Lpc2000

Dmitry:

__packed means that there will be no gaps left around that 
variable.  Normally a long would be put on a 32-bit boundary, however the 
__packed attribute tells the linker not to leave any adjacent space.

Note that on the ARM7 chips if you attempt to access a halfword or word 
that is not aligned properly the behaviour is "undefined".  If 'a' is not 
properly aligned that statement would not work.  In this case this __packed 
instructs the compiler that 'a' may not be aligned, so read/write it as a 
series of 4 bytes instead of a directly as a 32-bit word.

I had a bug in my app this way that happened because of this.  It was 
strange, I traced it down to writing a value almost like this, and my debug 
code did:

void dummyCall(long *lp)
{
   foo = *lp;
}

long foo, bar, *lp;

HexDump(b,4);

lp = (long *)b
bar = *lp;
dummyCall(&bar);
*lp = bar;

HexDump(b,4);

What was happening is the first and last halves of the 4-byte segment at b 
were swapping (i.e. 0xAABBCCDD became 0xCCDDAABB) and I couldn't for the 
life of me figure out why, until I realized that b wasn't on a 4-byte boundary.

The reason for the dummy call was because otherwise the compiler is because 
when I did this:

bar = *lp;
*lp = bar;

The compiler actually didn't compile that code.

-- Sean

At 04:26 2/7/2006, you wrote:
>Hi Bertrik,
>
> > Making the examples work with GCC involved byte-packing all USB related
> > structures and replacing compiler specific options with GCC equivalents.
>Thanks a lot...
>
>I looked into KEIL's examples and found __packed keyword which appeared in 
>the
>from of 'long'. I'm puzzled there, for example:
>
>char *a, *b;
>
>*(__packed long *) a = *(long *)b;
>
>how to interpret this???
>
>Cheers,
>Dmitry.
>
>
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