= Internal tricks =
During real and deep developing time, we faced some obstacles seeming hard to
overcome, but we did. That is because a few tricks helped us.

An early faced obstacle was the process queue tail pointer updating in `pcb.c`
module, avoiding to use a global-visible variable, but passing a parameter. To
do this, each involved function receives a pointer to that queue pointer (i.e. a
double pointer).

We use the same hack on a different situation, writing `st.c` source, where we
enjoined double advantages. Maintaining a tree after insertion/removal
operations, means updating a couple of pointers, and so knowing their location.
Because of the fact that tree is
[wiki:DataStructures#a3.Semaphorebinarytree single-linked], going back the tree
becomes impossible, and so, each of such operations must be performed starting
one level higher in the hierarchy. Here another problem comes up: starting from
a node, which of its child is the wanted one? As pointed up above, we found just
one solution for both problem: using a pointer to the father's field
(`s_leftC`/`s_rightC`), which point to the target node.

But, as you can see, there is another question: which is the ancestor of the
root element? If it does not exist, the solution just explained gets wrong and
need a small modification: the insertion of a dummy node, pointed to by
`semdRoot`, as root node, which makes the whole semaphore descriptors tree the
dummy's left-progeny.

This change suggests another hack, in order to find a suitable place to store
the free list. It contains descriptors of each inactive semaphore: we linked it
as a list, using `s_rightC` field, avoiding memory space wastes.

{{{

 Semaphore Tree structure:
                ______
               |      |
               | HEAD |
               |______|
               .'   '.
           __.'       '.__
          |__|         |__|
        __.''.__         '.__
       |__|  |__|         |__|

      |          |     |      |
       `--------'       `----'
          AST          Free List

  Further details can be found directly in source code :)

}}}