trunk/doc/pcrestack.3

.TH PCRESTACK 3 "24 June 2012" "PCRE 8.30"
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PCRE DISCUSSION OF STACK USAGE"
.rs
.sp
When you call \fBpcre[16|32]_exec()\fP, it makes use of an internal function
called \fBmatch()\fP. 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
\fBmatch()\fP function is also called in other circumstances, for example,
whenever a parenthesized sub-pattern is entered, and in certain cases of
repetition.
.P
Not all calls of \fBmatch()\fP 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
The above comments apply when \fBpcre[16|32]_exec()\fP is run in its normal
interpretive manner. If the pattern was studied with the
PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling was successful, and
the options passed to \fBpcre[16|32]_exec()\fP were not incompatible, the matching
process uses the JIT-compiled code instead of the \fBmatch()\fP function. In
this case, the memory requirements are handled entirely differently. See the
.\" HREF
\fBpcrejit\fP
.\"
documentation for details.
.P
The \fBpcre[16|32]_dfa_exec()\fP 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
\fBpcre[16|32]_dfa_exec()\fP 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 \fBpcre[16|32]_dfa_exec()\fP to run out of stack. At
present, there is no protection against this.
.P
The comments that follow do NOT apply to \fBpcre[16|32]_dfa_exec()\fP; they are
relevant only for \fBpcre[16|32]_exec()\fP without the JIT optimization.
.
.
.SS "Reducing \fBpcre[16|32]_exec()\fP's stack usage"
.rs
.sp
Each time that \fBmatch()\fP 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:
.sp
  ([^<]|<(?!inet))+
.sp
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:
.sp
  ([^<]++|<(?!inet))+
.sp
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
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.
.
.
.SS "Compiling PCRE to use heap instead of stack for \fBpcre[16|32]_exec()\fP"
.rs
.sp
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
\fBpcre[16|32]_exec()\fP is running. This makes it run a lot more slowly, however.
Details of how to do this are given in the
.\" HREF
\fBpcrebuild\fP
.\"
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
\fBpcre[16|32]_stack_malloc\fP and \fBpcre[16|32]_stack_free\fP variables. By
default, these point to \fBmalloc()\fP and \fBfree()\fP, 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.
.
.
.SS "Limiting \fBpcre[16|32]_exec()\fP's stack usage"
.rs
.sp
You can set limits on the number of times that \fBmatch()\fP is called, both in
total and recursively. If a limit is exceeded, \fBpcre[16|32]_exec()\fP 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
\fBpcre[16|32]_exec()\fP is called. For details of these interfaces, see the
.\" HREF
\fBpcrebuild\fP
.\"
documentation and the
.\" HTML <a href="pcreapi.html#extradata">
.\" </a>
section on extra data for \fBpcre[16|32]_exec()\fP
.\"
in the
.\" HREF
\fBpcreapi\fP
.\"
documentation.
.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
In Unix-like environments, the \fBpcretest\fP test program has a command line
option (\fB-S\fP) that can be used to increase the size of its stack. As long
as the stack is large enough, another option (\fB-M\fP) can be used to find the
smallest limits that allow a particular pattern to match a given subject
string. This is done by calling \fBpcre[16|32]_exec()\fP repeatedly with different
limits.
.
.
.SS "Obtaining an estimate of stack usage"
.rs
.sp
The actual amount of stack used per recursion can vary quite a lot, depending
on the compiler that was used to build PCRE and the optimization or debugging
options that were set for it. The rule of thumb value of 500 bytes mentioned
above may be larger or smaller than what is actually needed. A better
approximation can be obtained by running this command:
.sp
  pcretest -m -C
.sp
The \fB-C\fP option causes \fBpcretest\fP to output information about the
options with which PCRE was compiled. When \fB-m\fP is also given (before
\fB-C\fP), information about stack use is given in a line like this:
.sp
  Match recursion uses stack: approximate frame size = 640 bytes
.sp
The value is approximate because some recursions need a bit more (up to perhaps
16 more bytes).
.P
If the above command is given when PCRE is compiled to use the heap instead of
the stack for recursion, the value that is output is the size of each block
that is obtained from the heap.
