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hsearch(3c)
Standard C Library Functions hsearch(3C)
NAME
hsearch, hcreate, hdestroy - manage hash search tables
SYNOPSIS
#include <search.h>
ENTRY *hsearch(ENTRY item, ACTION action);
int hcreate(size_t nel);
void hdestroy(void);
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 indi‐
cating the location at which an entry can be found. The comparison
function used by hsearch() is strcmp() (see string(3C)). The item argu‐
ment is a structure of type ENTRY (defined in the <search.h> header)
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 void should be cast to pointer-to-void.)
The action argument is a member of an enumeration type ACTION (defined
in <search.h>) 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. Given a duplicate of an existing
item, the new item is not entered and hsearch() returns a pointer to
the existing item. FIND indicates that no entry should be made. Unsuc‐
cessful resolution is indicated by the return of a null pointer.
The hcreate() function allocates sufficient space for the table, and
must be called before hsearch() is used. The nel argument is an esti‐
mate of the maximum number of entries that the table will contain. This
number may be adjusted upward by the algorithm in order to obtain cer‐
tain mathematically favorable circumstances.
The hdestroy() function destroys the search table, and may be followed
by another call to hcreate().
RETURN VALUES
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 ta‐
ble is full.
The hcreate() function returns 0 if it cannot allocate sufficient space
for the table.
USAGE
The hsearch() and hcreate() functions use malloc(3C) to allocate space.
Only one hash search table may be active at any given time.
EXAMPLES
Example 1 Example to read in strings.
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.
#include <stdio.h>
#include <search.h>
#include <string.h>
#include <stdlib.h>
struct info { /* this is the info stored in 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;
/* 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 = (void *)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)
}
}
return 0;
}
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ Interface StabilityCommitted _ MT-LevelSafe _ Standard‐
See standards(7).
SEE ALSO
bsearch(3C), lsearch(3C), malloc(3C), string(3C), tsearch(3C), mal‐
loc(3MALLOC), attributes(7), standards(7)
The Art of Computer Programming, Volume 3, Sorting and Searching by
Donald E. Knuth, published by Addison-Wesley Publishing Company, 1973.
Oracle Solaris 11.4 17 Aug 2018 hsearch(3C)