- 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();
}
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