Warn about bitfields larger than union

Warn about bitfields larger than union

[Jon Grant]

Hello

I've uploaded a simple C program which shows my query:

http://pastebin.com/BQmG0m3g

Is there a way for GCC to warn when union size is exceeded by a bit-field?


Essentially a union of a type, and a bit-field. The bit-field is larger 
than the first member of the union. This means that the struct is larger 
than the uint8_t expected, it compiles as 2 bytes large. Output below.


jon@laptop:~$ gcc -Wall -o union union.c
jon@laptop:~$ ./union
sizeof(test_t): 4
t: 0x0x92f0008
reg0: 0x0x92f0008
reg1: 0x0x92f000a
sizeof(t): 4


Thank you for any feedback

Please keep my email address in any replies.

Best regards, Jon

Re: Warn about bitfields larger than union

[Kevin P. Fleming]

On 10/30/2011 01:12 PM, Jon Grant wrote:
> Hello
>
> I've uploaded a simple C program which shows my query:
>
> http://pastebin.com/BQmG0m3g
>
> Is there a way for GCC to warn when union size is exceeded by a bit-field?

The 'union size' is determined from the largest member of the union.

-- 
Kevin P. Fleming
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Re: Warn about bitfields larger than union

[Jonathan Wakely]

On 31 October 2011 15:15, Kevin P. Fleming wrote:
> On 10/30/2011 01:12 PM, Jon Grant wrote:
>>
>> Hello
>>
>> I've uploaded a simple C program which shows my query:
>>
>> http://pastebin.com/BQmG0m3g
>>
>> Is there a way for GCC to warn when union size is exceeded by a bit-field?
>
> The 'union size' is determined from the largest member of the union.

Indeed, I don't see how the compiler could know that you didn't mean
to make the union larger than the first member.

If you want to put checks in for that, use a static assertions of some kind:

char static_assert[(sizeof(REG0) == sizeof(REG0.reg8bit))-1];

Re: Warn about bitfields larger than union

[Jonathan Wakely]

Putting the OP back in the To field ...

On 31 October 2011 16:09, Jonathan Wakely wrote:
> On 31 October 2011 15:15, Kevin P. Fleming wrote:
>> On 10/30/2011 01:12 PM, Jon Grant wrote:
>>>
>>> Hello
>>>
>>> I've uploaded a simple C program which shows my query:
>>>
>>> http://pastebin.com/BQmG0m3g
>>>
>>> Is there a way for GCC to warn when union size is exceeded by a bit-field?
>>
>> The 'union size' is determined from the largest member of the union.
>
> Indeed, I don't see how the compiler could know that you didn't mean
> to make the union larger than the first member.
>
> If you want to put checks in for that, use a static assertions of some kind:
>
> char static_assert[(sizeof(REG0) == sizeof(REG0.reg8bit))-1];
>