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Diffstat (limited to 'secure/lib/libcrypto/man/lhash.3')
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diff --git a/secure/lib/libcrypto/man/lhash.3 b/secure/lib/libcrypto/man/lhash.3 new file mode 100644 index 0000000..619e4d9 --- /dev/null +++ b/secure/lib/libcrypto/man/lhash.3 @@ -0,0 +1,292 @@ +.\" Automatically generated by Pod::Man version 1.15 +.\" Thu May 9 13:19:27 2002 +.\" +.\" Standard preamble: +.\" ====================================================================== +.de Sh \" Subsection heading +.br +.if t .Sp +.ne 5 +.PP +\fB\\$1\fR +.PP +.. +.de Sp \" Vertical space (when we can't use .PP) +.if t .sp .5v +.if n .sp +.. +.de Ip \" List item +.br +.ie \\n(.$>=3 .ne \\$3 +.el .ne 3 +.IP "\\$1" \\$2 +.. +.de Vb \" Begin verbatim text +.ft CW +.nf +.ne \\$1 +.. +.de Ve \" End verbatim text +.ft R + +.fi +.. +.\" Set up some character translations and predefined strings. \*(-- will +.\" give an unbreakable dash, \*(PI will give pi, \*(L" will give a left +.\" double quote, and \*(R" will give a right double quote. | will give a +.\" real vertical bar. \*(C+ will give a nicer C++. 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Always turn off hyphenation; it +.\" makes way too many mistakes in technical documents. +.hy 0 +.if n .na +.\" +.\" Accent mark definitions (@(#)ms.acc 1.5 88/02/08 SMI; from UCB 4.2). +.\" Fear. Run. Save yourself. 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The hash table entries +can be arbitrary structures. Usually they consist of key and value +fields. +.PP +\&\fIlh_new()\fR creates a new \fB\s-1LHASH\s0\fR structure. \fBhash\fR 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. \fBcompare\fR takes two arguments, and returns 0 if their keys are +equal, non-zero otherwise. +.PP +\&\fIlh_free()\fR frees the \fB\s-1LHASH\s0\fR structure \fBtable\fR. Allocated hash table +entries will not be freed; consider using \fIlh_doall()\fR to deallocate any +remaining entries in the hash table. +.PP +\&\fIlh_insert()\fR inserts the structure pointed to by \fBdata\fR into \fBtable\fR. +If there already is an entry with the same key, the old value is +replaced. Note that \fIlh_insert()\fR stores pointers, the data are not +copied. +.PP +\&\fIlh_delete()\fR deletes an entry from \fBtable\fR. +.PP +\&\fIlh_retrieve()\fR looks up an entry in \fBtable\fR. Normally, \fBdata\fR is +a structure with the key \fIfield\fR\|(s) set; the function will return a +pointer to a fully populated structure. +.PP +\&\fIlh_doall()\fR will, for every entry in the hash table, call \fBfunc\fR with +the data item as parameters. +This function can be quite useful when used as follows: + void cleanup(\s-1STUFF\s0 *a) + { \fISTUFF_free\fR\|(a); } + lh_doall(hash,cleanup); + lh_free(hash); +This can be used to free all the entries. \fIlh_free()\fR then cleans up the +\&'buckets' that point to nothing. When doing this, be careful if you +delete entries from the hash table in \fBfunc\fR: 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. +.PP +\&\fIlh_doall_arg()\fR is the same as \fIlh_doall()\fR except that \fBfunc\fR will +be called with \fBarg\fR as the second argument. +.PP +\&\fIlh_error()\fR can be used to determine if an error occurred in the last +operation. \fIlh_error()\fR is a macro. +.SH "RETURN VALUES" +.IX Header "RETURN VALUES" +\&\fIlh_new()\fR returns \fB\s-1NULL\s0\fR on error, otherwise a pointer to the new +\&\fB\s-1LHASH\s0\fR structure. +.PP +When a hash table entry is replaced, \fIlh_insert()\fR returns the value +being replaced. \fB\s-1NULL\s0\fR is returned on normal operation and on error. +.PP +\&\fIlh_delete()\fR returns the entry being deleted. \fB\s-1NULL\s0\fR is returned if +there is no such value in the hash table. +.PP +\&\fIlh_retrieve()\fR returns the hash table entry if it has been found, +\&\fB\s-1NULL\s0\fR otherwise. +.PP +\&\fIlh_error()\fR returns 1 if an error occurred in the last operation, 0 +otherwise. +.PP +\&\fIlh_free()\fR, \fIlh_doall()\fR and \fIlh_doall_arg()\fR return no values. +.SH "BUGS" +.IX Header "BUGS" +\&\fIlh_insert()\fR returns \fB\s-1NULL\s0\fR both for success and error. +.SH "INTERNALS" +.IX Header "INTERNALS" +The following description is based on the SSLeay documentation: +.PP +The \fBlhash\fR library implements a hash table described in the +\&\fICommunications of the \s-1ACM\s0\fR in 1991. What makes this hash table +different is that as the table fills, the hash table is increased (or +decreased) in size via \fIOPENSSL_realloc()\fR. 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'. +.PP +The state for a particular hash table is kept in the \fB\s-1LHASH\s0\fR 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 +\&\fBup_load\fR has a default value of 1 and \fBdown_load\fR has a default value +of 2. These numbers can be modified by the application by just +playing with the \fBup_load\fR and \fBdown_load\fR 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. +.PP +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 \fBunsigned long\fR compares and 10 linked list traverses. This +will be much less expensive that 10 calls to you compare function. +.PP +\&\fIlh_strhash()\fR is a demo string hashing function: +.PP +.Vb 1 +\& unsigned long lh_strhash(const char *c); +.Ve +Since the \fB\s-1LHASH\s0\fR routines would normally be passed structures, this +routine would not normally be passed to \fIlh_new()\fR, rather it would be +used in the function passed to \fIlh_new()\fR. +.SH "SEE ALSO" +.IX Header "SEE ALSO" +lh_stats(3) +.SH "HISTORY" +.IX Header "HISTORY" +The \fBlhash\fR library is available in all versions of SSLeay and OpenSSL. +\&\fIlh_error()\fR was added in SSLeay 0.9.1b. +.PP +This manpage is derived from the SSLeay documentation. |
