Re: usb: descriptor structures have to be packed

On Wednesday 07 February 2007 3:04 am, Phil Endecott wrote:
> I won't pretend to understand all the subtleties of 
> __attribute__((packed)), but I did learn something about it when I 
> hacked USB/IP to run on ARM, for the NSLU2, last year.  The issue 
> boiled down to this:
> 
> struct S {
>    int intfield;
>    char charfield;
> } /* maybe add: __attribute__((packed)) here */ ;
> 
> f(struct S * s) {
>    ...
>    (s->intfield)++;
>    ...
> }
> 
> ...
> 
> What happens when we add __attribute__((packed)) to struct S?  ...
> will work, but a lot more slowly.  However, the ++ in f() WILL NOT 
> WORK!  The compiler assumes that the pointer that is passed IS 
> NATURALLY ALIGNED, and when you pass an unaligned pointer from g() IT 
> DOES NOT ISSUE A WARNING!  

Looks like a bug in your version of the toolchain.  I just tried
this not a minute ago (gcc 3.4.3) and it works correctly:  not only
do both versions of the function handle the "intfield" misaligned
for loads, they also do so for the store.


> Based on my experiece - it took me a long time to debug this -

Compiler bugs are that way, yes.  Maybe that was associated with
using the big-endian compile options?

- Dave


-------------------------------------------------------------------------
Using Tomcat but need to do more? Need to support web services, security?
Get stuff done quickly with pre-integrated technology to make your job easier.
Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo
http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642
_______________________________________________
[email protected]
To unsubscribe, use the last form field at:
https://lists.sourceforge.net/lists/listinfo/linux-usb-devel