_id semaphore_id; |
---|
882 | rtems_id scheduler_a_id; |
---|
883 | rtems_id scheduler_b_id; |
---|
884 | rtems_task_priority prio; |
---|
885 | |
---|
886 | /* Get the scheduler identifiers */ |
---|
887 | sc = rtems_scheduler_ident(SCHED_A, &scheduler_a_id); |
---|
888 | assert(sc == RTEMS_SUCCESSFUL); |
---|
889 | sc = rtems_scheduler_ident(SCHED_B, &scheduler_b_id); |
---|
890 | assert(sc == RTEMS_SUCCESSFUL); |
---|
891 | |
---|
892 | /* Create a MrsP semaphore object */ |
---|
893 | sc = rtems_semaphore_create( |
---|
894 | rtems_build_name('M', 'R', 'S', 'P'), |
---|
895 | 1, |
---|
896 | RTEMS_MULTIPROCESSOR_RESOURCE_SHARING | RTEMS_BINARY_SEMAPHORE, |
---|
897 | 1, |
---|
898 | &semaphore_id |
---|
899 | ); |
---|
900 | assert(sc == RTEMS_SUCCESSFUL); |
---|
901 | |
---|
902 | /* |
---|
903 | * The ceiling priority values per scheduler are equal to the value specified |
---|
904 | * for object creation. |
---|
905 | */ |
---|
906 | prio = RTEMS_CURRENT_PRIORITY; |
---|
907 | sc = rtems_semaphore_set_priority(semaphore_id, scheduler_a_id, prio, &prio); |
---|
908 | assert(sc == RTEMS_SUCCESSFUL); |
---|
909 | assert(prio == 1); |
---|
910 | |
---|
911 | /* Check the old value and set a new ceiling priority for scheduler B */ |
---|
912 | prio = 2; |
---|
913 | sc = rtems_semaphore_set_priority(semaphore_id, scheduler_b_id, prio, &prio); |
---|
914 | assert(sc == RTEMS_SUCCESSFUL); |
---|
915 | assert(prio == 1); |
---|
916 | |
---|
917 | /* Check the ceiling priority values */ |
---|
918 | prio = RTEMS_CURRENT_PRIORITY; |
---|
919 | sc = rtems_semaphore_set_priority(semaphore_id, scheduler_a_id, prio, &prio); |
---|
920 | assert(sc == RTEMS_SUCCESSFUL); |
---|
921 | assert(prio == 1); |
---|
922 | prio = RTEMS_CURRENT_PRIORITY; |
---|
923 | sc = rtems_semaphore_set_priority(semaphore_id, scheduler_b_id, prio, &prio); |
---|
924 | assert(sc == RTEMS_SUCCESSFUL); |
---|
925 | assert(prio == 2); |
---|
926 | sc = rtems_semaphore_delete(semaphore_id); |
---|
927 | assert(sc == RTEMS_SUCCESSFUL); |
---|
928 | exit(0); |
---|
929 | } |
---|
930 | |
---|
931 | #define CONFIGURE_APPLICATION_NEEDS_CLOCK_DRIVER |
---|
932 | #define CONFIGURE_APPLICATION_NEEDS_CONSOLE_DRIVER |
---|
933 | #define CONFIGURE_MAXIMUM_TASKS 1 |
---|
934 | #define CONFIGURE_MAXIMUM_SEMAPHORES 1 |
---|
935 | #define CONFIGURE_MAXIMUM_MRSP_SEMAPHORES 1 |
---|
936 | #define CONFIGURE_MAXIMUM_PROCESSORS 2 |
---|
937 | #define CONFIGURE_SCHEDULER_SIMPLE_SMP |
---|
938 | |
---|
939 | #include <rtems/scheduler.h> |
---|
940 | |
---|
941 | RTEMS_SCHEDULER_CONTEXT_SIMPLE_SMP(a); |
---|
942 | RTEMS_SCHEDULER_CONTEXT_SIMPLE_SMP(b); |
---|
943 | |
---|
944 | #define CONFIGURE_SCHEDULER_CONTROLS \ |
---|
945 | RTEMS_SCHEDULER_CONTROL_SIMPLE_SMP(a, SCHED_A), \ |
---|
946 | RTEMS_SCHEDULER_CONTROL_SIMPLE_SMP(b, SCHED_B) |
---|
947 | #define CONFIGURE_SMP_SCHEDULER_ASSIGNMENTS \ |
---|
948 | RTEMS_SCHEDULER_ASSIGN(0, RTEMS_SCHEDULER_ASSIGN_PROCESSOR_MANDATORY), \ |
---|
949 | RTEMS_SCHEDULER_ASSIGN(1, RTEMS_SCHEDULER_ASSIGN_PROCESSOR_MANDATORY) |
---|
950 | #define CONFIGURE_RTEMS_INIT_TASKS_TABLE |
---|
951 | #define CONFIGURE_INIT |
---|
952 | #include <rtems/confdefs.h> |
---|