dist2/doc/html/pcre2stack.html - nest-cam/4320010/pcre2 - Git at Google

 <html>
 <head>
 <title>pcre2stack specification</title>
 </head>
 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
 <h1>pcre2stack man page</h1>
 <p>
 Return to the <a href="index.html">PCRE2 index page</a>.
 </p>
 <p>
 This page is part of the PCRE2 HTML documentation. It was generated
 automatically from the original man page. If there is any nonsense in it,
 please consult the man page, in case the conversion went wrong.
 <br>
 <br><b>
 PCRE2 DISCUSSION OF STACK USAGE
 </b><br>
 <P>
 When you call <b>pcre2_match()</b>, it makes use of an internal function called
 <b>match()</b>. This calls itself recursively at branch points in the pattern,
 in order to remember the state of the match so that it can back up and try a
 different alternative after a failure. As matching proceeds deeper and deeper
 into the tree of possibilities, the recursion depth increases. The
 <b>match()</b> function is also called in other circumstances, for example,
 whenever a parenthesized sub-pattern is entered, and in certain cases of
 repetition.
 </P>
 <P>
 Not all calls of <b>match()</b> increase the recursion depth; for an item such
 as a* it may be called several times at the same level, after matching
 different numbers of a's. Furthermore, in a number of cases where the result of
 the recursive call would immediately be passed back as the result of the
 current call (a "tail recursion"), the function is just restarted instead.
 </P>
 <P>
 Each time the internal <b>match()</b> function is called recursively, it uses
 memory from the process stack. For certain kinds of pattern and data, very
 large amounts of stack may be needed, despite the recognition of "tail
 recursion". Note that if PCRE2 is compiled with the -fsanitize=address option
 of the GCC compiler, the stack requirements are greatly increased.
 </P>
 <P>
 The above comments apply when <b>pcre2_match()</b> is run in its normal
 interpretive manner. If the compiled pattern was processed by
 <b>pcre2_jit_compile()</b>, and just-in-time compiling was successful, and the
 options passed to <b>pcre2_match()</b> were not incompatible, the matching
 process uses the JIT-compiled code instead of the <b>match()</b> function. In
 this case, the memory requirements are handled entirely differently. See the
 <a href="pcre2jit.html"><b>pcre2jit</b></a>
 documentation for details.
 </P>
 <P>
 The <b>pcre2_dfa_match()</b> function operates in a different way to
 <b>pcre2_match()</b>, and uses recursion only when there is a regular expression
 recursion or subroutine call in the pattern. This includes the processing of
 assertion and "once-only" subpatterns, which are handled like subroutine calls.
 Normally, these are never very deep, and the limit on the complexity of
 <b>pcre2_dfa_match()</b> is controlled by the amount of workspace it is given.
 However, it is possible to write patterns with runaway infinite recursions;
 such patterns will cause <b>pcre2_dfa_match()</b> to run out of stack. At
 present, there is no protection against this.
 </P>
 <P>
 The comments that follow do NOT apply to <b>pcre2_dfa_match()</b>; they are
 relevant only for <b>pcre2_match()</b> without the JIT optimization.
 </P>
 <br><b>
 Reducing <b>pcre2_match()</b>'s stack usage
 </b><br>
 <P>
 You can often reduce the amount of recursion, and therefore the
 amount of stack used, by modifying the pattern that is being matched. Consider,
 for example, this pattern:
 <pre>
   ([^&#60;]|&#60;(?!inet))+
 </pre>
 It matches from wherever it starts until it encounters "&#60;inet" or the end of
 the data, and is the kind of pattern that might be used when processing an XML
 file. Each iteration of the outer parentheses matches either one character that
 is not "&#60;" or a "&#60;" that is not followed by "inet". However, each time a
 parenthesis is processed, a recursion occurs, so this formulation uses a stack
 frame for each matched character. For a long string, a lot of stack is
 required. Consider now this rewritten pattern, which matches exactly the same
 strings:
 <pre>
   ([^&#60;]++|&#60;(?!inet))+
 </pre>
 This uses very much less stack, because runs of characters that do not contain
 "&#60;" are "swallowed" in one item inside the parentheses. Recursion happens only
 when a "&#60;" character that is not followed by "inet" is encountered (and we
 assume this is relatively rare). A possessive quantifier is used to stop any
 backtracking into the runs of non-"&#60;" characters, but that is not related to
 stack usage.
