Blame - jni/libpcre/sources/doc/html/pcrestack.html - jami-client-android

blob: 78484dee2cabdf0ac91e2c65ac7172ce44afefca [file] [log] [blame]

Tristan Matthews	0461646	2013-11-14 16:09:34 -0500	[diff] [blame]	1	<html>
				2	<head>
				3	<title>pcrestack specification</title>
				4	</head>
				5	<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
				6	<h1>pcrestack man page</h1>
				7	<p>
				8	Return to the <a href="index.html">PCRE index page</a>.
				9	</p>
				10	<p>
				11	This page is part of the PCRE HTML documentation. It was generated automatically
				12	from the original man page. If there is any nonsense in it, please consult the
				13	man page, in case the conversion went wrong.
				14	<br>
				15	<br><b>
				16	PCRE DISCUSSION OF STACK USAGE
				17	</b><br>
				18	<P>
				19	When you call <b>pcre_exec()</b>, it makes use of an internal function called
				20	<b>match()</b>. This calls itself recursively at branch points in the pattern,
				21	in order to remember the state of the match so that it can back up and try a
				22	different alternative if the first one fails. As matching proceeds deeper and
				23	deeper into the tree of possibilities, the recursion depth increases. The
				24	<b>match()</b> function is also called in other circumstances, for example,
				25	whenever a parenthesized sub-pattern is entered, and in certain cases of
				26	repetition.
				27	</P>
				28	<P>
				29	Not all calls of <b>match()</b> increase the recursion depth; for an item such
				30	as a* it may be called several times at the same level, after matching
				31	different numbers of a's. Furthermore, in a number of cases where the result of
				32	the recursive call would immediately be passed back as the result of the
				33	current call (a "tail recursion"), the function is just restarted instead.
				34	</P>
				35	<P>
				36	The above comments apply when <b>pcre_exec()</b> is run in its normal
				37	interpretive manner. If the pattern was studied with the
				38	PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling was successful, and
				39	the options passed to <b>pcre_exec()</b> were not incompatible, the matching
				40	process uses the JIT-compiled code instead of the <b>match()</b> function. In
				41	this case, the memory requirements are handled entirely differently. See the
				42	<a href="pcrejit.html"><b>pcrejit</b></a>
				43	documentation for details.
				44	</P>
				45	<P>
				46	The <b>pcre_dfa_exec()</b> function operates in an entirely different way, and
				47	uses recursion only when there is a regular expression recursion or subroutine
				48	call in the pattern. This includes the processing of assertion and "once-only"
				49	subpatterns, which are handled like subroutine calls. Normally, these are never
				50	very deep, and the limit on the complexity of <b>pcre_dfa_exec()</b> is
				51	controlled by the amount of workspace it is given. However, it is possible to
				52	write patterns with runaway infinite recursions; such patterns will cause
				53	<b>pcre_dfa_exec()</b> to run out of stack. At present, there is no protection
				54	against this.
				55	</P>
				56	<P>
				57	The comments that follow do NOT apply to <b>pcre_dfa_exec()</b>; they are
				58	relevant only for <b>pcre_exec()</b> without the JIT optimization.
				59	</P>
				60	<br><b>
				61	Reducing <b>pcre_exec()</b>'s stack usage
				62	</b><br>
				63	<P>
				64	Each time that <b>match()</b> is actually called recursively, it uses memory
				65	from the process stack. For certain kinds of pattern and data, very large
				66	amounts of stack may be needed, despite the recognition of "tail recursion".
				67	You can often reduce the amount of recursion, and therefore the amount of stack
				68	used, by modifying the pattern that is being matched. Consider, for example,
				69	this pattern:
				70	<pre>
				71	([^<]\|<(?!inet))+
				72	</pre>
				73	It matches from wherever it starts until it encounters "<inet" or the end of
				74	the data, and is the kind of pattern that might be used when processing an XML
				75	file. Each iteration of the outer parentheses matches either one character that
				76	is not "<" or a "<" that is not followed by "inet". However, each time a
				77	parenthesis is processed, a recursion occurs, so this formulation uses a stack
				78	frame for each matched character. For a long string, a lot of stack is
				79	required. Consider now this rewritten pattern, which matches exactly the same
				80	strings:
				81	<pre>
				82	([^<]++\|<(?!inet))+
				83	</pre>
				84	This uses very much less stack, because runs of characters that do not contain
				85	"<" are "swallowed" in one item inside the parentheses. Recursion happens only
				86	when a "<" character that is not followed by "inet" is encountered (and we
				87	assume this is relatively rare). A possessive quantifier is used to stop any
				88	backtracking into the runs of non-"<" characters, but that is not related to
				89	stack usage.
