Operand of postfix operator ->

[Rajesh S R]

Consider the following code:
int main()
{
struct name
{
long a;
int b;
long c;
}s={3,4,5},*p;
p=&s;

printf("%d",*(i nt *)((char *)p+(unsigned int)&(((struct name*)0)-

>b)));

return 0;
}

Is the expression (unsigned int)&(((struct name*)0)->b) valid as per
C99?

That is,
Can an operand of postfix operator -be a null pointer?

Please clarify.

Thanks in advance for the reply.

[Richard Tobin]

In article <11************ **********@n35g 2000prd.googleg roups.com>,
Rajesh S R <SR**********@g mail.comwrote:

>Is the expression (unsigned int)&(((struct name*)0)->b) valid as per
C99?

No.

>That is,
Can an operand of postfix operator -be a null pointer?

-is not a postfix operator. It has two operands and appears between
them, so it's an infix operator.

-- Richard
--
"Considerat ion shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.

[Chris Dollin]

Rajesh S R wrote:

Consider the following code:
int main()
{
struct name
{
long a;
int b;
long c;
}s={3,4,5},*p;
p=&s;

Were we pairing, I'd not let that stay like that. Oh no.

printf("%d",*(i nt *)((char *)p+(unsigned int)&(((struct name*)0)-

>>b)));

At this point I'd ask you what on earth you were trying to do.
I wouldn't much care whether it was legal-by-the-Standard; I'd
care that it was indecently obscure.

return 0;
}

Is the expression (unsigned int)&(((struct name*)0)->b) valid as per
C99?

I don't think so.

That is,
Can an operand of postfix operator -be a null pointer?

Yes, it can be. The result is undefined. So don't do that.

(Your clarification asks a different question from your original.)

--
Nit-picking is best done among friends.

Hewlett-Packard Limited registered office: Cain Road, Bracknell,
registered no: 690597 England Berks RG12 1HN

[Richard Tobin]

In article <f0**********@m urdoch.hpl.hp.c om>,
Chris Dollin <ch**********@h p.comwrote:

>printf("%d",*( int *)((char *)p+(unsigned int)&(((struct name*)0)-

>>>b)));

>At this point I'd ask you what on earth you were trying to do.

Presumably he's trying to implement sizeof(), as apparently everyone
in India is doing these days. I assume he's on the same course as all
the others asking similar questions.

-- Richard
--
"Considerat ion shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.

[Stephen Sprunk]

"Rajesh S R" <SR**********@g mail.comwrote in message
news:11******** **************@ n35g2000prd.goo glegroups.com.. .

printf("%d",*(i nt *)((char *)p+(unsigned int)&(((struct name*)0)-

>>b)));

....

Is the expression (unsigned int)&(((struct name*)0)->b) valid as per
C99?

No. What you're seeing is one rather common implementation of offsetof(),
but it's officially UB for _you_ to use this construct unless you're writing
the implementation itself. Use offsetof() and you don't have to worry about
those systems where it doesn't actually work.

That is,
Can an operand of postfix operator -be a null pointer?

It's an infix operator, and the first operand must be a pointer to an
object. Since a null pointer constant does not point to an object by
definition, it's UB.

S

--
Stephen Sprunk "Those people who think they know everything
CCIE #3723 are a great annoyance to those of us who do."
K5SSS --Isaac Asimov
--
Posted via a free Usenet account from http://www.teranews.com

[Rajesh S R]

Thanks for ur reply.

-is not a postfix operator. It has two operands and appears between
them, so it's an infix operator.

But C standards categorises an -as postfix operator( C99 6.5.2 ) .
So what's wrong in stating it as a postfix operator?
Even C99 calls it that way!

[Richard Tobin]

In article <11************ **********@b40g 2000prd.googleg roups.com>,
Rajesh S R <SR**********@g mail.comwrote:

>-is not a postfix operator. It has two operands and appears between
them, so it's an infix operator.

>But C standards categorises an -as postfix operator( C99 6.5.2 ) .

You're right.

I can't imagine why they decided to misuse a well-defined term in that
way.

-- Richard
--
"Considerat ion shall be given to the need for as many as 32 characters
in some alphabets" - X3.4, 1963.

[Rajesh S R]

Thanks for your reply.

-is not a postfix operator. It has two operands and appears between
them, so it's an infix operator.

(C99 6.5.2) categorises -as postfix operator. So whats wrong in
calling it that way?

[Martin Ambuhl]

Rajesh S R wrote:

Consider the following code:
int main()
{
struct name
{
long a;
int b;
long c;
}s={3,4,5},*p;
p=&s;

printf("%d",*(i nt *)((char *)p+(unsigned int)&(((struct name*)0)-

>b)));

return 0;
}

Is the expression (unsigned int)&(((struct name*)0)->b) valid as per
C99?

There are otherwise enough errors in your code to make "valid as per
C99" a sick joke. And this is the most perverse way I have ever seen of
spelling the expression "p->b".

That is,
Can an operand of postfix operator -be a null pointer?

-is not a postfix operator. Strangely, your code claims it isn't a
token at all.

[Army1987]

"Rajesh S R" <SR**********@g mail.comha scritto nel messaggio
news:11******** **************@ n35g2000prd.goo glegroups.com.. .

Consider the following code:
int main()
{
struct name
{
long a;
int b;
long c;
}s={3,4,5},*p;
p=&s;

printf("%d",*(i nt *)((char *)p+(unsigned int)&(((struct name*)0)-

>>b)));

You've managed to beat my lecturer (and the book he copied from) in the
contest of the ugliest line of code ever:
http://groups.google.c om/group/comp.lang.c/msg/f0************* *************** ********@tdi.cu .mi.it

return 0;
}

Is the expression (unsigned int)&(((struct name*)0)->b) valid as per
C99?

What the hell are you trying to do?