Look the subject,that's my problem!
I hope someone can help me, thanks
I hope someone can help me, thanks
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Re: What's the differences between REGISTER and AUTO?
JOYCE wrote:Putting the question in the subject line is a problem. You should stateLook the subject,that's my problem!
the full question in the body of your message; the subject line should,
at most, contain a summary of the question, not the question itself.
Before asking us, what have you figured out for yourself from readingI hope someone can help me, thanks
your textbook? If you can't understand your text book, you're not likely
to understand the responses given in this newsgroup any better. However,
if you explain what you're having difficulty understanding from your
textbook's explanation, we can help correct your understanding.
Final note: C is a case-sensitive language. Make sure, whenever you
write down C identifiers, that you always write them in the correct
case. The identifier "register" is a keyword with a very specific
meaning defined by the standard. On the other hand "REGISTER" is a
completely unrelated identifier, which can only be declared or defined
in user code.
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Re: What's the differences between REGISTER and AUTO?
JOYCE wrote, On 04/11/08 11:58:It is better to put the entire question in the body of the post.Look the subject,that's my problem!
What does your text book say? What is the difference between a car and aI hope someone can help me, thanks
road? Anyway, register is a hint to the compiler (which normally is not
needed and has not been needed for many years) which affects what you
can do with a variable, auto has not been needed as part of the language
for even longer since where it is legal the variable would have
automatic storage duration without using it.
Another hint is that many processors have registers but I'm not aware of
any that have autos.
--
Flash Gordon
If spamming me sent it to smap@spam.cause way.com
If emailing me use my reply-to address
See the comp.lang.c Wiki hosted by me at http://clc-wiki.net/
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Re: What's the differences between REGISTER and AUTO?
always leave the subject in the body of your message:
Subject: What's the differences between REGISTER and AUTO?
On 4 Nov, 11:58, JOYCE <zzzzzz90...@12 6.comwrote:C is case sensitive so you mean auto and register.Look the subject,that's my problem!
I hope someone can help me, thanks
They are both somewhat obselete and hardly ever used.
auto is used with variables to indicate they are not static
or external. That they are local to a function and are
(conceptually) created on entry and (conceptually) destroyed
on exit. A stack is often used to implement this.
int fred (void)
{
auto int i;
auto j
}
the above are both legal. j is implicitly int (at least in the 1989
standard for C).
Note this is completly unecessary. Just use
int i;
int j;
register indicates to the C compiler that the indicated variable
is heavily used (in the programmer's opinion) and should be placed
in a hardware register for faster code. You cannot take the address
of a register variable.
int bill (void)
{
register int i;
int *pi;
pi = &i; /* ILLEGAL *?
}
register is only a request to the compiler and the compiler is
permitted
to ignore it. For instance there may not actually be any spare
registers.
Programmers are often not good at correctly identifying the important
variables and modern compilers are good at it. Hence modern compilers
used ignore the "register" hint (except for disallowing taking the
address
of a register variable).
In <large-numyears of C programming I have never used either
register or auto. I've worked with code with register in it.
I've no idea if the register keyword did any good (or any harm).
I have only seen in auto in old books (I think K&R-one might use
it).
--
Nick Keighley
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Re: What's the differences between REGISTER and AUTO?
On 4 Nov, 12:51, Flash Gordon <s...@spam.caus eway.comwrote:[what is the difference between register and auto]JOYCE wrote, On 04/11/08 11:58:
auto was still present in the 89 standardregister is a hint to the compiler (which normally is not
needed and has not been needed for many years) which affects what you
can do with a variable, auto has not been needed as part of the language
for even longer since where it is legal the variable would have
automatic storage duration without using it.
--
Nick Keighley
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Re: What's the differences between REGISTER and AUTO?
Nick Keighley wrote:
Present but not needed.On 4 Nov, 12:51, Flash Gordon <s...@spam.caus eway.comwrote:>>JOYCE wrote, On 04/11/08 11:58:
[what is the difference between register and auto]
>>>register is a hint to the compiler (which normally is not
>needed and has not been needed for many years) which affects what you
>can do with a variable, auto has not been needed as part of the language
>for even longer since where it is legal the variable would have
>automatic storage duration without using it.
auto was still present in the 89 standard
--
'Don't be afraid: /Electra City/
there will be minimal destruction.' - Panic Room
Hewlett-Packard Limited registered no:
registered office: Cain Road, Bracknell, Berks RG12 1HN 690597 England
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Re: What's the differences between REGISTER and AUTO?
