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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Why C has Pointers. |
| Date | 2011-12-17 13:55 -0800 |
| Organization | None to speak of |
| Message-ID | <lnpqfm50db.fsf@nuthaus.mib.org> (permalink) |
| References | <curixinfotech.54dl3z@no-mx.forums.yourdomain.com.au> <10353483.597.1324140693693.JavaMail.geo-discussion-forums@prez5> <golpe7lj73rnopq19ndti2hq1k02v02cat@4ax.com> <vqrpe7pkb15dek159fddlo2kfepn9dtcvl@4ax.com> |
ralph <nt_consulting64@yahoo.net> writes:
[...]
> I have trouble understanding the constant confusion between arrays and
> pointers. Way back when (and we are talking many moons ago <g>) when I
> started out in C my mentor explained it very simply in these terms in
> the first couple of days.
>
> 1) There is a difference between declarations and definitions.
> Remember which you are dealing with and when.
> 2) A pointer is a "variable", an array is a "constant"
That doesn't really make sense. An array variable is not a constant:
int arr[10];
arr[5] = 42;
I suppose what it means is that an array name, when it appears in an
expression (in most but not all context) decays to a pointer to the
array's first element, and that pointer value cannot be modified. But I
think that saying an array is a constant just reinforces the confusion
between arrays and pointers.
> 3) The array name evaluates to the address of the first element in the
> array.
This applies to any expression of array type, not just the name of a
declared array object. But it doesn't apply in all contexts.
Specifically, an expression of array type is converted to a pointer to
the array object's first element *unless* it's:
The operand of a unary "sizeof" operator (sizeof arr gives you the
same of the array, not of a pointer);
The operand of a unary "&" operator (&arr gives you the address of
the array object; it's the same memory location as the address of
its first element, but it's of a different type);
A string literal in an initializer used to initialize an array
object. In
char arr[6] = "hello";
the *value* of "hello" is copied into arr.
> 4) A pointer is a variable that holds the address of another variable
> or an array.
The distinction between variables an arrays is unnecessary. Variables
can be of any object type: integer, floating-point, struct, union,
array, pointer. There's nothing special about arrays in that particular
context.
An array object is an object whose value consists of the values of its
elements, just as a struct object is an object whose value consists of
the values of its members.
> 5) All pointers must be intialized before you use them.
The same applies to any object.
> 6) Since an array name is an address it can be used as a pointer that
> is initialized to point to the first element in the array.
In most contexts; see above. It would be clearer to say that an array
name *decays to* an address (or pointer value). And I wouldn't use the
word "initialized".
> 7) *variable means the contents of the variable.
No, it means the contents of the object to which "variable" points (and
is valid only if "variable" is of pointer type).
> 8) &variable means the address of the variable.
> 9) The array operator evaluates to a storage map equation, eg.
> type ar[i] := *(a + (i * sizeof(type))
I'm not sure what syntax you're using. What does "type" mean here? And
C pointer arithmetic is already scaled by the size of the pointed-to
type.
arr[i] is by definition equivalent to *(arr + i).
> 10) In all "pointer" operations you are either chewing on an address
> or the contents (value) at that address. Simple deconstrution of the
> even the most complex expression will quickly reveal which it is.
>
> I've since learned how to restate the above in fancier terminology,
> but those simple rules have stuck and served me well from my second
> week in C to today - thirty plus years later.
Section 6 of the comp.lang.c FAQ, <http://c-faq.com>, has an excellent
explanation of the relationship between arrays and pointers.
A digression: The rule that an array expression decays into a pointer to
the array object's first element leads to an interesting little hole in
the language. It assumes that there is an array *object* to which you
can point. This is usually the case, but not always. For example:
#include <stdio.h>
struct foo {
int arr[3];
};
struct foo func(void) {
struct foo result = { { 10, 20, 30 } };
return result;
}
int main(void) {
int *ptr = func().arr;
printf("*ptr = %d\n", *ptr);
return 0;
}
func() returns a *value* of type struct foo; it doesn't refer to any
object ("result" ceases to exist when the function returns). So we have
an array value, a member of the struct value returned by func(), that is
not the value of any array object.
C201X addresses this.
