1 | @c |
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2 | @c COPYRIGHT (c) 1988-2002. |
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3 | @c On-Line Applications Research Corporation (OAR). |
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4 | @c All rights reserved. |
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5 | @c |
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6 | @c $Id$ |
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7 | @c |
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8 | |
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9 | @chapter Event Manager |
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10 | |
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11 | @cindex events |
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12 | |
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13 | @section Introduction |
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14 | |
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15 | The event manager provides a high performance method |
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16 | of intertask communication and synchronization. The directives |
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17 | provided by the event manager are: |
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18 | |
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19 | @itemize @bullet |
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20 | @item @code{@value{DIRPREFIX}event_send} - Send event set to a task |
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21 | @item @code{@value{DIRPREFIX}event_receive} - Receive event condition |
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22 | @end itemize |
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23 | |
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24 | @section Background |
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25 | |
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26 | @subsection Event Sets |
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27 | |
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28 | @cindex event flag, definition |
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29 | @cindex event set, definition |
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30 | @findex rtems_event_set |
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31 | |
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32 | An event flag is used by a task (or ISR) to inform |
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33 | another task of the occurrence of a significant situation. |
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34 | Thirty-two event flags are associated with each task. A |
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35 | collection of one or more event flags is referred to as an event |
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36 | set. The data type @code{@value{DIRPREFIX}event_set} is used to manage |
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37 | event sets. |
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38 | |
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39 | The application developer should remember the following |
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40 | key characteristics of event operations when utilizing the event |
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41 | manager: |
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42 | |
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43 | @itemize @bullet |
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44 | @item Events provide a simple synchronization facility. |
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45 | |
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46 | @item Events are aimed at tasks. |
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47 | |
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48 | @item Tasks can wait on more than one event simultaneously. |
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49 | |
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50 | @item Events are independent of one another. |
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51 | |
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52 | @item Events do not hold or transport data. |
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53 | |
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54 | @item Events are not queued. In other words, if an event is |
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55 | sent more than once to a task before being received, the second and |
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56 | subsequent send operations to that same task have no effect. |
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57 | @end itemize |
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58 | |
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59 | An event set is posted when it is directed (or sent) to a task. A |
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60 | pending event is an event that has been posted but not received. An event |
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61 | condition is used to specify the event set which the task desires to receive |
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62 | and the algorithm which will be used to determine when the request is |
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63 | satisfied. An event condition is satisfied based upon one of two |
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64 | algorithms which are selected by the user. The |
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65 | @code{@value{RPREFIX}EVENT_ANY} algorithm states that an event condition |
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66 | is satisfied when at least a single requested event is posted. The |
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67 | @code{@value{RPREFIX}EVENT_ALL} algorithm states that an event condition |
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68 | is satisfied when every requested event is posted. |
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69 | |
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70 | @subsection Building an Event Set or Condition |
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71 | |
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72 | @cindex event condition, building |
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73 | @cindex event set, building |
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74 | |
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75 | An event set or condition is built by a bitwise OR of |
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76 | the desired events. The set of valid events is @code{@value{RPREFIX}EVENT_0} through |
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77 | @code{@value{RPREFIX}EVENT_31}. If an event is not explicitly specified in the set or |
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78 | condition, then it is not present. Events are specifically |
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79 | designed to be mutually exclusive, therefore bitwise OR and |
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80 | addition operations are equivalent as long as each event appears |
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81 | exactly once in the event set list. |
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82 | |
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83 | For example, when sending the event set consisting of |
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84 | @code{@value{RPREFIX}EVENT_6}, @code{@value{RPREFIX}EVENT_15}, and @code{@value{RPREFIX}EVENT_31}, |
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85 | the event parameter to the @code{@value{DIRPREFIX}event_send} |
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86 | directive should be @code{@value{RPREFIX}EVENT_6 @value{OR} |
