297 lines
10 KiB
Groff
297 lines
10 KiB
Groff
.\" $NetBSD: openssl_lhash.3,v 1.6 2002/08/09 16:15:45 itojun Exp $
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.\" Sat Aug 10 00:57:34 2002
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.\" ======================================================================
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.\"
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.IX Title "lhash 3"
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.TH lhash 3 "0.9.6g" "2001-04-12" "OpenSSL"
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.UC
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.SH "NAME"
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lh_new, lh_free, lh_insert, lh_delete, lh_retrieve, lh_doall,
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lh_doall_arg, lh_error \- dynamic hash table
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.SH "LIBRARY"
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libcrypto, -lcrypto
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/lhash.h>
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.Ve
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.Vb 3
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\& LHASH *lh_new(unsigned long (*hash)(/*void *a*/),
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\& int (*compare)(/*void *a,void *b*/));
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\& void lh_free(LHASH *table);
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.Ve
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.Vb 3
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\& void *lh_insert(LHASH *table, void *data);
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\& void *lh_delete(LHASH *table, void *data);
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\& void *lh_retrieve(LHASH *table, void *data);
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.Ve
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.Vb 3
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\& void lh_doall(LHASH *table, void (*func)(/*void *b*/));
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\& void lh_doall_arg(LHASH *table, void (*func)(/*void *a,void *b*/),
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\& void *arg);
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.Ve
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.Vb 1
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\& int lh_error(LHASH *table);
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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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This library implements dynamic hash tables. The hash table entries
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can be arbitrary structures. Usually they consist of key and value
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fields.
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.PP
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\&\fIlh_new()\fR creates a new \fB\s-1LHASH\s0\fR structure. \fBhash\fR takes a pointer to
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the structure and returns an unsigned long hash value of its key
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field. The hash value is normally truncated to a power of 2, so make
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sure that your hash function returns well mixed low order
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bits. \fBcompare\fR takes two arguments, and returns 0 if their keys are
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equal, non-zero otherwise.
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.PP
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\&\fIlh_free()\fR frees the \fB\s-1LHASH\s0\fR structure \fBtable\fR. Allocated hash table
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entries will not be freed; consider using \fIlh_doall()\fR to deallocate any
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remaining entries in the hash table.
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.PP
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\&\fIlh_insert()\fR inserts the structure pointed to by \fBdata\fR into \fBtable\fR.
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If there already is an entry with the same key, the old value is
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replaced. Note that \fIlh_insert()\fR stores pointers, the data are not
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copied.
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.PP
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\&\fIlh_delete()\fR deletes an entry from \fBtable\fR.
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.PP
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\&\fIlh_retrieve()\fR looks up an entry in \fBtable\fR. Normally, \fBdata\fR is
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a structure with the key \fIfield\fR\|(s) set; the function will return a
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pointer to a fully populated structure.
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.PP
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\&\fIlh_doall()\fR will, for every entry in the hash table, call \fBfunc\fR with
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the data item as parameters.
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This function can be quite useful when used as follows:
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void cleanup(\s-1STUFF\s0 *a)
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{ \fISTUFF_free\fR\|(a); }
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lh_doall(hash,cleanup);
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lh_free(hash);
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This can be used to free all the entries. \fIlh_free()\fR then cleans up the
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\&'buckets' that point to nothing. When doing this, be careful if you
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delete entries from the hash table in \fBfunc\fR: the table may decrease
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in size, moving item that you are currently on down lower in the hash
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table. This could cause some entries to be skipped. The best
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solution to this problem is to set hash->down_load=0 before you
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start. This will stop the hash table ever being decreased in size.
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.PP
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\&\fIlh_doall_arg()\fR is the same as \fIlh_doall()\fR except that \fBfunc\fR will
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be called with \fBarg\fR as the second argument.
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.PP
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\&\fIlh_error()\fR can be used to determine if an error occurred in the last
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operation. \fIlh_error()\fR is a macro.
