RE: RE: [Dev-C++] [Dev-C++]: Errors in structures

[Ioannis V. <no...@ya...>]

Now if i understood correctly this is a question for me. Yes a union
occupies the space of its biggest member, that's why the minimum size of
a struct is a multiple of the sizeof() this union. Of course this was a
simplified implementation detail and details in memory management differ
in every implementation. That was about my last posting concerning the
sizeof a structure.


Ioannis

* Ioannis Vranos
* Programming pages: http://www.noicys.f2s.com
* Alternative URL: http://run.to/noicys


-----Original Message-----
From: dev...@li...
[mailto:dev...@li...] On Behalf Of Johannes
Pretorius
Sent: Wednesday, July 04, 2001 7:08 AM
To: dev...@li...
Cc: dev...@li...
Subject: Fwd: RE: [Dev-C++] [Dev-C++]: Errors in structures



>Date: Tue, 03 Jul 2001 14:46:07 +0200
>To: dev...@li...
>From: Johannes Pretorius <joh...@pr...>
>Subject: RE: [Dev-C++] [Dev-C++]: Errors in structures
>Cc: dev...@li...
>Bcc: dev...@li...
>
>Please for give if I am incorrect but does a union not allocate the=20
>memory
>space
>of the biggest type inside it. for instace in your example the biggest=20
>type is the Align (I think) so the whole
>union will be that size. Note that a union enables varaibles to share a

>memory block. and the last one used has writen into that
>memory block .. IF I am correct , changes are I am understanding this
wrong.
>
>If so the problem still persist that the sizeof() property that is=20
>suppose
>to give
>you a result of the memory size that is take up for the variable is not

>correct, AND if this
>value is used in READING a file for instance.. This will result that if
a=20
>file was writen in a diffent program
>by a different compiler. You will not read the exact amount of bytes
into=20
>your structure to corrosponde to
>your structure. Meaning you dont have the correct values in.
>
>Example :
>A file header writen in MSVC looks as follows in the start of the file
>(note comments in [] are not in file)
>[START OF FILE]34^^^^^^^^[8 BYTES]Hallo world^^^^^[16 BYTES]1[1
BYTES]0[1=20
>BYTES]
>
>The structure for this file looks something like (not correct maybe but
>this is just to give the Idea out)
>
>struct header{
>     int version;
>     char comment[16];
>     char code;
>     char sub_code;
>}oHeader;
>
>Now if this is writen to the File we do it as
>
>fwrite(&oHeader,sizeof(oHeader),1,outfile);
>
>Meaning the size that we have occrding to our compiler.,.,
>
>NOW -- we go and write a App to read this in DevCpp and we have the=20
>same
>structure BUT
>DevCpp (or the gcc or mingW) has a different value of the sizeof() so
it=20
>will end up as follows
>
>we read the file in
>
>fread(&oHeader,sizeof(oHeader),1,infile);
>
>and we expect the result to look the same. BUT the sizeof this program
>returned 24 forinstance and
>
>This will result in the past that the LAST byte wont be read..or wont=20
>be
>read into the correct value
>considering that this is the same (almost ) as doing a memcpy from the=20
>file to the structure memory alocation..
>
>Please help me or correct me if I cam wrong..
>
>I hope I am wrong and misunderstand this..
>I do believe the packed solution may help us here.,
>
>
>Thanks for All your help
>
>Johannes
>
>
>
>
>
>At 14:29 03/07/2001 +0200, you wrote:
>>This is not an error. A structure's space in memory is not the summary

>>of the sizeof() of everyone of its members. It may occupy more memory=20
>>due to compiler's memory management technique. For more information=20
>>read section 8.7 in K&R 2. For example a compiler's memory management=20
>>can be a connected list of structs (compilers, operating systems=20
>>e.t.c. are written in C/C++ too). So in the example of K&R it has a=20
>>list of unions connected structs:
>>
>>typedef long Align;
>>
>>union header
>>{
>>   struct
>>   {
>>     union header *ptr; /* points to next block */
>>     unsigned size;
>>   }s;
>>
>>   Align x;
>>};
>>
>>typedef union header Header;
>>
>>
>>Ioannis
>>
>>* Ioannis Vranos
>>* Programming pages: http://www.noicys.f2s.com
>>* Alternative URL: http://run.to/noicys
>>
>>
>>-----Original Message-----
>>From: dev...@li...
>>[mailto:dev...@li...] On Behalf Of Jeethu

>>Rao
>>Sent: Tuesday, July 03, 2001 5:52 AM
>>To: Dev CPP Mailing List
>>Subject: [Dev-C++] [Dev-C++]: Errors in structures
>>
>>
>>Hi,
>>
>>I've noticed this strange little Bug.I'm not certain weather its a bug

>>or a feature.
>>
>>The size of this structure is actually 13 bytes.
>>
>>But sizeof(header) says its 16.
>>
>>struct tea_hdr
>>  {
>>  char jte[3];
>>  unsigned char version;
>>  DWORD infile_size;
>>  DWORD infile_crc;
>>  unsigned char fname_sz;
>>  }header;
>>
>>No, if I comment out the fname_sz member,
>>
>>struct tea_hdr
>>  {
>>  char jte[3];
>>  unsigned char version;
>>  DWORD infile_size;
>>  DWORD infile_crc;
>>  //unsigned char fname_sz;
>>  }header;
>>
>>sizeof(header) returns 12.
>>
>>Is the compiler trying to align structure on DWORD boundaries?
>>
>>How do I get rid of this Bug/Feature ?
>>
>>Thnx,
>>Jeethu Rao
>>
>>
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