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Simple hash table implementation for C.
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Simple hash table implementation for C.

I just wanted a simple and straightforward hash table implementation that I could drop into my own C-based projects on whatever platform.

I haven't implemented one of these before, so it may be super naive, but it does appear to work pretty well.

NOTE: After exposure on HN, and seeing other hash implementations, I'm planning to restructure the code to a macro-based style, which should cut down the duplication.


You can create a hash table, and add strings, long ints, doubles and pointers to it, keyed by strings or long ints.

You can retrieve strings, long ints, doubles and pointers via the get functions.

All strings saved in a hash table are copied, and copies of strings are returned on retrieval.

I added locking on hash buckets which only minorly affects performance, and allows safe retrieval and storing of key value pairs.

Performance seems decent. Saving 1,000,000 int values by string key is done in about 0.28 secs, and multi-threaded performance scales pretty closely with number of processors.

Performance Chart

Building and Using

Type make in the folder to build the code. Type ./test to run the demo.


The following were key to getting various aspects working:

Hash function for integer keys.

Hash function for string keys.

Efficient lock when low-contention is expected.

Data Structures

Hash Table

typedef struct jwHashTable jwHashTable;
struct jwHashTable
	jwHashEntry **bucket;			// pointer to array of buckets
	size_t buckets;
	size_t bucketsinitial;			// if we resize, may need to hash multiple times
	HASHRESULT lastError;
	volatile int *locks;			// array of locks
	volatile int lock;				// lock for entire table

Hash Entry

typedef struct jwHashEntry jwHashEntry;
struct jwHashEntry
		char  *strValue;
		double dblValue;
		int	   intValue;
	} key;
		char  *strValue;
		double dblValue;
		int	   intValue;
		void  *ptrValue;
	} value;
	jwHashEntry *next;


Creating a Hash Table

jwHashTable *create_hash( size_t buckets );
void *delete_hash( jwHashTable *table );		// clean up all memory

Storing by String Key

HASHRESULT add_str_by_str( jwHashTable*, char *key, char *value );
HASHRESULT add_int_by_str( jwHashTable*, char *key, long int value );
HASHRESULT add_dbl_by_str( jwHashTable*, char *key, double value );
HASHRESULT add_ptr_by_str( jwHashTable*, char *key, void *value );

Deleting by String

HASHRESULT del_by_str( jwHashTable*, char *key );

Retrieving by String

HASHRESULT get_str_by_str( jwHashTable *table, char *key, char **value );
HASHRESULT get_int_by_str( jwHashTable *table, char *key, int *i );
HASHRESULT get_dbl_by_str( jwHashTable *table, char *key, double *val );
HASHRESULT get_ptr_by_str( jwHashTable *table, char *key, void **val );

[Similar for long int keys]


  1. Support multi-threading, -- this started, and implemented for the test
  2. Implement clean-up,
  3. Implement re-hashing to a larger hash table,
  4. Implement a callback to allow iterating through keys, values


Example 1 - Save and Retrieve Some Values

// Test hashing by string
char * strv1 = "Jonathan";
char * strv2 = "Zevi";
char * strv3 = "Jude";
char * strv4 = "Voldemort";


char * sstrv1; get_str_by_str(table,"oldest",&sstrv1);
char * sstrv2; get_str_by_str(table,"2ndoldest",&sstrv2);
char * sstrv3; get_str_by_str(table,"3rdoldest",&sstrv3);
char * sstrv4; get_str_by_str(table,"4tholdest",&sstrv4);
printf("got strings:\noldest->%s \n2ndoldest->%s \n3rdoldest->%s \n4tholdest->%s\n",

Example 2 - Write and Read Key Value Pairs on Multiple Threads

#define NUMTHREADS 8
#define HASHCOUNT 1000000

typedef struct threadinfo {jwHashTable *table; int start;} threadinfo;
void * thread_func(void *arg)
	threadinfo *info = arg;
	char buffer[512];
	int i = info->start;
	jwHashTable *table = info->table;

int thread_test()
	// create
	jwHashTable * table = create_hash(HASHCOUNT>>2);

	// hash a million strings into various sizes of table
	struct timeval tval_before, tval_done1, tval_done2, tval_writehash, tval_readhash;
	gettimeofday(&tval_before, NULL);
	int t;
	pthread_t * threads[NUMTHREADS];
	for(t=0;t<NUMTHREADS;++t) {
		pthread_t * pth = malloc(sizeof(pthread_t));
		threads[t] = pth;
		threadinfo *info = (threadinfo*)malloc(sizeof(threadinfo));
		info->table = table; info->start = t;
	for(t=0;t<NUMTHREADS;++t) {
		pthread_join(*threads[t], NULL);
	gettimeofday(&tval_done1, NULL);
	int i,j;
	int error = 0;
	char buffer[512];
	for(i=0;i<HASHCOUNT;++i) {
		if(i!=j) {
			printf("Error: %d != %d\n",i,j);
			error = 1;
	if(!error) {
		printf("No errors.\n"); 
	gettimeofday(&tval_done2, NULL);
	timersub(&tval_done1, &tval_before, &tval_writehash);
	timersub(&tval_done2, &tval_done1, &tval_readhash);
	printf("\n%d threads.\n",NUMTHREADS);
	printf("Store %d ints by string: %ld.%06ld sec, read %d ints: %ld.%06ld sec\n",HASHCOUNT,
		(long int)tval_writehash.tv_sec, (long int)tval_writehash.tv_usec,HASHCOUNT,
		(long int)tval_readhash.tv_sec, (long int)tval_readhash.tv_usec);

	return 0;
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