.
.
.SS "Changing stack size in Unix-like systems"
.rs
.sp
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:
.sp
  ulimit -s
.sp
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:
.sp
  struct rlimit rlim;
  getrlimit(RLIMIT_STACK, &rlim);
  rlim.rlim_cur = 100*1024*1024;
  setrlimit(RLIMIT_STACK, &rlim);
.sp
This reads the current limits (soft and hard) using \fBgetrlimit()\fP, then
attempts to increase the soft limit to 100Mb using \fBsetrlimit()\fP. You must
do this before calling \fBpcre[16|32]_exec()\fP.
.
.
.SS "Changing stack size in Mac OS X"
.rs
.sp
Using \fBsetrlimit()\fP, 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:
.\" HTML <a href="http://developer.apple.com/qa/qa2005/qa1419.html">
.\" </a>
http://developer.apple.com/qa/qa2005/qa1419.html.
.\"
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 24 June 2012
Copyright (c) 1997-2012 University of Cambridge.
.fi
1	chpe	1055	.TH PCRESTACK 3 "24 June 2012" "PCRE 8.30"
2	nigel	91	.SH NAME
3			PCRE - Perl-compatible regular expressions
4			.SH "PCRE DISCUSSION OF STACK USAGE"
5			.rs
6			.sp
7	chpe	1055	When you call \fBpcre[16\|32]_exec()\fP, it makes use of an internal function
8	ph10	861	called \fBmatch()\fP. This calls itself recursively at branch points in the
9			pattern, in order to remember the state of the match so that it can back up and
10			try a different alternative if the first one fails. As matching proceeds deeper
11			and deeper into the tree of possibilities, the recursion depth increases. The
12	ph10	654	\fBmatch()\fP function is also called in other circumstances, for example,
13	ph10	631	whenever a parenthesized sub-pattern is entered, and in certain cases of
14			repetition.
15	nigel	91	.P
16			Not all calls of \fBmatch()\fP increase the recursion depth; for an item such
17			as a* it may be called several times at the same level, after matching
18			different numbers of a's. Furthermore, in a number of cases where the result of
19			the recursive call would immediately be passed back as the result of the
20			current call (a "tail recursion"), the function is just restarted instead.
21			.P
22	chpe	1055	The above comments apply when \fBpcre[16\|32]_exec()\fP is run in its normal
23	ph10	691	interpretive manner. If the pattern was studied with the
24			PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling was successful, and
25	chpe	1055	the options passed to \fBpcre[16\|32]_exec()\fP were not incompatible, the matching
26	ph10	691	process uses the JIT-compiled code instead of the \fBmatch()\fP function. In
27	ph10	678	this case, the memory requirements are handled entirely differently. See the
28			.\" HREF
29			\fBpcrejit\fP
30			.\"
31			documentation for details.
32			.P
33	chpe	1055	The \fBpcre[16\|32]_dfa_exec()\fP function operates in an entirely different way,
34	ph10	861	and uses recursion only when there is a regular expression recursion or
35			subroutine call in the pattern. This includes the processing of assertion and
36			"once-only" subpatterns, which are handled like subroutine calls. Normally,
37			these are never very deep, and the limit on the complexity of
38	chpe	1055	\fBpcre[16\|32]_dfa_exec()\fP is controlled by the amount of workspace it is given.
39	ph10	861	However, it is possible to write patterns with runaway infinite recursions;
40	chpe	1055	such patterns will cause \fBpcre[16\|32]_dfa_exec()\fP to run out of stack. At
41	ph10	861	present, there is no protection against this.
42	nigel	91	.P
43	chpe	1055	The comments that follow do NOT apply to \fBpcre[16\|32]_dfa_exec()\fP; they are
44			relevant only for \fBpcre[16\|32]_exec()\fP without the JIT optimization.
45	ph10	480	.
46			.
47	chpe	1055	.SS "Reducing \fBpcre[16\|32]_exec()\fP's stack usage"
48	ph10	480	.rs
49			.sp
50	nigel	91	Each time that \fBmatch()\fP is actually called recursively, it uses memory
51			from the process stack. For certain kinds of pattern and data, very large
52			amounts of stack may be needed, despite the recognition of "tail recursion".
