libc hcreate and hsearch.

ajpug · 06-03-2005, 11:20 AM

Hi all,

I am writing a program that uses a hash table in C.
I have been using hcreate and hsearch encountering in some weird behavior.
In the man pages it says that when I add an entry inthe hash table using hseearch, a copy of the item is made and put in the hash table. I found this to be not true since if I free the memory of the item that I just added to the memory, also the item in the table is lost. This means that there is no copy in the hash table but only a pointer pointing to the structure I allocated.
Another problem is that I used hcreate(10000) and even though I have only 10 entries in the table I get collisions (different keys give me the same data) and the keys are unique since each key is a MAC address.
Anyone can help in anyway? Do you know of any other hash table implementation/API in C?

Thank you,
ajpug

jim mcnamara · 06-03-2005, 04:55 PM

Here is a separate example that works.

Code:

#include <stdio.h>
#include <search.h>
#include <string.h>

struct info 
{           /* this is the info stored in the table */
     int age; 
     int room;     /* other than the key. */
};
#define NUM_EMPL    5000    /* # of elements in search table */

int main(int argc, char *argv[] )
{
     /* space to store strings employee name 19 char */
     char string_space[NUM_EMPL*20];

     /* space to store employee info */
     struct info info_space[NUM_EMPL];

     /* next avail space in string_space */
     char *str_ptr = string_space;

     /* next avail space in info_space */
     struct info *info_ptr = info_space;     
     ENTRY item, *found_item, *hsearch( );
          
     /* name to look for in table */
     char name_to_find[30];
     int i = 0;

     /* create table */
     (void) hcreate(NUM_EMPL);
     while (scanf("%s%d%d", str_ptr, &info_ptr->age,
            &info_ptr->room) != EOF && i++ < NUM_EMPL) {

          /* put info in structure, and structure in item */
          item.key = str_ptr;
          item.data = (char *)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 */
              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;
}