Development Guidelines

• Available APIs
• How to Develop
• Development Example
  • Example Description
  • Sample Code
  • Verification

Available APIs

How to Develop

1. Call LOS_SemCreate to create a semaphore. To create a binary semaphore, call LOS_BinarySemCreate.
2. Call LOS_SemPend to request a semaphore.
3. Call LOS_SemPost to release a semaphore.
4. Call LOS_SemDelete to delete a semaphore.

NOTE: As interrupts cannot be blocked, semaphores cannot be requested in block mode for interrupts.

Development Example

Example Description

This example implements the following:

1. Create a semaphore in task ExampleSem and lock task scheduling. Create two tasks ExampleSemTask1 and ExampleSemTask2 (with higher priority). Enable the two tasks to request the same semaphore. Unlock task scheduling. Enable task ExampleSem to enter sleep mode for 400 ticks. Release the semaphore in task ExampleSem.
2. Enable** ExampleSemTask2** to enter sleep mode for 20 ticks after acquiring the semaphore. (When ExampleSemTask2 is delayed, ExampleSemTask1 is woken up.)
3. Enable ExampleSemTask1 to request the semaphore in scheduled block mode, with a wait timeout period of 10 ticks. (Because the semaphore is still held by ExampleSemTask2, ExampleSemTask1 is suspended. ExampleSemTask1 is woken up after 10 ticks.) Enable ExampleSemTask1 to request the semaphore in permanent block mode after it is woken up 10 ticks later. (Because the semaphore is still held by ExampleSemTask2, ExampleSemTask1 is suspended.)** **
4. After 20 ticks, ExampleSemTask2 is woken up and releases the semaphore. ExampleSemTask1 acquires the semaphore and is scheduled to run. When ExampleSemTask1 is complete, it releases the semaphore.
5. Task ExampleSem is woken up after 400 ticks and deletes the semaphore.

Sample Code

The sample code is as follows:

#include "los_sem.h"
#include "securec.h"

/* Task ID*/
static UINT32 g_testTaskId01;
static UINT32 g_testTaskId02;

/* Task priority */
#define TASK_PRIO_TEST  5

/* Semaphore structure ID*/ 
static UINT32 g_semId;

VOID ExampleSemTask1(VOID)
{
    UINT32 ret;

    printf("ExampleSemTask1 try get sem g_semId, timeout 10 ticks.\n");

    /* Request the semaphore in scheduled block mode, with a wait timeout period of 10 ticks.*/ 
    ret = LOS_SemPend(g_semId, 10);

    /* The semaphore is acquired.*/ 
    if (ret == LOS_OK) {
         LOS_SemPost(g_semId);
         return;
    }
    /* The semaphore is not acquired when the timeout period has expired.*/ 
    if (ret == LOS_ERRNO_SEM_TIMEOUT) {
        printf("ExampleSemTask1 timeout and try get sem g_semId wait forever.\n");

        /* Request the semaphore in permanent block mode.*/ 
        ret = LOS_SemPend(g_semId, LOS_WAIT_FOREVER);
        printf("ExampleSemTask1 wait_forever and get sem g_semId.\n");
        if (ret == LOS_OK) {
            LOS_SemPost(g_semId);
            return;
        }
    }
}

VOID ExampleSemTask2(VOID)
{
    UINT32 ret;
    printf("ExampleSemTask2 try get sem g_semId wait forever.\n");

    /* Request the semaphore in permanent block mode.*/ 
    ret = LOS_SemPend(g_semId, LOS_WAIT_FOREVER);

    if (ret == LOS_OK) {
        printf("ExampleSemTask2 get sem g_semId and then delay 20 ticks.\n");
    }

  /* Enable the task to enter sleep mode for 20 ticks.*/ 
    LOS_TaskDelay(20);

    printf("ExampleSemTask2 post sem g_semId.\n");
    /* Release the semaphore.*/ 
    LOS_SemPost(g_semId);
    return;
}

UINT32 ExampleSem(VOID)
{
    UINT32 ret;
    TSK_INIT_PARAM_S task1;
    TSK_INIT_PARAM_S task2;

   /* Create a semaphore.*/ 
    LOS_SemCreate(0, &g_semId);

    /* Lock task scheduling.*/
    LOS_TaskLock();

    /* Create task 1.*/
    (VOID)memset_s(&task1, sizeof(TSK_INIT_PARAM_S), 0, sizeof(TSK_INIT_PARAM_S));
    task1.pfnTaskEntry = (TSK_ENTRY_FUNC)ExampleSemTask1;
    task1.pcName       = "TestTask1";
    task1.uwStackSize  = LOSCFG_BASE_CORE_TSK_DEFAULT_STACK_SIZE;
    task1.usTaskPrio   = TASK_PRIO_TEST;
    ret = LOS_TaskCreate(&g_testTaskId01, &task1);
    if (ret != LOS_OK) {
        printf("task1 create failed.\n");
        return LOS_NOK;
    }

    /* Create task 2.*/ 
    (VOID)memset_s(&task2, sizeof(TSK_INIT_PARAM_S), 0, sizeof(TSK_INIT_PARAM_S));
    task2.pfnTaskEntry = (TSK_ENTRY_FUNC)ExampleSemTask2;
    task2.pcName       = "TestTask2";
    task2.uwStackSize  = LOSCFG_BASE_CORE_TSK_DEFAULT_STACK_SIZE;
    task2.usTaskPrio   = (TASK_PRIO_TEST - 1);
    ret = LOS_TaskCreate(&g_testTaskId02, &task2);
    if (ret != LOS_OK) {
        printf("task2 create failed.\n");
        return LOS_NOK;
    }

    /* Unlock task scheduling.*/ 
    LOS_TaskUnlock();

    ret = LOS_SemPost(g_semId);

 /* Enable the task to enter sleep mode for 400 ticks.*/
    LOS_TaskDelay(400);

 /* Delete the semaphore. */
    LOS_SemDelete(g_semId);
    return LOS_OK;
}

Verification

The development is successful if the return result is as follows:

ExampleSemTask2 try get sem g_semId wait forever.
ExampleSemTask2 get sem g_semId and then delay 20 ticks.
ExampleSemTask1 try get sem g_semId, timeout 10 ticks.

ExampleSemTask1 timeout and try get sem g_semId wait forever.
ExampleSemTask2 post sem g_semId.
ExampleSemTask1 wait_forever and get sem g_semId.