53			You can often reduce the amount of recursion, and therefore the amount of stack
54			used, by modifying the pattern that is being matched. Consider, for example,
55			this pattern:
56			.sp
57			([^<]\|<(?!inet))+
58			.sp
59			It matches from wherever it starts until it encounters "<inet" or the end of
60			the data, and is the kind of pattern that might be used when processing an XML
61			file. Each iteration of the outer parentheses matches either one character that
62			is not "<" or a "<" that is not followed by "inet". However, each time a
63			parenthesis is processed, a recursion occurs, so this formulation uses a stack
64			frame for each matched character. For a long string, a lot of stack is
65			required. Consider now this rewritten pattern, which matches exactly the same
66			strings:
67			.sp
68	ph10	122	([^<]++\|<(?!inet))+
69	nigel	91	.sp
70			This uses very much less stack, because runs of characters that do not contain
71			"<" are "swallowed" in one item inside the parentheses. Recursion happens only
72			when a "<" character that is not followed by "inet" is encountered (and we
73			assume this is relatively rare). A possessive quantifier is used to stop any
74			backtracking into the runs of non-"<" characters, but that is not related to
75			stack usage.
76			.P
77	nigel	93	This example shows that one way of avoiding stack problems when matching long
78			subject strings is to write repeated parenthesized subpatterns to match more
79			than one character whenever possible.
80	ph10	358	.
81	ph10	480	.
82	chpe	1055	.SS "Compiling PCRE to use heap instead of stack for \fBpcre[16\|32]_exec()\fP"
83	ph10	358	.rs
84			.sp
85	nigel	91	In environments where stack memory is constrained, you might want to compile
86	ph10	480	PCRE to use heap memory instead of stack for remembering back-up points when
87	chpe	1055	\fBpcre[16\|32]_exec()\fP is running. This makes it run a lot more slowly, however.
88	ph10	480	Details of how to do this are given in the
89	nigel	91	.\" HREF
90			\fBpcrebuild\fP
91			.\"
92	ph10	174	documentation. When built in this way, instead of using the stack, PCRE obtains
93			and frees memory by calling the functions that are pointed to by the
94	chpe	1055	\fBpcre[16\|32]_stack_malloc\fP and \fBpcre[16\|32]_stack_free\fP variables. By
95	ph10	861	default, these point to \fBmalloc()\fP and \fBfree()\fP, but you can replace
96			the pointers to cause PCRE to use your own functions. Since the block sizes are
97			always the same, and are always freed in reverse order, it may be possible to
98			implement customized memory handlers that are more efficient than the standard
99			functions.
100	ph10	358	.
101	ph10	480	.
102	chpe	1055	.SS "Limiting \fBpcre[16\|32]_exec()\fP's stack usage"
103	ph10	358	.rs
104			.sp
105	ph10	480	You can set limits on the number of times that \fBmatch()\fP is called, both in
106	chpe	1055	total and recursively. If a limit is exceeded, \fBpcre[16\|32]_exec()\fP returns an
107	ph10	480	error code. Setting suitable limits should prevent it from running out of
108			stack. The default values of the limits are very large, and unlikely ever to
109			operate. They can be changed when PCRE is built, and they can also be set when
110	chpe	1055	\fBpcre[16\|32]_exec()\fP is called. For details of these interfaces, see the
111	ph10	358	.\" HREF
112			\fBpcrebuild\fP
113			.\"
114	ph10	480	documentation and the
115			.\" HTML <a href="pcreapi.html#extradata">
116			.\" </a>
117	chpe	1055	section on extra data for \fBpcre[16\|32]_exec()\fP
118	ph10	480	.\"
119			in the
120	ph10	358	.\" HREF
121			\fBpcreapi\fP
122			.\"
123			documentation.
