How early can I use a mutex?

Kernel
1

vespaman2 wrote on Friday, October 02, 2009:

Are there dependencies for mutexes ? I’d like to use a mutex very early on in boot process (main), prior to starting threads. Is this allowed? (the mutexes are created, but vTaskStartScheduler() has not been called)

((I know that the concept of mutexes are not needed at this point, but it makes my code nicer, if I can use existing functions this early in the startup))

Thanks
Micael

2

davedoors wrote on Friday, October 02, 2009:

You can call functions that do not attempt to block or cause a context switch.

3

vespaman2 wrote on Friday, October 02, 2009:

Excellent

Thank you for this quick response!!

- Micael

4

destremps wrote on Monday, February 22, 2010:

So just to be clear - it is safe to call ‘xSemaphoreTake’ and ‘xSemaphoreGive’ -after- the Mutex has been created but -before- the scheduler has been started? I find myself in the same position as the original poster.

Suggestion: perhaps a list of which OS API functions are safe to use before the scheduler has been started would be handy/interesting. Probably not many of them are! I have seen in writing that taskENTER_CRITICAL() and EXIT are safe to use. I wonder which others?

5

edwards3 wrote on Monday, February 22, 2010:

I think the semaphore give and take functions will be ok to use before the scheduler is started provided you do not try and block while giving or taking. Keep the block time at zero.

6

destremps wrote on Wednesday, February 24, 2010:

Sorry I’ve been sidetracked for a few days.

Thanks edwards for your input.

Richard,  May I ask that you please definitively state if it is okay to call the xSemaphore Take and Give functions after creating the semaphore but prior to starting the scheduler? I’ve looked in my copy of both your books hoping to see my answer but couldn’t find it.

Thank so much.
-rmd

7

rtel wrote on Wednesday, February 24, 2010:

vSemaphoreCreateBinary() is implemented as:

{                                                                    \
    xSemaphore = xQueueCreate( ( unsigned portBASE_TYPE ) 1, 0 );    \
    if( xSemaphore != NULL )                                         \
    {                                                                \
        xSemaphoreGive( xSemaphore );                                \
    }                                                                \
}

so that would seem to provide the answer as far as xSemaphoreGive() is concerned (yes it is ok to call it).  I think the same will be true for taking a semaphore too, provided a block time is never specified (as was previously mentioned).

Regards.

8

destremps wrote on Wednesday, February 24, 2010:

Okay. Thanks guys. I get it now.