NAME
hcreate, hdestroy, hsearch - manage hash search table
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 shall manage hash
search tables.
The hcreate() function shall allocate sufficient space for the table,
and the application shall ensure it is called before hsearch() is used.
The nel argument is an estimate of the maximum number of entries that
the table shall contain. This number may be adjusted upward by the
algorithm in order to obtain certain mathematically favorable
circumstances.
The hdestroy() function shall dispose 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 hsearch() function is a hash-table search routine. It shall return
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(). 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.
These functions need not be reentrant. A function that is not required
to be reentrant is not required to be thread-safe.
RETURN VALUE
The hcreate() function shall return 0 if it cannot allocate sufficient
space for the table; otherwise, it shall return non-zero.
The hdestroy() function shall not return a value.
The hsearch() function shall return 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.
ERRORS
The hcreate() and hsearch() functions may fail if:
ENOMEM Insufficient storage space is available.
The following sections are informative.
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>
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 string_space[NUM_EMPL*20]; /* Space to store strings. */
struct info info_space[NUM_EMPL]; /* Space to store employee info. */
char *str_ptr = string_space; /* Next space in string_space. */
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_ptr, &info_ptr->age,
&info_ptr->room) != EOF && i++ < NUM_EMPL) {
/* Put information in structure, and structure in item. */
item.key = str_ptr;
item.data = 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;
}
APPLICATION USAGE
The hcreate() and hsearch() functions may use malloc() to allocate
space.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
bsearch() , lsearch() , malloc() , strcmp() , tsearch() , the Base
Definitions volume of IEEE Std 1003.1-2001, <search.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .