structure packing

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?

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.

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

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