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.

Thanks.

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...

Hey,

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:

[CODE]
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 -

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
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.