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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>
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.
24 </P>
25 <P>
26 Not all calls of <b>match()</b> increase the recursion depth; for an item such
27 as a* it may be called several times at the same level, after matching
28 different numbers of a's. Furthermore, in a number of cases where the result of
29 the recursive call would immediately be passed back as the result of the
30 current call (a "tail recursion"), the function is just restarted instead.
31 </P>
32 <P>
33 The <b>pcre_dfa_exec()</b> function operates in an entirely different way, and
34 hardly uses recursion at all. The limit on its complexity is the amount of
35 workspace it is given. The comments that follow do NOT apply to
36 <b>pcre_dfa_exec()</b>; they are relevant only for <b>pcre_exec()</b>.
37 </P>
38 <P>
39 You can set limits on the number of times that <b>match()</b> is called, both in
40 total and recursively. If the limit is exceeded, an error occurs. For details,
41 see the
42 <a href="pcreapi.html#extradata">section on extra data for <b>pcre_exec()</b></a>
43 in the
44 <a href="pcreapi.html"><b>pcreapi</b></a>
45 documentation.
46 </P>
47 <P>
48 Each time that <b>match()</b> is actually called recursively, it uses memory
49 from the process stack. For certain kinds of pattern and data, very large
50 amounts of stack may be needed, despite the recognition of "tail recursion".
51 You can often reduce the amount of recursion, and therefore the amount of stack
52 used, by modifying the pattern that is being matched. Consider, for example,
53 this pattern:
54 <pre>
55 ([^&#60;]|&#60;(?!inet))+
56 </pre>
57 It matches from wherever it starts until it encounters "&#60;inet" or the end of
58 the data, and is the kind of pattern that might be used when processing an XML
59 file. Each iteration of the outer parentheses matches either one character that
60 is not "&#60;" or a "&#60;" that is not followed by "inet". However, each time a
61 parenthesis is processed, a recursion occurs, so this formulation uses a stack
62 frame for each matched character. For a long string, a lot of stack is
63 required. Consider now this rewritten pattern, which matches exactly the same
64 strings:
65 <pre>
66 ([^&#60;]++|&#60;(?!inet))
67 </pre>
68 This uses very much less stack, because runs of characters that do not contain
69 "&#60;" are "swallowed" in one item inside the parentheses. Recursion happens only
70 when a "&#60;" character that is not followed by "inet" is encountered (and we
71 assume this is relatively rare). A possessive quantifier is used to stop any
72 backtracking into the runs of non-"&#60;" characters, but that is not related to
73 stack usage.
74 </P>
75 <P>
76 This example shows that one way of avoiding stack problems when matching long
77 subject strings is to write repeated parenthesized subpatterns to match more
78 than one character whenever possible.
79 </P>
80 <P>
81 In environments where stack memory is constrained, you might want to compile
82 PCRE to use heap memory instead of stack for remembering back-up points. This
83 makes it run a lot more slowly, however. Details of how to do this are given in
84 the
85 <a href="pcrebuild.html"><b>pcrebuild</b></a>
86 documentation.
87 </P>
88 <P>
89 In Unix-like environments, there is not often a problem with the stack unless
90 very long strings are involved, though the default limit on stack size varies
91 from system to system. Values from 8Mb to 64Mb are common. You can find your
92 default limit by running the command:
93 <pre>
94 ulimit -s
95 </pre>
96 Unfortunately, the effect of running out of stack is often SIGSEGV, though
97 sometimes a more explicit error message is given. You can normally increase the
98 limit on stack size by code such as this:
99 <pre>
100 struct rlimit rlim;
101 getrlimit(RLIMIT_STACK, &rlim);
102 rlim.rlim_cur = 100*1024*1024;
103 setrlimit(RLIMIT_STACK, &rlim);
104 </pre>
105 This reads the current limits (soft and hard) using <b>getrlimit()</b>, then
106 attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must
107 do this before calling <b>pcre_exec()</b>.
108 </P>
109 <P>
110 PCRE has an internal counter that can be used to limit the depth of recursion,
111 and thus cause <b>pcre_exec()</b> to give an error code before it runs out of
112 stack. By default, the limit is very large, and unlikely ever to operate. It
113 can be changed when PCRE is built, and it can also be set when
114 <b>pcre_exec()</b> is called. For details of these interfaces, see the
115 <a href="pcrebuild.html"><b>pcrebuild</b></a>
116 and
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. The <b>pcretest</b> test program has a command
125 line option (<b>-S</b>) that can be used to increase the size of its stack.
126 </P>
127 <P>
128 Last updated: 14 September 2006
129 <br>
130 Copyright &copy; 1997-2006 University of Cambridge.
131 <p>
132 Return to the <a href="index.html">PCRE index page</a>.
133 </p>


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