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       tsearch, tfind, tdelete, twalk, tdestroy - manage a binary tree


       #include <search.h>

       void *tsearch(const void *key, void **rootp,
                       int (*compar)(const void *, const void *));

       void *tfind(const void *key, void *const *rootp,
                       int (*compar)(const void *, const void *));

       void *tdelete(const void *key, void **rootp,
                       int (*compar)(const void *, const void *));

       void twalk(const void *root, void (*action)(const void *nodep,
                                          const VISIT which,
                                          const int depth));

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <search.h>

       void tdestroy(void *root, void (*free_node)(void *nodep));


       tsearch(),  tfind(),  twalk(),  and  tdelete() manage a binary 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 it.  If it is not found, then tsearch() adds it,
       and returns a pointer to the newly added item.

       tfind() is like tsearch(), except that if the item is  not  found,  then  tfind()  returns

       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.)

       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 item in the tree, or to the newly added item, or
       NULL if there was insufficient memory to add the item.  tfind() returns a pointer  to  the
       item,  or  NULL  if no match is found.  If there are multiple elements that match the key,
       the element returned is unspecified.

       tdelete() returns a pointer to the parent of the item deleted, or NULL if the item was not

       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).

       │InterfaceAttributeValue              │
       │tsearch(), tfind(), │ Thread safety │ MT-Safe race:rootp │
       │tdelete()           │               │                    │
       │twalk()             │ Thread safety │ MT-Safe race:root  │
       │tdestroy()          │ Thread safety │ MT-Safe            │


       POSIX.1-2001, POSIX.1-2008, SVr4.  The function tdestroy() is a GNU extension.


       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 <stdlib.h>
       #include <stdio.h>
       #include <time.h>

       static void *root = NULL;

       static void *
       xmalloc(unsigned n)
           void *p;
           p = malloc(n);
           if (p)
               return p;
           fprintf(stderr, "insufficient memory\n");

       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, const VISIT which, const int depth)
           int *datap;

           switch (which) {
           case preorder:
           case postorder:
               datap = *(int **) nodep;
               printf("%6d\n", *datap);
           case endorder:
           case leaf:
               datap = *(int **) nodep;
               printf("%6d\n", *datap);

           int i, *ptr;
           void *val;

           for (i = 0; i < 12; i++) {
               ptr = xmalloc(sizeof(int));
               *ptr = rand() & 0xff;
               val = tsearch((void *) ptr, &root, compare);
               if (val == NULL)
               else if ((*(int **) val) != ptr)
           twalk(root, action);
           tdestroy(root, free);


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


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