 </P>
 <P>
 This example shows that one way of avoiding stack problems when matching long
 subject strings is to write repeated parenthesized subpatterns to match more
 than one character whenever possible.
 </P>
 <br><b>
 Compiling PCRE2 to use heap instead of stack for <b>pcre2_match()</b>
 </b><br>
 <P>
 In environments where stack memory is constrained, you might want to compile
 PCRE2 to use heap memory instead of stack for remembering back-up points when
 <b>pcre2_match()</b> is running. This makes it run more slowly, however. Details
 of how to do this are given in the
 <a href="pcre2build.html"><b>pcre2build</b></a>
 documentation. When built in this way, instead of using the stack, PCRE2
 gets memory for remembering backup points from the heap. By default, the memory
 is obtained by calling the system <b>malloc()</b> function, but you can arrange
 to supply your own memory management function. For details, see the section
 entitled
 <a href="pcre2api.html#matchcontext">"The match context"</a>
 in the
 <a href="pcre2api.html"><b>pcre2api</b></a>
 documentation. Since the block sizes are always the same, it may be possible to
 implement customized a memory handler that is more efficient than the standard
 function. The memory blocks obtained for this purpose are retained and re-used
 if possible while <b>pcre2_match()</b> is running. They are all freed just
 before it exits.
 </P>
 <br><b>
 Limiting <b>pcre2_match()</b>'s stack usage
 </b><br>
 <P>
 You can set limits on the number of times the internal <b>match()</b> function
 is called, both in total and recursively. If a limit is exceeded,
 <b>pcre2_match()</b> returns an error code. Setting suitable limits should
 prevent it from running out of stack. The default values of the limits are very
 large, and unlikely ever to operate. They can be changed when PCRE2 is built,
 and they can also be set when <b>pcre2_match()</b> is called. For details of
 these interfaces, see the
 <a href="pcre2build.html"><b>pcre2build</b></a>
 documentation and the section entitled
 <a href="pcre2api.html#matchcontext">"The match context"</a>
 in the
 <a href="pcre2api.html"><b>pcre2api</b></a>
 documentation.
 </P>
 <P>
 As a very rough rule of thumb, you should reckon on about 500 bytes per
 recursion. Thus, if you want to limit your stack usage to 8Mb, you should set
 the limit at 16000 recursions. A 64Mb stack, on the other hand, can support
 around 128000 recursions.
 </P>
 <P>
 The <b>pcre2test</b> test program has a modifier called "find_limits" which, if
 applied to a subject line, causes it to find the smallest limits that allow a a
 pattern to match. This is done by calling <b>pcre2_match()</b> repeatedly with
 different limits.
 </P>
 <br><b>
 Changing stack size in Unix-like systems
 </b><br>
 <P>
 In Unix-like environments, there is not often a problem with the stack unless
 very long strings are involved, though the default limit on stack size varies
 from system to system. Values from 8Mb to 64Mb are common. You can find your
 default limit by running the command:
 <pre>
   ulimit -s
 </pre>
 Unfortunately, the effect of running out of stack is often SIGSEGV, though
 sometimes a more explicit error message is given. You can normally increase the
 limit on stack size by code such as this:
 <pre>
   struct rlimit rlim;
   getrlimit(RLIMIT_STACK, &rlim);
   rlim.rlim_cur = 100*1024*1024;
   setrlimit(RLIMIT_STACK, &rlim);
 </pre>
 This reads the current limits (soft and hard) using <b>getrlimit()</b>, then
 attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must
 do this before calling <b>pcre2_match()</b>.
 </P>
 <br><b>
 Changing stack size in Mac OS X
 </b><br>
 <P>
 Using <b>setrlimit()</b>, as described above, should also work on Mac OS X. It
 is also possible to set a stack size when linking a program. There is a
 discussion about stack sizes in Mac OS X at this web site:
 <a href="http://developer.apple.com/qa/qa2005/qa1419.html">http://developer.apple.com/qa/qa2005/qa1419.html.</a>
 </P>
 <br><b>
 AUTHOR
 </b><br>
 <P>
 Philip Hazel
 <br>
 University Computing Service
 <br>
 Cambridge, England.
 <br>
 </P>
 <br><b>
 REVISION
 </b><br>
 <P>
 Last updated: 21 November 2014
 <br>
 Copyright &copy; 1997-2014 University of Cambridge.
