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Groups > comp.lang.c > #163885
| From | Ben Bacarisse <ben.usenet@bsb.me.uk> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: on understanding & and pointer arithmetic |
| Date | 2021-12-16 21:34 +0000 |
| Organization | A noiseless patient Spider |
| Message-ID | <87wnk4utcs.fsf@bsb.me.uk> (permalink) |
| References | <86a6h0eiy9.fsf@levado.to> <spfi5g$6ko$1@dont-email.me> <865yrod2cx.fsf@levado.to> <87h7b8wind.fsf@bsb.me.uk> <86ee6c9w1a.fsf@levado.to> |
Meredith Montgomery <mmontgomery@levado.to> writes:
> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>
>> Meredith Montgomery <mmontgomery@levado.to> writes:
>>
>>> Bart <bc@freeuk.com> writes:
>>>> On 16/12/2021 14:13, Meredith Montgomery wrote:
>>
>>>>> I also
>>>>> know that /&a/ is the same as /a/.
>>>>
>>>> The same /what/? Those have the same values but different types
>>>> ((T*)[] vs T* where T is a's element type).
>>
>> In this specific case, char (*)[4] and char *. (T*)[] is not a valid
>> type (for some base type T), and even with the syntax corrected (it's
>> T(*)[]), it's still wrong because the size is missing. C does have
>> array types with no size, but none are relevant to your case.
>
> Thanks for the correction. I'll repeat what you say in my words just to
> get a chance that you might verify what I'm saying. When I have an
> array such as
>
> char a[] = "hello, world"
>
> which occupies 13 bytes, then a pointer to it is a pointer of type
>
> char (*a)[13]
>
> and that's how we write its type --- although I could omit the letter a
> there, perhaps I should, I also think it's okay to write it there.
Not quite. When you write a type (for example, to use in a cast
operator) you must omit the name. If you include a name, then what you
are writing a declaration, not a type.
So, the object `a` has type char [13]. When used in a most contexts,
`a` is converted to an expression of type char *, but when you apply the
& operator, this conversion is not done and the pointer you get is of
type char (*)[13].
> Bart
> effectively wrote (char*)[], which is not the same or perhaps not a
> valid type at all. That's my understanding now.
Yes, that was just (I assume) a typo. Bart knows how to write pointer-
to-array types because his software uses them more than most C code
does. But the empty [] are wrong here. Your [13] is correct.
<cut>
>>> #include <stdio.h>
>>> int main() {
>>> char a[] = "abc";
>>> printf("a: %lu\n", a);
>>> printf("a: %lu\n", a + 3);
>>> printf("a: %lu\n", &a + 3);
>>> }
>>
>> With any luck, you compiler should have had lots to say about this
>> code. If it did not, turn up the warnings. I'll write it my way with
>> comments.
>
> Yes, the warnings were there. In my new experiment I added casts, but
> kept the %lu because I wanted to see things in base 10.
OK, so you converted the pointers to unsigned long. That will often
work, but "modern" C (since 1999), has an integer type that, if the
conversion is possible at all, is guaranteed to be the right width:
uintptr_t. It, along with a whole lot of other useful types, is
declared in <stdint.h>.
You might ask, how does one print such a type? Well there is another
header, <inttypes.h>, that defines macros with the right width:
printf("a is at %"PRIuPTR"\n", (uintptr_t)a);
A bit of a mouthful, but the result will work on any system where
pointers can be converted to some unsigned integer type. (There is also
a signed integer type intptr_t and a corresponding macro, PRIiPTR, for
printing it.)
--
Ben.
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on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-16 11:13 -0300
Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-16 14:23 +0000
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-16 11:57 -0300
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 17:42 +0000
Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-16 18:14 +0000
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 21:14 +0000
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-16 16:42 -0300
Re: on understanding & and pointer arithmetic Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-16 12:44 -0800
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:06 -0300
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 21:34 +0000
Re: on understanding & and pointer arithmetic Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-16 14:14 -0800
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 23:44 +0000
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:09 -0300
Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-16 16:23 +0100
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-16 16:25 -0300
Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-16 21:08 +0100
Re: on understanding & and pointer arithmetic Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-16 12:54 -0800
Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-17 10:53 +0100
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 11:39 -0300
Re: on understanding & and pointer arithmetic scott@slp53.sl.home (Scott Lurndal) - 2021-12-18 16:31 +0000
Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-18 18:56 +0100
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-25 21:29 -0300
Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-18 18:29 +0000
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 11:32 -0300
Re: on understanding & and pointer arithmetic Manfred <noname@add.invalid> - 2021-12-16 21:34 +0100
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 22:20 +0000
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:02 -0300
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:34 -0300
Re: on understanding & and pointer arithmetic Manfred <noname@add.invalid> - 2021-12-18 18:36 +0100
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-18 21:17 +0000
Re: on understanding & and pointer arithmetic James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-12-16 18:59 -0500
Re: on understanding & and pointer arithmetic James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-12-16 18:38 -0500
Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-16 23:50 +0000
Re: on understanding & and pointer arithmetic "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2021-12-17 07:34 -0800
Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-17 17:48 +0000
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 23:56 +0000
Re: on understanding & and pointer arithmetic James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-12-16 19:06 -0500
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-17 00:10 +0000
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:32 -0300
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:30 -0300
Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-17 11:08 +0100
Re: on understanding & and pointer arithmetic Manfred <noname@add.invalid> - 2021-12-17 16:24 +0100
Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-17 18:17 +0100
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:11 -0300
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