CubeMx中HAL库函数的调用不同于库函数调用，在学习CubeMx串口通信时，不理解HAL库中的回调函数是怎么被调用的，于是查看每个的定义，参考其他人写的博客，总算弄明白了HAL库中断调用与库函数不同之处。写下这篇博客一是加深自己的理解，二是希望对不理解HAL库中回调函数调用机制的朋友有所帮助。

        工程代码参考：【STM32】-CubeMX-HAL库-UART-串口通信-STM32F103C8T6-收发测试

        在库函数中，UART串口发生中断时，我们直接将业务代码写在void USART1_IRQHandler(void)中，如下图：

void USART1_IRQHandler(void)                	//串口1中断服务程序
	{
	u8 Res;

	if(USART_GetITStatus(USART1, USART_IT_RXNE) != RESET)  //接收中断(接收到的数据必须是0x0d 0x0a结尾)
		{
		Res =USART_ReceiveData(USART1);	//读取接收到的数据
		
		if((USART_RX_STA&0x8000)==0)//接收未完成
			{
			if(USART_RX_STA&0x4000)//接收到了0x0d
				{
				if(Res!=0x0a)USART_RX_STA=0;//接收错误,重新开始
				else USART_RX_STA|=0x8000;	//接收完成了 
				}
			else //还没收到0X0D
				{	
				if(Res==0x0d)USART_RX_STA|=0x4000;
				else
					{
					USART_RX_BUF[USART_RX_STA&0X3FFF]=Res ;
					USART_RX_STA++;
					if(USART_RX_STA>(USART_REC_LEN-1))USART_RX_STA=0;//接收数据错误,重新开始接收	  
					}		 
				}
			}   		 
     } 

} 

        对于CubeMX生成的代码，USART1_IRQHandler(void)函数为了提高中断效率采用了回调机制。（业务代码可以等中断关闭了再去处理，这样中断处理不会占用太多时间影响程序的执行效率）

        USART1_IRQHandler(void)函数中只调用了HAL_UART_IRQHandler(&huart1)（可以在STM32f1xx_it.c中找到），参数为uart1的句柄huart1，句柄可以理解为通过huart1访问到uart1的各种寄存器和数据类型，不理解的话，可以去看UART_HandleTypeDef结构体的定义。

        在HAL_UART_IRQHandler(UART_HandleTypeDef *huart)函数内又调用了UART_Receive_IT(huart);（回调函数在这个函数中被调用）看注释理解该函数的作用

static HAL_StatusTypeDef UART_Receive_IT(UART_HandleTypeDef *huart)
{
    //用这个指针指向我们用于接收数据的变量或数组，在收发测试例程中定义的是char Res
  uint8_t  *pdata8bits;
  uint16_t *pdata16bits;

  /* Check that a Rx process is ongoing */
  if (huart->RxState == HAL_UART_STATE_BUSY_RX)
  {
    if ((huart->Init.WordLength == UART_WORDLENGTH_9B) && (huart->Init.Parity == UART_PARITY_NONE))
    {
      pdata8bits  = NULL;
       
      pdata16bits = (uint16_t *) huart->pRxBuffPtr;//指向Res,相当于pdata16bits=&Res
        //具体原因参考HAL_UART_Receive_IT(&huart1, &Res, 1);
      *pdata16bits = (uint16_t)(huart->Instance->DR & (uint16_t)0x01FF);
      huart->pRxBuffPtr += 2U;
    }
    else
    {
      pdata8bits = (uint8_t *) huart->pRxBuffPtr;//指向Res,相当于pdata8bits=&Res
      pdata16bits  = NULL;

      if ((huart->Init.WordLength == UART_WORDLENGTH_9B) || ((huart->Init.WordLength == UART_WORDLENGTH_8B) && (huart->Init.Parity == UART_PARITY_NONE)))
      {
        //指针操作 相当于Res= (uint8_t)(huart->Instance->DR & (uint8_t)0x00FF);
        *pdata8bits = (uint8_t)(huart->Instance->DR & (uint8_t)0x00FF);
      }
      else
      {
        //指针操作 相当于Res= (uint8_t)(huart->Instance->DR & (uint8_t)0x00FF);
        *pdata8bits = (uint8_t)(huart->Instance->DR & (uint8_t)0x007F);
      }
      huart->pRxBuffPtr += 1U;
    }

    if (--huart->RxXferCount == 0U)//关闭中断，准备回调，对串口接收到的数据保存
    {
      /* Disable the UART Data Register not empty Interrupt */
      __HAL_UART_DISABLE_IT(huart, UART_IT_RXNE);

      /* Disable the UART Parity Error Interrupt */
      __HAL_UART_DISABLE_IT(huart, UART_IT_PE);

      /* Disable the UART Error Interrupt: (Frame error, noise error, overrun error) */
      __HAL_UART_DISABLE_IT(huart, UART_IT_ERR);

      /* Rx process is completed, restore huart->RxState to Ready */
      huart->RxState = HAL_UART_STATE_READY;

