trunk/doc/pcrestack.3

.TH PCRESTACK 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PCRE DISCUSSION OF STACK USAGE"
.rs
.sp
When you call \fBpcre_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.
.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 \fBpcre_dfa_exec()\fP 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
\fBpcre_dfa_exec()\fP; they are relevant only for \fBpcre_exec()\fP.
.P
You can set limits on the number of times that \fBmatch()\fP is called, both in
total and recursively. If the limit is exceeded, an error occurs. For details,
see the
.\" HTML <a href="pcreapi.html#extradata">
.\" </a>
section on extra data for \fBpcre_exec()\fP
.\"
in the
.\" HREF
\fBpcreapi\fP
.\"
documentation.
.P
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"
.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. 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_stack_malloc\fP and \fBpcre_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 PCRE's stack usage"
.rs
.sp
PCRE has an internal counter that can be used to limit the depth of recursion,
and thus cause \fBpcre_exec()\fP 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
\fBpcre_exec()\fP is called. For details of these interfaces, see the
.\" HREF
\fBpcrebuild\fP
.\"
and
.\" 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. The \fBpcretest\fP test program has a command
line option (\fB-S\fP) that can be used to increase the size of its stack.
.
.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_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: 09 July 2008
Copyright (c) 1997-2008 University of Cambridge.
.fi
1	.TH PCRESTACK 3
2	.SH NAME
3	PCRE - Perl-compatible regular expressions
4	.SH "PCRE DISCUSSION OF STACK USAGE"
5	.rs
6	.sp
7	When you call \fBpcre_exec()\fP, it makes use of an internal function called
8	\fBmatch()\fP. This calls itself recursively at branch points in the pattern,
9	in order to remember the state of the match so that it can back up and try a
10	different alternative if the first one fails. As matching proceeds deeper and
11	deeper into the tree of possibilities, the recursion depth increases.
12	.P
13	Not all calls of \fBmatch()\fP increase the recursion depth; for an item such
14	as a* it may be called several times at the same level, after matching
15	different numbers of a's. Furthermore, in a number of cases where the result of
16	the recursive call would immediately be passed back as the result of the
17	current call (a "tail recursion"), the function is just restarted instead.
18	.P
19	The \fBpcre_dfa_exec()\fP function operates in an entirely different way, and
20	hardly uses recursion at all. The limit on its complexity is the amount of
21	workspace it is given. The comments that follow do NOT apply to
22	\fBpcre_dfa_exec()\fP; they are relevant only for \fBpcre_exec()\fP.
23	.P
24	You can set limits on the number of times that \fBmatch()\fP is called, both in
25	total and recursively. If the limit is exceeded, an error occurs. For details,
26	see the
27	.\" HTML <a href="pcreapi.html#extradata">
28	.\" </a>
29	section on extra data for \fBpcre_exec()\fP
30	.\"
31	in the
32	.\" HREF
33	\fBpcreapi\fP
34	.\"
35	documentation.
36	.P
37	Each time that \fBmatch()\fP is actually called recursively, it uses memory
38	from the process stack. For certain kinds of pattern and data, very large
39	amounts of stack may be needed, despite the recognition of "tail recursion".
40	You can often reduce the amount of recursion, and therefore the amount of stack
41	used, by modifying the pattern that is being matched. Consider, for example,
42	this pattern:
43	.sp
44	([^<]\|<(?!inet))+
45	.sp
46	It matches from wherever it starts until it encounters "<inet" or the end of
47	the data, and is the kind of pattern that might be used when processing an XML
48	file. Each iteration of the outer parentheses matches either one character that
49	is not "<" or a "<" that is not followed by "inet". However, each time a
50	parenthesis is processed, a recursion occurs, so this formulation uses a stack
51	frame for each matched character. For a long string, a lot of stack is
52	required. Consider now this rewritten pattern, which matches exactly the same
53	strings:
54	.sp
55	([^<]++\|<(?!inet))+
56	.sp
57	This uses very much less stack, because runs of characters that do not contain
58	"<" are "swallowed" in one item inside the parentheses. Recursion happens only
59	when a "<" character that is not followed by "inet" is encountered (and we
60	assume this is relatively rare). A possessive quantifier is used to stop any
61	backtracking into the runs of non-"<" characters, but that is not related to
62	stack usage.