--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Will write code for food.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Why C has Pointers. curixinfotech <curixinfotech.54dl3z@no-mx.forums.yourdomain.com.au> - 2011-12-07 07:22 -0500
Re: Why C has Pointers. Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-12-07 04:35 -0800
Re: Why C has Pointers. Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-07 07:58 -0500
Re: Why C has Pointers. Mike Manilone <zhtx10@gmail.com> - 2011-12-07 13:15 +0000
Re: Why C has Pointers. Bill Reid <hormelfree@gmail.com> - 2011-12-08 06:29 -0800
Re: Why C has Pointers. Vincenzo Mercuri <vincenzo.mercuri@hotmail.it> - 2011-12-07 14:50 +0100
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-07 11:01 -0800
Re: Why C has Pointers. Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2011-12-07 20:49 +0100
Re: Why C has Pointers. Brian <nospam@nospam.com> - 2011-12-07 20:00 +0000
Re: Why C has Pointers. James Kuyper <jameskuyper@verizon.net> - 2011-12-07 15:30 -0500
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-07 12:43 -0800
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-07 13:09 -0800
Re: Why C has Pointers. Vincenzo Mercuri <vincenzo.mercuri@hotmail.it> - 2011-12-08 19:39 +0100
Re: Why C has Pointers. James Kuyper <jameskuyper@verizon.net> - 2011-12-08 14:16 -0500
Re: Why C has Pointers. Vincenzo Mercuri <vincenzo.mercuri@hotmail.it> - 2011-12-08 20:23 +0100
Re: Why C has Pointers. Kaz Kylheku <kaz@kylheku.com> - 2011-12-07 20:48 +0000
Re: Why C has Pointers. Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2011-12-07 22:07 +0100
Re: Why C has Pointers. Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-12-10 07:34 +0000
Re: Why C has Pointers. August Karlstrom <fusionfile@gmail.com> - 2011-12-07 16:16 +0100
Re: Why C has Pointers. Borax Man <rotflol2@hotmail.com> - 2011-12-08 18:32 +1100
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-08 00:20 -0800
Re: Why C has Pointers. André Gillibert <MetaEntropy.removeThis@gmail.com> - 2011-12-08 13:44 +0100
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-09 05:04 -0800
Re: Why C has Pointers. gazelle@shell.xmission.com (Kenny McCormack) - 2011-12-09 14:59 +0000
Re: Why C has Pointers. Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-12-09 20:19 -0800
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-10 07:36 -0800
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-08 00:12 -0800
Re: Why C has Pointers. Cong Wang <xiyou.wangcong@gmail.com> - 2011-12-15 07:26 +0000
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 08:51 -0800
Re: Why C has Pointers. Barry Schwarz <schwarzb@dqel.com> - 2011-12-17 10:10 -0800
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-17 14:28 -0600
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-17 15:00 -0600
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-17 13:55 -0800
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-17 18:27 -0600
Re: Why C has Pointers. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-18 01:26 +0000
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-17 22:34 -0600
Re: Why C has Pointers. "Philip Lantz" <prl@canterey.us> - 2011-12-17 23:48 -0800
Re: Why C has Pointers. superpollo <superpollo@tznvy.pbz> - 2011-12-18 11:06 +0100
Re: Why C has Pointers. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-18 13:44 +0000
Re: Why C has Pointers. pete <pfiland@mindspring.com> - 2011-12-18 11:05 -0500
Re: Why C has Pointers. James Kuyper <jameskuyper@verizon.net> - 2011-12-18 11:50 -0500
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-18 13:10 -0800
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-27 16:03 -0600
Re: Why C has Pointers. Barry Schwarz <schwarzb@dqel.com> - 2011-12-27 14:40 -0800
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-27 15:08 -0800
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-27 20:09 -0600
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-27 23:00 -0800
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-28 02:53 -0800
Re: Why C has Pointers. James Kuyper <jameskuyper@verizon.net> - 2011-12-28 08:39 -0500
Re: Why C has Pointers. James Kuyper <jameskuyper@verizon.net> - 2011-12-27 18:14 -0500
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-27 15:36 -0800
Re: Why C has Pointers. James Kuyper <jameskuyper@verizon.net> - 2011-12-27 18:53 -0500
Re: Why C has Pointers. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-28 13:20 +0000
Re: Why C has Pointers. John Bode <jfbode1029@gmail.com> - 2011-12-28 08:42 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 20:36 -0800
Re: Why C has Pointers. Seebs <usenet-nospam@seebs.net> - 2011-12-18 04:56 +0000
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 21:37 -0800
Re: Why C has Pointers. Seebs <usenet-nospam@seebs.net> - 2011-12-18 05:42 +0000
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-18 08:08 -0800
Re: Why C has Pointers. Robert Wessel <robertwessel2@yahoo.com> - 2011-12-18 10:36 -0600
Re: Why C has Pointers. Seebs <usenet-nospam@seebs.net> - 2011-12-18 18:50 +0000
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-19 02:39 -0800
Re: Why C has Pointers. gordonb.1i27q@burditt.org (Gordon Burditt) - 2011-12-21 22:34 -0600
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-19 07:56 -0800
Re: Why C has Pointers. John Tsiombikas <nuclear@member.fsf.org> - 2011-12-19 18:38 +0000
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-19 13:47 -0800
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-28 02:58 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-28 03:21 -0800
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-18 13:07 -0800
Re: Why C has Pointers. "io_x" <a@b.c.invalid> - 2011-12-19 04:02 +0100
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 15:35 -0800
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-17 18:41 -0600
Re: Why C has Pointers. John Bode <jfbode1029@gmail.com> - 2011-12-19 05:14 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-18 09:50 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-09 15:48 -0800
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-01-10 00:21 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-10 01:03 -0800
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-17 11:45 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 15:18 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-19 14:10 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-20 10:46 -0800
Re: Why C has Pointers. "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2011-12-17 19:15 +0100
Re: Why C has Pointers. "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2011-12-17 19:19 +0100
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-19 02:48 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-18 11:05 -0800
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-19 02:48 -0800
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