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NAME
tsearch, tfind, tdelete, twalk, twalk_r, tdestroy - manage a binary search tree
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <search.h>
typedef enum { preorder, postorder, endorder, leaf } VISIT;
void *tsearch(const void *key, void **rootp,
int (*compar)(const void *, const void *));
void *tfind(const void *key, void *const *rootp,
int (*compar)(const void *, const void *));
void *tdelete(const void *restrict key, void **restrict rootp,
int (*compar)(const void *, const void *));
void twalk(const void *root,
void (*action)(const void *nodep, VISIT which,
int depth));
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <search.h>
void twalk_r(const void *root,
void (*action)(const void *nodep, VISIT which,
void *closure),
void *closure);
void tdestroy(void *root, void (*free_node)(void *nodep));
DESCRIPTION
tsearch(), tfind(), twalk(), and tdelete() manage a binary search tree. They are
generalized from Knuth (6.2.2) Algorithm T. The first field in each node of the tree is a
pointer to the corresponding data item. (The calling program must store the actual data.)
compar points to a comparison routine, which takes pointers to two items. It should
return an integer which is negative, zero, or positive, depending on whether the first
item is less than, equal to, or greater than the second.
tsearch() searches the tree for an item. key points to the item to be searched for.
rootp points to a variable which points to the root of the tree. If the tree is empty,
then the variable that rootp points to should be set to NULL. If the item is found in the
tree, then tsearch() returns a pointer to the corresponding tree node. (In other words,
tsearch() returns a pointer to a pointer to the data item.) If the item is not found,
then tsearch() adds it, and returns a pointer to the corresponding tree node.
tfind() is like tsearch(), except that if the item is not found, then tfind() returns
NULL.
tdelete() deletes an item from the tree. Its arguments are the same as for tsearch().
twalk() performs depth-first, left-to-right traversal of a binary tree. root points to
the starting node for the traversal. If that node is not the root, then only part of the
tree will be visited. twalk() calls the user function action each time a node is visited
(that is, three times for an internal node, and once for a leaf). action, in turn, takes
three arguments. The first argument is a pointer to the node being visited. The
structure of the node is unspecified, but it is possible to cast the pointer to a pointer-
to-pointer-to-element in order to access the element stored within the node. The
application must not modify the structure pointed to by this argument. The second
argument is an integer which takes one of the values preorder, postorder, or endorder
depending on whether this is the first, second, or third visit to the internal node, or
the value leaf if this is the single visit to a leaf node. (These symbols are defined in
<search.h>.) The third argument is the depth of the node; the root node has depth zero.
(More commonly, preorder, postorder, and endorder are known as preorder, inorder, and
postorder: before visiting the children, after the first and before the second, and after
visiting the children. Thus, the choice of name postorder is rather confusing.)
twalk_r() is similar to twalk(), but instead of the depth argument, the closure argument
pointer is passed to each invocation of the action callback, unchanged. This pointer can
be used to pass information to and from the callback function in a thread-safe fashion,
without resorting to global variables.
tdestroy() removes the whole tree pointed to by root, freeing all resources allocated by
the tsearch() function. For the data in each tree node the function free_node is called.
The pointer to the data is passed as the argument to the function. If no such work is
necessary, free_node must point to a function doing nothing.
RETURN VALUE
tsearch() returns a pointer to a matching node in the tree, or to the newly added node, or
NULL if there was insufficient memory to add the item. tfind() returns a pointer to the
node, or NULL if no match is found. If there are multiple items that match the key, the
item whose node is returned is unspecified.
tdelete() returns a pointer to the parent of the node deleted, or NULL if the item was not
found. If the deleted node was the root node, tdelete() returns a dangling pointer that
must not be accessed.
tsearch(), tfind(), and tdelete() also return NULL if rootp was NULL on entry.
VERSIONS
twalk_r() is available since glibc 2.30.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌────────────────────────────────────────────────────┬───────────────┬────────────────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────────────┼───────────────┼────────────────────┤
│tsearch(), tfind(), tdelete() │ Thread safety │ MT-Safe race:rootp │
├────────────────────────────────────────────────────┼───────────────┼────────────────────┤
│twalk() │ Thread safety │ MT-Safe race:root │
├────────────────────────────────────────────────────┼───────────────┼────────────────────┤
│twalk_r() │ Thread safety │ MT-Safe race:root │
├────────────────────────────────────────────────────┼───────────────┼────────────────────┤
│tdestroy() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────────────┴───────────────┴────────────────────┘
STANDARDS
POSIX.1-2001, POSIX.1-2008, SVr4. The functions tdestroy() and twalk_r() are GNU
extensions.
NOTES
twalk() takes a pointer to the root, while the other functions take a pointer to a
variable which points to the root.
tdelete() frees the memory required for the node in the tree. The user is responsible for
freeing the memory for the corresponding data.
The example program depends on the fact that twalk() makes no further reference to a node
after calling the user function with argument "endorder" or "leaf". This works with the
GNU library implementation, but is not in the System V documentation.
EXAMPLES
The following program inserts twelve random numbers into a binary tree, where duplicate
numbers are collapsed, then prints the numbers in order.
#define _GNU_SOURCE /* Expose declaration of tdestroy() */
#include <search.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
static void *root = NULL;
static void *
xmalloc(size_t n)
{
void *p;
p = malloc(n);
if (p)
return p;
fprintf(stderr, "insufficient memory\n");
exit(EXIT_FAILURE);
}
static int
compare(const void *pa, const void *pb)
{
if (*(int *) pa < *(int *) pb)
return -1;
if (*(int *) pa > *(int *) pb)
return 1;
return 0;
}
static void
action(const void *nodep, VISIT which, int depth)
{
int *datap;
switch (which) {
case preorder:
break;
case postorder:
datap = *(int **) nodep;
printf("%6d\n", *datap);
break;
case endorder:
break;
case leaf:
datap = *(int **) nodep;
printf("%6d\n", *datap);
break;
}
}
int
main(void)
{
int *ptr;
int **val;
srand(time(NULL));
for (unsigned int i = 0; i < 12; i++) {
ptr = xmalloc(sizeof(*ptr));
*ptr = rand() & 0xff;
val = tsearch(ptr, &root, compare);
if (val == NULL)
exit(EXIT_FAILURE);
if (*val != ptr)
free(ptr);
}
twalk(root, action);
tdestroy(root, free);
exit(EXIT_SUCCESS);
}
SEE ALSO
bsearch(3), hsearch(3), lsearch(3), qsort(3)