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>
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, 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>
The effect of running out of stack is often SIGSEGV, though sometimes an 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 its stack.
</P>
<P>
Last updated: 29 June 2006
<br>
Copyright &copy; 1997-2006 University of Cambridge.
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
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.
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	In environments where stack memory is constrained, you might want to compile
77	PCRE to use heap memory instead of stack for remembering back-up points. This
78	makes it run a lot more slowly, however. Details of how to do this are given in
79	the
80	<a href="pcrebuild.html"><b>pcrebuild</b></a>
81	documentation.
82	</P>
83	<P>
84	In Unix-like environments, there is not often a problem with the stack, though
85	the default limit on stack size varies from system to system. Values from 8Mb
86	to 64Mb are common. You can find your default limit by running the command:
87	<pre>
88	ulimit -s
89	</pre>
90	The effect of running out of stack is often SIGSEGV, though sometimes an error
91	message is given. You can normally increase the limit on stack size by code
92	such as this:
93	<pre>
94	struct rlimit rlim;
95	getrlimit(RLIMIT_STACK, &rlim);
96	rlim.rlim_cur = 10010241024;
97	setrlimit(RLIMIT_STACK, &rlim);
98	</pre>
99	This reads the current limits (soft and hard) using <b>getrlimit()</b>, then
100	attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must
101	do this before calling <b>pcre_exec()</b>.
102	</P>
103	<P>
104	PCRE has an internal counter that can be used to limit the depth of recursion,
105	and thus cause <b>pcre_exec()</b> to give an error code before it runs out of
106	stack. By default, the limit is very large, and unlikely ever to operate. It
107	can be changed when PCRE is built, and it can also be set when
108	<b>pcre_exec()</b> is called. For details of these interfaces, see the
109	<a href="pcrebuild.html"><b>pcrebuild</b></a>
110	and
111	<a href="pcreapi.html"><b>pcreapi</b></a>
112	documentation.
113	</P>
114	<P>
115	As a very rough rule of thumb, you should reckon on about 500 bytes per
116	recursion. Thus, if you want to limit your stack usage to 8Mb, you
117	should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can
118	support around 128000 recursions. The <b>pcretest</b> test program has a command
119	line option (<b>-S</b>) that can be used to increase its stack.
120	</P>
121	<P>
122	Last updated: 29 June 2006
123	<br>
124	Copyright © 1997-2006 University of Cambridge.
125	<p>
126	Return to the <a href="index.html">PCRE index page</a>.
127	</p>