Re: usb: descriptor structures have to be packed
Alan Stern <[email protected]>
| Newsgroups | gmane.linux.usb.devel |
|---|---|
| Message-ID | <[email protected]> |
On Thu, 8 Feb 2007, Phil Endecott wrote: > Alan Stern wrote: > > On Thu, 8 Feb 2007, Phil Endecott wrote: > >> > From a > >> > higher-level (source code) point of view, the alignment doesn't affect the > >> > set of operations which can be applied to the data type. > >> > >> It affects whether you can legally use & to take its address. > > > > Are you sure about that? I would think that if you take the address of an > > unaligned quantity, you simply end up with an unaligned pointer, not with > > a compiler error. (Remember, we're talking specifically about GCC here. > > The official C language doesn't include any of these concepts.) > > I'm not personally sure, but I'm repeating what I was told by the gcc > people when I asked. Search for my name in gcc-help in December 2006. > I'm probably mis-remembering some of the details. See e.g. > > http://thread.gmane.org/gmane.comp.gcc.help/17551/focus=17554 > > In that message, I assert that it is unsafe "on some architectures" to > take the address of an unaligned packed structure member, and am told > that it is unsafe on *all* architectures because the compiler is > allowed by the C standard to assume that the least significant bits of > an int* are zero. I can agree with this. But above you said it wasn't _legal_ to take the address of an unaligned quantity. That's very different from being _unsafe_. You have to watch out when talking about computer languages, especially when compiler writers get involved. The language lawyers will get you every time... Alan Stern ------------------------------------------------------------------------- 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