bsearch(3C)

NAME

     bsearch - binary search a sorted table

SYNOPSIS

     #include <stdlib.h>

     void *bsearch(const void *key, const void *base, size_t nel,
     size_t size, int (*compar)(const void *,const void *));

DESCRIPTION

     The bsearch() function is a binary search  routine  general-
     ized  from  Knuth  (6.2.1) Algorithm B. It returns a pointer
     into a table (an array) indicating  where  a  datum  may  be
     found  or  a  null pointer if the datum cannot be found. The
     table must be previously sorted in increasing order  accord-
     ing to a comparison function pointed to by compar.

     The key argument points to a datum instance to be sought  in
     the  table.   The base argument points to the element at the
     base of the table.  The nel argument is the number  of  ele-
     ments  in  the  table.   The  size argument is the number of
     bytes in each element.

     The comparison function pointed to by compar is called  with
     two  arguments  that point to the key object and to an array
     element, in that order.  The function must return an integer
     less  than, equal to, or greater than 0 if the key object is
     considered, respectively, to be  less  than,  equal  to,  or
     greater than the array element.

RETURN VALUES

     The bsearch() function  returns  a  pointer  to  a  matching
     member of the array, or a null pointer if no match is found.
     If two or  more  members  compare  equal,  which  member  is
     returned is unspecified.

USAGE

     The pointers to the key and the element at the base  of  the
     table should be of type pointer-to-element.

     The comparison function need  not  compare  every  byte,  so
     arbitrary  data may be contained in the elements in addition
     to the values being compared.

     If the number of elements in the table is less than the size
     reserved for the table, nel should be the lower number.

EXAMPLES

     Example  1:  Examples  for  searching  a  table   containing
     pointers to nodes.

     The example below searches a table  containing  pointers  to
     nodes  consisting  of  a string and its length. The table is
     ordered alphabetically on the string in the node pointed  to
     by each entry.

     This  program  reads  in  strings  and  either   finds   the
     corresponding node and prints out the string and its length,
     or prints an error message.

     #include <stdio.h>
     #include <stdlib.h>
     #include <string.h>
     struct node {  /* these are stored in the table */
         char *string;
         int length;
     };
     static struct node table[] = {    /* table to be searched */
         { "asparagus", 10 },
         { "beans", 6 },
         { "tomato", 7 },
         { "watermelon", 11 },
     };

     main()
     {
         struct node *node_ptr, node;
         /* routine to compare 2 nodes */
         static int node_compare(const void *, const void *);
         char str_space[20];   /* space to read string into */

         node.string = str_space;
         while (scanf("%20s", node.string) != EOF) {
             node_ptr = bsearch( &node,
                 table, sizeof(table)/sizeof(struct node),
                 sizeof(struct node), node_compare);
             if (node_ptr != NULL) {
                 (void) printf("string = %20s, length = %d\n",
                     node_ptr->string, node_ptr->length);
             } else {
                 (void)printf("not found: %20s\n", node.string);
             }
         }
         return(0);
     }

     /* routine to compare two nodes based on an  */
     /* alphabetical ordering of the string field */
     static int
     node_compare(const void *node1, const void *node2) {
         return (strcmp(
                 ((const struct node *)node1)->string,
                 ((const struct node *)node2)->string));

     }

ATTRIBUTES

     See attributes(5) for descriptions of the  following  attri-
     butes:

     ____________________________________________________________
    |       ATTRIBUTE TYPE        |       ATTRIBUTE VALUE       |
    |_____________________________|_____________________________|
    | MT-Level                    | Safe                        |
    |_____________________________|_____________________________|

SEE ALSO

     hsearch(3C),  lsearch(3C),  qsort(3C),  tsearch(3C),  attri-
     butes(5)