Sometimes I add a new preference option by just adding it to the end of the
structure so that when I read in the preferences, all the existing ones will
be in the correct locations. I also generally want the new fields to
initially be zero. So this is what I was doing:
t_PrefsSize = sizeof(W_PREFS);
MemSet(&w_Prefs, sizeof(W_PREFS), 0);
s_PrefVer = PrefGetAppPreferences(GPID_CREATOR, 0, &w_Prefs,
&t_PrefsSize, true);
In a recent case, I added a new short int field to the structure. The
initial value of t_PrefsSize was 52; after the GetAppPreferences call the
first time after "upgrading" from the previous version, t_PrefsSize was now
set to 50, which was as expected. HOWEVER, the contents of the short int
field that had initially been zeroed by the MemSet call actually contained
garbage after the GetPreferences call. In other words, even though the
function knew that the current saved preferences were shorter than the size
that I passed in and it returned the actual current size, it actually wrote
to memory for the full extent of the passed-in size! This seems a bit
strange, to return a size of 50, but to actually have written 52 bytes.
I got around this behavior something like this:
t_PrefsSize = sizeof(W_PREFS);
s_PrefVer = PrefGetAppPreferences(GPID_CREATOR, 0, &w_Prefs,
&t_PrefsSize, true);
// Zero any new preference fields
if (t_PrefsSize < sizeof(W_PREFS)) {
MemSet(((char *) &w_Prefs) + t_PrefsSize, sizeof(W_PREFS) -
t_PrefsSize, 0);
Kludgey IMO, but it works!
Doug Gordon
GHCS Software
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