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1 nigel 91 <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>
16     PCRE DISCUSSION OF STACK USAGE
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 nigel 93 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 nigel 91 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 nigel 93 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 nigel 91 <pre>
94     ulimit -s
95     </pre>
96 nigel 93 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 nigel 91 <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 nigel 93 line option (<b>-S</b>) that can be used to increase the size of its stack.
126 nigel 91 </P>
127     <P>
128 nigel 93 Last updated: 14 September 2006
129 nigel 91 <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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