1 # Event 2 3 ## Basic Concepts<a name="section122115620816"></a> 4 5 An event is a mechanism for communication between tasks. It can be used to synchronize tasks. 6 7 In multi-task environment, synchronization is required between tasks. Events can be used for synchronization in the following cases: 8 9 - One-to-many synchronization: A task waits for the triggering of multiple events. A task is woken up by one or multiple events. 10 - Many-to-many synchronization: Multiple tasks wait for the triggering of multiple events. 11 12 The event mechanism provided by the OpenHarmony LiteOS-A event module has the following features: 13 14 - A task triggers or waits for an event by creating an event control block. 15 - Events are independent of each other. The internal implementation is a 32-bit unsigned integer, and each bit indicates an event type. The 25th bit is unavailable. Therefore, a maximum of 31 event types are supported. 16 - Events are used only for synchronization between tasks, but not for data transmission. 17 - Writing the same event type to the event control block for multiple times is equivalent to writing the event type only once before the event control block is cleared. 18 - Multiple tasks can read and write the same event. 19 - The event read/write timeout mechanism is supported. 20 21 ## Working Principles<a name="section94611116593"></a> 22 23 ### Event Control Block<a name="section1161415384467"></a> 24 25 ``` 26 /** 27 * Event control block data structure 28 */ 29 typedef struct tagEvent { 30 UINT32 uwEventID; /* Event set, which is a collection of events processed (written and cleared). */ 31 LOS_DL_LIST stEventList; /* List of tasks waiting for specific events */ 32 } EVENT_CB_S, *PEVENT_CB_S; 33 ``` 34 35 ### Working Principles<a name="section187761153144617"></a> 36 37 **Initializing an event**: An event control block is created to maintain a collection of processed events and a linked list of tasks waiting for specific events. 38 39 **Writing an event**: When a specified event is written to the event control block, the event control block updates the event set, traverses the task linked list, and determines whether to wake up related task based on the task conditions. 40 41 **Reading an event**: If the read event already exists, it is returned synchronously. In other cases, the return time is determined based on the timeout period and event triggering status. If the wait event condition is met before the timeout period expires, the blocked task will be directly woken up. Otherwise, the blocked task will be woken up only after the timeout period has expired. 42 43 The input parameters **eventMask** and **mode** determine whether the condition for reading an event is met. **eventMask** indicates the mask of the event. **mode** indicates the handling mode, which can be any of the following: 44 45 - **LOS\_WAITMODE\_AND**: Event reading is successful only when all the events corresponding to **eventMask** occur. Otherwise, the task will be blocked, or an error code will be returned. 46 - **LOS\_WAITMODE\_OR**: Event reading is successful when any of the events corresponding to **eventMask** occurs. Otherwise, the task will be blocked, or an error code will be returned. 47 - **LOS\_WAITMODE\_CLR**: This mode must be used with **LOS\_WAITMODE\_AND** or **LOS\_WAITMODE\_OR** \(LOS\_WAITMODE\_AND | LOS\_WAITMODE\_CLR or LOS\_WAITMODE\_OR | LOS\_WAITMODE\_CLR\). In this mode, if **LOS\_WAITMODE\_AND** or **LOS\_WAITMODE\_OR** is successful, the corresponding event type bit in the event control block will be automatically cleared. 48 49 **Clearing events**: Clear the event set of the event control block based on the specified mask. If the mask is **0**, the event set will be cleared. If the mask is **0xffff**, no event will be cleared, and the event set remains unchanged. 50 51 **Destroying an event**: Destroy the specified event control block. 52 53 **Figure 1** Event working mechanism for small systems<a name="fig17799175324612"></a> 54  55 56 ## Development Guidelines<a name="section44744471891"></a> 57 58 ### Available APIs<a name="section172373513919"></a> 59 60 The following table describes APIs available for the OpenHarmony LiteOS-A event module. 