hcreate(3) BSD Library Functions Manual hcreate(3)
NAME
hcreate, hdestroy, hsearch -- manage hash search table
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <search.h>
int
hcreate(size_t nel);
void
hdestroy(void);
ENTRY *
hsearch(ENTRY item, ACTION action);
DESCRIPTION
The hcreate(), hdestroy(), and hsearch() functions manage hash search
tables.
The hcreate() function allocates sufficient space for the table, and the
application should ensure it is called before hsearch() is used. The nel
argument is an estimate of the maximum number of entries that the table
should contain. This number may be adjusted upward by the algorithm in
order to obtain certain mathematically favorable circumstances.
The hdestroy() function disposes of the search table, and may be followed
by another call to hcreate(). After the call to hdestroy(), the data can
no longer be considered accessible. The hdestroy() function calls
free(3) for each comparison key in the search table but not the data item
associated with the key.
The hsearch() function is a hash-table search routine. It returns a
pointer into a hash table indicating the location at which an entry can
be found. The item argument is a structure of type ENTRY (defined in the
<search.h> header) containing two pointers: item.key points to the com-
parison key (a char *), and item.data (a void *) points to any other data
to be associated with that key. The comparison function used by
hsearch() is strcmp(3). The action argument is a member of an enumera-
tion 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.
The comparison key (passed to hsearch() as item.key) must be allocated
using malloc(3) if action is ENTER and hdestroy() is called.
RETURN VALUES
The hcreate() function returns 0 if the table creation failed and the
global variable errno is set to indicate the error; otherwise, a non-zero
value is returned.
The hdestroy() function does not return a value.
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.
EXAMPLES
The following example reads in strings followed by two numbers and stores
them in a hash table, discarding duplicates. It then reads in strings
and finds the matching entry in the hash table and prints it out.
#include <stdio.h>
#include <search.h>
#include <string.h>
#include <stdlib.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. */
int
main(void)
{
char str[BUFSIZ]; /* Space to read string */
struct info info_space[NUM_EMPL]; /* Space to store employee info. */
struct info *info_ptr = info_space; /* Next space in info_space. */
ENTRY item;
ENTRY *found_item; /* Name to look for in table. */
char name_to_find[30];
int i = 0;
/* Create table; no error checking is performed. */
(void) hcreate(NUM_EMPL);
while (scanf("%s%d%d", str, &info_ptr->age,
&info_ptr->room) != EOF && i++ < NUM_EMPL) {
/* Put information in structure, and structure in item. */
item.key = strdup(str);
item.data = info_ptr;
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);
}
hdestroy();
return 0;
}
ERRORS
The hcreate() and hsearch() functions may fail if:
[ENOMEM] Insufficient storage space is available.
[EINVAL] A table already exists.
SEE ALSO
bsearch(3), lsearch(3), malloc(3), strcmp(3), tsearch(3)
STANDARDS
The hcreate(), hdestroy(), and hsearch() functions conform to X/Open
Portability Guide Issue 4, Version 2 (``XPG4.2'').
HISTORY
The hcreate(), hdestroy(), and hsearch() functions first appeared in AT&T
System V UNIX.
BUGS
The interface permits the use of only one hash table at a time.
BSD July 6, 2008 BSD
Mac OS X 10.8 - Generated Tue Aug 28 05:48:52 CDT 2012