October 2001
Intermediate to advanced
704 pages
19h 33m
English
Content preview from Palm OS Programming, 2nd Edition
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Portability Issues
There are two important portability issues to take into account when moving data back and forth from the handheld to the desktop: byte ordering and structure packing.
Byte Ordering
The Palm OS currently runs on a Motorola platform, which stores bytes differently from Windows running on an Intel platform. This crucial difference can mess up data transfers royally if you are not careful.
Tip
It’s been announced that a future version of the OS will run on the ARM processor in little-endian mode (which will match the Intel ordering). Right now, it’s not clear what the implication will be for conduits. To be safe, use the Sync Manager routines for swapping rather than writing your own.
On the handheld, the 16-bit number 0x0102 is
stored with the high byte, 0x01, first, and the
low byte, 0x02, second. In the conduit on Windows,
the same number is stored with the low byte, 0x02,
first, and the high byte, 0x01, second. As a
result, any 2-byte values stored in your records or in your
AppInfo block must be swapped when transferred
between the two systems. (If you fail to swap, a simple request for 3
boxes of toys on the handheld would be processed on the desktop as a
request for 768 boxes!) A similar problem occurs with 4-byte values;
they are also stored in switched forms (see Table 15-2).
Table 15-2. Comparison of byte orderings for the 4-byte value 0x01020304
|
Palm handheld byte order |
Wintel byte order |
|---|---|
0x01 |
0x04 |
0x02 |
0x03 |
0x03 |
0x02 |
0x04 |
0x01 |
The Sync Manager ...
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