Re: different way of finding out offsetof a member in structure

On Mon, 5 Nov 2007 18:59:41 +1000, "MisterE" <MisterE@xxxxxxxxx> wrote
in comp.lang.c:


<abhimanyu.v@xxxxxxxxx> wrote in message
news:1194249397.600624.89570@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi Guys,

I have one doubt. The test program is given below. It uses two way of
finding out the offset of a variable in structure. I executed the
program and found the same result.

My question is what is difference between

1) (unsigned long) &((struct foobar *)0)->foo
and
2) (unsigned long)((char*)&tmp.boo - (char*)&tmp)

? I assume you know the difference. The 0 one is just assigning the pointer
value 0 (Address 0) and the compiler does the offset from the struct.

I have to assume that you don't know much about C. Assigning 0 to a
pointer creates a null pointer, which does not point to address 0, and
may not be all bits 0 in its representation.

The first one can load the value 0 to a reigster as a direct value. The 2nd
one cannot load its values directly because they are variable.
The second one also uses a subtraction operation.
The difference is that the first one is going to require less machine
instructions and will execute faster.

....the real difference is that the first one produces undefined
behavior and is completely non-portable.

--
Jack Klein
Home: http://JK-Technology.Com
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