Question

Possible Duplicate:
context switch during doubly linked list creation

I was reading The Design of the UNIX Operating System (Maurice Bach).

He gives the following example of a doubly linked-list.

struct queue{
    //pointers (back and forward)
    //other data
}*bp, *bp1

bp1->forp = bp->forp; //1

bp1->backp = bp; //makes sense
bp->forp = bp1; //makes sense

bp1->forp->backp = bp1; //2

I can’t understand the purpose of the statements marked 1 and 2. 1 seems wrong, 2 looks to be redundant.

Is this a valid way to create a doubly linked-list?

Answer 1

The code is correct.

bp is a doubly linked list.
You want to insert bp1 into bp as the second item in the list (that is what the code does).

To do that you need to set 4 pointers:

bp1->forp should point to the second item in the list, bp->forp (//1 above)

bp1->backp should point to the first item in the list, bp

bp->forp should point to the inserted item, bp1

The back pointer of the second item, bp1->forp->backp should point to the inserted item, bp1. (//2 above)

Edit:

Let’s call the structs A, B, C, D…
The list (pointed to by bp consists of A, C, D… before the insert. We want to insert B (pointed to by bp1.
<-> denotes the forward and backwards pointers.

Before:

bp --> A <-> C <-> D <-> E <-> ...
bp1--> B

After:

bp--> A <-> B <-> C <-> D <-> E <-> ...