Experimental Physics and
Industrial Control System

"J. Frederick Bartlett" <[email protected]> · Thu, 02 Sep 2004 15:13:49 -0500

  If we had stayed with the Pascal/Modula/Oberon family, this particular
problem would be solved by the pack and unpack built-in functions. In this
era of C++ dominance I continue to pine for the clarity of Niklaus Wirth's
syntax design.

											Fritz

> -----Original Message-----
> From: Kay-Uwe Kasemir [mailto:[email protected]]
> Sent: Thursday, September 02, 2004 2:28 PM
> To: Lawrence T. Hoff
> Cc: Fairley, Diane; EPICS Tech Talk
> Subject: Re: Compiler switches on RISC byte alignment
>
>
> Hi:
>
> I'm surprised Jeff Hill hasn't jumped onto this thread.
> He and I had talked about similar issues before.
> Maybe he's getting some work done and ignoring tech-talk.
>
> Anyway, the thing about using packed structures
> in order to access memory mapped I/O or
> raw network packages is that you often run
> into compiler-specific "pack" instructions.
> Larry's copy works around the alignment,
> next you'd run into byte order issues when
> you change the CPU type.
>
> The most portable way of coding might be
> to always use byte pointers and assemble/disassemble
> the data yourself. So instead of
> .....
> typedef struct __attribute__ ((packed)){
>       unsigned char  uCh;
>       short          sInt;
> .....
> you use
> .....
>    volatile char *buf = <some address>;
>    unsigned char uCh = *(buf++);
>    unsigned short low = *(buf++);
>    unsigned short high = *(buf++);
>    short sInt = (high << 8) | low;
> .....
>
> You can somewhat beautify this by defining routines
> unpack_short(), ...
>
> -Kay
>
> On Sep 2, 2004, at 14:56, Lawrence T. Hoff wrote:
> >    As far as I know, that is a characteristic of the SPARC processor,
> > and cannot be
> > influenced by compiler directives. You *can* use byte-oriented
> > instructions on
> > data which is not on "natural" boundaries. I.e. you could access
> > "lInt" as follows
> > (from a packed array):
> >
> > unsigned long int temp;
> > memcpy(&temp, &testdata.lInt, sizeof(temp));
> >
> > printf("testdata.lInt = %lu", temp)
> >
> > HTH -- Larry
> >
> >>
> >> does not print the right value.  A pointer to an unaligned structure
> >> member results in a segment violation.
> >>
> >
> >
>
>

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System