Experimental Physics and
Industrial Control System

Kay-Uwe Kasemir <[email protected]> · Thu, 02 Sep 2004 15:27:50 -0400

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