 <br>
 <p>
 Return to the <a href="index.html">PCRE2 index page</a>.
 </p>
	<html>
	<head>
	<title>pcre2stack specification</title>
	</head>
	<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
	<h1>pcre2stack man page</h1>
	<p>
	Return to the <a href="index.html">PCRE2 index page</a>.
	</p>
	<p>
	This page is part of the PCRE2 HTML documentation. It was generated
	automatically from the original man page. If there is any nonsense in it,
	please consult the man page, in case the conversion went wrong.
	<br>
	<br><b>
	PCRE2 DISCUSSION OF STACK USAGE
	</b><br>
	<P>
	When you call <b>pcre2_match()</b>, it makes use of an internal function called
	<b>match()</b>. This calls itself recursively at branch points in the pattern,
	in order to remember the state of the match so that it can back up and try a
	different alternative after a failure. As matching proceeds deeper and deeper
	into the tree of possibilities, the recursion depth increases. The
	<b>match()</b> function is also called in other circumstances, for example,
	whenever a parenthesized sub-pattern is entered, and in certain cases of
	repetition.
	</P>
	<P>
	Not all calls of <b>match()</b> increase the recursion depth; for an item such
	as a* it may be called several times at the same level, after matching
	different numbers of a's. Furthermore, in a number of cases where the result of
	the recursive call would immediately be passed back as the result of the
	current call (a "tail recursion"), the function is just restarted instead.
	</P>
	<P>
	Each time the internal <b>match()</b> function is called recursively, it uses
	memory from the process stack. For certain kinds of pattern and data, very
	large amounts of stack may be needed, despite the recognition of "tail
	recursion". Note that if PCRE2 is compiled with the -fsanitize=address option
	of the GCC compiler, the stack requirements are greatly increased.
	</P>
	<P>
	The above comments apply when <b>pcre2_match()</b> is run in its normal
	interpretive manner. If the compiled pattern was processed by
	<b>pcre2_jit_compile()</b>, and just-in-time compiling was successful, and the
	options passed to <b>pcre2_match()</b> were not incompatible, the matching
	process uses the JIT-compiled code instead of the <b>match()</b> function. In
	this case, the memory requirements are handled entirely differently. See the
	<a href="pcre2jit.html"><b>pcre2jit</b></a>
	documentation for details.
	</P>
	<P>
	The <b>pcre2_dfa_match()</b> function operates in a different way to
	<b>pcre2_match()</b>, and uses recursion only when there is a regular expression
	recursion or subroutine call in the pattern. This includes the processing of
	assertion and "once-only" subpatterns, which are handled like subroutine calls.
	Normally, these are never very deep, and the limit on the complexity of
	<b>pcre2_dfa_match()</b> is controlled by the amount of workspace it is given.
	However, it is possible to write patterns with runaway infinite recursions;
	such patterns will cause <b>pcre2_dfa_match()</b> to run out of stack. At
	present, there is no protection against this.
	</P>
	<P>
	The comments that follow do NOT apply to <b>pcre2_dfa_match()</b>; they are
	relevant only for <b>pcre2_match()</b> without the JIT optimization.
	</P>
	<br><b>
	Reducing <b>pcre2_match()</b>'s stack usage
	</b><br>
	<P>
	You can often reduce the amount of recursion, and therefore the
	amount of stack used, by modifying the pattern that is being matched. Consider,
	for example, this pattern:
	<pre>
	([^<]\|<(?!inet))+
	</pre>
	It matches from wherever it starts until it encounters "<inet" or the end of
	the data, and is the kind of pattern that might be used when processing an XML
	file. Each iteration of the outer parentheses matches either one character that
	is not "<" or a "<" that is not followed by "inet". However, each time a
	parenthesis is processed, a recursion occurs, so this formulation uses a stack
	frame for each matched character. For a long string, a lot of stack is
	required. Consider now this rewritten pattern, which matches exactly the same
	strings:
	<pre>
	([^<]++\|<(?!inet))+
	</pre>
	This uses very much less stack, because runs of characters that do not contain
	"<" are "swallowed" in one item inside the parentheses. Recursion happens only
	when a "<" character that is not followed by "inet" is encountered (and we
	assume this is relatively rare). A possessive quantifier is used to stop any
	backtracking into the runs of non-"<" characters, but that is not related to
	stack usage.
