int a[17]; int b = -1; /* a[17] same as b guaranteed in ANSI C? */

[bq]

In the code
int a[17];
int b = -1;
does ANSI C guarantee that "b" is located in memory right after "a[16]"
so that "a[17]" refers to "b"?

Thanks.
bq

[ark]

"bq" <fo**********@y ahoo.com> wrote in message
news:6c******** *************** ***@posting.goo gle.com...

In the code
int a[17];
int b = -1;
does ANSI C guarantee that "b" is located in memory right after "a[16]"
so that "a[17]" refers to "b"?

Thanks.
bq

Since a and b belong (whatever th official terminology) to different
categories -- uninitialized and initialized data respectively, a compiler
may have sound reasons to put them in different, e.g., named segments, to
optimize the startup code, for instance.

However, if both are initialized (int a=1,b=-1;) or both are not (int a,
b;), both are arrays or both are not (and both have identical types and
qualifiers) it is rather hard to imagine reasons why a compiler would prefer
an allocation different from the order of occurrence. (For the heck of it,
it could be the opposite order, but then it would break some existing code,
would it not? :)

Now, the standard quoting in this thread... undefined behavior and such...
If a function receives an int *, it has no idea where it came from, so it
(function) must allow adding any int to it (pointer). (As is usual in C, it
is the programmer's job to care about overflows.) So this "undefinedn ess"
looks like a lip service.

I wonder if I am correct in this assessment and also if indeed for all
practical purposes (&a+1==&b) holds true for statements <qual><type>a ;
<qual><type>b ;?

Thanks
- Ark

[Keith Thompson]

"ark" <ar****@comcast .net> writes:

"bq" <fo**********@y ahoo.com> wrote in message
news:6c******** *************** ***@posting.goo gle.com...

In the code
int a[17];
int b = -1;
does ANSI C guarantee that "b" is located in memory right after "a[16]"
so that "a[17]" refers to "b"?

Thanks.
bq
Since a and b belong (whatever th official terminology) to different
categories -- uninitialized and initialized data respectively, a compiler
may have sound reasons to put them in different, e.g., named segments, to
optimize the startup code, for instance.

However, if both are initialized (int a=1,b=-1;) or both are not (int a,
b;), both are arrays or both are not (and both have identical types and
qualifiers) it is rather hard to imagine reasons why a compiler would prefer
an allocation different from the order of occurrence. (For the heck of it,
it could be the opposite order, but then it would break some existing code,
would it not? :)

Any existing code that assumes declared variables are allocated
consecutively is already badly broken.

I just tried the above code with gcc; the address of b was 4 bytes
"lower" than the address of a.

There are any number of reasons why a compiler might choose to
allocate declared variables in a seemingly arbitrary order. For
example, on some architectures it's cheaper to access variables in the
first N bytes of the current activation frame; in that case, the
compiler is likely to group smaller and/or frequently accessed
variables together.
Now, the standard quoting in this thread... undefined behavior and such...
If a function receives an int *, it has no idea where it came from, so it
(function) must allow adding any int to it (pointer). (As is usual in C, it
is the programmer's job to care about overflows.) So this "undefinedn ess"
looks like a lip service.
No, undefined behavior in this case is really undefined behavior. A
function receiving an int* had better be written so it won't try to
access memory outside the object it points to. If goes beyond the
bounds of the original object and steps on neighboring objects, that's
a bug. (And yes, it is the programmer's job to care about overflows;
if the programmer makes a mistake, the compiler isn't obligated to
clean up the resulting mess.)
I wonder if I am correct in this assessment and also if indeed for all
practical purposes (&a+1==&b) holds true for statements <qual><type>a ;
<qual><type>b ;?

Nope.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://www.sdsc.edu/~kst>
Schroedinger does Shakespeare: "To be *and* not to be"
(Note new e-mail address)

[Toni Uusitalo]

"Joona I Palaste" <pa*****@cc.hel sinki.fi> wrote in message
news:br******** **@oravannahka. helsinki.fi...

Toni Uusitalo <to************ **@luukkudot.ko m> scribbled the following:

"Default User" <fi********@boe ing.com.invalid > wrote in message
news:3F******** *******@boeing. com.invalid...

Toni Uusitalo wrote:
> oh, to ensure struct variables to be contiguous you must turn off struct > padding of course.

I just checked the standard, and I failed to see the section that talks
about turning off struct padding. Perhaps you could enlighten me?

Well I guess that isn't defined in the standard, sorry. It's only

slightly less
"wise guy trick" to use something like
#pragma packed
than what's proposed in the subject line ;-)
No real gain, only possible pain.

Those "wise guy tricks" are compiler-dependent and not supported by the
C standard.

Yes, you're right.
--
/-- Joona Palaste (pa*****@cc.hel sinki.fi) ------------- Finland --------\
\-- http://www.helsinki.fi/~palaste --------------------- rules! --------/
"You have moved your mouse, for these changes to take effect you must shut down and restart your computer. Do you want to restart your computer now?"
- Karri Kalpio

Poor guy, I've seen some windows boxes that are in really bad shape, but
this Karri guy's machine is something ;-) must be beta of upcoming Windows
Longboot BE (bluescreen edition)?

with respect,
Toni Uusitalo

[Joona I Palaste]

Toni Uusitalo <to************ **@luukkudot.ko m> scribbled the following:

"Joona I Palaste" <pa*****@cc.hel sinki.fi> wrote in message
news:br******** **@oravannahka. helsinki.fi...

