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
comparison 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 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.
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
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.