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Co-routines
[More about co-routines...]

Limitations and Restrictions

The benefit of a co-routines lower RAM usage when compared to an equivalent task comes at the cost of some restrictions on how a co-routine can be used. Co-routines are more restrictive and complex to use than tasks.
  • Sharing a stack

    The stack of a co-routine is not maintained when a co-routine blocks. This means variables allocated on the stack will most probably loose their values. To overcome this a variable that must maintain its value across a blocking call must be declared as static. For example:

    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();
    }
     

    Another consequence of sharing a stack is that calls to API functions that could cause the co-routine to block can only be made from the co-routine function itself - not from within a function called by the co-routine. For example:

    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!
    }
     
  • Use of switch statements

    The default co-routine implementation included in the FreeRTOS download does not permit a blocking call to be made from within a switch statement. For example:

    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();
    }
    





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