				90	</P>
				91	<P>
				92	This example shows that one way of avoiding stack problems when matching long
				93	subject strings is to write repeated parenthesized subpatterns to match more
				94	than one character whenever possible.
				95	</P>
				96	<br><b>
				97	Compiling PCRE to use heap instead of stack for <b>pcre_exec()</b>
				98	</b><br>
				99	<P>
				100	In environments where stack memory is constrained, you might want to compile
				101	PCRE to use heap memory instead of stack for remembering back-up points when
				102	<b>pcre_exec()</b> is running. This makes it run a lot more slowly, however.
				103	Details of how to do this are given in the
				104	<a href="pcrebuild.html"><b>pcrebuild</b></a>
				105	documentation. When built in this way, instead of using the stack, PCRE obtains
				106	and frees memory by calling the functions that are pointed to by the
				107	<b>pcre_stack_malloc</b> and <b>pcre_stack_free</b> variables. By default, these
				108	point to <b>malloc()</b> and <b>free()</b>, but you can replace the pointers to
				109	cause PCRE to use your own functions. Since the block sizes are always the
				110	same, and are always freed in reverse order, it may be possible to implement
				111	customized memory handlers that are more efficient than the standard functions.
				112	</P>
				113	<br><b>
				114	Limiting <b>pcre_exec()</b>'s stack usage
				115	</b><br>
				116	<P>
				117	You can set limits on the number of times that <b>match()</b> is called, both in
				118	total and recursively. If a limit is exceeded, <b>pcre_exec()</b> returns an
				119	error code. Setting suitable limits should prevent it from running out of
				120	stack. The default values of the limits are very large, and unlikely ever to
				121	operate. They can be changed when PCRE is built, and they can also be set when
				122	<b>pcre_exec()</b> is called. For details of these interfaces, see the
				123	<a href="pcrebuild.html"><b>pcrebuild</b></a>
				124	documentation and the
				125	<a href="pcreapi.html#extradata">section on extra data for <b>pcre_exec()</b></a>
				126	in the
				127	<a href="pcreapi.html"><b>pcreapi</b></a>
				128	documentation.
				129	</P>
				130	<P>
				131	As a very rough rule of thumb, you should reckon on about 500 bytes per
				132	recursion. Thus, if you want to limit your stack usage to 8Mb, you
				133	should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can
				134	support around 128000 recursions.
				135	</P>
				136	<P>
				137	In Unix-like environments, the <b>pcretest</b> test program has a command line
				138	option (<b>-S</b>) that can be used to increase the size of its stack. As long
				139	as the stack is large enough, another option (<b>-M</b>) can be used to find the
				140	smallest limits that allow a particular pattern to match a given subject
				141	string. This is done by calling <b>pcre_exec()</b> repeatedly with different
				142	limits.
				143	</P>
				144	<br><b>
				145	Changing stack size in Unix-like systems
				146	</b><br>
				147	<P>
				148	In Unix-like environments, there is not often a problem with the stack unless
				149	very long strings are involved, though the default limit on stack size varies
				150	from system to system. Values from 8Mb to 64Mb are common. You can find your
				151	default limit by running the command:
				152	<pre>
				153	ulimit -s
				154	</pre>
				155	Unfortunately, the effect of running out of stack is often SIGSEGV, though
				156	sometimes a more explicit error message is given. You can normally increase the
				157	limit on stack size by code such as this:
				158	<pre>
				159	struct rlimit rlim;
				160	getrlimit(RLIMIT_STACK, &rlim);
				161	rlim.rlim_cur = 10010241024;
				162	setrlimit(RLIMIT_STACK, &rlim);
				163	</pre>
				164	This reads the current limits (soft and hard) using <b>getrlimit()</b>, then
				165	attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must
				166	do this before calling <b>pcre_exec()</b>.
				167	</P>
				168	<br><b>
				169	Changing stack size in Mac OS X
				170	</b><br>
				171	<P>
				172	Using <b>setrlimit()</b>, as described above, should also work on Mac OS X. It
				173	is also possible to set a stack size when linking a program. There is a
				174	discussion about stack sizes in Mac OS X at this web site:
				175	<a href="http://developer.apple.com/qa/qa2005/qa1419.html">http://developer.apple.com/qa/qa2005/qa1419.html.</a>
				176	</P>
				177	<br><b>
				178	AUTHOR
				179	</b><br>
				180	<P>
				181	Philip Hazel
				182	<br>
				183	University Computing Service
				184	<br>
				185	Cambridge CB2 3QH, England.
				186	<br>
				187	</P>
				188	<br><b>
				189	REVISION
				190	</b><br>
				191	<P>
				192	Last updated: 26 August 2011
				193	<br>
				194	Copyright © 1997-2011 University of Cambridge.
				195	<br>
				196	<p>
				197	Return to the <a href="index.html">PCRE index page</a>.
				198	</p>