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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     In environments where stack memory is constrained, you might want to compile
77     PCRE to use heap memory instead of stack for remembering back-up points. This
78     makes it run a lot more slowly, however. Details of how to do this are given in
79     the
80     <a href="pcrebuild.html"><b>pcrebuild</b></a>
81     documentation.
82     </P>
83     <P>
84     In Unix-like environments, there is not often a problem with the stack, though
85     the default limit on stack size varies from system to system. Values from 8Mb
86     to 64Mb are common. You can find your default limit by running the command:
87     <pre>
88     ulimit -s
89     </pre>
90     The effect of running out of stack is often SIGSEGV, though sometimes an error
91     message is given. You can normally increase the limit on stack size by code
92     such as this:
93     <pre>
94     struct rlimit rlim;
95     getrlimit(RLIMIT_STACK, &rlim);
96     rlim.rlim_cur = 100*1024*1024;
97     setrlimit(RLIMIT_STACK, &rlim);
98     </pre>
99     This reads the current limits (soft and hard) using <b>getrlimit()</b>, then
100     attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must
101     do this before calling <b>pcre_exec()</b>.
102     </P>
103     <P>
104     PCRE has an internal counter that can be used to limit the depth of recursion,
105     and thus cause <b>pcre_exec()</b> to give an error code before it runs out of
106     stack. By default, the limit is very large, and unlikely ever to operate. It
107     can be changed when PCRE is built, and it can also be set when
108     <b>pcre_exec()</b> is called. For details of these interfaces, see the
109     <a href="pcrebuild.html"><b>pcrebuild</b></a>
110     and
111     <a href="pcreapi.html"><b>pcreapi</b></a>
112     documentation.
113     </P>
114     <P>
115     As a very rough rule of thumb, you should reckon on about 500 bytes per
116     recursion. Thus, if you want to limit your stack usage to 8Mb, you
117     should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can
118     support around 128000 recursions. The <b>pcretest</b> test program has a command
119     line option (<b>-S</b>) that can be used to increase its stack.
120     </P>
121     <P>
122     Last updated: 29 June 2006
123     <br>
124     Copyright &copy; 1997-2006 University of Cambridge.
125     <p>
126     Return to the <a href="index.html">PCRE index page</a>.
127     </p>

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