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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
tdelete, tfind, tsearch, twalk — manage a binary search tree
SYNOPSIS
#include <search.h> void *tdelete(const void *restrict key, void **restrict rootp, int(*compar)(const void *, const void *)); void *tfind(const void *key, void *const *rootp, int(*compar)(const void *, const void *)); void *tsearch(const void *key, void **rootp, int (*compar)(const void *, const void *)); void twalk(const void *root, void (*action)(const void *, VISIT, int));
DESCRIPTION
The tdelete(), tfind(), tsearch(), and twalk() functions manipulate binary search trees. Comparisons are
made with a user-supplied routine, the address of which is passed as the compar argument. This routine is
called with two arguments, which are the pointers to the elements being compared. The application shall
ensure that the user-supplied routine returns an integer less than, equal to, or greater than 0,
according to whether the first argument is to be considered less than, equal to, or greater than the
second argument. The comparison function need not compare every byte, so arbitrary data may be contained
in the elements in addition to the values being compared.
The tsearch() function shall build and access the tree. The key argument is a pointer to an element to be
accessed or stored. If there is a node in the tree whose element is equal to the value pointed to by key,
a pointer to this found node shall be returned. Otherwise, the value pointed to by key shall be inserted
(that is, a new node is created and the value of key is copied to this node), and a pointer to this node
returned. Only pointers are copied, so the application shall ensure that the calling routine stores the
data. The rootp argument points to a variable that points to the root node of the tree. A null pointer
value for the variable pointed to by rootp denotes an empty tree; in this case, the variable shall be set
to point to the node which shall be at the root of the new tree.
Like tsearch(), tfind() shall search for a node in the tree, returning a pointer to it if found.
However, if it is not found, tfind() shall return a null pointer. The arguments for tfind() are the same
as for tsearch().
The tdelete() function shall delete a node from a binary search tree. The arguments are the same as for
tsearch(). The variable pointed to by rootp shall be changed if the deleted node was the root of the
tree. If the deleted node was the root of the tree and had no children, the variable pointed to by rootp
shall be set to a null pointer. The tdelete() function shall return a pointer to the parent of the
deleted node, or an unspecified non-null pointer if the deleted node was the root node, or a null pointer
if the node is not found.
If tsearch() adds an element to a tree, or tdelete() successfully deletes an element from a tree, the
concurrent use of that tree in another thread, or use of pointers produced by a previous call to tfind()
or tsearch(), produces undefined results.
The twalk() function shall traverse a binary search tree. The root argument is a pointer to the root node
of the tree to be traversed. (Any node in a tree may be used as the root for a walk below that node.)
The argument action is the name of a routine to be invoked at each node. This routine is, in turn, called
with three arguments. The first argument shall be the address of the node being visited. The structure
pointed to by this argument is unspecified and shall not be modified by the application, but it shall be
possible to cast a pointer-to-node into a pointer-to-pointer-to-element to access the element stored in
the node. The second argument shall be a value from an enumeration data type:
typedef enum { preorder, postorder, endorder, leaf } VISIT;
(defined in <search.h>), depending on whether this is the first, second, or third time that the node is
visited (during a depth-first, left-to-right traversal of the tree), or whether the node is a leaf. The
third argument shall be the level of the node in the tree, with the root being level 0.
If the calling function alters the pointer to the root, the result is undefined.
If the functions pointed to by action or compar (for any of these binary search functions) change the
tree, the results are undefined.
These functions are thread-safe only as long as multiple threads do not access the same tree.
RETURN VALUE
If the node is found, both tsearch() and tfind() shall return a pointer to it. If not, tfind() shall
return a null pointer, and tsearch() shall return a pointer to the inserted item.
A null pointer shall be returned by tsearch() if there is not enough space available to create a new
node.
A null pointer shall be returned by tdelete(), tfind(), and tsearch() if rootp is a null pointer on
entry.
The tdelete() function shall return a pointer to the parent of the deleted node, or an unspecified non-
null pointer if the deleted node was the root node, or a null pointer if the node is not found.
The twalk() function shall not return a value.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
The following code reads in strings and stores structures containing a pointer to each string and a count
of its length. It then walks the tree, printing out the stored strings and their lengths in alphabetical
order.
#include <limits.h>
#include <search.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
struct element { /* Pointers to these are stored in the tree. */
int count;
char string[];
};
void *root = NULL; /* This points to the root. */
int main(void)
{
char str[_POSIX2_LINE_MAX+1];
int length = 0;
struct element *elementptr;
void *node;
void print_node(const void *, VISIT, int);
int node_compare(const void *, const void *),
delete_root(const void *, const void *);
while (fgets(str, sizeof(str), stdin)) {
/* Set element. */
length = strlen(str);
if (str[length-1] == '\n')
str[--length] = '\0';
elementptr = malloc(sizeof(struct element) + length + 1);
strcpy(elementptr->string, str);
elementptr->count = 1;
/* Put element into the tree. */
node = tsearch((void *)elementptr, &root, node_compare);
if (node == NULL) {
fprintf(stderr,
"tsearch: Not enough space available\n");
exit(EXIT_FAILURE);
}
else if (*(struct element **)node != elementptr) {
/* A node containing the element already exists */
(*(struct element **)node)->count++;
free(elementptr);
}
}
twalk(root, print_node);
/* Delete all nodes in the tree */
while (root != NULL) {
elementptr = *(struct element **)root;
printf("deleting node: string = %s, count = %d\n",
elementptr->string,
elementptr->count);
tdelete((void *)elementptr, &root, delete_root);
free(elementptr);
}
return 0;
}
/*
* This routine compares two nodes, based on an
* alphabetical ordering of the string field.
*/
int
node_compare(const void *node1, const void *node2)
{
return strcmp(((const struct element *) node1)->string,
((const struct element *) node2)->string);
}
/*
* This comparison routine can be used with tdelete()
* when explicitly deleting a root node, as no comparison
* is necessary.
*/
int
delete_root(const void *node1, const void *node2)
{
return 0;
}
/*
* This routine prints out a node, the second time
* twalk encounters it or if it is a leaf.
*/
void
print_node(const void *ptr, VISIT order, int level)
{
const struct element *p = *(const struct element **) ptr;
if (order == postorder || order == leaf) {
(void) printf("string = %s, count = %d\n",
p->string, p->count);
}
}
APPLICATION USAGE
The root argument to twalk() is one level of indirection less than the rootp arguments to tdelete() and
tsearch().
There are two nomenclatures used to refer to the order in which tree nodes are visited. The twalk()
function uses preorder, postorder, and endorder to refer respectively to visiting a node before any of
its children, after its left child and before its right, and after both its children. The alternative
nomenclature uses preorder, inorder, and postorder to refer to the same visits, which could result in
some confusion over the meaning of postorder.
Since the return value of tdelete() is an unspecified non-null pointer in the case that the root of the
tree has been deleted, applications should only use the return value of tdelete() as indication of
success or failure and should not assume it can be dereferenced. Some implementations in this case will
return a pointer to the new root of the tree (or to an empty tree if the deleted root node was the only
node in the tree); other implementations return arbitrary non-null pointers.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
hcreate(), lsearch()
The Base Definitions volume of POSIX.1‐2017, <search.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .