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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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tsearch(3) Library Functions Manual tsearch(3)
tsearch, tfind, tdelete, twalk, twalk_r, tdestroy - manage a
binary search tree
Standard C library (libc, -lc)
#include <search.h>
typedef enum { preorder, postorder, endorder, leaf } VISIT;
void *tfind(const void *key, void *const *rootp,
typeof(int (const void *, const void *)) *compar);
void *tsearch(const void *key, void **rootp,
typeof(int (const void *, const void *)) *compar);
void *tdelete(const void *restrict key, void **restrict rootp,
typeof(int (const void *, const void *)) *compar);
void twalk(const void *root,
typeof(void (const void *nodep, VISIT which, int depth))
*action);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <search.h>
void twalk_r(const void *root,
typeof(void (const void *nodep, VISIT which, void *closure))
*action,
void *closure);
void tdestroy(void *root,
typeof(void (void *nodep)) *free_node);
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.
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.
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────────────────┬───────────────┬────────────────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────┼───────────────┼────────────────────┤
│ tsearch(), tfind(), │ Thread safety │ MT-Safe race:rootp │
│ tdelete() │ │ │
├───────────────────────────┼───────────────┼────────────────────┤
│ twalk() │ Thread safety │ MT-Safe race:root │
├───────────────────────────┼───────────────┼────────────────────┤
│ twalk_r() │ Thread safety │ MT-Safe race:root │
├───────────────────────────┼───────────────┼────────────────────┤
│ tdestroy() │ Thread safety │ MT-Safe │
└───────────────────────────┴───────────────┴────────────────────┘
tsearch()
tfind()
tdelete()
twalk()
POSIX.1-2008.
tdestroy()
twalk_r()
GNU.
tsearch()
tfind()
tdelete()
twalk()
POSIX.1-2001, POSIX.1-2008, SVr4.
twalk_r()
glibc 2.30.
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.
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);
}
bsearch(3), hsearch(3), lsearch(3), qsort(3)
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Linux man-pages 6.15 2025-05-17 tsearch(3)
Pages that refer to this page: bsearch(3), hsearch(3), lsearch(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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