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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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HCREATE(3P) POSIX Programmer's Manual HCREATE(3P)
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.
hcreate, hdestroy, hsearch — manage hash search table
#include <search.h>
int hcreate(size_t nel);
void hdestroy(void);
ENTRY *hsearch(ENTRY item, ACTION action);
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.
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.
The hcreate() and hsearch() functions may fail if:
ENOMEM Insufficient storage space is available.
The following sections are informative.
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;
}
The hcreate() and hsearch() functions may use malloc() to allocate
space.
None.
None.
bsearch(3p), lsearch(3p), malloc(3p), strcmp(3p), tdelete(3p)
The Base Definitions volume of POSIX.1‐2017, search.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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 .
IEEE/The Open Group 2017 HCREATE(3P)
Pages that refer to this page: search.h(0p), bsearch(3p), lsearch(3p), tdelete(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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