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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of
this interface may differ (consult the corresponding Linux manual page for details of
Linux behavior), or the interface may not be implemented on Linux.
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 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(), tdelete()
The Base Definitions volume of POSIX.1‐2008, <search.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .