NAME

hsearch, hcreate, hdestroy − manage hash search tables

SYNOPSIS

#include <search.h>

ENTRY ∗hsearch (item, action)
ENTRY item;
ACTION action;

int hcreate (nel)
unsigned nel;

void hdestroy ( )

DESCRIPTION

hsearch is a hash-table search routine generalized from Knuth (6.4) Algorithm D.  It returns a pointer into a hash table indicating the location at which an entry can be found.  item is a structure of type ENTRY (defined in the <search.h> header file) containing two pointers: item.key points to the comparison key, and item.data points to any other data to be associated with that key.  (Pointers to types other than character should be cast to pointer-to-character.)  action is a member of an enumeration type ACTION indicating the disposition of the entry if it cannot be found in the table.  ENTER indicates that the item should be inserted in the table at an appropriate point.  FIND indicates that no entry should be made.  Unsuccessful resolution is indicated by the return of a NULL pointer. 

hcreate allocates sufficient space for the table, and must be called before hsearch is used.  nel is an estimate of the maximum number of entries that the table will contain.  This number may be adjusted upward by the algorithm in order to obtain certain mathematically favorable circumstances. 

hdestroy destroys the search table, and may be followed by another call to hcreate.

NOTES

hsearch uses open addressing with a multiplicative hash function.  However, its source code has many other options available which the user may select by compiling the hsearch source with the following symbols defined to the preprocessor:

DIV Use the remainder modulo table size as the hash function instead of the multiplicative algorithm. 

USCR Use a User Supplied Comparison Routine for ascertaining table membership.  The routine should be named hcompar and should behave in a mannner similar to strcmp [see string(3C)].

CHAINED
Use a linked list to resolve collisions. If this option is selected, the following other options become available.

START Place new entries at the beginning of the linked list (default is at the end). 

SORTUP Keep the linked list sorted by key in ascending order. 

SORTDOWN
Keep the linked list sorted by key in descending order.

Additionally, there are preprocessor flags for obtaining debugging printout (−DDEBUG) and for including a test driver in the calling routine (−DDRIVER).  Consult the source code for further details. 

EXAMPLE

The following example reads in strings followed by two numbers and stores them in a hash table, discarding duplicates.  It then reads in strings and finds the matching entry in the hash table and prints it out. 

#include <stdio.h>
#include <search.h>
struct info {/∗ this is the info stored in the table ∗/
int age, room;/∗ other than the key. ∗/
};
#define NUM_EMPL    5000    /∗ # of elements in search table ∗/
 main( )
{
/∗ space to store strings ∗/
char string_space[NUM_EMPL∗20];
/∗ space to store employee info ∗/
struct info info_space[NUM_EMPL];
/∗ next avail space in string_space ∗/
char ∗str_ptr = string_space;
/∗ next avail space in info_space ∗/
struct info ∗info_ptr = info_space;
ENTRY item, ∗found_item, ∗hsearch( );
/∗ name to look for in table ∗/
char name_to_find[30];
int i = 0;
 /∗ create table ∗/
(void) hcreate(NUM_EMPL);
while (scanf("%s%d%d", str_ptr, &info_ptr−>age,
       &info_ptr−>room) != EOF && i++ < NUM_EMPL) {
/∗ put info in structure, and structure in item ∗/
item.key = str_ptr;
item.data = (char ∗)info_ptr;
str_ptr += strlen(str_ptr) + 1;
info_ptr++;
/∗ put item into table ∗/
(void) hsearch(item, ENTER);
}
 /∗ access table ∗/
item.key = name_to_find;
while (scanf("%s", item.key) != EOF) {
    if ((found_item = hsearch(item, FIND)) != NULL) {
/∗ if item is in the table ∗/
(void)printf("found %s, age = %d, room = %d\n",
found_item−>key,
((struct info ∗)found_item−>data)−>age,
((struct info ∗)found_item−>data)−>room);
    } else {
(void)printf("no such employee %s\n",
name_to_find)
    }
}
}

SEE ALSO

bsearch(3C), lsearch(3C), malloc(3C), malloc(3X), string(3C), tsearch(3C)

DIAGNOSTICS

hsearch returns a NULL pointer either if the action is FIND and the item could not be found or the action is ENTER and the table is full. 

hcreate returns zero if it cannot allocate sufficient space for the table. 

WARNING

hsearch and hcreate use malloc(3C) to allocate space.

CAVEAT

Only one hash search table may be active at any given time. 

September 02, 1992