HSEARCH(3C) HSEARCH(3C)
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
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HSEARCH(3C) HSEARCH(3C)
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). The source code should
be consulted for further details.
EXAMPLE
The following example will read in strings followed by two
numbers and store them in a hash table, discarding
duplicates. It will then read in strings and find the
matching entry in the hash table and print 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;
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/* 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 if either 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.
ORIGIN
AT&T V.3
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