JOYCE wrote:What subject? You didn't ask a question in your message. The questionLook the subject,that's my problem!
you asked in your Subject line (which many people can't see when they
open your post) refers to REGISTER and AUTO, which are not defined by
ISO C; remember, C is case sensitive. I can see why you're having problems.
If you meant to ask what the difference is between "register" andI hope someone can help me, thanks
"auto", the only guaranteed difference is that you cannot take the
address of a variable of "register" storage class with the unary & operator.
In ancient compilers, there was a chance that "register" variables might
have been faster to use, but modern optimizing compilers will
automatically put variables in registers without needing a hint from the
programmer.
S
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Re: What's the differences between REGISTER and AUTO?
Nick Keighley <nick_keighley_ nospam@hotmail. comwrites:auto is still in C99 as well; it's in the language, but either illegalOn 4 Nov, 12:51, Flash Gordon <s...@spam.caus eway.comwrote:>>JOYCE wrote, On 04/11/08 11:58:
[what is the difference between register and auto]
>>>register is a hint to the compiler (which normally is not
>needed and has not been needed for many years) which affects what you
>can do with a variable, auto has not been needed as part of the language
>for even longer since where it is legal the variable would have
>automatic storage duration without using it.
auto was still present in the 89 standard
or redundant in all contexts. There's a proposal to drop it.
--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
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Re: What's the differences between REGISTER and AUTO?
Keith Thompson <kst-u@mib.orgwrites :
Was there ever a time when `auto' did something useful?Nick Keighley <nick_keighley_ nospam@hotmail. comwrites:>>On 4 Nov, 12:51, Flash Gordon <s...@spam.caus eway.comwrote:>>>>JOYCE wrote, On 04/11/08 11:58:
>[what is the difference between register and auto]
>>>>>>register is a hint to the compiler (which normally is not
>>needed and has not been needed for many years) which affects what you
>>can do with a variable, auto has not been needed as part of the language
>>for even longer since where it is legal the variable would have
>>automatic storage duration without using it.
>auto was still present in the 89 standard
auto is still in C99 as well; it's in the language, but either illegal
or redundant in all contexts. There's a proposal to drop it.
It seems to have been redundant even in K&R I. Was it just included for
consistency, so that every storage class had a corresponding keyword?
It could maybe be useful to emphasize that a particular variable was
auto, where the context might suggest something else, but I can't really
think of a good example, and in any case a comment would probably be
just as good.
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Re: What's the differences between REGISTER and AUTO?
Nate Eldredge <nate@vulcan.la nwrites:[...]Keith Thompson <kst-u@mib.orgwrites :Yes. Pre-ANSI C allowed implicit int for object declarations, so>>auto is still in C99 as well; it's in the language, but either illegal
>or redundant in all contexts. There's a proposal to drop it.
Was there ever a time when `auto' did something useful?
"auto i;" was equivalent to "auto int i;", but "i;" was illegal (if it
appeared before a declaration in the same block) or a reference to a
variable ``i''.
In K&R C, one might write:
foo(x, y)
{
auto i;
extern j;
/* ... */
}
I *think* that "auto i;" is illegal in C90.
[...]
--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
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Re: What's the differences between REGISTER and AUTO?
Nate Eldredge <nate@vulcan.la nwrites:
Well, it is more a case of not wanting to remove it, I imagine. autoKeith Thompson <kst-u@mib.orgwrites :
>>>Nick Keighley <nick_keighley_ nospam@hotmail. comwrites:>>>>On 4 Nov, 12:51, Flash Gordon <s...@spam.caus eway.comwrote:
>>>JOYCE wrote, On 04/11/08 11:58:
>>>
>>[what is the difference between register and auto]
>>>
>>>register is a hint to the compiler (which normally is not
>>>needed and has not been needed for many years) which affects what you
>>>can do with a variable, auto has not been needed as part of the language
>>>for even longer since where it is legal the variable would have
>>>automatic storage duration without using it.
>>>
>>auto was still present in the 89 standard
>auto is still in C99 as well; it's in the language, but either illegal
>or redundant in all contexts. There's a proposal to drop it.
Was there ever a time when `auto' did something useful?