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87 | @value{RPREFIX}EVENT_15 @value{OR} @value{RPREFIX}EVENT_31}. |
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88 | |
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89 | @subsection Building an EVENT_RECEIVE Option Set |
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90 | |
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91 | In general, an option is built by a bitwise OR of the |
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92 | desired option components. The set of valid options for the |
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93 | @code{@value{DIRPREFIX}event_receive} directive are listed |
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94 | in the following table: |
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95 | |
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96 | @itemize @bullet |
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97 | @item @code{@value{RPREFIX}WAIT} - task will wait for event (default) |
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98 | @item @code{@value{RPREFIX}NO_WAIT} - task should not wait |
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99 | @item @code{@value{RPREFIX}EVENT_ALL} - return after all events (default) |
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100 | @item @code{@value{RPREFIX}EVENT_ANY} - return after any events |
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101 | @end itemize |
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102 | |
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103 | Option values are specifically designed to be |
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104 | mutually exclusive, therefore bitwise OR and addition operations |
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105 | are equivalent as long as each option appears exactly once in |
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106 | the component list. An option listed as a default is not |
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107 | required to appear in the option list, although it is a good |
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108 | programming practice to specify default options. If all |
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109 | defaults are desired, the option @code{@value{RPREFIX}DEFAULT_OPTIONS} should be |
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110 | specified on this call. |
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111 | |
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112 | This example demonstrates the option parameter needed |
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113 | to poll for all events in a particular event condition to |
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114 | arrive. The option parameter passed to the |
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115 | @code{@value{DIRPREFIX}event_receive} directive should be either |
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116 | @code{@value{RPREFIX}EVENT_ALL @value{OR} @value{RPREFIX}NO_WAIT} |
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117 | or @code{@value{RPREFIX}NO_WAIT}. The option parameter can be set to |
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118 | @code{@value{RPREFIX}NO_WAIT} because @code{@value{RPREFIX}EVENT_ALL} is the |
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119 | default condition for @code{@value{DIRPREFIX}event_receive}. |
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120 | |
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121 | @section Operations |
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122 | |
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123 | @subsection Sending an Event Set |
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124 | |
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125 | The @code{@value{DIRPREFIX}event_send} directive allows a task (or an ISR) to |
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126 | direct an event set to a target task. Based upon the state of |
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127 | the target task, one of the following situations applies: |
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128 | |
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129 | @itemize @bullet |
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130 | @item Target Task is Blocked Waiting for Events |
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131 | |
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132 | @itemize - |
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133 | |
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134 | @item If the waiting task's input event condition is |
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135 | satisfied, then the task is made ready for execution. |
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136 | |
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137 | @item If the waiting task's input event condition is not |
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138 | satisfied, then the event set is posted but left pending and the |
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139 | task remains blocked. |
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140 | |
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141 | @end itemize |
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142 | |
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143 | @item Target Task is Not Waiting for Events |
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144 | |
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145 | @itemize - |
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146 | @item The event set is posted and left pending. |
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147 | @end itemize |
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148 | |
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149 | @end itemize |
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150 | |
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151 | @subsection Receiving an Event Set |
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152 | |
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153 | The @code{@value{DIRPREFIX}event_receive} directive is used by tasks to |
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154 | accept a specific input event condition. The task also |
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155 | specifies whether the request is satisfied when all requested |
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156 | events are available or any single requested event is available. |
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157 | If the requested event condition is satisfied by pending |
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158 | events, then a successful return code and the satisfying event |
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159 | set are returned immediately. If the condition is not |
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160 | satisfied, then one of the following situations applies: |
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161 | |
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162 | @itemize @bullet |
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163 | @item By default, the calling task will wait forever for the |
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164 | event condition to be satisfied. |
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165 | |
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166 | @item Specifying the @code{@value{RPREFIX}NO_WAIT} option forces an immediate return |
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167 | with an error status code. |
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168 | |
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169 | @item Specifying a timeout limits the period the task will |
