HSEARCH(S) XENIX System V HSEARCH(S)
Name
hsearch, hcreate, hdestroy - Manages hash search tables.
Syntax
#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. This routine 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
item.data points to any other data associated with the
comparison 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 the appropriate point.
FIND indicates that no entry should be made. The return of
a NULL pointer indicates unsuccessful resolution.
hcreate makes sufficient space for the table, and must be
called before hsearch is used. nel is an estimate of the
highest number of entries the table will contain. The
algorithm can adjust this number upwards in order to obtain
mathematically favorable circumstances.
hdestroy destroys the search table, and may be followed by
another call to hcreate.
Page 1 (printed 8/7/87)
HSEARCH(S) XENIX System V HSEARCH(S)
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 determining
table membership. The routine should be named hcompar
and should behave in a manner similar to strcmp (see
string(S)).
CHAINED
Use a linked list to resolve collisions. If this
option is selected, the user has the following options:
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.
In addition, 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.
Return Value
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.
Example
The following fragment of code 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 ( )
{
Page 2 (printed 8/7/87)
HSEARCH(S) XENIX System V HSEARCH(S)
/* 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 (scan(``%s%d%d'', str_ptr, &infor_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(S), lsearch(S), malloc(S), string(S), tsearch(S).
Diagnostics
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.
Notes
Only one hash search table may be active at any given time.
Page 3 (printed 8/7/87)
HSEARCH(S) XENIX System V HSEARCH(S)
Warning
hsearch and hcreate use malloc(S) to allocate space.
Page 4 (printed 8/7/87)