61 62 **Table 1** Event module APIs 63 64 <a name="table1415203765610"></a> 65 <table><thead align="left"><tr id="row134151837125611"><th class="cellrowborder" valign="top" width="12.85128512851285%" id="mcps1.2.4.1.1"><p id="p16415637105612"><a name="p16415637105612"></a><a name="p16415637105612"></a>Function</p> 66 </th> 67 <th class="cellrowborder" valign="top" width="29.8029802980298%" id="mcps1.2.4.1.2"><p id="p11415163718562"><a name="p11415163718562"></a><a name="p11415163718562"></a>API</p> 68 </th> 69 <th class="cellrowborder" valign="top" width="57.34573457345735%" id="mcps1.2.4.1.3"><p id="p1641533755612"><a name="p1641533755612"></a><a name="p1641533755612"></a>Description</p> 70 </th> 71 </tr> 72 </thead> 73 <tbody><tr id="row0415737175610"><td class="cellrowborder" valign="top" width="12.85128512851285%" headers="mcps1.2.4.1.1 "><p id="p9598124913544"><a name="p9598124913544"></a><a name="p9598124913544"></a>Initializing events</p> 74 </td> 75 <td class="cellrowborder" valign="top" width="29.8029802980298%" headers="mcps1.2.4.1.2 "><p id="p77891354175812"><a name="p77891354175812"></a><a name="p77891354175812"></a>LOS_EventInit</p> 76 </td> 77 <td class="cellrowborder" valign="top" width="57.34573457345735%" headers="mcps1.2.4.1.3 "><p id="p2334141425515"><a name="p2334141425515"></a><a name="p2334141425515"></a>Initializes an event control block.</p> 78 </td> 79 </tr> 80 <tr id="row421753455514"><td class="cellrowborder" rowspan="2" valign="top" width="12.85128512851285%" headers="mcps1.2.4.1.1 "><p id="p13441112105813"><a name="p13441112105813"></a><a name="p13441112105813"></a>Reading/Writing events</p> 81 </td> 82 <td class="cellrowborder" valign="top" width="29.8029802980298%" headers="mcps1.2.4.1.2 "><p id="p17234205011559"><a name="p17234205011559"></a><a name="p17234205011559"></a>LOS_EventRead</p> 83 </td> 84 <td class="cellrowborder" valign="top" width="57.34573457345735%" headers="mcps1.2.4.1.3 "><p id="p1621275475517"><a name="p1621275475517"></a><a name="p1621275475517"></a>Reads a specified type of event, with the timeout period of a relative time period in ticks.</p> 85 </td> 86 </tr> 87 <tr id="row13129193718555"><td class="cellrowborder" valign="top" headers="mcps1.2.4.1.1 "><p id="p17477615564"><a name="p17477615564"></a><a name="p17477615564"></a>LOS_EventWrite</p> 88 </td> 89 <td class="cellrowborder" valign="top" headers="mcps1.2.4.1.2 "><p id="p10271958567"><a name="p10271958567"></a><a name="p10271958567"></a>Writes a specified type of event.</p> 90 </td> 91 </tr> 92 <tr id="row1831124035511"><td class="cellrowborder" valign="top" width="12.85128512851285%" headers="mcps1.2.4.1.1 "><p id="p1313401559"><a name="p1313401559"></a><a name="p1313401559"></a>Clearing events</p> 93 </td> 94 <td class="cellrowborder" valign="top" width="29.8029802980298%" headers="mcps1.2.4.1.2 "><p id="p7788152419567"><a name="p7788152419567"></a><a name="p7788152419567"></a>LOS_EventClear</p> 95 </td> 96 <td class="cellrowborder" valign="top" width="57.34573457345735%" headers="mcps1.2.4.1.3 "><p id="p14862153525620"><a name="p14862153525620"></a><a name="p14862153525620"></a>Clears a specified type of event.</p> 97 </td> 98 </tr> 99 <tr id="row1525316428553"><td class="cellrowborder" valign="top" width="12.85128512851285%" headers="mcps1.2.4.1.1 "><p id="p4253144265519"><a name="p4253144265519"></a><a name="p4253144265519"></a>Checking the event mask</p> 100 </td> 101 <td class="cellrowborder" valign="top" width="29.8029802980298%" headers="mcps1.2.4.1.2 "><p id="p768611115563"><a name="p768611115563"></a><a name="p768611115563"></a>LOS_EventPoll</p> 102 </td> 103 <td class="cellrowborder" valign="top" width="57.34573457345735%" headers="mcps1.2.4.1.3 "><p id="p13998115465617"><a name="p13998115465617"></a><a name="p13998115465617"></a>Returns whether the event input by the user meets the expectation based on the event ID, event mask, and read mode passed by the user.</p> 104 </td> 105 </tr> 106 <tr id="row6447135825614"><td class="cellrowborder" valign="top" width="12.85128512851285%" headers="mcps1.2.4.1.1 "><p id="p104471658155615"><a name="p104471658155615"></a><a name="p104471658155615"></a>Destroying events</p> 107 </td> 108 <td class="cellrowborder" valign="top" width="29.8029802980298%" headers="mcps1.2.4.1.2 "><p id="p15259169573"><a name="p15259169573"></a><a name="p15259169573"></a>LOS_EventDestroy</p> 109 </td> 110 <td class="cellrowborder" valign="top" width="57.34573457345735%" headers="mcps1.2.4.1.3 "><p id="p32592615573"><a name="p32592615573"></a><a name="p32592615573"></a>Destroys a specified event control block.