doc/html/pcrestack.html

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<head>
<title>pcrestack specification</title>
</head>
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
<h1>pcrestack man page</h1>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
<p> 
This page is part of the PCRE 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>
PCRE DISCUSSION OF STACK USAGE
</b><br>
<P>
When you call <b>pcre_exec()</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 if the first one fails. As matching proceeds deeper and
deeper into the tree of possibilities, the recursion depth increases.
</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>
The <b>pcre_dfa_exec()</b> function operates in an entirely different way, and
hardly uses recursion at all. The limit on its complexity is the amount of
workspace it is given. The comments that follow do NOT apply to
<b>pcre_dfa_exec()</b>; they are relevant only for <b>pcre_exec()</b>.
</P>
<P>
You can set limits on the number of times that <b>match()</b> is called, both in
total and recursively. If the limit is exceeded, an error occurs. For details,
see the
<a href="pcreapi.html#extradata">section on extra data for <b>pcre_exec()</b></a>
in the
<a href="pcreapi.html"><b>pcreapi</b></a>
documentation.
</P>
<P>
Each time that <b>match()</b> is actually 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".
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>
<P>
In environments where stack memory is constrained, you might want to compile
PCRE to use heap memory instead of stack for remembering back-up points. This
makes it run a lot more slowly, however. Details of how to do this are given in
the
<a href="pcrebuild.html"><b>pcrebuild</b></a>
documentation.
</P>
<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>pcre_exec()</b>.
</P>
<P>
PCRE has an internal counter that can be used to limit the depth of recursion,
and thus cause <b>pcre_exec()</b> to give an error code before it runs out of
stack. By default, the limit is very large, and unlikely ever to operate. It
can be changed when PCRE is built, and it can also be set when
<b>pcre_exec()</b> is called. For details of these interfaces, see the
<a href="pcrebuild.html"><b>pcrebuild</b></a>
and
<a href="pcreapi.html"><b>pcreapi</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. The <b>pcretest</b> test program has a command
line option (<b>-S</b>) that can be used to increase the size of its stack.
</P>
<br><b>
AUTHOR
</b><br>
<P>
Philip Hazel
<br>
University Computing Service
<br>
Cambridge CB2 3QH, England.
<br>
</P>
<br><b>
REVISION
</b><br>
<P>
Last updated: 06 March 2007
<br>
Copyright &copy; 1997-2007 University of Cambridge.
<br>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
1	nigel	91	<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	ph10	99	<p>
11	nigel	91	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	ph10	99	<br>
15	nigel	91	<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.
24			</P>
25			<P>
26			Not all calls of <b>match()</b> increase the recursion depth; for an item such
27			as a* it may be called several times at the same level, after matching
28			different numbers of a's. Furthermore, in a number of cases where the result of
29			the recursive call would immediately be passed back as the result of the
30			current call (a "tail recursion"), the function is just restarted instead.
31			</P>
32			<P>
33			The <b>pcre_dfa_exec()</b> function operates in an entirely different way, and
34			hardly uses recursion at all. The limit on its complexity is the amount of
35			workspace it is given. The comments that follow do NOT apply to
36			<b>pcre_dfa_exec()</b>; they are relevant only for <b>pcre_exec()</b>.
37			</P>
38			<P>
39			You can set limits on the number of times that <b>match()</b> is called, both in
40			total and recursively. If the limit is exceeded, an error occurs. For details,
41			see the
42			<a href="pcreapi.html#extradata">section on extra data for <b>pcre_exec()</b></a>
43			in the
44			<a href="pcreapi.html"><b>pcreapi</b></a>
45			documentation.
46			</P>
47			<P>
48			Each time that <b>match()</b> is actually called recursively, it uses memory
49			from the process stack. For certain kinds of pattern and data, very large
50			amounts of stack may be needed, despite the recognition of "tail recursion".
