segger embedded studio with gcc volatile problem

[Ian]

Hi,

I use SES import my keil project, and my keel project work well. With SES, i meed a problem, that is , i have a global variable like this:

uint32_t               g_ulMainLoopCounter = 0;

and i decrease it in timmer isr:

if  (g_ulMainLoopCounter > 0)
{
    g_ulMainLoopCounter--;
}

in my main function, i wait until 'g_ulMainLoopCounter' decrease to 0:

int main(void)
{   
    system_init();                                      //init timer etc...
   
    g_ulMainLoopCounter = 500;
    while (g_ulMainLoopCounter)
    {
    }

   ....
}

now the problem is , my code after 'while' will never be executioned, even 'g_ulMainLoopCounter' decrease to 0. This work well in keil.

Then, if i use 'volatile' qualifier for 'g_ulMainLoopCounter', it works, the code then like this:

volatile uint32_t g_ulMainLoopCounter = 0;

My optimization level set to none, means do not optimize my code.

I know use 'volatile' qualifier is a better way, but 'g_ulMainLoopCounter' is just only a example, there are lots of variables which used like "g_ulMainLoopCounter" (means multi-access variable, change the value in a function or isr and comparison in other function)in my program, must i check every variable in my program and determine if the variable is needed use 'volatile' qualifier? If so, i think that's too difficult.Is there any easy solutions?

[Aleksei Vegner]

I'm a little bit confused by your description. You are decreasing "g_ulUartTimeoutCounter" variable in the isr but the loop in main has "while (g_ulMainLoopCounter)". Is it right?

[Nguyen Hoan Hoang]

Did you mean it worked by adding volatile in front ? That is the correct behaviour. There is no bug with GCC but might be a bug in Keil if it is compiled with full optimization. The volatile key word tells the compiler to always load data from memory. This is for this type of multi-access to guarantee that the value is loaded before comparison. Without it, optimization will preload the value in register and use locally in the loop. Hence when the interrupt updates the value it will not reflect in the main loop. So you will loop endlessly.

[Ian]

Yes, "g_ulUartTimeoutCounter" is a global variable, in the isr, check if "g_ulUartTimeoutCounter" is not zero, decrease it, in main function, wait until "g_ulUartTimeoutCounter" decrease to zero.

[Ian]

Yes, i agree with you about this, and i understand what the effect of "volatile" is. But, in my program, there are lots of variables which used like "g_ulUartTimeoutCounter" , for example:

if ((!g_ulMainLoopCounter) && (!g_tMainLoopInterface.iap_state))
{
    //do something
}

When i use keil MDK, it works well，and when i use SES, the code in this "if" segment never be executioned, even "g_ulMainLoopCounter" used 'volatile' qualifier, because the member "iap_state" of struct "g_tMainLoopInterface" is not 'volatile', and i change the value of 'iap_state' in other function(not isr).

So, if i want use SES(or other IDE used gcc),must i check every variable in my program and determine if the variable is needed use 'volatile' qualifier(it's very difficult i think)? Is there any easy solutions?

[Nguyen Hoan Hoang]

If you only have one writer and many readers, volatile is sufficient. If you have multiple writers then atomic access is required. No matter which compiler you are using. You may not get the issue now with Keil does not mean you will not get it later with optimization turn on or with newer version because this is a C language thing not compiler thing. The behaviour of GCC complies with it. That is why you get the issue.