[freertos - Open Discussion and Support] why freertos use mostly static variables?

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By: blackflowers

why does freertos define most of its members as statics, as for example, tasks,
semaphores etc.?

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By: edwards3

That is normal good software engineering practice. Global variables are not
a good thing, making them static ensures they are not global.

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By: blackflowers

But i have a problem because i need to use a semaphore in two different modules.

I have this in the [b]main.c[/b]:
[code]
static xSemaphoreHandle xSemaphore;
xSemaphore = xSemaphoreCreateMutex();
if(xSemaphoreTake( xSemaphore, ( portTickType ) 1)==pdTRUE)
{
//here is some code
xSemaphoreGive( xSemaphore );
}
[/code]

and i need to use the same semaphore in this module, the same way, in order
to protect a global variable shared by both modules. (i know it should't be
used): [b]port.c, on the timer2 interrupt function[/b]
[code]
void __attribute__((__interrupt__, auto_psv)) _T2Interrupt( void )
{
IFS0bits.T2IF = 0;

if(xSemaphoreTake( xSemaphore, ( portTickType ) 1)==pdTRUE)
{
//here is some code
xSemaphoreGive( xSemaphore );
}

PR2 = 0x1cca;
}
[/code]

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By: richardbarry

I'm not sure I understand your question. You asked why [b]FreeRTOS[/b] uses
static [b]members[/b], and received a reasonable reply. However, the code you
have posted is your own code, not FreeRTOS code. You can write your own code
however you want.

To use the same semaphore in two files you could make the semaphore declaration
static, the provide a 'get' function for the other file to receive a reference
to it - but that would seem overkill. It would be better just to make
it global.

A comment on the code you posted though. It is breaking two very fundamental
rules. First, it is calling a FreeRTOS API function that does not end in "FromISR"
from an ISR. Second, the API function has a block time specified. You cannot
block in an interrupt!

Regards.

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By: blackflowers

ok, i got it working using global variables. Now it's time to do it right, so
i'm reading about sofware timer, and another basic freertos stuff. So i'll keep
on with this thread in order to get my application running freertos correctly.

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By: blackflowers

by the way, where can i find documentation about those "fundamental rules"?
I mean, i couldn't find any information about what you have said, about ""calling

a FreeRTOS API function that does not end in "FromISR" from an ISR""

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By: blackflowers

besides, if i can't use a semaphore into an interrupt (because it has a block
time specified)... then how can i protect the global variable from being changed
in the main() and in the interrupt() simultaneously?

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By: blackflowers

i've just found out the documentation about apis and isr's here:
http://www.freertos.org/index.html?http://www.freertos.org/a00110.html#kernel_p
riority

and there's a "xSemaphoreGiveFromIsr" function indeed, but i couldn't find
"xSemaphoreTakeFromIsr" function anywhere.

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By: richard_damon

If you need to protect a variable used in both tasks and interrupts, you need
to use a critical section (which basically disables interrupts). Because of
how they work, they need to be kept short. The other option is to move the code
in the interrupt that needs to access the variable into a high priority task,
and have it waiting on a semaphore/queue from the interrupt.

As to where the rule on API usage in interrupts, it is at
http://www.freertos.org/api.html, in the paragraph marked "API Usage"

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By: richardbarry

[quote] http://www.freertos.org/api.html[/quote]

Gosh - I didn't realise that page was still live. It is a little out of date
as it refers to the FreeRTOS V5 API. Some maintenance work required on my part!

Regards.

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By: richardbarry

...if you look at the reference for the API function (macro) xSemaphoreTake()
http://www.freertos.org/a00122.html it does states: "This macro must not be
called from an ISR. xQueueReceiveFromISR() can be used to take a semaphore from
within an interrupt if required, although this would not be a normal operation
[because it only makes sense with a block time of zero, and even then it would
not be recommended]. Semaphores use queues as their underlying mechanism, so
functions are to some extent interoperable."

There are other references too, for example, item 3 here:
http://www.freertos.org/FAQHelp.html

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By: blackflowers

i finally did it using the task sincronized with the interrupt routine, by
a semaphore. I suppose this is the right way to do it, since with critical regions
i give priority to the main program instead of the interrupt routine, and this
is not what i needed.

Thanks!