63	.P
64	This example shows that one way of avoiding stack problems when matching long
65	subject strings is to write repeated parenthesized subpatterns to match more
66	than one character whenever possible.
67	.
68	.SS "Compiling PCRE to use heap instead of stack"
69	.rs
70	.sp
71	In environments where stack memory is constrained, you might want to compile
72	PCRE to use heap memory instead of stack for remembering back-up points. This
73	makes it run a lot more slowly, however. Details of how to do this are given in
74	the
75	.\" HREF
76	\fBpcrebuild\fP
77	.\"
78	documentation. When built in this way, instead of using the stack, PCRE obtains
79	and frees memory by calling the functions that are pointed to by the
80	\fBpcre_stack_malloc\fP and \fBpcre_stack_free\fP variables. By default, these
81	point to \fBmalloc()\fP and \fBfree()\fP, but you can replace the pointers to
82	cause PCRE to use your own functions. Since the block sizes are always the
83	same, and are always freed in reverse order, it may be possible to implement
84	customized memory handlers that are more efficient than the standard functions.
85	.
86	.SS "Limiting PCRE's stack usage"
87	.rs
88	.sp
89	PCRE has an internal counter that can be used to limit the depth of recursion,
90	and thus cause \fBpcre_exec()\fP to give an error code before it runs out of
91	stack. By default, the limit is very large, and unlikely ever to operate. It
92	can be changed when PCRE is built, and it can also be set when
93	\fBpcre_exec()\fP is called. For details of these interfaces, see the
94	.\" HREF
95	\fBpcrebuild\fP
96	.\"
97	and
98	.\" HREF
99	\fBpcreapi\fP
100	.\"
101	documentation.
102	.P
103	As a very rough rule of thumb, you should reckon on about 500 bytes per
104	recursion. Thus, if you want to limit your stack usage to 8Mb, you
105	should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can
106	support around 128000 recursions. The \fBpcretest\fP test program has a command
107	line option (\fB-S\fP) that can be used to increase the size of its stack.
108	.
109	.SS "Changing stack size in Unix-like systems"
110	.rs
111	.sp
112	In Unix-like environments, there is not often a problem with the stack unless
113	very long strings are involved, though the default limit on stack size varies
114	from system to system. Values from 8Mb to 64Mb are common. You can find your
115	default limit by running the command:
116	.sp
117	ulimit -s
118	.sp
119	Unfortunately, the effect of running out of stack is often SIGSEGV, though
120	sometimes a more explicit error message is given. You can normally increase the
121	limit on stack size by code such as this:
122	.sp
123	struct rlimit rlim;
124	getrlimit(RLIMIT_STACK, &rlim);
125	rlim.rlim_cur = 10010241024;
126	setrlimit(RLIMIT_STACK, &rlim);
127	.sp
128	This reads the current limits (soft and hard) using \fBgetrlimit()\fP, then
129	attempts to increase the soft limit to 100Mb using \fBsetrlimit()\fP. You must
130	do this before calling \fBpcre_exec()\fP.
131	.
132	.SS "Changing stack size in Mac OS X"
133	.rs
134	.sp
135	Using \fBsetrlimit()\fP, as described above, should also work on Mac OS X. It
136	is also possible to set a stack size when linking a program. There is a
137	discussion about stack sizes in Mac OS X at this web site:
138	.\" HTML <a href="http://developer.apple.com/qa/qa2005/qa1419.html">
139	.\" </a>
140	http://developer.apple.com/qa/qa2005/qa1419.html.
141	.\"
142	.
143	.
144	.SH AUTHOR
145	.rs
146	.sp
147	.nf
148	Philip Hazel
149	University Computing Service
150	Cambridge CB2 3QH, England.
151	.fi
152	.
153	.
154	.SH REVISION
155	.rs
156	.sp
157	.nf
158	Last updated: 09 July 2008
159	Copyright (c) 1997-2008 University of Cambridge.
160	.fi
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