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170 | wait before returning with an error status code. |
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171 | @end itemize |
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172 | |
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173 | @subsection Determining the Pending Event Set |
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174 | |
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175 | A task can determine the pending event set by calling |
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176 | the @code{@value{DIRPREFIX}event_receive} directive with a value of |
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177 | @code{@value{RPREFIX}PENDING_EVENTS} for the input event condition. |
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178 | The pending events are returned to the calling task but the event |
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179 | set is left unaltered. |
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180 | |
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181 | @subsection Receiving all Pending Events |
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182 | |
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183 | A task can receive all of the currently pending |
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184 | events by calling the @code{@value{DIRPREFIX}event_receive} |
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185 | directive with a value of @code{@value{RPREFIX}ALL_EVENTS} |
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186 | for the input event condition and |
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187 | @code{@value{RPREFIX}NO_WAIT @value{OR} @value{RPREFIX}EVENT_ANY} |
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188 | for the option set. The pending events are returned to the |
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189 | calling task and the event set is cleared. If no events are |
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190 | pending then the @code{@value{RPREFIX}UNSATISFIED} status code will be returned. |
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191 | |
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192 | @section Directives |
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193 | |
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194 | This section details the event manager's directives. |
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195 | A subsection is dedicated to each of this manager's directives |
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196 | and describes the calling sequence, related constants, usage, |
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197 | and status codes. |
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198 | |
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199 | @c |
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200 | @c |
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201 | @c |
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202 | @page |
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203 | @subsection EVENT_SEND - Send event set to a task |
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204 | |
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205 | @cindex send event set to a task |
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206 | |
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207 | @subheading CALLING SEQUENCE: |
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208 | |
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209 | @ifset is-C |
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210 | @findex rtems_event_send |
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211 | @example |
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212 | rtems_status_code rtems_event_send ( |
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213 | rtems_id id, |
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214 | rtems_event_set event_in |
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215 | ); |
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216 | @end example |
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217 | @end ifset |
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218 | |
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219 | @ifset is-Ada |
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220 | @example |
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221 | procedure Event_Send ( |
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222 | ID : in RTEMS.ID; |
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223 | Event_In : in RTEMS.Event_Set; |
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224 | Result : out RTEMS.Status_Codes |
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225 | ); |
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226 | @end example |
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227 | @end ifset |
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228 | |
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229 | @subheading DIRECTIVE STATUS CODES: |
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230 | @code{@value{RPREFIX}SUCCESSFUL} - event set sent successfully@* |
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231 | @code{@value{RPREFIX}INVALID_ID} - invalid task id |
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232 | |
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233 | @subheading DESCRIPTION: |
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234 | |
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235 | This directive sends an event set, event_in, to the |
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236 | task specified by id. If a blocked task's input event condition |
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237 | is satisfied by this directive, then it will be made ready. If |
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238 | its input event condition is not satisfied, then the events |
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239 | satisfied are updated and the events not satisfied are left |
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240 | pending. If the task specified by id is not blocked waiting for |
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241 | events, then the events sent are left pending. |
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242 | |
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243 | @subheading NOTES: |
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244 | |
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245 | Specifying @code{@value{RPREFIX}SELF} for id results in the event set being |
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246 | sent to the calling task. |
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247 | |
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248 | Identical events sent to a task are not queued. In |
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249 | other words, the second, and subsequent, posting of an event to |
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250 | a task before it can perform an @code{@value{DIRPREFIX}event_receive} |
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251 | has no effect. |
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252 | |
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253 | The calling task will be preempted if it has |
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254 | preemption enabled and a higher priority task is unblocked as |
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255 | the result of this directive. |
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256 | |
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257 | Sending an event set to a global task which does not |
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258 | reside on the local node will generate a request telling the |
