# Development Guidelines - [Available APIs](#section158501652121514) - [How to Develop](#section783435801510) - [Development Example](#section460018317164) - [Example Description](#section896412438910) - [Sample Code](#section149077554912) - [Verification](#section4461439172017) ## Available APIs

Category	API	Description
Checking events	LOS_EventPoll	Checks whether the expected event occurs based on eventID, eventMask, and mode. NOTICE: If mode contains LOS_WAITMODE_CLR and the expected event occurs, the event that meets the requirements in eventID will be cleared. In this case, eventID is an input parameter and an output parameter. In other cases, eventID is used only as an input parameter.
Initializing events	LOS_EventInit	Initializes an event control block.
Reading events	LOS_EventRead	Reads an event (wait event). The task is blocked to wait based on the timeout period (in ticks). If no event is read, 0 is returned. If an event is successfully read, a positive value (event set) is returned. In other cases, a specific error code is returned.
Writing events	LOS_EventWrite	Writes a specific event to the event control block.
Clearing events	LOS_EventClear	Clears an event in the event control block based on the event mask.
Destroying events	LOS_EventDestroy	Destroys an event control block.

## How to Develop The typical event development process is as follows: 1. Initialize an event control block. 2. Block a read event control block. 3. Write related events. 4. Wake up a blocked task, read the event, and check whether the event meets conditions. 5. Handle the event control block. 6. Destroy an event control block. >![](../public_sys-resources/icon-note.gif) **NOTE:** >- When an event is read or written, the 25th bit of the event is reserved and cannot be set. >- Repeated writes of the same event are treated as one write. ## Development Example ### Example Description 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. 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. 1. Create the **Example\_Event** task in the **Example\_TaskEntry** task with a higher priority than the **Example\_TaskEntry** task. 2. Run the **Example\_Event** task to read event **0x00000001**. Task switching occurs to execute the **Example\_TaskEntry** task. 3. Run the **Example\_TaskEntry** task to write event **0x00000001**. Task switching occurs to execute the **Example\_Event** task. 4. The **Example\_Event** task is executed. 5. The **Example\_TaskEntry** task is executed. ### Sample Code The sample code is as follows: ``` #include "los_event.h" #include "los_task.h" #include "securec.h" /* Task ID*/ UINT32 g_testTaskId; /* Event control structure*/ EVENT_CB_S g_exampleEvent; /* Type of the wait event*/ #define EVENT_WAIT 0x00000001 /* Example task entry function*/ VOID Example_Event(VOID) { UINT32 ret; UINT32 event; /* 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.*/ printf("Example_Event wait event 0x%x \n", EVENT_WAIT); event = LOS_EventRead(&g_exampleEvent, EVENT_WAIT, LOS_WAITMODE_AND, 100); if (event == EVENT_WAIT) { printf("Example_Event,read event :0x%x\n", event); } else { printf("Example_Event,read event timeout\n"); } } UINT32 Example_TaskEntry(VOID) { UINT32 ret; TSK_INIT_PARAM_S task1; /* Initialize the event.*/ ret = LOS_EventInit(&g_exampleEvent); if (ret != LOS_OK) { printf("init event failed .\n"); return -1; } /* Create a task.*/ (VOID)memset_s(&task1, sizeof(TSK_INIT_PARAM_S), 0, sizeof(TSK_INIT_PARAM_S)); task1.pfnTaskEntry = (TSK_ENTRY_FUNC)Example_Event; task1.pcName = "EventTsk1"; task1.uwStackSize = OS_TSK_DEFAULT_STACK_SIZE; task1.usTaskPrio = 5; ret = LOS_TaskCreate(&g_testTaskId, &task1); if (ret != LOS_OK) { printf("task create failed.\n"); return LOS_NOK; } /* Write the task wait event (g_testTaskId). */ printf("Example_TaskEntry write event.\n"); ret = LOS_EventWrite(&g_exampleEvent, EVENT_WAIT); if (ret != LOS_OK) { printf("event write failed.\n"); return LOS_NOK; } /* Clear the flag.*/ printf("EventMask:%d\n", g_exampleEvent.uwEventID); LOS_EventClear(&g_exampleEvent, ~g_exampleEvent.uwEventID); printf("EventMask:%d\n", g_exampleEvent.uwEventID); /* Delete the task.*/ ret = LOS_TaskDelete(g_testTaskId); if (ret != LOS_OK) { printf("task delete failed.\n"); return LOS_NOK; } return LOS_OK; } ``` ### Verification The development is successful if the return result is as follows: ``` Example_Event wait event 0x1 Example_TaskEntry write event. Example_Event,read event :0x1 EventMask:1 EventMask:0 ```