Another common form to organize data for efficient search is to use
trees. The tsearch function family provides a nice interface to
functions to organize possibly large amounts of data by providing a mean
access time proportional to the logarithm of the number of elements.
The GNU C library implementation even guarantees that this bound is
never exceeded even for input data which cause problems for simple
binary tree implementations.

The functions described in the chapter are all described in the System V and X/Open specifications and are therefore quite portable.

In contrast to the hsearch functions the tsearch functions
can be used with arbitrary data and not only zero-terminated strings.

The tsearch functions have the advantage that no function to
initialize data structures is necessary. A simple pointer of type
void * initialized to NULL is a valid tree and can be
extended or searched.

The tsearch function searches in the tree pointed to by
*rootp for an element matching key. The function
pointed to by compar is used to determine whether two elements
match. See Comparison Functions, for a specification of the functions
which can be used for the compar parameter.

If the tree does not contain a matching entry the key value will
be added to the tree. tsearch does not make a copy of the object
pointed to by key (how could it since the size is unknown).
Instead it adds a reference to this object which means the object must
be available as long as the tree data structure is used.

The tree is represented by a pointer to a pointer since it is sometimes
necessary to change the root node of the tree. So it must not be
assumed that the variable pointed to by rootp has the same value
after the call. This also shows that it is not safe to call the
tsearch function more than once at the same time using the same
tree. It is no problem to run it more than once at a time on different
trees.

The return value is a pointer to the matching element in the tree. If a
new element was created the pointer points to the new data (which is in
fact key). If an entry had to be created and the program ran out
of space NULL is returned.

The tfind function is similar to the tsearch function. It
locates an element matching the one pointed to by key and returns
a pointer to this element. But if no matching element is available no
new element is entered (note that the rootp parameter points to a
constant pointer). Instead the function returns NULL.

Another advantage of the tsearch function in contrast to the
hsearch functions is that there is an easy way to remove
elements.

To remove a specific element matching key from the tree
tdelete can be used. It locates the matching element using the
same method as tfind. The corresponding element is then removed
and a pointer to the parent of the deleted node is returned by the
function. If there is no matching entry in the tree nothing can be
deleted and the function returns NULL. If the root of the tree
is deleted tdelete returns some unspecified value not equal to
NULL.

If the complete search tree has to be removed one can use
tdestroy. It frees all resources allocated by the tsearch
function to generate the tree pointed to by vroot.

For the data in each tree node the function freefct is called.
The pointer to the data is passed as the argument to the function. If
no such work is necessary freefct must point to a function doing
nothing. It is called in any case.

This function is a GNU extension and not covered by the System V or
X/Open specifications.

In addition to the function to create and destroy the tree data
structure, there is another function which allows you to apply a
function to all elements of the tree. The function must have this type:

void __action_fn_t (const void *nodep, VISIT value, int level);

The nodep is the data value of the current node (once given as the
key argument to tsearch). level is a numeric value
which corresponds to the depth of the current node in the tree. The
root node has the depth 0 and its children have a depth of
1 and so on. The VISIT type is an enumeration type.

— Data Type: VISIT

The VISIT value indicates the status of the current node in the
tree and how the function is called. The status of a node is either
`leaf' or `internal node'. For each leaf node the function is called
exactly once, for each internal node it is called three times: before
the first child is processed, after the first child is processed and
after both children are processed. This makes it possible to handle all
three methods of tree traversal (or even a combination of them).

preorder

The current node is an internal node and the function is called before
the first child was processed.

postorder

The current node is an internal node and the function is called after
the first child was processed.

endorder

The current node is an internal node and the function is called after
the second child was processed.

For each node in the tree with a node pointed to by root, the
twalk function calls the function provided by the parameter
action. For leaf nodes the function is called exactly once with
value set to leaf. For internal nodes the function is
called three times, setting the value parameter or action to
the appropriate value. The level argument for the action
function is computed while descending the tree with increasing the value
by one for the descend to a child, starting with the value 0 for
the root node.

Since the functions used for the action parameter to twalk
must not modify the tree data, it is safe to run twalk in more
than one thread at the same time, working on the same tree. It is also
safe to call tfind in parallel. Functions which modify the tree
must not be used, otherwise the behavior is undefined.

Published under the terms of the GNU General Public License