	</P>
	<P>
	This example shows that one way of avoiding stack problems when matching long
	subject strings is to write repeated parenthesized subpatterns to match more
	than one character whenever possible.
	</P>
	<br><b>
	Compiling PCRE2 to use heap instead of stack for <b>pcre2_match()</b>
	</b><br>
	<P>
	In environments where stack memory is constrained, you might want to compile
	PCRE2 to use heap memory instead of stack for remembering back-up points when
	<b>pcre2_match()</b> is running. This makes it run more slowly, however. Details
	of how to do this are given in the
	<a href="pcre2build.html"><b>pcre2build</b></a>
	documentation. When built in this way, instead of using the stack, PCRE2
	gets memory for remembering backup points from the heap. By default, the memory
	is obtained by calling the system <b>malloc()</b> function, but you can arrange
	to supply your own memory management function. For details, see the section
	entitled
	<a href="pcre2api.html#matchcontext">"The match context"</a>
	in the
	<a href="pcre2api.html"><b>pcre2api</b></a>
	documentation. Since the block sizes are always the same, it may be possible to
	implement customized a memory handler that is more efficient than the standard
	function. The memory blocks obtained for this purpose are retained and re-used
	if possible while <b>pcre2_match()</b> is running. They are all freed just
	before it exits.
	</P>
	<br><b>
	Limiting <b>pcre2_match()</b>'s stack usage
	</b><br>
	<P>
	You can set limits on the number of times the internal <b>match()</b> function
	is called, both in total and recursively. If a limit is exceeded,
	<b>pcre2_match()</b> returns an error code. Setting suitable limits should
	prevent it from running out of stack. The default values of the limits are very
	large, and unlikely ever to operate. They can be changed when PCRE2 is built,
	and they can also be set when <b>pcre2_match()</b> is called. For details of
	these interfaces, see the
	<a href="pcre2build.html"><b>pcre2build</b></a>
	documentation and the section entitled
	<a href="pcre2api.html#matchcontext">"The match context"</a>
	in the
	<a href="pcre2api.html"><b>pcre2api</b></a>
	documentation.
	</P>
	<P>
	As a very rough rule of thumb, you should reckon on about 500 bytes per
	recursion. Thus, if you want to limit your stack usage to 8Mb, you should set
	the limit at 16000 recursions. A 64Mb stack, on the other hand, can support
	around 128000 recursions.
	</P>
	<P>
	The <b>pcre2test</b> test program has a modifier called "find_limits" which, if
	applied to a subject line, causes it to find the smallest limits that allow a a
	pattern to match. This is done by calling <b>pcre2_match()</b> repeatedly with
	different limits.
	</P>
	<br><b>
	Changing stack size in Unix-like systems
	</b><br>
	<P>
	In Unix-like environments, there is not often a problem with the stack unless
	very long strings are involved, though the default limit on stack size varies
	from system to system. Values from 8Mb to 64Mb are common. You can find your
	default limit by running the command:
	<pre>
	ulimit -s
	</pre>
	Unfortunately, the effect of running out of stack is often SIGSEGV, though
	sometimes a more explicit error message is given. You can normally increase the
	limit on stack size by code such as this:
	<pre>
	struct rlimit rlim;
	getrlimit(RLIMIT_STACK, &rlim);
	rlim.rlim_cur = 10010241024;
	setrlimit(RLIMIT_STACK, &rlim);
	</pre>
	This reads the current limits (soft and hard) using <b>getrlimit()</b>, then
	attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must
	do this before calling <b>pcre2_match()</b>.
	</P>
	<br><b>
	Changing stack size in Mac OS X
	</b><br>
	<P>
	Using <b>setrlimit()</b>, as described above, should also work on Mac OS X. It
	is also possible to set a stack size when linking a program. There is a
	discussion about stack sizes in Mac OS X at this web site:
	<a href="http://developer.apple.com/qa/qa2005/qa1419.html">http://developer.apple.com/qa/qa2005/qa1419.html.</a>
	</P>
	<br><b>
	AUTHOR
	</b><br>
	<P>
	Philip Hazel
	<br>
	University Computing Service
	<br>
	Cambridge, England.
	<br>
	</P>
	<br><b>
	REVISION
	</b><br>
	<P>
	Last updated: 21 November 2014
	<br>
	Copyright © 1997-2014 University of Cambridge.
	<br>
	<p>
	Return to the <a href="index.html">PCRE2 index page</a>.
	</p>