doc/html/pcrestack.html

<html>
<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. 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>
The <b>pcre_dfa_exec()</b> function operates in an entirely different way, 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>pcre_dfa_exec()</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>pcre_dfa_exec()</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>pcre_dfa_exec()</b>; they are
relevant only for <b>pcre_exec()</b>.
</P>
<br><b>
Reducing <b>pcre_exec()</b>'s stack usage
</b><br>
<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>
<br><b>
Compiling PCRE to use heap instead of stack for <b>pcre_exec()</b>
</b><br>
<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 when
<b>pcre_exec()</b> is running. 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. When built in this way, instead of using the stack, PCRE obtains
and frees memory by calling the functions that are pointed to by the
<b>pcre_stack_malloc</b> and <b>pcre_stack_free</b> variables. By default, these
point to <b>malloc()</b> and <b>free()</b>, but you can replace the pointers to
cause PCRE to use your own functions. Since the block sizes are always the
same, and are always freed in reverse order, it may be possible to implement
customized memory handlers that are more efficient than the standard functions.
</P>
<br><b>
Limiting <b>pcre_exec()</b>'s stack usage
</b><br>
<P>
You can set limits on the number of times that <b>match()</b> is called, both in
total and recursively. If a limit is exceeded, <b>pcre_exec()</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 PCRE is built, and they 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>
documentation and 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>
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>
In Unix-like environments, 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. As long
as the stack is large enough, another option (<b>-M</b>) can be used to find the
smallest limits that allow a particular pattern to match a given subject
string. This is done by calling <b>pcre_exec()</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>pcre_exec()</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 CB2 3QH, England.
<br>
</P>
<br><b>
REVISION
</b><br>
<P>
Last updated: 22 July 2011
<br>
Copyright &copy; 1997-2011 University of Cambridge.
<br>
<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. 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 <b>pcre_dfa_exec()</b> function operates in an entirely different way, and
37	uses recursion only when there is a regular expression recursion or subroutine
38	call in the pattern. This includes the processing of assertion and "once-only"
39	subpatterns, which are handled like subroutine calls. Normally, these are never
40	very deep, and the limit on the complexity of <b>pcre_dfa_exec()</b> is
41	controlled by the amount of workspace it is given. However, it is possible to
42	write patterns with runaway infinite recursions; such patterns will cause
43	<b>pcre_dfa_exec()</b> to run out of stack. At present, there is no protection
44	against this.
45	</P>
46	<P>
47	The comments that follow do NOT apply to <b>pcre_dfa_exec()</b>; they are
48	relevant only for <b>pcre_exec()</b>.
49	</P>
50	<br><b>
51	Reducing <b>pcre_exec()</b>'s stack usage
52	</b><br>
53	<P>
54	Each time that <b>match()</b> is actually called recursively, it uses memory
55	from the process stack. For certain kinds of pattern and data, very large
56	amounts of stack may be needed, despite the recognition of "tail recursion".
57	You can often reduce the amount of recursion, and therefore the amount of stack
58	used, by modifying the pattern that is being matched. Consider, for example,
59	this pattern:
60	<pre>
61	([^<]\|<(?!inet))+
62	</pre>
63	It matches from wherever it starts until it encounters "<inet" or the end of
64	the data, and is the kind of pattern that might be used when processing an XML
65	file. Each iteration of the outer parentheses matches either one character that
66	is not "<" or a "<" that is not followed by "inet". However, each time a
67	parenthesis is processed, a recursion occurs, so this formulation uses a stack
68	frame for each matched character. For a long string, a lot of stack is
69	required. Consider now this rewritten pattern, which matches exactly the same
70	strings:
71	<pre>
72	([^<]++\|<(?!inet))+
73	</pre>
74	This uses very much less stack, because runs of characters that do not contain
75	"<" are "swallowed" in one item inside the parentheses. Recursion happens only
76	when a "<" character that is not followed by "inet" is encountered (and we
77	assume this is relatively rare). A possessive quantifier is used to stop any
78	backtracking into the runs of non-"<" characters, but that is not related to
79	stack usage.
80	</P>
81	<P>
82	This example shows that one way of avoiding stack problems when matching long
83	subject strings is to write repeated parenthesized subpatterns to match more
84	than one character whenever possible.
