TamsPalm – the Palm OS / web OS Blog

Some PalmOS API functions require pointers to structures. Usually, this is a standard operation-until APIs start calculating and returning junk.
Lets take the following piece of code as an example. It is a time limiter that lets a beta expire after a defined day, or rather on a fixed date.
DateTimeType date;
date.day=2;
date.month=2;
date.year=2005;
if(TimGetSeconds()>TimDateTimeToSeconds(&date))
{
FrmCustomAlert(1000,"Beta Period expired!!n", " "," ");
return 222;
}
Looks perfect-doesn’t it? Common sense finds no error. In tests, the routine didn’t work well. The program did not expire at the date specified, but rather at a random date a few years later. After rewriting the code for a few times, the structure was nulled out. And alas, the code worked well(see below)
DateTimeType date;
date.minute=0;//All Values must be defined!!!
date.second=0;
date.hour=0;
date.day=2;
date.month=2;
date.year=2005;
if(TimGetSeconds()>TimDateTimeToSeconds(&date))
{
FrmCustomAlert(1000,"Beta Period expired!!n", " "," ");
return 222;
}
As we all know, C compilers do not initialize local variables while they are declared. Thus, random values were contained in the hour, minute and second fields, each of which was defined as a UInt16(I don’t know why->waste of memory without apparent bonus in simplicity). Thus, values of up to 2^16 could be contained. The API did not check for the sanity of the input, and thus the displacement occurred.
Discussing the matter with the developer community, three possibilities for nulling out structures emerged. The first one was used in the example above. Although it works well and makes a nice, fast fix, most programmers will prefer a tidier way of duking it out with the structure.
A more advanced way of programming would be the use of structure constants. ANSI C allows the programmer to preload structures with definite values. You just use a list of elements, like shown in the example below:
struct{
int a;
int b;
int c;}foo;
foo={ValueA, ValueB, ValueC};
As you see, each structure element can have a value assigned-if the types match. If there are less values than struct elements, the ones defined first get values, and the rest stays undefined.
However, this solution still has a disadvantage. Every type of struct needs a different list.
The ultimate solution is the use of the MemSet function. If you aren’t familiar with the call, look it up here.
If you now have a structure foo and want to zero it out, all you need is a call to
MemSet(&foo,sizeof(foo),0);
This can be packed into a parameterized macro if you plan to use it a lot. Use the pre-processor, you paid for it after all!

UPDATE-when zeroing out structures, think about what the caller expects to find there… . You an provoke a fatal alert easily.