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Last site update Oct 5 2014

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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