hcreate, hdestroy, hsearch - hash table management
#include <search.h>
ENTRY *hsearch(ENTRY item, ACTION action);
int hcreate(unsigned nel);
void hdestroy(void);
These three functions allow the user to create a hash table which associates
a key with any data.
First the table must be created with the function hcreate(). nel is an
estimate of the number of entries in the table. hcreate() may adjust
this value upward to improve the performance of the resulting hash table.
The GNU implementation of hsearch() will also enlarge the table
if it gets nearly full. malloc(3) is used to allocate space for the
table.
The corresponding function hdestroy() frees the memory occupied by the
hash table so that a new table can be constructed.
item is of type ENTRY, which is a typedef defined in <search.h> and
includes these elements:
typedef struct entry
{
char *key;
char *data;
} ENTRY;
key points to the zero-terminated ASCII string which is the search key.
data points to the data associated with that key. (A pointer to a type
other than character should be cast to pointer-to-character.)
hsearch() searches the hash table for an item with the same key as
item, and if successful returns a pointer to it. action determines
what hsearch() does after an unsuccessful search. A value of ENTER
instructs it to insert the new item, while a value of FIND means to
return NULL.
hcreate() returns NULL if the hash table cannot be successfully
installed.
hsearch() returns NULL if action is ENTER and there is insufficient
memory to expand the hash table, or action is FIND and item cannot be
found in the hash table.
SVID, except that in SysV, the hash table cannot grow.
The implementation can manage only one hash table at a time. Individual
hash table entries can be added, but not deleted.
The following program inserts 24 items in to a hash table, then prints
some of them.
#include <stdio.h>
#include <search.h>
char *data[]={ "alpha", "bravo", "charlie", "delta",
"echo", "foxtrot", "golf", "hotel", "india", "juliet",
"kilo", "lima", "mike", "november", "oscar", "papa",
"quebec", "romeo", "sierra", "tango", "uniform",
"victor", "whisky", "x-ray", "yankee", "zulu"
};
int main()
{
ENTRY e, *ep;
int i;
/* start with small table, and let it grow */
hcreate(3);
for (i = 0; i < 24; i++)
{
e.key = data[i];
/* data is just an integer, instead of a pointer
to something */
e.data = (char *)i;
ep = hsearch(e, ENTER);
/* there should be no failures */
if(ep == NULL) {fprintf(stderr, "entry failed\n"); exit(1);}
}
for (i = 22; i < 26; i++)
/* print two entries from the table, and show that
two are not in the table */
{
e.key = data[i];
ep = hsearch(e, FIND);
printf("%9.9s -> %9.9s:%d\n", e.key, ep?ep->key:"NULL",
ep?(int)(ep->data):0);
}
return 0;
}
bsearch(3), lsearch(3), tsearch(3), malloc(3)
GNU 1995-09-30 HSEARCH(3)
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