structure packing

**rajkumarmadhani** · 11-26-2007

hi,

structure padding is the way to put padding bits/bytes depening on byte boundary.

but where structure packing comes, as i know that it supresses the padding bits.if it so, is the processor able to read that paricular memory efficiently.
supressing means what actually will happen in memory.

Code:

struct 
{
        char a;
        char b;
        char c;
        int i;
};
--->here     1 byte+1 byte + 1 byte + padding byte + 4 bytes = 8 bytes

sturct 
{
      char a;
      char b;
      char c;
      int i;
}__attribute__((packed));

--->here 1 byte+1 byte+1 byte+4 bytes=7 bytes.

supressing of padding bytes means what?.but then no byte boundary here.

please tell me what is it?

**MacGyver** · 11-26-2007

Some CPUs, particular from Intel, don't have to have everything lined up along boundaries, although performance significantly suffers.

This is a rather system specific and even compiler specific topic.

**matsp** · 11-26-2007

Right, the padding is there for a reason:
reading an int from an address that is aligned with the size of the int makes it much faster.
On some CPU's it's even invalid to read unaligned data types - so the member "i" in your packed struct could not be read directly on a 68000 machine, for example, an attempt to do so would trap into a "unaligned access" handler - which of course COULD fix up the problem, but now reading a single integer, which is a relatively simple operation, is taking hundreds of clock-cycles. Most machines with this type of architecture will CRASH when an unaligned access occurs, rather than fix it up [because it's probably not intentional].

There are good reasons to pack data structures for compatibility with old architectures, but it's bad for performance and portability.

--
Mats

**rajkumarmadhani** · 11-26-2007

thank you for reply. from this i got something on structure padding and packing

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