HSEARCH(S) UNIX System V HSEARCH(S)
Name
hsearch, hcreate, hdestroy - manage hash search tables
Syntax
#include <search.h>
ENTRY *hsearch (item, action)
ENTRY item;
ACTION action;
int hcreate (nel)
unsigned nel;
void hdestroy ( )
Description
The hsearch function 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.
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;
/* 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(S), lsearch(S), malloc(S), string(S), tsearch(S)
Diagnostics
The hsearch function 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.
Notes
The hsearch function 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(S)).
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.
Warning
hsearch and hcreate use malloc(S) to allocate space.
Note
Only one hash search table may be active at any given time.
Standards Conformance
hcreate, hdestroy and hsearch are conformant with:
AT&T SVID Issue 2, Select Code 307-127;
and The X/Open Portability Guide II of January 1987.
(printed 6/20/89)