druid wrote:
Yea, but that brings up another question
If the structure of a record is actualy a series on
teminated strings, when you retrieve records, how do you
know the length of any field in that record
You just have to figure out a way to do this yourself.
The most obvious way is to include null characters as
terminators, then put all the strings in sequence. That
means you can't access each string with the record
independently, but that may not be a big problem.
Let's say you wanted to be able to read and write a
series of strings into a single record. The code might
look like this:
Int32 SizeNeeded (Char *strings[])
{
Char **currentstring;
Int32 bytesneeded = 0;
for (currentstring = strings;
*currentstring != NULL;
currentstring++)
{
bytesneeded += 1 + StrLen (*currentstring);
}
return (bytesneeded);
}
void WriteStrings (Char *strings[],
MemPtr storageheapptr, Int32 startoffset)
{
Int32 offset = startoffset;
for (currentstring = strings;
*currentstring != NULL;
currentstring++)
{
DmWrite (storageheapptr, offset,
(void *) *currentstring, 1 + StrLen (*currentstring));
offset += 1 + StrLen (*currenstring);
}
}
Then, you'd call SizeNeeded() to figure out how big of a record to
create (or how big to resize a record to, if you're re-writing an
existing record) and afterwards call WriteStrings() to actually
write them.
Reading is similar to writing. If you want to be lazy (and not
check for records that were truncated in the middle of a string),
you can call StrLen() directly on a pointer into the record, since
the storage heap is addressable memory, and then you can call
StrCopy() to copy directly from the record to some memory you
allocated using MemPtrNew() or something. Or, if you only need
the strings for a short time (while you still have the DB record
locked), you could even just compute pointers into the record
itself. That is, you could fill the strings[] argument with
pointers into the storage heap. That method could be error prone
if the caller doesn't realize that the pointers may become invalid
once the record is unlocked, but it could be useful in some cases
(such as if you are sorting the database, and you need speed,
or something like that).
- Logan
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