C programming: hashtable insertion/search
- by Ricardo Campos
Hello i have a problem with my hash table its implemented like this:
#define HT_SIZE 10
typedef struct _list_t_ {
char key[20];
char string[20];
char prevValue[20];
struct _list_t_ *next;
} list_t;
typedef struct _hash_table_t_ {
int size; /* the size of the table */
list_t ***table; /* first */
sem_t lock;
} hash_table_t;
I have a Linked list with 3 pointers because i want a hash table with several partitions (shards), here is my initialization of my Hash table:
hash_table_t *create_hash_table(int NUM_SERVER_THREADS, int num_shards){
hash_table_t *new_table;
int j,i;
if (HT_SIZE<1) return NULL; /* invalid size for table */
/* Attempt to allocate memory for the hashtable structure */
new_table = (hash_table_t*)malloc(sizeof(hash_table_t)*HT_SIZE);
/* Attempt to allocate memory for the table itself */
new_table->table = (list_t ***)calloc(1,sizeof(list_t **));
/* Initialize the elements of the table */
for(j=0; j<num_shards; j++){
new_table->table[j] = (list_t **)calloc(1,sizeof(list_t *));
for(i=0; i<HT_SIZE; i++){
new_table->table[j][i] = (list_t *)calloc(1,sizeof(list_t ));
}
}
/* Set the table's size */
new_table->size = HT_SIZE;
sem_init(&new_table->lock, 0, 1);
return new_table;
}
Here is my search function to search in the hash table
list_t *lookup_string(hash_table_t *hashtable, char *key, int shardId){
list_t *list ;
int hashval = hash(key);
/* Go to the correct list based on the hash value and see if key is
* in the list. If it is, return return a pointer to the list element.
* If it isn't, the item isn't in the table, so return NULL.
*/
sem_wait(&hashtable->lock);
for(list = hashtable->table[shardId][hashval]; list != NULL; list =list->next) {
if (strcmp(key, list->key) == 0){
sem_post(&hashtable->lock);
return list;
}
}
sem_post(&hashtable->lock);
return NULL;
}
And my insert function:
char *add_string(hash_table_t *hashtable, char *str,char *key, int shardId){
list_t *new_list;
list_t *current_list;
unsigned int hashval = hash(key);
/*printf("|%d|%d|%s|\n",hashval,shardId,key);*/
/* Lock for concurrency */
sem_wait(&hashtable->lock);
/* Attempt to allocate memory for list */
new_list = (list_t*)malloc(sizeof(list_t));
/* Does item already exist? */
sem_post(&hashtable->lock);
current_list = lookup_string(hashtable, key,shardId);
sem_wait(&hashtable->lock);
/* item already exists, don't insert it again. */
if (current_list != NULL){
strcpy(new_list->prevValue,current_list->string);
strcpy(new_list->string,str);
strcpy(new_list->key,key);
new_list->next = hashtable->table[shardId][hashval];
hashtable->table[shardId][hashval] = new_list;
sem_post(&hashtable->lock);
return new_list->prevValue;
}
/* Insert into list */
strcpy(new_list->string,str);
strcpy(new_list->key,key);
new_list->next = hashtable->table[shardId][hashval];
hashtable->table[shardId][hashval] = new_list;
/* Unlock */
sem_post(&hashtable->lock);
return new_list->prevValue;
}
My main class runs some of tests by executing the insertion / reading / delete from the elements of the hash table the problem is when i have more than 4 partitions/shards the tests stop at the first reading element saying it returned the wrong value NULL on the search function, when its less than 4 it runs perfectly well and passes all the tests.
You can see my main.c in here if you want to give a look:
http://hostcode.sourceforge.net/view/1105
My complete Hash table code:
http://hostcode.sourceforge.net/view/1103
And other functions where hash table code is executed:
.c file http://hostcode.sourceforge.net/view/1104
.h file http://hostcode.sourceforge.net/view/1106
Thank for you time, i appreciate any help you can give to me this is a college important project that I'm trying to solve and I'm stuck here for 2 days.
