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Re: on understanding & and pointer arithmetic

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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.  Bart
effectively wrote (char*)[], which is not the same or perhaps not a
valid type at all.  That's my understanding now.

>> Oh, so that's at least part of what I didn't know.  I'm trying to use
>> this new information to explain the numbers, but I haven't made it
>> yet.
>
> Take care with Bart's replies.  He loves to make things sound more
> complicated than then really are, and in this case the types he wrote
> are wrong.  Also, pointer to array types are rarely used in C, so while
> it's worth knowing about them, must people learn a lot of C before
> coming across them.

Thanks for the advice.  I'm still going to study declarations.  I do
think they're pretty difficult, but I believe they don't appear too
often.  Brian Kernighan says the same in the book.

>> #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.

> #include <stdio.h>
> int main(void) {  // Empty ()s here have a special, archaic, meaning.
>   char a[] = "abc";
>   // Declare a to be an array of 4 characters initialised with
>   // 'a', 'b', 'c' and '\0'.
>   printf("a: %p\n", (void *)a);
>   // C has a format for pointers (%p), but you must convert the
>   // pointer to the expected type of void *.
>   printf("a: %p\n", (void *)(a + 3));
>   printf("a: %p\n", (void *)(&a + 3));
> }
>
>> %./hello.exe
>> a: 4294954044
>
> This is the address of a -- the address of the first element.  Pointers
> in C are addresses with type information.

That's good info!  I must keep this in mind --- addresses with type
information.  Thank you!

>> a: 4294954047
>
> This is the address of the null at the end of the array.  An array name,
> in most contexts, converts to a pointer to the first element, so a here
> is of type char *.  Pointer arithmetic is done by adding units of the
> size of the pointed-to type and, since sizeof(char) is 1, that adds
> three.
>
>> a: 4294954056
>
> An array as the operand of & is one of very few cases where the
> conversion to a pointer to the first element does not occur.  The result
> is a pointer to the whole four-byte array.  Adding 3 therefore adds 3
> times the size of that object, i.e. 12.

Thank you.  That's right --- I was expecting a conversion to a pointer
to the first element.  I'm much better prepared now.  Thanks a lot!

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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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