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259 | remote node to send the event set to the appropriate task. |
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260 | |
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261 | @c |
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262 | @c |
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263 | @c |
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264 | @page |
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265 | @subsection EVENT_RECEIVE - Receive event condition |
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266 | |
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267 | @cindex receive event condition |
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268 | |
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269 | @subheading CALLING SEQUENCE: |
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270 | |
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271 | @ifset is-C |
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272 | @findex rtems_event_receive |
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273 | @example |
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274 | rtems_status_code rtems_event_receive ( |
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275 | rtems_event_set event_in, |
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276 | rtems_option option_set, |
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277 | rtems_interval ticks, |
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278 | rtems_event_set *event_out |
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279 | ); |
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280 | @end example |
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281 | @end ifset |
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282 | |
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283 | @ifset is-Ada |
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284 | @example |
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285 | procedure Event_Receive ( |
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286 | Event_In : in RTEMS.Event_Set; |
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287 | Option_Set : in RTEMS.Option; |
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288 | Ticks : in RTEMS.Interval; |
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289 | Event_Out : out RTEMS.Event_Set; |
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290 | Result : out RTEMS.Status_Codes |
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291 | ); |
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292 | @end example |
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293 | @end ifset |
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294 | |
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295 | @subheading DIRECTIVE STATUS CODES: |
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296 | @code{@value{RPREFIX}SUCCESSFUL} - event received successfully@* |
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297 | @code{@value{RPREFIX}UNSATISFIED} - input event not satisfied (@code{@value{RPREFIX}NO_WAIT})@* |
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298 | @code{@value{RPREFIX}TIMEOUT} - timed out waiting for event |
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299 | |
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300 | @subheading DESCRIPTION: |
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301 | |
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302 | This directive attempts to receive the event |
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303 | condition specified in event_in. If event_in is set to |
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304 | @code{@value{RPREFIX}PENDING_EVENTS}, then the current pending events are returned in |
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305 | event_out and left pending. The @code{@value{RPREFIX}WAIT} and @code{@value{RPREFIX}NO_WAIT} options in the |
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306 | option_set parameter are used to specify whether or not the task |
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307 | is willing to wait for the event condition to be satisfied. |
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308 | @code{@value{RPREFIX}EVENT_ANY} and @code{@value{RPREFIX}EVENT_ALL} are used in the option_set parameter are |
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309 | used to specify whether a single event or the complete event set |
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310 | is necessary to satisfy the event condition. The event_out |
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311 | parameter is returned to the calling task with the value that |
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312 | corresponds to the events in event_in that were satisfied. |
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313 | |
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314 | If pending events satisfy the event condition, then |
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315 | event_out is set to the satisfied events and the pending events |
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316 | in the event condition are cleared. If the event condition is |
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317 | not satisfied and @code{@value{RPREFIX}NO_WAIT} is specified, then event_out is set to |
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318 | the currently satisfied events. If the calling task chooses to |
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319 | wait, then it will block waiting for the event condition. |
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320 | |
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321 | If the calling task must wait for the event condition |
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322 | to be satisfied, then the timeout parameter is used to specify |
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323 | the maximum interval to wait. If it is set to @code{@value{RPREFIX}NO_TIMEOUT}, then |
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324 | the calling task will wait forever. |
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325 | |
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326 | @subheading NOTES: |
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327 | |
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328 | This directive only affects the events specified in |
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329 | event_in. Any pending events that do not correspond to any of |
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330 | the events specified in event_in will be left pending. |
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331 | |
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332 | The following event receive option constants are defined by |
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333 | RTEMS: |
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334 | |
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335 | @itemize @bullet |
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336 | @item @code{@value{RPREFIX}WAIT} task will wait for event (default) |
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337 | @item @code{@value{RPREFIX}NO_WAIT} task should not wait |
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338 | @item @code{@value{RPREFIX}EVENT_ALL} return after all events (default) |
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339 | @item @code{@value{RPREFIX}EVENT_ANY} return after any events |
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340 | @end itemize |
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341 | |
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342 | A clock tick is required to support the functionality of this directive. |
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