Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #21997

Re: typedef struct

Show all headers | View raw

> On Saturday, June 9, 2012 2:31:00 AM UTC+8, James Kuyper wrote:
>> On 06/08/2012 01:20 PM, 张源 西北工业大学 wrote:
...
>> Being able to typedef incompletely is just a side effect of being able
>> to declare an incomplete struct type:
>>
>> struct incomplete_type;
>> struct another_incomplete_type *pointer_to_incomplete_type;
>>
>> The key point is that for some purposes there's no need to know the
>> actual type of the struct. This is feasible because the standard
>> requires that all pointers to structs have the same representation and
>> alignment requirements, so you don't need to tell the compiler the exact
>> type, so long as all you're doing is receiving pointers to the type, and
>> passing them on to other functions that do know what the exact type is.
> 
> thank you.i know a lot .
> what do the same representation and alignment requirements mean?

Every C object is stored in a series of bits; except for bit-fields,
those bits usually fill up entire bytes. Every C type has an
implementation-dependent mapping which connects bit patterns to values.
That mapping is called the representation used by that type. Unsigned
integer types have a simple binary representation. C signed integers
have one of three types of representation: 1's complement, 2's
complement, and sign-magnitude. They all store positive numbers the same
way as the corresponding unsigned type, they differ only in the way they
represent negative values. The C standard imposes no requirements on the
representation of floating point types, but does say that if
__STDC_IEC_559__ is pre-defined by the implementation, the
implementation of floating point types should conform to the
requirements of annex F, which details the ways in which they are
required to conform to the requirements of IEEE/IEC 60559.

Byte ordering (bigendian, littleendian, middleendian, etc.) is also part
of the representation. The standard imposes no requirements on byte
ordering. The C standard also says nothing about how pointers are
represented. That's entirely up to the implementation. However, it does
require that, in some cases, pointers to different types must be
represented the same way.

The alignment of a type is a requirement that objects of that type be
allocated only at particular types of addresses. For instance, if an
object may only be allocated at even addresses, it has an alignment
requirement of 2. If it can only be allocated at addresses which are
integer multiples of 4, it has an alignment requirement of 4.

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Re: typedef struct Kulin <remailer@reece.net.au> - 2012-05-31 19:10 +0000
  Re: typedef struct Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-05-31 15:14 -0400
    Re: typedef struct Nomen Nescio <nobody@dizum.com> - 2012-06-01 02:50 +0200
    Re: typedef struct 张源 西北工业大学 <zhabgyuan1993@gmail.com> - 2012-06-08 10:02 -0700
      Re: typedef struct Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-08 13:20 -0400
        Re: typedef struct 张源 西北工业大学 <zhabgyuan1993@gmail.com> - 2012-06-08 10:27 -0700
          Re: typedef struct Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-08 13:32 -0400
            Re: typedef struct 张源 西北工业大学 <zhabgyuan1993@gmail.com> - 2012-06-08 10:37 -0700
              Re: typedef struct Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-08 13:55 -0400
                Re: typedef struct Zhang Yuan <zhabgyuan1993@gmail.com> - 2012-06-08 11:22 -0700
                Re: typedef struct James Kuyper <jameskuyper@verizon.net> - 2012-06-08 14:35 -0400
                Re: typedef struct Zhang Yuan <zhabgyuan1993@gmail.com> - 2012-06-08 11:41 -0700
                Re: typedef struct Keith Thompson <kst-u@mib.org> - 2012-06-08 14:22 -0700
                Re: typedef struct Zhang Yuan <zhabgyuan1993@gmail.com> - 2012-06-08 18:28 -0700
                Re: typedef struct Zhang Yuan <zhabgyuan1993@gmail.com> - 2012-06-08 11:08 -0700
  Re: typedef struct Xavier Roche <xroche@free.fr.NOSPAM.invalid> - 2012-05-31 21:14 +0200
    Re: typedef struct 张源 西北工业大学 <zhabgyuan1993@gmail.com> - 2012-06-08 10:20 -0700
      Re: typedef struct James Kuyper <jameskuyper@verizon.net> - 2012-06-08 14:31 -0400
        Re: typedef struct Zhang Yuan <zhabgyuan1993@gmail.com> - 2012-06-08 11:45 -0700
          Re: typedef struct James Kuyper <jameskuyper@verizon.net> - 2012-06-08 15:10 -0400
            Re: typedef struct Zhang Yuan <zhabgyuan1993@gmail.com> - 2012-06-08 12:27 -0700
        Re: typedef struct Zhang Yuan <zhabgyuan1993@gmail.com> - 2012-06-08 11:48 -0700
  Re: typedef struct Keith Thompson <kst-u@mib.org> - 2012-05-31 14:24 -0700
    Re: typedef struct Nomen Nescio <nobody@dizum.com> - 2012-06-01 10:20 +0200
      Re: typedef struct Keith Thompson <kst-u@mib.org> - 2012-06-01 11:45 -0700
        Re: typedef struct Ian Collins <ian-news@hotmail.com> - 2012-06-02 07:52 +1200
        Re: typedef struct Ike Naar <ike@sverige.freeshell.org> - 2012-06-01 21:54 +0000
        Re: typedef struct "BartC" <bc@freeuk.com> - 2012-06-01 23:36 +0100
        Re: typedef struct Fritz Wuehler <fritz@spamexpire-201206.rodent.frell.theremailer.net> - 2012-06-02 23:31 +0200
          Re: typedef struct Keith Thompson <kst-u@mib.org> - 2012-06-02 18:30 -0700
        Re: typedef struct Robert Miles <milesrf@Usenet-News.net> - 2012-06-29 16:29 -0500
          Re: typedef struct Keith Thompson <kst-u@mib.org> - 2012-06-29 14:58 -0700
            Re: typedef struct Robert Miles <milesrf@Usenet-News.net> - 2012-07-01 22:55 -0500
              Re: typedef struct Keith Thompson <kst-u@mib.org> - 2012-07-01 23:00 -0700
                Re: typedef struct James Kuyper <jameskuyper@verizon.net> - 2012-07-02 07:12 -0400

csiph-web