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Fri Jul 22 10:13:28 2011 UTC (8 years, 4 months ago) by ph10
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Documentation update.
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. The
12 \fBmatch()\P function is also called in other circumstances, for example,
13 whenever a parenthesized sub-pattern is entered, and in certain cases of
14 repetition.
15 .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 The \fBpcre_dfa_exec()\fP function operates in an entirely different way, and
23 uses recursion only when there is a regular expression recursion or subroutine
24 call in the pattern. This includes the processing of assertion and "once-only"
25 subpatterns, which are handled like subroutine calls. Normally, these are never
26 very deep, and the limit on the complexity of \fBpcre_dfa_exec()\fP is
27 controlled by the amount of workspace it is given. However, it is possible to
28 write patterns with runaway infinite recursions; such patterns will cause
29 \fBpcre_dfa_exec()\fP to run out of stack. At present, there is no protection
30 against this.
31 .P
32 The comments that follow do NOT apply to \fBpcre_dfa_exec()\fP; they are
33 relevant only for \fBpcre_exec()\fP.
34 .
35 .
36 .SS "Reducing \fBpcre_exec()\fP's stack usage"
37 .rs
38 .sp
39 Each time that \fBmatch()\fP is actually called recursively, it uses memory
40 from the process stack. For certain kinds of pattern and data, very large
41 amounts of stack may be needed, despite the recognition of "tail recursion".
42 You can often reduce the amount of recursion, and therefore the amount of stack
43 used, by modifying the pattern that is being matched. Consider, for example,
44 this pattern:
45 .sp
46 ([^<]|<(?!inet))+
47 .sp
48 It matches from wherever it starts until it encounters "<inet" or the end of
49 the data, and is the kind of pattern that might be used when processing an XML
50 file. Each iteration of the outer parentheses matches either one character that
51 is not "<" or a "<" that is not followed by "inet". However, each time a
52 parenthesis is processed, a recursion occurs, so this formulation uses a stack
53 frame for each matched character. For a long string, a lot of stack is
54 required. Consider now this rewritten pattern, which matches exactly the same
55 strings:
56 .sp
57 ([^<]++|<(?!inet))+
58 .sp
59 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
61 when a "<" character that is not followed by "inet" is encountered (and we
62 assume this is relatively rare). A possessive quantifier is used to stop any
63 backtracking into the runs of non-"<" characters, but that is not related to
64 stack usage.
65 .P
66 This example shows that one way of avoiding stack problems when matching long
67 subject strings is to write repeated parenthesized subpatterns to match more
68 than one character whenever possible.
69 .
70 .
71 .SS "Compiling PCRE to use heap instead of stack for \fBpcre_exec()\fP"
72 .rs
73 .sp
74 In environments where stack memory is constrained, you might want to compile
75 PCRE to use heap memory instead of stack for remembering back-up points when
76 \fBpcre_exec()\fP is running. This makes it run a lot more slowly, however.
77 Details of how to do this are given in the
78 .\" HREF
79 \fBpcrebuild\fP
80 .\"
81 documentation. When built in this way, instead of using the stack, PCRE obtains
82 and frees memory by calling the functions that are pointed to by the
83 \fBpcre_stack_malloc\fP and \fBpcre_stack_free\fP variables. By default, these
84 point to \fBmalloc()\fP and \fBfree()\fP, but you can replace the pointers to
85 cause PCRE to use your own functions. Since the block sizes are always the
86 same, and are always freed in reverse order, it may be possible to implement
87 customized memory handlers that are more efficient than the standard functions.
88 .
89 .
90 .SS "Limiting \fBpcre_exec()\fP's stack usage"
91 .rs
92 .sp
93 You can set limits on the number of times that \fBmatch()\fP is called, both in
94 total and recursively. If a limit is exceeded, \fBpcre_exec()\fP returns an
95 error code. Setting suitable limits should prevent it from running out of
96 stack. The default values of the limits are very large, and unlikely ever to
97 operate. They can be changed when PCRE is built, and they can also be set when
98 \fBpcre_exec()\fP is called. For details of these interfaces, see the
99 .\" HREF
100 \fBpcrebuild\fP
101 .\"
102 documentation and the
103 .\" HTML <a href="pcreapi.html#extradata">
104 .\" </a>
105 section on extra data for \fBpcre_exec()\fP
106 .\"
107 in the
108 .\" HREF
109 \fBpcreapi\fP
110 .\"
111 documentation.
112 .P
113 As a very rough rule of thumb, you should reckon on about 500 bytes per
114 recursion. Thus, if you want to limit your stack usage to 8Mb, you
115 should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can
116 support around 128000 recursions.
117 .P
118 In Unix-like environments, the \fBpcretest\fP test program has a command line
119 option (\fB-S\fP) that can be used to increase the size of its stack. As long
120 as the stack is large enough, another option (\fB-M\fP) can be used to find the
121 smallest limits that allow a particular pattern to match a given subject
122 string. This is done by calling \fBpcre_exec()\fP repeatedly with different
123 limits.
124 .
125 .
126 .SS "Changing stack size in Unix-like systems"
127 .rs
128 .sp
129 In Unix-like environments, there is not often a problem with the stack unless
130 very long strings are involved, though the default limit on stack size varies
131 from system to system. Values from 8Mb to 64Mb are common. You can find your
132 default limit by running the command:
133 .sp
134 ulimit -s
135 .sp
136 Unfortunately, the effect of running out of stack is often SIGSEGV, though
137 sometimes a more explicit error message is given. You can normally increase the
138 limit on stack size by code such as this:
139 .sp
140 struct rlimit rlim;
141 getrlimit(RLIMIT_STACK, &rlim);
142 rlim.rlim_cur = 100*1024*1024;
143 setrlimit(RLIMIT_STACK, &rlim);
144 .sp
145 This reads the current limits (soft and hard) using \fBgetrlimit()\fP, then
146 attempts to increase the soft limit to 100Mb using \fBsetrlimit()\fP. You must
147 do this before calling \fBpcre_exec()\fP.
148 .
149 .
150 .SS "Changing stack size in Mac OS X"
151 .rs
152 .sp
153 Using \fBsetrlimit()\fP, as described above, should also work on Mac OS X. It
154 is also possible to set a stack size when linking a program. There is a
155 discussion about stack sizes in Mac OS X at this web site:
156 .\" HTML <a href="http://developer.apple.com/qa/qa2005/qa1419.html">
157 .\" </a>
158 http://developer.apple.com/qa/qa2005/qa1419.html.
159 .\"
160 .
161 .
162 .SH AUTHOR
163 .rs
164 .sp
165 .nf
166 Philip Hazel
167 University Computing Service
168 Cambridge CB2 3QH, England.
169 .fi
170 .
171 .
172 .SH REVISION
173 .rs
174 .sp
175 .nf
176 Last updated: 22 July 2011
177 Copyright (c) 1997-2011 University of Cambridge.
178 .fi

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