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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\&\fIlh_new()\fR returns \fB\s-1NULL\s0\fR on error, otherwise a pointer to the new
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\&\fB\s-1LHASH\s0\fR structure.
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.PP
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When a hash table entry is replaced, \fIlh_insert()\fR returns the value
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being replaced. \fB\s-1NULL\s0\fR is returned on normal operation and on error.
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.PP
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\&\fIlh_delete()\fR returns the entry being deleted. \fB\s-1NULL\s0\fR is returned if
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there is no such value in the hash table.
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.PP
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\&\fIlh_retrieve()\fR returns the hash table entry if it has been found,
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\&\fB\s-1NULL\s0\fR otherwise.
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.PP
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\&\fIlh_error()\fR returns 1 if an error occurred in the last operation, 0
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otherwise.
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.PP
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\&\fIlh_free()\fR, \fIlh_doall()\fR and \fIlh_doall_arg()\fR return no values.
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.SH "BUGS"
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.IX Header "BUGS"
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\&\fIlh_insert()\fR returns \fB\s-1NULL\s0\fR both for success and error.
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.SH "INTERNALS"
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.IX Header "INTERNALS"
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The following description is based on the SSLeay documentation:
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.PP
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The \fBlhash\fR library implements a hash table described in the
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\&\fICommunications of the \s-1ACM\s0\fR in 1991. What makes this hash table
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different is that as the table fills, the hash table is increased (or
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decreased) in size via \fIOPENSSL_realloc()\fR. When a 'resize' is done, instead of
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all hashes being redistributed over twice as many 'buckets', one
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bucket is split. So when an 'expand' is done, there is only a minimal
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cost to redistribute some values. Subsequent inserts will cause more
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single 'bucket' redistributions but there will never be a sudden large
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cost due to redistributing all the 'buckets'.
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.PP
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The state for a particular hash table is kept in the \fB\s-1LHASH\s0\fR structure.
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The decision to increase or decrease the hash table size is made
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depending on the 'load' of the hash table. The load is the number of
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items in the hash table divided by the size of the hash table. The
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default values are as follows. If (hash->up_load < load) =>
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expand. if (hash->down_load > load) => contract. The
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\&\fBup_load\fR has a default value of 1 and \fBdown_load\fR has a default value
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of 2. These numbers can be modified by the application by just
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playing with the \fBup_load\fR and \fBdown_load\fR variables. The 'load' is
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kept in a form which is multiplied by 256. So
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hash->up_load=8*256; will cause a load of 8 to be set.
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.PP
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If you are interested in performance the field to watch is
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num_comp_calls. The hash library keeps track of the 'hash' value for
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each item so when a lookup is done, the 'hashes' are compared, if
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there is a match, then a full compare is done, and
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hash->num_comp_calls is incremented. If num_comp_calls is not equal
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to num_delete plus num_retrieve it means that your hash function is
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generating hashes that are the same for different values. It is
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probably worth changing your hash function if this is the case because
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even if your hash table has 10 items in a 'bucket', it can be searched
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with 10 \fBunsigned long\fR compares and 10 linked list traverses. This
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will be much less expensive that 10 calls to you compare function.
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.PP
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\&\fIlh_strhash()\fR is a demo string hashing function:
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.PP
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.Vb 1
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\& unsigned long lh_strhash(const char *c);
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.Ve
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Since the \fB\s-1LHASH\s0\fR routines would normally be passed structures, this
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routine would not normally be passed to \fIlh_new()\fR, rather it would be
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used in the function passed to \fIlh_new()\fR.
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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lh_stats(3)
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.SH "HISTORY"
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.IX Header "HISTORY"
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The \fBlhash\fR library is available in all versions of SSLeay and OpenSSL.
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\&\fIlh_error()\fR was added in SSLeay 0.9.1b.
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.PP
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This manpage is derived from the SSLeay documentation.
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