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TSEARCH(3) FreeBSD Library Functions Manual TSEARCH(3)

tsearch, tfind, tdelete, twalk
manipulate binary search trees

#include <search.h>
void *
tdelete(const void * restrict key, posix_tnode ** restrict rootp, int (*compar) (const void *, const void *));
posix_tnode *
tfind(const void *key, posix_tnode * const *rootp, int (*compar) (const void *, const void *));
posix_tnode *
tsearch(const void *key, posix_tnode **rootp, int (*compar) (const void *, const void *));
twalk(const posix_tnode *root, void (*action) (const posix_tnode *, VISIT, int));

The tdelete(), tfind(), tsearch(), and twalk() functions manage binary search trees. This implementation uses a balanced AVL tree, which due to its strong theoretical limit on the height of the tree has the advantage of calling the comparison function relatively infrequently.
The comparison function passed in by the user has the same style of return values as strcmp(3).
The tfind() function searches for the datum matched by the argument key in the binary tree rooted at rootp, returning a pointer to the datum if it is found and NULL if it is not.
The tsearch() function is identical to tfind() except that if no match is found, key is inserted into the tree and a pointer to it is returned. If rootp points to a NULL value a new binary search tree is created.
The tdelete() function deletes a node from the specified binary search tree and returns a pointer to the parent of the node to be deleted. It takes the same arguments as tfind() and tsearch(). If the node to be deleted is the root of the binary search tree, rootp will be adjusted.
The twalk() function walks the binary search tree rooted in root and calls the function action on each node. The action function is called with three arguments: a pointer to the current node, a value from the enum typedef enum { preorder, postorder, endorder, leaf } VISIT; specifying the traversal type, and a node level (where level zero is the root of the tree).

The tsearch() function returns NULL if allocation of a new node fails (usually due to a lack of free memory).
The tfind(), tsearch(), and tdelete() functions return NULL if rootp is NULL or the datum cannot be found.
The twalk() function returns no value.

This example uses tsearch() to search for four strings in root. Because the strings are not already present, they are added. tsearch() is called twice on the fourth string to demonstrate that a string is not added when it is already present. tfind() is used to find the single instance of the fourth string, and tdelete() removes it. Finally, twalk() is used to return and display the resulting binary search tree.
#include <stdio.h> 
#include <search.h> 
#include <string.h> 
comp(const void *a, const void *b) 
	return strcmp(a, b); 
printwalk(const posix_tnode * node, VISIT v, int __unused0) 
	if (v == postorder || v == leaf) { 
		printf("node: %s\n", *(char **)node); 
	posix_tnode *root = NULL; 
	char one[] = "blah1"; 
	char two[] = "blah-2"; 
	char three[] = "blah-3"; 
	char four[] = "blah-4"; 
	tsearch(one, &root, comp); 
	tsearch(two, &root, comp); 
	tsearch(three, &root, comp); 
	tsearch(four, &root, comp); 
	tsearch(four, &root, comp); 
	printf("four: %s\n", *(char **)tfind(four, &root, comp)); 
	tdelete(four, &root, comp); 
	twalk(root, printwalk); 
	return 0; 

bsearch(3), hsearch(3), lsearch(3)

These functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).
The posix_tnode type is not part of IEEE Std 1003.1-2008 (“POSIX.1”), but is expected to be standardized by future versions of the standard. It is defined as void for source-level compatibility. Using posix_tnode makes distinguishing between nodes and keys easier.
June 4, 2017 FreeBSD 12.0-RELEASE

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