51			You can often reduce the amount of recursion, and therefore the amount of stack
52			used, by modifying the pattern that is being matched. Consider, for example,
53			this pattern:
54			<pre>
55			([^<]\|<(?!inet))+
56			</pre>
57			It matches from wherever it starts until it encounters "<inet" or the end of
58			the data, and is the kind of pattern that might be used when processing an XML
59			file. Each iteration of the outer parentheses matches either one character that
60			is not "<" or a "<" that is not followed by "inet". However, each time a
61			parenthesis is processed, a recursion occurs, so this formulation uses a stack
62			frame for each matched character. For a long string, a lot of stack is
63			required. Consider now this rewritten pattern, which matches exactly the same
64			strings:
65			<pre>
66			([^<]++\|<(?!inet))
67			</pre>
68			This uses very much less stack, because runs of characters that do not contain
69			"<" are "swallowed" in one item inside the parentheses. Recursion happens only
70			when a "<" character that is not followed by "inet" is encountered (and we
71			assume this is relatively rare). A possessive quantifier is used to stop any
72			backtracking into the runs of non-"<" characters, but that is not related to
73			stack usage.
74			</P>
75			<P>
76	nigel	93	This example shows that one way of avoiding stack problems when matching long
77			subject strings is to write repeated parenthesized subpatterns to match more
78			than one character whenever possible.
79			</P>
80			<P>
81	nigel	91	In environments where stack memory is constrained, you might want to compile
82			PCRE to use heap memory instead of stack for remembering back-up points. This
83			makes it run a lot more slowly, however. Details of how to do this are given in
84			the
85			<a href="pcrebuild.html"><b>pcrebuild</b></a>
86			documentation.
87			</P>
88			<P>
89	nigel	93	In Unix-like environments, there is not often a problem with the stack unless
90			very long strings are involved, though the default limit on stack size varies
91			from system to system. Values from 8Mb to 64Mb are common. You can find your
92			default limit by running the command:
93	nigel	91	<pre>
94			ulimit -s
95			</pre>
96	nigel	93	Unfortunately, the effect of running out of stack is often SIGSEGV, though
97			sometimes a more explicit error message is given. You can normally increase the
98			limit on stack size by code such as this:
99	nigel	91	<pre>
100			struct rlimit rlim;
101			getrlimit(RLIMIT_STACK, &rlim);
102			rlim.rlim_cur = 10010241024;
103			setrlimit(RLIMIT_STACK, &rlim);
104			</pre>
105			This reads the current limits (soft and hard) using <b>getrlimit()</b>, then
106			attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must
107			do this before calling <b>pcre_exec()</b>.
108			</P>
109			<P>
110			PCRE has an internal counter that can be used to limit the depth of recursion,
111			and thus cause <b>pcre_exec()</b> to give an error code before it runs out of
112			stack. By default, the limit is very large, and unlikely ever to operate. It
113			can be changed when PCRE is built, and it can also be set when
114			<b>pcre_exec()</b> is called. For details of these interfaces, see the
115			<a href="pcrebuild.html"><b>pcrebuild</b></a>
116			and
117			<a href="pcreapi.html"><b>pcreapi</b></a>
118			documentation.
119			</P>
120			<P>
121			As a very rough rule of thumb, you should reckon on about 500 bytes per
122			recursion. Thus, if you want to limit your stack usage to 8Mb, you
123			should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can
124			support around 128000 recursions. The <b>pcretest</b> test program has a command
125	nigel	93	line option (<b>-S</b>) that can be used to increase the size of its stack.
126	nigel	91	</P>
127	ph10	99	<br><b>
128			AUTHOR
129			</b><br>
130	nigel	91	<P>
131	ph10	99	Philip Hazel
132	nigel	91	<br>
133	ph10	99	University Computing Service
134			<br>
135			Cambridge CB2 3QH, England.
136			<br>
137			</P>
138			<br><b>
139			REVISION
140			</b><br>
141			<P>
142			Last updated: 06 March 2007
143			<br>
144			Copyright © 1997-2007 University of Cambridge.
145			<br>
146	nigel	91	<p>
147			Return to the <a href="index.html">PCRE index page</a>.
148			</p>
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