</p> 111 </td> 112 </tr> 113 </tbody> 114 </table> 115 116 ### How to Develop<a name="section1118215161013"></a> 117 118 The typical event development process is as follows: 119 120 1. Initialize an event control block. 121 2. Block a read event control block. 122 3. Write related events. 123 4. Wake up a blocked task, read the event, and check whether the event meets conditions. 124 5. Handle the event control block. 125 6. Destroy an event control block. 126 127 > **NOTE:** 128 >- When an event is read or written, the 25th bit of the event is reserved and cannot be set. 129 >- Repeated writes of the same event are treated as one write. 130 131 ## Development Example<a name="section5837165132911"></a> 132 133 ### Example Description<a name="section128221510145718"></a> 134 135 In this example, run the **Example\_TaskEntry** task to create the **Example\_Event** task, run the **Example\_Event** task to read an event to trigger task switching, and run the **Example\_TaskEntry** task to write an event. You can understand the task switching during event operations based on the sequence in which logs are recorded. 136 137 1. Create the **Example\_Event** task in the **Example\_TaskEntry** task with a higher priority than the **Example\_TaskEntry** task. 138 2. Run the **Example\_Event** task to read event **0x00000001**. Task switching is triggered to execute the **Example\_TaskEntry** task. 139 3. Run the **Example\_TaskEntry** task to write event **0x00000001**. Task switching is triggered to execute the **Example\_Event** task. 140 4. The **Example\_Event** task is executed. 141 5. The **Example\_TaskEntry** task is executed. 142 143 ### Sample Code<a name="section71507479577"></a> 144 145 The sample code is as follows: 146 147 ``` 148 #include "los_event.h" 149 #include "los_task.h" 150 #include "securec.h" 151 152 /* Task ID*/ 153 UINT32 g_testTaskId; 154 155 /* Event control structure*/ 156 EVENT_CB_S g_exampleEvent; 157 158 /* Type of the wait event*/ 159 #define EVENT_WAIT 0x00000001 160 161 /* Example task entry function*/ 162 VOID Example_Event(VOID) 163 { 164 UINT32 event; 165 166 /* Set a timeout period for event reading to 100 ticks. If the specified event is not read within 100 ticks, the read operation times out and the task is woken up.*/ 167 printf("Example_Event wait event 0x%x \n", EVENT_WAIT); 168 169 event = LOS_EventRead(&g_exampleEvent, EVENT_WAIT, LOS_WAITMODE_AND, 100); 170 if (event == EVENT_WAIT) { 171 printf("Example_Event,read event :0x%x\n", event); 172 } else { 173 printf("Example_Event,read event timeout\n"); 174 } 175 } 176 177 UINT32 Example_EventEntry(VOID) 178 { 179 UINT32 ret; 180 TSK_INIT_PARAM_S task1; 181 182 /* Initialize the event.*/ 183 ret = LOS_EventInit(&g_exampleEvent); 184 if (ret != LOS_OK) { 185 printf("init event failed .\n"); 186 return -1; 187 } 188 189 /* Create a task.*/ 190 (VOID)memset_s(&task1, sizeof(TSK_INIT_PARAM_S), 0, sizeof(TSK_INIT_PARAM_S)); 191 task1.pfnTaskEntry = (TSK_ENTRY_FUNC)Example_Event; 192 task1.pcName = "EventTsk1"; 193 task1.uwStackSize = LOSCFG_BASE_CORE_TSK_DEFAULT_STACK_SIZE; 194 task1.usTaskPrio = 5; 195 ret = LOS_TaskCreate(&g_testTaskId, &task1); 196 if (ret != LOS_OK) { 197 printf("task create failed.\n"); 198 return LOS_NOK; 199 } 200 201 /* Write the task wait event (g_testTaskId). */ 202 printf("Example_TaskEntry write event.\n"); 203 204 ret = LOS_EventWrite(&g_exampleEvent, EVENT_WAIT); 205 if (ret != LOS_OK) { 206 printf("event write failed.\n"); 207 return LOS_NOK; 208 } 209 210 /* Clear the flag.*/ 211 printf("EventMask:%d\n", g_exampleEvent.uwEventID); 212 LOS_EventClear(&g_exampleEvent, ~g_exampleEvent.uwEventID); 213 printf("EventMask:%d\n", g_exampleEvent.uwEventID); 214 215 /* Delete the task.*/ 216 ret = LOS_TaskDelete(g_testTaskId); 217 if (ret != LOS_OK) { 218 printf("task delete failed.\n"); 219 return LOS_NOK; 220 } 221 222 return LOS_OK; 223 } 224 ``` 225 226 ### Verification<a name="section16570171645813"></a> 227 228 The development is successful if the return result is as follows: 229 230 ``` 231 Example_Event wait event 0x1 232 Example_TaskEntry write event. 233 Example_Event,read event :0x1 234 EventMask:1 235 EventMask:0 236 ``` 237 238
