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Old 01-25-2010, 08:53 AM   #1
Registered: Jul 2009
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Question how to disable structure alignment using gcc compiler options ?

Hi All,

How can I disable structure alignment feature of gcc using command-line options ?

I recently migrated to 64-bit OS, and doubt that I might be experiencing a structure alignment problem due to the new 64-bit architecture.

I checked the sizes of the same C-style struct in both x86 and x86_64, and found out that they appear to be different by 20 bytes.

I am not sure if this is due to structure alignment or the differences in data type lengths between two platforms.

Hence, I will first disable the structure alignment feature, and then check the struct sizes again.

Old 01-25-2010, 11:02 AM   #2
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Are you looking for:

#pragma pack(push, 1)

This will set the compiler to pack structures in 1 byte chunks (no fillers)

#pragma pack(pop)

will get you back to default.

Edit -- Sorry, I replied to quickly. Yes, you are correct. The size of individual datatypes change. long in 32bit = 4bytes, 8 bytes in 64bit OS from my experience. If you cannot change the structure to use variables with set sizes (uint32_t for ex.), then you need to d/l the 32-bit libs (Google is your friend here, as I cant recall the names of the libs req'd, buildessentials is one--*I think*) and compile with the -m32 flag to get 32bit object code (and hence datatypes).

Edit again: bad name for 32 bit int datatype...

Last edited by Xyro; 01-26-2010 at 10:22 AM.
Old 01-25-2010, 01:26 PM   #3
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it is basically both, the data type sizes (long equals 4bytes on 32bit vs 8bytes on 64bit) change AND the alignment requirements change as well. 4-byte data types are aligned on a 4-byte boundary, while 8-byte data types are aligned on an 8-byte boundary. So, I doubt that packing would help here. By the way, just for completeness and to avoid the #pragma directives of the compiler, you could do the following to specify the "pack" attribute per structure definition:

struct a
} __attribute__((packed));

But note, the __attribute__ syntax is a GNU extension which you wouldn't find on any other compiler.

Lastly, why do you think your application will crash due to the port to 64-bit architecture? How do you fill your structures? If you would like to, you could post your structure and extracts of your code which you think will crash ...

- Andi -
Old 01-26-2010, 09:21 AM   #4
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If your structure represents a hardware layout, then you can force the sizes of components of the structure using the predefined types from stdint.h (such as uint16_t etc), or by using the ':n' notation, eg
unsigned int foo:16;
Both the packing and alignment can be set using the packed and aligned attributes as demonstrated in the above posts.

However, you should be careful of such controlled structures; they are seldom portable (such as when moving between little endian to big endian architectures), and often represent a failure to serialize data. The best reason for packing structures is to conserve space (usually at the expense of speed), not to force a consistent byte format.

Last edited by neonsignal; 01-26-2010 at 09:43 AM.


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