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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 .