openssl/doc/crypto/lhash.pod

=pod

=head1 NAME

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

=head1 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);

=head1 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 B<LHASH> structure. B<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. B<compare> takes two arguments, and returns 0 if their keys are
equal, non-zero otherwise.

lh_free() frees the B<LHASH> structure B<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 B<data> into B<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 B<table>.

lh_retrieve() looks up an entry in B<table>. Normally, B<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 B<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 B<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-E<gt>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 B<func> will
be called with B<arg> as the second argument.

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

=head1 RETURN VALUES

lh_new() returns B<NULL> on error, otherwise a pointer to the new
B<LHASH> structure.

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

lh_delete() returns the entry being deleted.  B<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,
B<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.

=head1 BUGS

lh_insert() returns B<NULL> both for success and error.

=head1 INTERNALS

The following description is based on the SSLeay documentation:

The B<lhash> library implements a hash table described in the
I<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 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 B<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 E<lt> load) =E<gt>
expand.  if (hash-E<gt>down_load E<gt> load) =E<gt> contract.  The
B<up_load> has a default value of 1 and B<down_load> has a default value
of 2.  These numbers can be modified by the application by just
playing with the B<up_load> and B<down_load> variables.  The 'load' is
kept in a form which is multiplied by 256.  So
hash-E<gt>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-E<gt>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 B<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 B<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().

=head1 SEE ALSO

L<lh_stats(3)|lh_stats(3)>

=head1 HISTORY

The B<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.

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