Re: [linux-usb-devel] usb: descriptor structures have to be packed

[Phil Endecott]

David Brownell wrote:
On Wednesday 07 February 2007 10:41 am, Phil Endecott wr
void inc(int* i) {
   (*i)++;
}

here's a quote from Paul Brook 
(http://gcc.gnu.org/ml/gcc-help/2006-12/msg00115.html):

   " the compiler is allowed to assume that the low 2 bits of an int* 
are zero. "

Unless it's declared as packed ... a fact which is 100% clear in the
definition of "packed".

Dave, how would you propose to re-write my "inc" function (above) to 
declare i as a "packed int*"?  I have tried various things like

void inc(int __attribute__((packed)) * i)
void inc(int * __attribute__((packed)) i)
void inc(int * i __attribute__((packed)))
void inc(int __attribute__((aligned(1))) * i)
void inc(int * __attribute__((aligned(1))) i)
void inc(int * i __attribute__((aligned(1))))

and I get variously

warning: ‘packed’ attribute ignored
error: alignment may not be specified for ‘i’

or no message at all, and I only ever get loadword-add-storeword code 
generated.  My feeling is that there is no way to declare a "packed" or 
"unaligned" pointer to an int, and so Paul is right.

Phil.





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