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Re: Address of array && address of pointer to array

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Stanley Rice Wrote:

> 
> Err, I seems that I am just a novice and I don't fully understand
> the pointer. Why should a pointer have to an object that has a
> specific type, even,(void). Every code and variable and anything else
> just store in the memory. Does the pointer with the specific type to
> point to locate how much memory space to charge?

It might be true for the computer that you are sitting at that everything is
stored in a single big pool of memory, but C is defined to be flexible
enough that it also can deal with really weird systems.
It is, for example, possible to use C on a system with these
characteristics:
- code and data are stored in physically different areas of memory.
- code memory has 2^48 locations where functions can be located. (^ used in
the meaning of 'power of')
- data memory has 2^16 16-bit wide cells.
- the C compiler provides byte (octet) access through emulation.
On this system, you would have:
* CHAR_BITS == 8
* sizeof(char) == 1
* sizeof(short) == sizeof(int) == 2
* sizeof(long) == 4
* sizeof(void*) == sizeof(char*) == 4
* sizeof(int*) == 2
* sizeof(void (*)() /* function pointer */) == 6

As you can see, the size of the pointers themselves is not always the same
and depends on the type of data that the pointer points at.
In this case, void* and char* need to be larger to store the additional
information which portion of a memory-cell the pointer refers to.

> 
> For example, a pointer points to type int means that 4 bytes of
> memory is in the charge of the pointer, and a pointer points to type
> char means that 1 byte of memory is in the charge of the pointer. In
> this way, when we dereference a pointer that points to type int,
> we can get a 4 byte integer, and when we dereference a pointer
> to char, we can get one byte character. Is that right? And I wrote
> a testing program, the snippet is as below:
> 
> int a = 48;           // 48 represents character '0' in ASCII
> int *b = &a;
> char *c = (char *)b;
> printf("%c\n", c);      //2
> 
> The printf statement prints the character '0'. I guess the reason is
> that,according to the little-endian priciple, integer 48 stores in the
> side of least significant bit, but its memory address is lowest.

This may happen to be true for your computer, but it is not universally
true. It is equally well possible that the processor uses big-endian storage
and the integer 48 is stored in the higest byte of the memory used by the
variable a.
In that case, the printf would have resulted in the character '\0' (which is
typically invisible).

<snip> 

> So I guess the use of pointer pointed type is to determine how
> many byte could this pointer in charge from the least significant byte.
> But I don't know whether I'm right or not.

You are partially right. Another very important use of the pointed-to type
is to determine how to interpret those bytes and which operations you can
perform on them.

> 
> If my assumption is correct, then may be I can guess why casting
> a pointer to function to void* is undefined. The pointer points to
> the starting address of the function, without knowing the whole
> length of the function, so the pointer doesn't know how many
> bytes could the pointer in charge of, right?

No. The size of the function is not a concern, because a void* is also in
charge of an unspecified number of bytes.
The reason lies in the support for architectures with separate code and data
memories, where you can have more functions than data items, or where a
function pointer needs additional information (for example security
attributes) that are not needed for data pointers.

Bart v Ingen Schenau

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Thread

Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-14 06:12 -0700
  Re: Address of array && address of pointer to array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-14 14:41 +0100
    Re: Address of array && address of pointer to array Paul N <gw7rib@aol.com> - 2011-09-14 14:15 -0700
    Re: Address of array && address of pointer to array Peter Nilsson <airia@acay.com.au> - 2011-09-14 19:25 -0700
      Re: Address of array && address of pointer to array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-15 10:54 +0100
        Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-15 05:36 -0700
          Re: Address of array && address of pointer to array Ike Naar <ike@sverige.freeshell.org> - 2011-09-15 13:46 +0000
          Re: Address of array && address of pointer to array Nobody <nobody@nowhere.com> - 2011-09-15 14:48 +0100
            Re: Address of array && address of pointer to array Keith Thompson <kst-u@mib.org> - 2011-09-15 10:28 -0700
              Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-15 18:23 -0700
                Re: Address of array && address of pointer to array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-16 12:58 +0100
                Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-16 08:03 -0400
              Re: Address of array && address of pointer to array Nobody <nobody@nowhere.com> - 2011-09-16 13:09 +0100
                Re: Address of array && address of pointer to array Keith Thompson <kst-u@mib.org> - 2011-09-16 12:05 -0700
                Re: Address of array && address of pointer to array gazelle@shell.xmission.com (Kenny McCormack) - 2011-09-16 19:35 +0000
                Re: Address of array && address of pointer to array Peter Nilsson <airia@acay.com.au> - 2011-09-19 15:45 -0700
                Re: Address of array && address of pointer to array Keith Thompson <kst-u@mib.org> - 2011-09-19 17:47 -0700
          Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-15 10:21 -0400
    Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-15 05:22 -0700
      Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-15 10:23 -0400
        Re: Address of array && address of pointer to array pete <pfiland@mindspring.com> - 2011-09-15 15:26 -0400
          Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-15 18:14 -0700
            Re: Address of array && address of pointer to array Keith Thompson <kst-u@mib.org> - 2011-09-16 02:09 -0700
              Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-16 08:39 -0400
              Re: Address of array && address of pointer to array pete <pfiland@mindspring.com> - 2011-09-16 18:08 -0400
            Re: Address of array && address of pointer to array Bart van Ingen Schenau <bart@ingen.ddns.info> - 2011-09-16 16:38 +0200
              Re: Address of array && address of pointer to array cri@tiac.net (Richard Harter) - 2011-09-16 22:04 +0000
          Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-15 18:08 -0700
            Re: Address of array && address of pointer to array Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-16 13:12 +0100
            Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-16 08:30 -0400
      Re: Address of array && address of pointer to array Keith Thompson <kst-u@mib.org> - 2011-09-15 11:51 -0700
        Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-15 18:13 -0700
          Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-15 21:18 -0400
            Re: Address of array && address of pointer to array Stanley Rice <heconghui@gmail.com> - 2011-09-16 05:37 -0700
              Re: Address of array && address of pointer to array James Kuyper <jameskuyper@verizon.net> - 2011-09-16 09:46 -0400
              Re: Address of array && address of pointer to array Keith Thompson <kst-u@mib.org> - 2011-09-16 12:08 -0700
          Re: Address of array && address of pointer to array Nobody <nobody@nowhere.com> - 2011-09-16 13:20 +0100

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