lhash(3)

NAME
lh_new, lh_free, lh_insert, lh_delete, lh_retrieve, lh_doall, lh_doall_arg, lh_error - dynamic hash table

SYNOPSIS
 #include <openssl/lhash.h>

 LHASH *lh_new(unsigned long (*hash)(/*void *a*/),
          int (*compare)(/*void *a,void *b*/));
 void lh_free(LHASH *table);

 void *lh_insert(LHASH *table, void *data);
 void *lh_delete(LHASH *table, void *data);
 void *lh_retrieve(LHASH *table, void *data);

 void lh_doall(LHASH *table, void (*func)(/*void *b*/));
 void lh_doall_arg(LHASH *table, void (*func)(/*void *a,void *b*/),
          void *arg);

 int lh_error(LHASH *table);

DESCRIPTION
This library implements dynamic hash tables. The hash table entries can be arbitrary structures. Usually they consist of key and value fields.

lh_new() creates a new LHASH structure. hash takes a pointer to the structure and returns an unsigned long hash value of its key field. The hash value is normally truncated to a power of 2, so make sure that your hash function returns well mixed low order bits. compare takes two arguments, and returns 0 if their keys are equal, non-zero otherwise.

lh_free() frees the LHASH structure table. Allocated hash table entries will not be freed; consider using lh_doall() to deallocate any remaining entries in the hash table.

lh_insert() inserts the structure pointed to by data into table. If there already is an entry with the same key, the old value is replaced. Note that lh_insert() stores pointers, the data are not copied.

lh_delete() deletes an entry from table.

lh_retrieve() looks up an entry in table. Normally, data is a structure with the key field(s) set; the function will return a pointer to a fully populated structure.

lh_doall() will, for every entry in the hash table, call func with the data item as parameters. This function can be quite useful when used as follows:
 void cleanup(STUFF *a)
  { STUFF_free(a); }
 lh_doall(hash,cleanup);
 lh_free(hash); This can be used to free all the entries. lh_free() then cleans up the 'buckets' that point to nothing. When doing this, be careful if you delete entries from the hash table in func: the table may decrease in size, moving item that you are currently on down lower in the hash table. This could cause some entries to be skipped. The best solution to this problem is to set hash->down_load=0 before you start. This will stop the hash table ever being decreased in size.

lh_doall_arg() is the same as lh_doall() except that func will be called with arg as the second argument.

lh_error() can be used to determine if an error occurred in the last operation. lh_error() is a macro.

RETURN VALUES
lh_new() returns NULL on error, otherwise a pointer to the new LHASH structure.

When a hash table entry is replaced, lh_insert() returns the value being replaced. NULL is returned on normal operation and on error.

lh_delete() returns the entry being deleted. NULL is returned if there is no such value in the hash table.

lh_retrieve() returns the hash table entry if it has been found, NULL otherwise.

lh_error() returns 1 if an error occurred in the last operation, 0 otherwise.

lh_free(), lh_doall() and lh_doall_arg() return no values.

BUGS
lh_insert() returns NULL both for success and error.

INTERNALS
The following description is based on the SSLeay documentation:

The lhash library implements a hash table described in the Communications of the ACM in 1991. What makes this hash table different is that as the table fills, the hash table is increased (or decreased) in size via OPENSSL_realloc(). When a 'resize' is done, instead of all hashes being redistributed over twice as many 'buckets', one bucket is split. So when an 'expand' is done, there is only a minimal cost to redistribute some values. Subsequent inserts will cause more single 'bucket' redistributions but there will never be a sudden large cost due to redistributing all the 'buckets'.

The state for a particular hash table is kept in the LHASH structure. The decision to increase or decrease the hash table size is made depending on the 'load' of the hash table. The load is the number of items in the hash table divided by the size of the hash table. The default values are as follows. If (hash->up_load < load) => expand. if (hash->down_load > load) => contract. The up_load has a default value of 1 and down_load has a default value of 2. These numbers can be modified by the application by just playing with the up_load and down_load variables. The 'load' is kept in a form which is multiplied by 256. So hash->up_load=8*256; will cause a load of 8 to be set.

If you are interested in performance the field to watch is num_comp_calls. The hash library keeps track of the 'hash' value for each item so when a lookup is done, the 'hashes' are compared, if there is a match, then a full compare is done, and hash->num_comp_calls is incremented. If num_comp_calls is not equal to num_delete plus num_retrieve it means that your hash function is generating hashes that are the same for different values. It is probably worth changing your hash function if this is the case because even if your hash table has 10 items in a 'bucket', it can be searched with 10 unsigned long compares and 10 linked list traverses. This will be much less expensive that 10 calls to you compare function.

lh_strhash() is a demo string hashing function:

 unsigned long lh_strhash(const char *c);

Since the LHASH routines would normally be passed structures, this routine would not normally be passed to lh_new(), rather it would be used in the function passed to lh_new().

SEE ALSO
lh_stats(3)

HISTORY
The lhash library is available in all versions of SSLeay and OpenSSL. lh_error() was added in SSLeay 0.9.1b.

This manpage is derived from the SSLeay documentation.