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 lose 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();
    }
    




Copyright (C) Amazon Web Services, Inc. or its affiliates. All rights reserved.

Latest News

FreeRTOS v10.2.1 is available for immediate download. MIT licensed, includes 64-bit RISC-V, NXP Cortex-M33 demo & Nuvoton Cortex-M23 demo & STM32H745 dual core (AMP) demo.

New MIT licensed IoT MQTT and IoT Task Pool libraries now available. Includes demo projects.

View a recording of the “OTA Update Security and Reliability” webinar, presented by TI and AWS.

Careers

FreeRTOS and other embedded software careers at AWS.

FreeRTOS Partners
ARM Connected RTOS partner for all ARM microcontroller cores
Espressif ESP32
IAR Partner
Microchip Premier RTOS Partner
Renesas
STMicro RTOS partner supporting ARM7, ARM Cortex-M3, ARM Cortex-M4 and ARM Cortex-M0
Texas Instruments MCU Developer Network RTOS partner for ARM and MSP430 microcontrollers
OpenRTOS and SafeRTOS
Xilinx Microblaze and Zynq partner