85	</P>
86	<br><b>
87	Compiling PCRE to use heap instead of stack for <b>pcre_exec()</b>
88	</b><br>
89	<P>
90	In environments where stack memory is constrained, you might want to compile
91	PCRE to use heap memory instead of stack for remembering back-up points when
92	<b>pcre_exec()</b> is running. This makes it run a lot more slowly, however.
93	Details of how to do this are given in the
94	<a href="pcrebuild.html"><b>pcrebuild</b></a>
95	documentation. When built in this way, instead of using the stack, PCRE obtains
96	and frees memory by calling the functions that are pointed to by the
97	<b>pcre_stack_malloc</b> and <b>pcre_stack_free</b> variables. By default, these
98	point to <b>malloc()</b> and <b>free()</b>, but you can replace the pointers to
99	cause PCRE to use your own functions. Since the block sizes are always the
100	same, and are always freed in reverse order, it may be possible to implement
101	customized memory handlers that are more efficient than the standard functions.
102	</P>
103	<br><b>
104	Limiting <b>pcre_exec()</b>'s stack usage
105	</b><br>
106	<P>
107	You can set limits on the number of times that <b>match()</b> is called, both in
108	total and recursively. If a limit is exceeded, <b>pcre_exec()</b> returns an
109	error code. Setting suitable limits should prevent it from running out of
110	stack. The default values of the limits are very large, and unlikely ever to
111	operate. They can be changed when PCRE is built, and they can also be set when
112	<b>pcre_exec()</b> is called. For details of these interfaces, see the
113	<a href="pcrebuild.html"><b>pcrebuild</b></a>
114	documentation and the
115	<a href="pcreapi.html#extradata">section on extra data for <b>pcre_exec()</b></a>
116	in the
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.
125	</P>
126	<P>
127	In Unix-like environments, the <b>pcretest</b> test program has a command line
128	option (<b>-S</b>) that can be used to increase the size of its stack. As long
129	as the stack is large enough, another option (<b>-M</b>) can be used to find the
130	smallest limits that allow a particular pattern to match a given subject
131	string. This is done by calling <b>pcre_exec()</b> repeatedly with different
132	limits.
133	</P>
134	<br><b>
135	Changing stack size in Unix-like systems
136	</b><br>
137	<P>
138	In Unix-like environments, there is not often a problem with the stack unless
139	very long strings are involved, though the default limit on stack size varies
140	from system to system. Values from 8Mb to 64Mb are common. You can find your
141	default limit by running the command:
142	<pre>
143	ulimit -s
144	</pre>
145	Unfortunately, the effect of running out of stack is often SIGSEGV, though
146	sometimes a more explicit error message is given. You can normally increase the
147	limit on stack size by code such as this:
148	<pre>
149	struct rlimit rlim;
150	getrlimit(RLIMIT_STACK, &rlim);
151	rlim.rlim_cur = 10010241024;
152	setrlimit(RLIMIT_STACK, &rlim);
153	</pre>
154	This reads the current limits (soft and hard) using <b>getrlimit()</b>, then
155	attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must
156	do this before calling <b>pcre_exec()</b>.
157	</P>
158	<br><b>
159	Changing stack size in Mac OS X
160	</b><br>
161	<P>
162	Using <b>setrlimit()</b>, as described above, should also work on Mac OS X. It
163	is also possible to set a stack size when linking a program. There is a
164	discussion about stack sizes in Mac OS X at this web site:
165	<a href="http://developer.apple.com/qa/qa2005/qa1419.html">http://developer.apple.com/qa/qa2005/qa1419.html.</a>
166	</P>
167	<br><b>
168	AUTHOR
169	</b><br>
170	<P>
171	Philip Hazel
172	<br>
173	University Computing Service
174	<br>
175	Cambridge CB2 3QH, England.
176	<br>
177	</P>
178	<br><b>
179	REVISION
180	</b><br>
181	<P>
182	Last updated: 22 July 2011
183	<br>
184	Copyright © 1997-2011 University of Cambridge.
185	<br>
186	<p>
187	Return to the <a href="index.html">PCRE index page</a>.
188	</p>
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svn:keywords	"Author Date Id Revision Url"