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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



tsearch, tfind, tdelete, twalk, tdestroy - manage binary search trees


#include <search.h>

void *tsearch(const void *
key, void **
     int (*compar)(const void *, const void *));
void *tfind(const void *
key, void * const *
     int (*compar)(const void *, const void *));
void *tdelete(const void *restrict
 key, void **restrict 
     int (*compar)(const void *, const void *));
void twalk(const void *root
, void (*action)(const void *, VISIT, int));
void tdestroy(void *root, 
void (*free_action)(void *


The tsearch(), tfind(), tdelete(), twalk(), and tdestroy() functions are routines for manipulating binary search trees. They are generalized from Knuth (6.2.2) Algorithms T and D. All comparisons are done with a user-supplied routine. This routine is called with two arguments, the pointers to the elements being compared. It 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 is used to build and access the tree. The key argument is a pointer to a datum to be accessed or stored. If there is a datum in the tree equal to *key (the value pointed to by key), a pointer to this found datum is returned. Otherwise, *key is inserted, and a pointer to it returned. Only pointers are copied, so the calling routine must store the data. The rootp argument points to a variable that points to the root of the tree. A null value for the variable pointed to by rootp denotes an empty tree; in this case, the variable will be set to point to the datum which will be at the root of the new tree.

Like tsearch(), tfind() will search for a datum in the tree, returning a pointer to it if found. However, if it is not found, tfind() will return a null pointer. The arguments for tfind() are the same as for tsearch().

The tdelete() function deletes a node from a binary search tree. The arguments are the same as for tsearch (). The variable pointed to by rootp will be changed if the deleted node was the root of the tree. tdelete () returns a pointer to the parent of the deleted node, or a null pointer if the node is not found.

The twalk() function traverses a binary search tree. The root argument is the root of the tree to be traversed. (Any node in a tree may be used as the root for a walk below that node.) 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 is the address of the node being visited. The second argument is 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 has been visited (during a depth-first, left-to-right traversal of the tree), or whether the node is a leaf. The third argument is the level of the node in the tree, with the root being level zero.

The pointers to the key and the root of the tree should be of type pointer-to-element, and cast to type pointer-to-character. Similarly, although declared as type pointer-to-character, the value returned should be cast into type pointer-to-element.

The tdestroy() removes the whole tree pointed to by root, freeing all resources allocated by the tsearch() function. For the key datum in each tree node, the function free_action() is called. The pointer to the key is passed as the argument to the function. If no such work is necessary free_action can be NULL.

Return Values

If the node is found, both tsearch() and tfind() return a pointer to it. If not, tfind() returns a null pointer, and tsearch() returns a pointer to the inserted item.

A null pointer is returned by tsearch() if there is not enough space available to create a new node.

A null pointer is returned by tsearch(), tfind() and tdelete() if rootp is a null pointer on entry.

The tdelete() function returns a pointer to the parent of the deleted node, or a null pointer if the node is not found.

The twalk() and tdestroy() functions return no value.


No errors are defined.


The root argument to twalk() is one level of indirection less than the rootp arguments to tsearch() and tdelete().

There are two nomenclatures used to refer to the order in which tree nodes are visited. tsearch() 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 alternate nomenclature uses preorder, inorder and postorder to refer to the same visits, which could result in some confusion over the meaning of postorder.

If the calling function alters the pointer to the root, the results are unpredictable.

These functions safely allows concurrent access by multiple threads to disjoint data, such as overlapping subtrees or tables.


Example 1 A sample program of using tsearch() function.

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. The tree is destroyed at the end.

#include <string.h>
#include <stdio.h>
#include <search.h>
struct node {
        char *string;
        int length;
char string_space[10000];
struct node nodes[500];
void *root = NULL;

int node_compare(const void *node1, const void *node2) {
        return strcmp(((const struct node *) node1)->string,
                      ((const struct node *) node2)->string);

void print_node(const void *node, VISIT order, int level) {
        if (order == preorder || order == leaf) {
                printf("length=%d, string=%20s\n",
                (*(struct node **)node)->length,
                (*(struct node **)node)->string);

        char *strptr = string_space;
        struct node *nodeptr = nodes;
        int i = 0;

        while (gets(strptr) != NULL && i++ < 500) {
                nodeptr->string = strptr;
                nodeptr->length = strlen(strptr);
                (void) tsearch((void *)nodeptr,
                        &root, node_compare);
                strptr += nodeptr->length + 1;
        twalk(root, print_node);
				 tdestroy(root, NULL);


See attributes(7) for descriptions of the following attributes:

Interface Stability

See Also

bsearch(3C), hsearch(3C), lsearch(3C), attributes(7), standards(7)

The Art of Computer Programming, Volume 3, Sorting and Searching by Donald E. Knuth, published by Addison-Wesley Publishing Company, 1973.