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

[Johannes P. <joh...@pr...>]

I realize now that my example with the 24 bytes is correct for it is 24 
bytes. But if you remove the sub_code from the structure and
have two entries of the structure in a binary file forinstance.. like
[START OF FILE]34^^^^^^^^[8 BYTES]Hallo world^^^^^[16 BYTES]1[1 BYTES]0[1 
BYTES][START OF 2 STRUC]34^^^^^^^^[8 BYTES]Hallo world^^^^^[16 BYTES]1[1 
BYTES]0[1 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;
}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 same 
structure BUT
DevCpp (or the gcc or mingW) has a different value of the sizeof() so it 
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
we will start reading the beginning of the next entry of the second 
structure...

Now this will make my example more true..

I hope I understand this incorrect and that somebody can please explain 
this problem to me in more detail...

Thanks

Johannes




At 07:08 04/07/2001 +0200, you wrote:

>>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 
>>memory space
>>of the biggest type inside it. for instace in your example the biggest 
>>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 
>>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 
>>file was writen in a diffent program
>>by a different compiler. You will not read the exact amount of bytes into 
>>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 
>>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 same 
>>structure BUT
>>DevCpp (or the gcc or mingW) has a different value of the sizeof() so it 
>>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 be 
>>read into the correct value
>>considering that this is the same (almost ) as doing a memcpy from the 
>>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
>>>due to compiler's memory management technique. For more information read
>>>section 8.7 in K&R 2. For example a compiler's memory management can be
>>>a connected list of structs (compilers, operating systems e.t.c. are
>>>written in C/C++ too). So in the example of K&R it has a 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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