What I was trying to say, but in better form!

On 01/16/2013 08:11 PM, Byron Jeff wrote:
> On Wed, Jan 16, 2013 at 04:01:08PM -0800, Bob Blick wrote:
>> Is this a correct way to declare and call a function that uses a union
>> as the argument, and pass it a value that is not declared as union?
> No. They are different types, so it cannot be done.
>
> I always think of a union as a structure whose variables share the same
> memory. So the bottom line is that it's a structure. So you cannot impres=
s
> a simple type on top of it.
>
> The simplest way to resolve the problem is to convert the parameters to
> pointers. Even though pointers are type specific, they can be readily
> interchanged.
>
> Finally I would go ahead and typedef the union (and its pointer) so that
> you can specify the type by name.
>
>> //declaring the function
>> void myDumbFunction(union {unsigned int uint; signed int ssint; unsigned
>> char[2] unchar});
>>
>> // calling it from somewhere in my program
>>    unsigned int dumbvar;
>>    dumbvar =3D somedumbvalue;
>>    myDumbFunction(dumbvar);
>>
>> // the function
>> void myDumbFunction(somevar){
>>    if (somevar.unchar[1])
>>      do_something();
>> }
>>
>> Is that going to work, the compiler should know that somevar is an
>> unsigned int(not that it should matter since it's just memory space that
>> is the size of the union), so within my function I can access
>> somevar.uint somevar.ssint as well as somevar.unchar[0] and
>> somevar.unchar[1]
> Again the problem is that the parameter is typed, and the types do not
> match. I would not feel confortable trying to cast the simple types to th=
e
> union. Pointers are much more predictable in their behavior.
>
>> That all seem OK? I know that in most cases using a union serves no real
>> purpose other than to be dangerous, there are other ways to do things.
>> But my question is whether this is accurate or how it would be made
>> correct if it isn't.
>>
> Here's my crack at it given my explanation above:
>
> // Typedef the union and its pointer
> typedef union {unsigned int uint; signed int ssint; unsigned char[2] unch=
ar} my_u, *my_p;
>
> //declaring the function
> void myDumbFunction(void *somevar); // The void facilitates passing any t=
ype of pointer in.
>
> // calling it from somewhere in my program
>    unsigned int dumbvar;
>    dumbvar =3D somedumbvalue;
>    myDumbFunction(&dumbvar); // Pass in the unsigned int pointer to the u=
nion pointer of the function
>
> // the function
> void myDumbFunction(void *somevar){
>    my_p u1 =3D somevar; // Convert pointer into a union pointer
>
>    if (u1->unchar[1])
>      do_something();
> }
>
>> Thanks,
> Happy to help.
>
> BAJ
>
>> Bob
>>
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Joe Wronski
Stillwater Embedded Engineering
www.stillwatereng.net

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