hsearch(3)
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hsearch, hcreate, hdestroy Subroutine
manage hash search tables
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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. It returns a pointer into a hash table
indicating where 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 what
to do with 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 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
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hsearch(3)
Hsearch uses open addressing with a multiplicative hash function.
However, its source code has many other options. If you have the
source code, then you can select these options by compiling the
hsearch source with the following symbols defined to the
preprocessor:
DIV Use remainder-modulo-tablesize as the hash function
instead of the multiplicative algorithm.
USCR Use a user supplied comparison routine to see if an
entry is in the table. The routine should be named
hcompar and should behave like strcmp (see
string(3C)).
CHAINED Use a linked list to resolve collisions. These are
the options for CHAINED:
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.
There are also preprocessor flags to get debugging printout
(-DDEBUG) and to include a test driver in the calling routine
(-DDRIVER).
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, 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 */
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hsearch(3)
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 if the action is FIND and the item
could not be found, or if the action is ENTER and the table is
full.
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Hcreate returns zero if it cannot allocate enough space for the
table.
WARNING
Hsearch and hcreate use malloc(3C) to allocate space.
BUGS
Only one hash search table may be active at once.
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