pointer subtraction

[junky_fellow]

Guys,

I want to find out an offset of a member inside a structure.
Although, there is a macro (offsetof) available to do this. Still, I
have one question. Can I subtract the address of the member from the
address of structure to get the offset ? If not, what are the reasons
for not doing this subtraction ?

Thanks for any help ...

 

[Justin Spahr-Summers]

Guys,

I want to find out an offset of a member inside a structure.
Although, there is a macro (offsetof) available to do this. Still, I
have one question. Can I subtract the address of the member from the
address of structure to get the offset ? If not, what are the reasons
for not doing this subtraction ?

Thanks for any help ...

You acknowledge the existence of offsetof() but then proceed to ask
for ways to re-implement the exact same behavior... why do you need to
do this?

 

[Chris Dollin]

I want to find out an offset of a member inside a structure.
Although, there is a macro (offsetof) available to do this. Still, I
have one question. Can I subtract the address of the member from the
address of structure to get the offset ?
Yes.

If not, what are the reasons
for not doing this subtraction ?

(a) It's only defined to work if you /have/ an instance of the structure.
(Otherwise, there's nothing to take the address of.)

(b) offsetof exists. Someone's done the work for me. Laziness rules!

 

[Army1987]

Guys,

I want to find out an offset of a member inside a structure.
Although, there is a macro (offsetof) available to do this. Still, I
have one question. Can I subtract the address of the member from the
address of structure to get the offset ?

You have to convert them both to unsigned char *.

If not, what are the reasons for not doing this subtraction ?

Because there's no need to reinvent the wheel.

 

[Barry Schwarz]

You have to convert them both to unsigned char *.

If the only action is to subtract one pointer from another, any of the
char types will work.

Because there's no need to reinvent the wheel.

Remove del for email

 

[David Thompson]

Other way, obviously: subtract base address from member address.

If the only action is to subtract one pointer from another, any of the
char types will work.
Right.

Right.

- formerly david.thompson1 || achar(64) || worldnet.att.net