协程
[更多关于协程的信息……]

局限和限制

• 共享堆栈

  当协程阻塞时，协程的堆栈无法维持。 这意味着在堆栈上分配的变量 很可能丢失其值。 要克服这一问题，必须将一个必须在阻塞调用中保持其值的变量声明为静态 。 例如：

  void vACoRoutineFunction( CoRoutineHandle_t xHandle,
                      UBaseType_t uxIndex )
  {
  static char c = 'a';
  
     // Co-routines must start with a call to crSTART().
     crSTART( xHandle );
  
     for( ;; )
     {
        // If we set c to equal 'b' here ...
        c = 'b';
  
        // ... then make a blocking call ...
        crDELAY( xHandle, 10 );
  
        // ... c will only be guaranteed to still 
        // equal 'b' here if it is declared static
        // (as it is here).
     }
  
     // Co-routines must end with a call to crEND().
     crEND();
  }
   

  共享堆栈的另一个结果是，能从协程函数本身调用可能导致协程阻塞的 API 函数， 而不能从协程调用的函数中调用。 例如：

  void vACoRoutineFunction( CoRoutineHandle_t xHandle, UBaseType_t uxIndex )
  {
     // Co-routines must start with a call to crSTART().
     crSTART( xHandle );
  
     for( ;; )
     {
        // It is fine to make a blocking call here,
        crDELAY( xHandle, 10 );
  
        // but a blocking call cannot be made from within
        // vACalledFunction().
        vACalledFunction();
     }
  
     // Co-routines must end with a call to crEND().
     crEND();
  }
  
  void vACalledFunction( void )
  {
     // Cannot make a blocking call here!
  }
   

• switch 语句的使用

  FreeRTOS 下载中的默认协程实现不允许在 switch 语句中进行阻塞调用 。 例如：

  void vACoRoutineFunction( CoRoutineHandle_t xHandle, UBaseType_t uxIndex )
  {
     // Co-routines must start with a call to crSTART().
     crSTART( xHandle );
  
     for( ;; )
     {
        // It is fine to make a blocking call here,
        crDELAY( xHandle, 10 );
  
        switch( aVariable )
        {
           case 1 : // Cannot make a blocking call here!
                  break;
           default: // Or here!
        }
     }
  
     // Co-routines must end with a call to crEND().
     crEND();
  }