HCREATE (3) | C library functions | Unix Manual Pages

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HCREATE (3) | C library functions | Unix Manual Pages | :man▋

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
.Vt ENTRY (defined in the
.In search.h header) containing two pointers: item.key points to the comparison key (a
.Vt "char *" ) , and item.data (a
.Vt "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
.Vt 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 it cannot allocate sufficient space for the table; otherwise, it returns non-zero.

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;
}