Hi Bo.

19-Jan-04 14:33:08, Bo Simonsen wrote to Pascal Schmidt


 PS>> Hi Bo! :-)

 BS> Hi Pascal,

 BS>> I'm looking into this issue because it looks like
 BS>> that SquishMail has
 BS>> problems my reading a packed (__attribute__(packed)) structure from
 BS>> diskfile written on a little endian machine.

 PS>> No guesses please, show us the structure definition. :)

 BS> It would be no-sence to show you the whole structure, so here's just a
 BS> bit
 BS> of it:

 BS> struct _pkthdr
 BS> {
 BS>   sword orig_node;        /* originating node */
 BS>   sword dest_node;        /* destination node */
 BS> }  __attribute__((packed, aligned(2)));

OK, a fidonet .PKT file, there are little endian (opposite of network byte
order)

the easiest fix it to use do byte-swap operation on the words after/before
reading/writing them.

 &define  fido_swab(x)   (x=x>8 & 0xff )


 BS> read(fp, &pkthdr, sizeof(struct _pkthdr));

 fido_swab(pkthdr.orig_node)
 fido_swab(pkthdr.dest_node)

and before you write them do fido_swab on them again....

The trick for portable code is to get fido_swab to do nothing when htons
does something and vice versa. on little-endian hardware fido_swab should
be defined blank.

 #define fido_swab

probably it's a good idea to test it at runtime, or as part of the make
process.

  {
  short int x=0
  ((char*)x)[0]=1;
  fido_swab[x]
  if(x != 1) { fpriintf(stderr "fido_swab mis-defined in source";
               exit(1);
             }
  }

This isn't a truly general solution as it won;'t work on macines that don't
use two's complement numbers but those are practiacally unheard of these
days,



the other fix is to re-write it to use all bytes.

 BS> struct _pkthdr
 BS> {
 BS>   uchar orig_node_minor,orig_node_major;     /* originating node */
 BS>   uchar dest_node_minor,dest_node_major;     /* destination node */
 BS> }  __attribute__((packed, aligned(1)));

byt then you may need to do more edits to your code so that it can get the
node number

eg:

 unsigned short orig_node = (pkt.orig_node_major << 8 & 0xff00)
                           +(pkt.orig_node_minor      & 0x00FF)

 BS> Should I read every single variable in the structure to get it working?

no, you either need to address each byte separately or convert the
multi-byte values from file-format to host-format, and back to file format
before writing

 -=> Bye <=-

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