124	nigel	91	.P
125	ph10	358	As a very rough rule of thumb, you should reckon on about 500 bytes per
126	ph10	901	recursion. Thus, if you want to limit your stack usage to 8Mb, you should set
127			the limit at 16000 recursions. A 64Mb stack, on the other hand, can support
128			around 128000 recursions.
129	ph10	480	.P
130			In Unix-like environments, the \fBpcretest\fP test program has a command line
131			option (\fB-S\fP) that can be used to increase the size of its stack. As long
132			as the stack is large enough, another option (\fB-M\fP) can be used to find the
133			smallest limits that allow a particular pattern to match a given subject
134	chpe	1055	string. This is done by calling \fBpcre[16\|32]_exec()\fP repeatedly with different
135	ph10	480	limits.
136	ph10	358	.
137	ph10	480	.
138	ph10	901	.SS "Obtaining an estimate of stack usage"
139			.rs
140			.sp
141			The actual amount of stack used per recursion can vary quite a lot, depending
142			on the compiler that was used to build PCRE and the optimization or debugging
143	ph10	903	options that were set for it. The rule of thumb value of 500 bytes mentioned
144			above may be larger or smaller than what is actually needed. A better
145	ph10	901	approximation can be obtained by running this command:
146			.sp
147			pcretest -m -C
148			.sp
149	ph10	903	The \fB-C\fP option causes \fBpcretest\fP to output information about the
150			options with which PCRE was compiled. When \fB-m\fP is also given (before
151	ph10	901	\fB-C\fP), information about stack use is given in a line like this:
152			.sp
153			Match recursion uses stack: approximate frame size = 640 bytes
154	ph10	903	.sp
155			The value is approximate because some recursions need a bit more (up to perhaps
156	ph10	901	16 more bytes).
157			.P
158	ph10	903	If the above command is given when PCRE is compiled to use the heap instead of
159			the stack for recursion, the value that is output is the size of each block
160			that is obtained from the heap.
161	ph10	901	.
162			.
163	ph10	358	.SS "Changing stack size in Unix-like systems"
164			.rs
165			.sp
166	nigel	93	In Unix-like environments, there is not often a problem with the stack unless
167			very long strings are involved, though the default limit on stack size varies
168			from system to system. Values from 8Mb to 64Mb are common. You can find your
169			default limit by running the command:
170	nigel	91	.sp
171			ulimit -s
172			.sp
173	nigel	93	Unfortunately, the effect of running out of stack is often SIGSEGV, though
174			sometimes a more explicit error message is given. You can normally increase the
175			limit on stack size by code such as this:
176	nigel	91	.sp
177			struct rlimit rlim;
178			getrlimit(RLIMIT_STACK, &rlim);
179			rlim.rlim_cur = 10010241024;
180			setrlimit(RLIMIT_STACK, &rlim);
181			.sp
182			This reads the current limits (soft and hard) using \fBgetrlimit()\fP, then
183			attempts to increase the soft limit to 100Mb using \fBsetrlimit()\fP. You must
184	chpe	1055	do this before calling \fBpcre[16\|32]_exec()\fP.
185	ph10	358	.
186	ph10	480	.
187	ph10	358	.SS "Changing stack size in Mac OS X"
188			.rs
189			.sp
190			Using \fBsetrlimit()\fP, as described above, should also work on Mac OS X. It
191			is also possible to set a stack size when linking a program. There is a
192			discussion about stack sizes in Mac OS X at this web site:
193			.\" HTML <a href="http://developer.apple.com/qa/qa2005/qa1419.html">
194			.\" </a>
195			http://developer.apple.com/qa/qa2005/qa1419.html.
196	nigel	91	.\"
197	ph10	99	.
198			.
199			.SH AUTHOR
200			.rs
201			.sp
202			.nf
203			Philip Hazel
204			University Computing Service
205			Cambridge CB2 3QH, England.
206			.fi
207			.
208			.
209			.SH REVISION
210			.rs
211			.sp
212			.nf
213	chpe	1055	Last updated: 24 June 2012
214	ph10	861	Copyright (c) 1997-2012 University of Cambridge.
215	ph10	99	.fi
Name	Value
svn:eol-style	native
svn:keywords	"Author Date Id Revision Url"