C programming, stm32 uint64_t varibles not working

Asked Dec 17 '18 at 16:25

Active Dec 17 '18 at 16:25

Viewed 1,019 times

I am haveing troubles with the following. I have an array of uint8_t values.

I am trying to get the first 8 elements of the array into a uint64_t variable.

Therefore I used the following code to achieve that:

uint64_t dummy;
int i, x;
uint8_t pRxBuffer[32];

...

i = 0;
for(x = i; x < (i + 8); x++)
{
    dummy += pRxBuffer[x] << ((x - i) * 8); 
}

The problem now is that the variable dummy is treated like a 32 Bit variable. The result of dummy is only the first 32 Bits (so the first 3 elements of the array).

I am using a STM32F303 MCU where this runs on. It is a 32bit machine, but this should not be a problem in my opinion.

Any help is welcomed.

Benjamin

asked Dec 17 '18 at 16:25

Benjamin Graef

3

You need to cast `pRxBuffer[x]` to `uint64_t` as well. Otherwise it will get promoted to `int` and get shifted as one. – Eugene Sh. Dec 17 '18 at 16:26
1

It is not `dummy` that is treated as 32 bits. It is the value you add to it. – Gerhardh Dec 17 '18 at 16:28
answer, someone? or duplicate? – Jean-François Fabre Dec 17 '18 at 16:31
duplicate it is. – Jean-François Fabre Dec 17 '18 at 16:35
As others above have mentioned, the `pRxBuffer[x] << ((x - i) * 8)` subexpression will be evaluated as a 32-bit expression. In that case, when `x >= i + 4`, the shift amount will be greater than or equal to the width of the value being shifted, leading to *undefined behavior*. – Ian Abbott Dec 17 '18 at 16:36
It will be evaluated as *`int` (signed)* – Antti Haapala -- Слава Україні Dec 17 '18 at 16:46
Thanks to everyone, it is working now. But why is the the right side evaluated as a 32 bit expression and not as a 8 bit one, since pRxBuffer is a 8 bit array it should be treated like that. So why the 32 bit ? Is any calculation regardless of variable type treated as a 32 bit expression if no cast is used? I hope my question is clear – Benjamin Graef Dec 18 '18 at 19:55

C programming, stm32 uint64_t varibles not working

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