On Sat, 29 Sep 2001, Joe Smith wrote:
> On Friday 28 September 2001 16:42, you wrote:
> > On Thursday 27 September 2001 21:29, Wayne E. Van Loon Sr. wrote:
> >
> > [...copying structs...]
>
> Granted, structs mixing different-sized elements are best avoided.
>
> > A nearly fool-proof method is to use inline functions or macros to access
> > elements inside byte arrays. Correctly implemented, this method even
> > ensures endian correctness, and usually compiles into virtually no code,
> > unless byte order inversion is required. Unless you're going to send data
> > between different endian CPUs, you can just make sure that your "encoded"
> > endian is the same as the CPU's native endian.
> >
> > Example:
> >
> > /*
> > * Writes "val" in big endian format into 'buf'.
> > * Returns number of bytes written.
> > */
> > int write_int(char *buf, int val)
> > {
> > buf[0] = val & 0xff;
> > buf[1] = (val>>8) & 0xff;
> > buf[2] = (val>>16) & 0xff;
> > buf[3] = (val>>24) & 0xff;
> > return 4;
> > }
> >
> > Note that for total portability, your should verify that sizeof(int)
> > really is 4, as it might be smaller on some systems... (It's not a
> > problem if it's bigger - this code will still write a 32 bit int.)
>
> Is there really a situation where simple read/write of int's wouldn't work?
>
> Read/write do byte copies, so there's no problem with alignment in the FIFO
> buffer, and the program variables are declared 'int', so they'll be aligned.
> Both ends of an RT-FIFO are always going to be accessed by the same hardware
> arch (if not the same CPU), so whatever int size and byte order is used on
> the RT side will be correct for the user side, right?
Yeah.. for soemthing like a FIFO, that all runs on the same arch,
endianness is sooo not an issue it isn't funny.
>
> My code depends on this (and structs that contain only int members), so I'd
> really like to know if I've overlooked something.
I am so liberal with this stuff I even copy around structs that contain
all sorts of members. I think as long as you are careful to compile your
kernel-side and user-side together and/or release them together you will
be fine. It's only when you are designing a REALLY generic kernel task
that you expect everyone in the universe to use with all sorts of
applications and compilers that you really have to worry about copying
structs, IMHO.
-Calin
> > <Joe
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