"Unsigned / int mixup (?)"

**Hennet** · 10-26-2011

Hello.
In a struct:

Code:

<...>
unsigned upper : 4;
<...>

The above 'upper' field is populated by a int mySQL field whose value is a bit-field with 4 possible values (that stack, e.g. a row whose value is 15): 1|2|4|8. the value is introduced to the var as:

Code:

instance->upper = atoi(var-with-result-from-mysql);

I've faced this for the first time today, I didn't get why/how it works -- I thought it was broken and would leak/overflow.
Are you able to clarify this for me?
Thank you for your time.

**quzah** · 10-26-2011

I'm not sure what your question is. You should use unsigned int types for your bit fields. If you try to assign a bigger value, they will just be truncated. If you try to fit 16 in there, you should end up with 0 because unsigned overflow's behavior is defined in C.

Quzah.

**Hennet** · 10-26-2011

but later in the code, the following works:

Code:

if( instance->upper&8 ) { <print value of upper in console> }

and it prints 8 (in a entry whose upper value is 8) so yeah I don't get what could be going on
Thank you for your reply.

note: I'm compiling in GCC
note2: the code is not mine; I'm actually curious to know why it behaves like that.

**quzah** · 10-26-2011

I'm still not sure what your actual question is. But with a four bit unsigned value, the upper bit is 8, but the upper value is 15.

8 + 4 + 2 + 1 = 15

Quzah.

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