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
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.
.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:
.sp
  ulimit -s
.sp
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:
.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.
.P
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 its stack.
.P
.in 0
Last updated: 29 June 2006
.br
Copyright (c) 1997-2006 University of Cambridge.
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	In environments where stack memory is constrained, you might want to compile
65	PCRE to use heap memory instead of stack for remembering back-up points. This
66	makes it run a lot more slowly, however. Details of how to do this are given in
67	the
68	.\" HREF
69	\fBpcrebuild\fP
70	.\"
71	documentation.
72	.P
73	In Unix-like environments, there is not often a problem with the stack, though
74	the default limit on stack size varies from system to system. Values from 8Mb
75	to 64Mb are common. You can find your default limit by running the command:
76	.sp
77	ulimit -s
78	.sp
79	The effect of running out of stack is often SIGSEGV, though sometimes an error
80	message is given. You can normally increase the limit on stack size by code
81	such as this:
82	.sp
83	struct rlimit rlim;
84	getrlimit(RLIMIT_STACK, &rlim);
85	rlim.rlim_cur = 10010241024;
86	setrlimit(RLIMIT_STACK, &rlim);
87	.sp
88	This reads the current limits (soft and hard) using \fBgetrlimit()\fP, then
89	attempts to increase the soft limit to 100Mb using \fBsetrlimit()\fP. You must
90	do this before calling \fBpcre_exec()\fP.
91	.P
92	PCRE has an internal counter that can be used to limit the depth of recursion,
93	and thus cause \fBpcre_exec()\fP to give an error code before it runs out of
94	stack. By default, the limit is very large, and unlikely ever to operate. It
95	can be changed when PCRE is built, and it can also be set when
96	\fBpcre_exec()\fP is called. For details of these interfaces, see the
97	.\" HREF
98	\fBpcrebuild\fP
99	.\"
100	and
101	.\" HREF
102	\fBpcreapi\fP
103	.\"
104	documentation.
105	.P
106	As a very rough rule of thumb, you should reckon on about 500 bytes per
107	recursion. Thus, if you want to limit your stack usage to 8Mb, you
108	should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can
109	support around 128000 recursions. The \fBpcretest\fP test program has a command
110	line option (\fB-S\fP) that can be used to increase its stack.
111	.P
112	.in 0
113	Last updated: 29 June 2006
114	.br
115	Copyright (c) 1997-2006 University of Cambridge.