>
It seems to have been redundant even in K&R I.
comes from B, which is typeless, so the storage class must always be
there. As C evolved, the shadow of B remained in the guise of
implicit int:
auto x;
and
register y;
were ints just like
f() {...}
was an int-returning function. I imagine that for a while (pre-K&R I)
a lot of very early C looked a lot like B, except where the type
really mattered and then, after a while, it just seemed churlish to
remove the keyword. That last bit is a guess of course!
--
Ben.
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Re: What's the differences between REGISTER and AUTO?
Keith Thompson <kst-u@mib.orgwrites :
I see.Nate Eldredge <nate@vulcan.la nwrites:[...]>Keith Thompson <kst-u@mib.orgwrites :>>>>>auto is still in C99 as well; it's in the language, but either illegal
>>or redundant in all contexts. There's a proposal to drop it.
>Was there ever a time when `auto' did something useful?
Yes. Pre-ANSI C allowed implicit int for object declarations, so
"auto i;" was equivalent to "auto int i;", but "i;" was illegal (if it
appeared before a declaration in the same block) or a reference to a
variable ``i''.
>
In K&R C, one might write:
>
foo(x, y)
{
auto i;
extern j;
/* ... */
}
In a sense, it's still redundant, because writing `int i;' would be
equivalent. But I can see how that wouldn't fit the idiom.
Comment
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Re: What's the differences between REGISTER and AUTO?
Keith Thompson wrote, On 04/11/08 16:58:C90 still had implicit int so I think that "auto i;" is legal in C90.Nate Eldredge <nate@vulcan.la nwrites:[...]>Keith Thompson <kst-u@mib.orgwrites :>>Was there ever a time when `auto' did something useful?>>auto is still in C99 as well; it's in the language, but either illegal
>>or redundant in all contexts. There's a proposal to drop it.
Yes. Pre-ANSI C allowed implicit int for object declarations, so
"auto i;" was equivalent to "auto int i;", but "i;" was illegal (if it
appeared before a declaration in the same block) or a reference to a
variable ``i''.
>
In K&R C, one might write:
>
foo(x, y)
{
auto i;
extern j;
/* ... */
}
>
I *think* that "auto i;" is illegal in C90.
For what it's worth, gcc agrees with me and accepts it without complaint
with "-ansi -pedantic", you have to enable additional warnings or
(partial) C99 to get a warning.
I still don't think auto was useful in K&R C since one could always have
used "int i;" instead of "auto i;" with the same effect. Perhaps pre-K&R
there was a situation where nothing other than auto would do?
--
Flash Gordon
If spamming me sent it to smap@spam.cause way.com
If emailing me use my reply-to address
See the comp.lang.c Wiki hosted by me at http://clc-wiki.net/
Comment
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Re: What's the differences between REGISTER and AUTO?
Nick Keighley wrote, On 04/11/08 13:08:
<snip>
Local to the block, which can be a smaller scope than the entire function.auto is used with variables to indicate they are not static
or external. That they are local to a function and are
Missing semi-colon ;-)(conceptually) created on entry and (conceptually) destroyed
on exit. A stack is often used to implement this.
>
int fred (void)
{
auto int i;
auto j
Illegal in C99 of course.}
>
the above are both legal. j is implicitly int (at least in the 1989
standard for C).
<snip good stuff>
--
Flash Gordon
If spamming me sent it to smap@spam.cause way.com
If emailing me use my reply-to address
See the comp.lang.c Wiki hosted by me at http://clc-wiki.net/
Comment
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Re: What's the differences between REGISTER and AUTO?
Flash Gordon wrote:Remember that it was a very gradual evolution from B to NB to C. WhenI still don't think auto was useful in K&R C since one could always have
used "int i;" instead of "auto i;" with the same effect. Perhaps pre-K&R
there was a situation where nothing other than auto would do?
language features were added (on a weekly basis, as K&R found something
new that would make coding easier), it was desirable that old code still
compile correctly so that they wouldn't have to constantly rewrite
existing programs that worked just fine; only new programs that needed
those new features (like types) had to use them.
B was typeless and so "auto i;" was the only way to declare a local
variable. For B programs to be valid C, it was necessary that C have
implicit int, meaning that "auto i;" was legal in C as well.
C99 started removing some of the old compatibility features because the
amount of code that still relied on it had become insignificant and had
become a source of bugs, programmer confusion, and unnecessary compiler
complexity. B is finally dead.
S
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