Follow+Star Public Account Number, don’t miss wonderful content

Author | strongerHuang

WeChat Public Account | strongerHuang

The essence of a semaphore is to pass an “event”. For example: Task A completes sending data and notifies Task B through the semaphore.

OSSemPost(EventSem_SendOK);

We mainly want to convey the event of “data sent successfully”, further analysis shows that it is actually a “flag” or “variable”.

The principle of queues is somewhat similar to that of semaphores, but here it is a “variable”. For example: after receiving a complete frame of data through the serial port, it is sent to Task B through the queue.

OSQPost(UARTRcvQueue, RcvBuf);

Compared to semaphores, queues can transmit a larger amount of data, and the effective data transmitted by queues is generally an array.

There are also mailboxes, which are similar to queues and can be understood as “two-dimensional arrays”.

Writing to this point, you will find that whether it is semaphores or queues, the underlying essence is also to pass “variables” and “arrays”.

So the question arises: Why not use global variables for inter-task communication in RTOS?

This question is quite common, and I have also seen discussions about it in my technical exchange group, so I will simply share my views.

INT8U  OSSemPost (OS_EVENT *pevent){#if OS_CRITICAL_METHOD == 3u                          /* Allocate storage for CPU status register      */    OS_CPU_SR  cpu_sr = 0u;#endif
#if OS_ARG_CHK_EN > 0u    if (pevent == (OS_EVENT *)0) {                    /* Validate 'pevent'                             */        return (OS_ERR_PEVENT_NULL);    }#endif    if (pevent->OSEventType != OS_EVENT_TYPE_SEM) {   /* Validate event block type                     */        return (OS_ERR_EVENT_TYPE);    }    OS_ENTER_CRITICAL();    if (pevent->OSEventGrp != 0u) {                   /* See if any task waiting for semaphore         */                                                      /* Ready HPT waiting on event                    */        (void)OS_EventTaskRdy(pevent, (void *)0, OS_STAT_SEM, OS_STAT_PEND_OK);        OS_EXIT_CRITICAL();        OS_Sched();                                   /* Find HPT ready to run                         */        return (OS_ERR_NONE);    }    if (pevent->OSEventCnt < 65535u) {                /* Make sure semaphore will not overflow         */        pevent->OSEventCnt++;                         /* Increment semaphore count to register event   */        OS_EXIT_CRITICAL();        return (OS_ERR_NONE);    }    OS_EXIT_CRITICAL();                               /* Semaphore value has reached its maximum       */    return (OS_ERR_SEM_OVF);}

Although we only need to transmit one variable of effective information, it will manage the “critical section” and also anticipate some error conditions, etc.

Finally, RTOS source code can also be considered an excellent project, especially those RTOS with relatively high popularity and installation, such as µC/OS, FreeRTOS, RT-Thread, ThreadX.

Lastly, if you have time, you can read the RTOS source code, I recommend µC/OS for reading the source code, as it can help you grasp some software architecture knowledge and understand common issues in the development process.

———— END ————

● Column “Embedded Tools”

● Column “Embedded Development”

● Column “Keil Tutorial”

● Selected Tutorials for Embedded Columns

Click “Read the Original” to see more shares.