...Plus, if this is following Palm Computing's Address code, they don't
overwrite the existing record in the DB. They put a "new" record containing
the changes into the database then delete the old one. That is, if I
remember correctly :)
--Alan
> -----Original Message-----
> From: Neil Rhodes [mailto:[EMAIL PROTECTED]]
> Sent: Monday, June 07, 1999 11:19 AM
> To: [EMAIL PROTECTED]
> Subject: Re: Pack and Unpack question
>
>
> >
> > I got that far, Neil ... but what I don't understand
> > is this: if the ptrs in the unpacked customer record
> > are merely pointing into the packed customer record;
> > how can you change the city from "Taos" to "Dallas"
> > in the form without tromping all over the phone #?
> > If there is code to move the phone # up when changing
> > the city, I didn't see it. Did I just miss it, or is
> > there some magic being performed by the OS to do this?
>
> Here's what happens when we change the city from "Taos" to
> "Dallas". We have
> an unpacked customer record (containing four character pointers), and we
> change one of the pointers from pointing to "Taos" (the bytes of which are
> in the packed record in the DB) to "Dallas" (the bytes of which are
> somewhere in the dynamic heap). When we save the record, we
> convert it from
> unpacked to packed. It's at this point that we'll be repacking.
>
> Neil
> >
> >
> >>
> >>
> >>
> >> A packed customer (in a database record) is stored as a
> >> 4-byte ID, followed
> >> by 4 null-terminated strings (name, address, city, phone).
> >>
> >> Here's the declaration
> >>
> >> typedef struct {
> >> SDWord customerID;
> >> char name[1]; // actually longer than 1
> >> } PackedCustomer.
> >>
> >> Here's an example of the bytes of a (small) packed customer:
> >> 0x00 0x00 0x00 0x52 'N' 'e' 'i' 'l' '\0' 'M' 'a' 'i' 'n'
> >> '\0' 'R' 'e'
> >> 'd' 'l' 'a' 'n' 'd' 's' '\0' '7' '9' '3' '-' '5' '9' '9' '5' '\0'
> >>
> >> Notice that the name will always start at offset 4 within the
> >> packed record.
> >>
> >> An unpacked customer is used in memory, to actually access the name,
> >> address, city, and phone.
> >>
> >> typedef struct {
> >> SDWord customerID;
> >> const char *name;
> >> const char *address;
> >> const char *city;
> >> const char *phone;
> >> } Customer;
> >>
> >> It has four character pointers which point into the
> >> appropriate offsets
> >> within the associated (locked) PackedCustomer.
> >>
> >> If the example packed customer above were at memory address
> >> 0x1230, then the
> >> Customer would be
> >> 0x00000052 // id
> >> 0x1234 // offset 4 within packed customer
> >> 0x1239 // offset 9 within packed customer
> >> 0x123e // offset 14 within packed customer
> >> 0x1247 // offset 23 within packed customer
> >>
> >> The authors should have made it more clear in the book, but I
> >> think they
> >> (incorrectly) assumed this was a well-known, or obvious technique.
> >>
> >>
> >> Neil
> >>
> >>
> >
> >
>
>
>