      /* Check current reception Mode :
         If Reception till IDLE event has been selected : */
      if (huart->ReceptionType == HAL_UART_RECEPTION_TOIDLE)
      {
        /* Set reception type to Standard */
        huart->ReceptionType = HAL_UART_RECEPTION_STANDARD;

        /* Disable IDLE interrupt */
        CLEAR_BIT(huart->Instance->CR1, USART_CR1_IDLEIE);

        /* Check if IDLE flag is set */
        if (__HAL_UART_GET_FLAG(huart, UART_FLAG_IDLE))
        {
          /* Clear IDLE flag in ISR */
          __HAL_UART_CLEAR_IDLEFLAG(huart);
        }

#if (USE_HAL_UART_REGISTER_CALLBACKS == 1)
        /*Call registered Rx Event callback*/
        huart->RxEventCallback(huart, huart->RxXferSize);
#else
        /*Call legacy weak Rx Event callback*/
        HAL_UARTEx_RxEventCallback(huart, huart->RxXferSize);
#endif
      }
      else
      {
       /* Standard reception API called */
#if (USE_HAL_UART_REGISTER_CALLBACKS == 1)		  
       /*Call registered Rx complete callback*/
       huart->RxCpltCallback(huart);
#else
       /*Call legacy weak Rx complete callback*/
       HAL_UART_RxCpltCallback(huart);//正常情况下会执行这一条语句
        //我们可以自己定义这个函数内部的具体操作
#endif /* USE_HAL_UART_REGISTER_CALLBACKS */
      }

      return HAL_OK;
    }
    return HAL_OK;
  }
  else
  {
    return HAL_BUSY;
  }
}

表格中左右两边的操作为什么等价呢？

pdata8bits = (uint8_t *) huart->pRxBuffPtr;	pdata8bits=&Res
*pdata8bits = (uint8_t)(huart->Instance->DR & (uint8_t)0x00FF);	Res= (uint8_t)(huart->Instance->DR & (uint8_t)0x00FF);

可以从UART_Start_Receive_IT函数中找到答案

（UART_Start_Receive_IT被main函数中的HAL_UART_Receive_IT调用）

//函数参数相当于（&huart1, &Res, 1）
HAL_StatusTypeDef UART_Start_Receive_IT(UART_HandleTypeDef *huart, uint8_t *pData, uint16_t Size)
{
  //函数参数相当于（&huart1, &Res, 1）
  huart->pRxBuffPtr = pData;//pData==&Res 
  //在UART_Receive_IT函数中 pdata8bits = (uint8_t *) huart->pRxBuffPtr;
  //pdata8bits=&Res，其他都是同理，如果不理解，需要回顾一下指针操作。
  huart->RxXferSize = Size;//Size==1
  huart->RxXferCount = Size;

  huart->ErrorCode = HAL_UART_ERROR_NONE;
  huart->RxState = HAL_UART_STATE_BUSY_RX;

  /* Process Unlocked */
  __HAL_UNLOCK(huart);

  /* Enable the UART Parity Error Interrupt */
  __HAL_UART_ENABLE_IT(huart, UART_IT_PE);

  /* Enable the UART Error Interrupt: (Frame error, noise error, overrun error) */
  __HAL_UART_ENABLE_IT(huart, UART_IT_ERR);

  /* Enable the UART Data Register not empty Interrupt */
  __HAL_UART_ENABLE_IT(huart, UART_IT_RXNE);

  return HAL_OK;
}

回调函数：

void HAL_UART_RxCpltCallback(UART_HandleTypeDef *huart)//自定义回调函数 在UART_Receive_IT()中调用
{
	//判断是哪个串口触发的中断  huart1.Instance = USART1;定义在MX_USART1_UART_Init中
	if(huart==&huart1)//huart ->Instance == USART1两种判断条件等价
	{
		if((UART1_RX_STA & 0x8000)==0)//接收未完成&位运算符 &&短路与判断
		{
			if(UART1_RX_STA & 0x4000)//接收到 \r
			{
				if(Res==0x0a)//下一个必须是接收 \n
					UART1_RX_STA|=0x8000;
				else
					UART1_RX_STA=0;
			}
			else //未接收到\r
			{
				if(Res==0x0d)//Receive \r
				{
					UART1_RX_STA|=0x4000;
				}
				else
				{
					UART1_RX_Buffer[UART1_RX_STA&0X3FFF]=Res;
					UART1_RX_STA++;
					if(UART1_RX_STA>UART1_REC_LEN-1) UART1_RX_STA=0;//如果接收数据大于200Byte 重新开始接收
				}
			}
		}
		HAL_UART_Receive_IT(&huart1, &Res, 1);//完成一次接受，再此开启中断
	}
}

        如果我们自己没有定义回调函数的话，系统会调用自带的回调函数，函数类型为__weak，表示弱定义

        如果用户自己定义了函数就优先调用用户定义的回调函数

__weak void HAL_UART_RxCpltCallback(UART_HandleTypeDef *huart)
{
  /* Prevent unused argument(s) compilation warning */
  UNUSED(huart);
  /* NOTE: This function should not be modified, when the callback is needed,
           the HAL_UART_RxCpltCallback could be implemented in the user file
   */
}

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