[prev in list] [next in list] [prev in thread] [next in thread] 

List:       koffice-devel
Subject:    Re: Portable Code (am I dreaming? :))
From:       Nicolas Goutte <nicog () snafu ! de>
Date:       2002-01-16 20:09:46
[Download RAW message or body]

I have just noticed that I have made my examples, as if int was 16 bits wide. 
Sorry!

The example for QDataStream corrected for 32 bits int:

  struct
  {
 	int a;
 	int b;
  } mystruct;

  /* Add here some code to initialize the QDataStream stream */

  stream.setByteOrder(QDataStream::LittleEndian);
  Q_INT32 helper;
  stream >> helper;
  mystruct.a=helper;
  stream >> helper;
  mystruct.b=helper;

If you do not read or write the structure often, you can define your 
structure directly with QINT32. The example would become then:

  struct
  {
 	Q_INT32 a;
 	Q_INT32 b;
  } mystruct;

  /* Add here some code to initialize the QDataStream stream */

  stream.setByteOrder(QDataStream::LittleEndian);
  stream >> mystruct.a;
  stream >> mystruct.b;

Please just ask, if you want the other examples and explanations converted to 
32 bit int!

Have a nice day/evening/night!

On Tuesday 15 January 2002 21:13, Nicolas Goutte wrote:
> On Tuesday 15 January 2002 01:25, Clarence Dang wrote:
> > On Sun, 13 Jan 2002 21:54, shaheed wrote:
> > > Hi Clarence,
> > >
> > > > Do I have to care about making filter code portable?
> > >
> > > I think you should. The MS Word filter started off (consciously, see
> > > below) being little-endian and unaligned access dependent, and I soon
> > > found that Ihad both Sparc and Alpha users. The MS Word filter has
> > > hooks for both these platforms (see macro crap in mswordgenerated.h),
> > > but amazingly enough, there are platforms for which the appropriate
> > > compiler pragmas do not seem to exist (Irix and possibly HPUX).
> >
> > Just wondering but does #pragma pack(1) exist and work for all platforms?
>
> No, it does not, as gcc is not the compiler of all platforms.
>
> > I don't think it's ISO C++ but it's GNU so should work everywhere?
> > I notice that you have:
> > #ifndef __GNUC__
> > #define __attribute__(a)
> > #pragma pack(1)
> > #endif
> > So what do I, if I they're not using GNU (is anyone not?)?
>
> Use the old portability trick: read byte per byte and calculate the
> variables yourself.
>
> Please also note that with pack==1, you are generating unaligned accesses
> everytime you would access your structure. The structure must be packed
> like the CPU want it. You read one time from the file, but you access each
> variable many times (typically.)
>
> > Also, is this portable:?
>
> No! Size of int is unknown (could be 64 bits). Packing is unknown (the int
> could be aligned at addresses that can divide by 8.)
>
> > FILE *filepointer;
> >
> > struct
> > {
> > 	int a;
> > 	int b;
> > } mystruct;
> >
> > fread (&mystruct, sizeof (int), 2, filepointer);
> > if (mystruct.a == some_magic_number)
> > 	do_something ();
> >
> > Will bit order affect the "mystruct.a == some_magic_number" test?
>
> If you have bit order problems, you would have very big problems! I
> therefore suppose that you mean byte.
>
> Yes, your test will be affected. (For example: 0x5300 could become 0x0053.)
>
> If you want fully portable (non-QT) code, you would need something like
> reading four bytes and then calculate the two variables from them.
>
> Something like:
>
>  FILE *filepointer;
>
>  struct
>  {
>  	int a;
>  	int b;
>  } mystruct;
>
>  Q_INT8 helper[3];
>
>  fread (&helper, 1, 4, filepointer);
>  mystruct.a=helper[0]+256*helper[1];
>  mystruct.b=helper[2]+256*helper[3];
>  if (mystruct.a == some_magic_number)
>  	do_something ();
>
> Better use QT's QDataStream, like for example (not tested):
>
>  struct
>  {
> 	int a;
> 	int b;
>  } mystruct;
>
>  /* Add here some code to initialize the QDataStream stream */
>
>  stream.setByteOrder(QDataStream::LittleEndian);
>  Q_INT16 helper;
>  stream >> helper;
>  mystruct.a=helper;
>  stream >> helper;
>  mystruct.b=helper;
>
> > > I finally started work to fix this (it is "easy" in my case because I
> > > generated the code involved), but have not yet finished.
> >
> > How do I test my code, without having the target system?
> > I only have ix86 systems.
>
> No, idea! Sorry!
>
> > > > With non-portable expressions like these, which make assumptions on
> > > > data type sizes and byte ordering and bit ordering...
> > >
> > > The first step is to use Q_{U,}INT{8,16,32} as I believe you can trust
> > > these to be set correctly on any platform. Then, QDataStream in little
> > > endian mode may be your friend. If only I had known about these when I
> > > started...
> >
> > Thanks for the information.
> >
> > Thanks!
> > Clarence
>
> I hope that I was able to help you a little.
>
> Have a nice day/evening/night!
_______________________________________________
koffice-devel mailing list
koffice-devel@mail.kde.org
http://mail.kde.org/mailman/listinfo/koffice-devel
[prev in list] [next in list] [prev in thread] [next in thread]