/-- Joona Palaste (pa*****@cc.hel sinki.fi) ------------- Finland --------\
\-- http://www.helsinki.fi/~palaste --------------------- rules! --------/
"You have moved your mouse, for these changes to take effect you must shut down

and restart your computer. Do you want to restart your computer now?"
- Karri Kalpio

Poor guy, I've seen some windows boxes that are in really bad shape, but
this Karri guy's machine is something ;-) must be beta of upcoming Windows
Longboot BE (bluescreen edition)?

I happen to know Karri Kalpio in person (he was my team leader at my
former job, until I got fired), and he's a really adamant Linux
enthusiast and Windows hater. So it's no wonder he said that quote -
and even gave it to me on paper so I could stick it on my monitor at
work.

--
/-- Joona Palaste (pa*****@cc.hel sinki.fi) ------------- Finland --------\
\-- http://www.helsinki.fi/~palaste --------------------- rules! --------/
"A bee could, in effect, gather its junk. Llamas (no poor quadripeds) tune
and vow excitedly zooming."
- JIPsoft

[Christopher Benson-Manica]

Joona I Palaste <pa*****@cc.hel sinki.fi> spoke thus:

until I got fired)

*You* got fired? How'd you manage that?

(the point being that, if you, being much more competent than I, are
being fired, I may have some issues in my future...)

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

[Joona I Palaste]

Christopher Benson-Manica <at***@nospam.c yberspace.org> scribbled the following:

Joona I Palaste <pa*****@cc.hel sinki.fi> spoke thus:

until I got fired)

*You* got fired? How'd you manage that? (the point being that, if you, being much more competent than I, are
being fired, I may have some issues in my future...)

I got fired because the company had to subsidise (sp?) to avoid going
bankrupt, and I was chosen as "least productive programmer". Not "least
competent" - least *productive*. The code I was writing was fine, but it
could just as well have been written by any other programmer on our
team.

--
/-- Joona Palaste (pa*****@cc.hel sinki.fi) ------------- Finland --------\
\-- http://www.helsinki.fi/~palaste --------------------- rules! --------/
"Stronger, no. More seductive, cunning, crunchier the Dark Side is."
- Mika P. Nieminen

[Christopher Benson-Manica]

Joona I Palaste <pa*****@cc.hel sinki.fi> spoke thus:

I got fired because the company had to subsidise (sp?) to avoid going
bankrupt, and I was chosen as "least productive programmer". Not "least
competent" - least *productive*. The code I was writing was fine, but it
could just as well have been written by any other programmer on our
team.

So what you meant to say is that you got laid off, yes? That might be
a language thing, really. One might also say that you were
"downsized. " My condolences.

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

[Joona I Palaste]

Christopher Benson-Manica <at***@nospam.c yberspace.org> scribbled the following:

Joona I Palaste <pa*****@cc.hel sinki.fi> spoke thus:

I got fired because the company had to subsidise (sp?) to avoid going
bankrupt, and I was chosen as "least productive programmer". Not "least
competent" - least *productive*. The code I was writing was fine, but it
could just as well have been written by any other programmer on our
team.

So what you meant to say is that you got laid off, yes? That might be
a language thing, really. One might also say that you were
"downsized. " My condolences.

Yes, that's right. IANAL, so I use the term "get fired" for all
occurrences of quitting one's job by someone else's decision, not one's
own. Thanks for your condolences.

--
/-- Joona Palaste (pa*****@cc.hel sinki.fi) ------------- Finland --------\
\-- http://www.helsinki.fi/~palaste --------------------- rules! --------/
"B-but Angus! You're a dragon!"
- Mickey Mouse

[Grumble]

Christopher Benson-Manica wrote:

So what you meant to say is that you got laid off, yes?

Email would appear more appropriate for such a conversation.

[Kevin Goodsell]

Joona I Palaste wrote:

Christopher Benson-Manica <at***@nospam.c yberspace.org> scribbled the following:

Joona I Palaste <pa*****@cc.hel sinki.fi> spoke thus:

until I got fired)

*You* got fired? How'd you manage that?

(the point being that, if you, being much more competent than I, are
being fired, I may have some issues in my future...)

I got fired because the company had to subsidise (sp?) to avoid going
bankrupt, and I was chosen as "least productive programmer". Not "least
competent" - least *productive*. The code I was writing was fine, but it
could just as well have been written by any other programmer on our
team.

Man, that sucks. :(

I don't suppose they took into consideration anything other than amount
of code? So you might have been producing code that took a little longer
to write, but which saved a great deal of time in debugging and
maintenance, and was of overall better quality. But the managers
apparently aren't getting paid to look at the big picture, so you get
the boot.

I'm guessing they let go of one of their best programmers. Those kinds
of decisions are probably a major reason for their financial troubles.

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.