Re: Structure alignment with GCC

[gshubin <gshubin at wco dot com>]

Thanks, now I have a good example of why it is always a bad idea to use structures
for passing data. I usually use the example of inter-processor communication, but
now you have given me and example of inter-compiler communication (I assume it's
on the same processor since you didn't state otherwise). This is right up there
with imposing a structure overlay on top of hardware registers.

Oh, sorry, I can't help with your problem. I do think there is a command line
switch for using packed strucures, but I am not at my Linux machine with GCC
installed, so I can't look it up. I'm sure someone will post it.

Christopher Bahns wrote:

> Hello ppl,
>
> I'm having trouble with structure alignment using gcc. The problem is
> that if a structure contains only 1-byte fields, gcc will use 1-byte
> alignment for the entire structure. Consider the following example:
> _______________________________________________________
> typedef struct { char a; short b; char c; } DataTypeA;
> typedef struct { char a; char b; char c; } DataTypeB;
>
> const char Data1 = 10;
>
> const DataTypeB Data2 = { 5, 6, 7 };
>
> const short MyAlign = __alignof__ (Data2);
> _______________________________________________________
>
> If I build as shown above Data1 will be put at (e.g.) 0x10000 followed
> by Data2 at 0x10001. In this case, the alignment of Data2, MyAlign=1.
>
> However, if I change Data2 to be of type DataTypeA (containing a short)
> instead of DataTypeB (having all chars), then Data2 is placed at 0x10002
> and MyAlign=2. Of course, it puts a padding byte (0x00) after Data1 and
> before Data2.
>
> I know I can use GNU's 'aligned' attribute to force the alignment of a
> particular variable, as follows.
> _______________________________________________________
> typedef struct { char a; short b; char c; } DataTypeA;
> typedef struct { char a; char b; char c; } DataTypeB;
>
> const char Data1 = 10;
>
> const DataTypeB Data2 __attribute__ ((aligned (2))) = { 5, 6, 7 };
>
> const short MyAlign = __alignof__ (Data2);
> const short MySize = sizeof(Data2);
> _______________________________________________________
>
> This works just fine (and is currently what I'm doing), but this is VERY
> UNDESIRABLE. Here's why:
>
> I'm porting a large project from MRI (Microtek Research) to GNU. The MRI
> compiler apparently has a default structure alignment of 2, even for
> structures that contain only 1-byte fields. Part of the project requires
> that I build data tables whose format is identical to those built with
> MRI, because in some cases we will have to have MRI-built code access
> the tables that are being converted to GNU. The nature of the project
> demands that we convert the table generation to GNU before the main
> program. I *really* don't want to have to find all of these particular
> situations (an odd-sized data element followed by a structure with
> 1-byte alignment). This would be time-consuming and the risk of missing
> something (or something going wrong in the future) would be undesirable.
> I'd also prefer not to add the aligned attribute to EVERY single
> structure (although this would probably be most effective) -- note that
> this would have to be done for each array or structure variable. In
> either case the maintainability of the project is diminished because
> each time a new variable is declared that meets the above criteria, the
> maintainer will have to be aware of the problem and remember to insert
> the 'aligned' attribute.
>
> All of this is most undesirable and not at all elegant (as well as prone
> to failure). I'd like some kind of command line option to specify the
> default alignment of structures, but have searched the documentation (a
> bit outdated) and found nothing. The only possible solution I've seen
> mentioned so far is:
> _______________________________________________________
> Add a line to ./gcc-2.95.2/gcc/config/i386/linux.h:
>    #define HANDLE_PRAGMA_PACK_PUSH_POP 1
> Rebuild the compiler. Now you can use #pragma pack(push,<n>) and #pragma
> pack(push) to change the maxiumum alignment (in bytes) of fields within
> a structure.
> _______________________________________________________
>
> Based on the description, it does not sound like the use of #pragma
> pack() will have the desired effect, but I suppose I'll try it (compiler
> is building now). In the above case it seems the person was building an
> i386 target on a linux host, so I don't know if it would even work for
> me...
>
> Here is my setup:
>    Target=m68k-coff
>    Windows NT 4.0 Workstation
>    Cygnus Cygwin 1.1.2
>    binutils-2.10
>    gcc-2.95.2
>    newlib-1.8.2
>
> So, at the moment I'm stuck and really don't like the approach I've had
> to take so far. If anyone can help (including confirming for me that
> there is no better solution), I'd appreciate it.
>
> Thanks
> Chris
>
>   ------------------------------------------------------------------------
> ------
> Want more information?  See the CrossGCC FAQ, http://www.objsw.com/CrossGCC/
> Want to unsubscribe? Send a note to crossgcc-unsubscribe@sourceware.cygnus.com


------
Want more information?  See the CrossGCC FAQ, http://www.objsw.com/CrossGCC/
Want to unsubscribe? Send a note to crossgcc-unsubscribe@sourceware.cygnus.com