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Wed Jul 9 11:03:07 2008 UTC (13 years, 2 months ago) by ph10
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Comments about stack usage added.
3 PCRE - Perl-compatible regular expressions
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 This example shows that one way of avoiding stack problems when matching long
65 subject strings is to write repeated parenthesized subpatterns to match more
66 than one character whenever possible.
67 .
68 .SS "Compiling PCRE to use heap instead of stack"
69 .rs
70 .sp
71 In environments where stack memory is constrained, you might want to compile
72 PCRE to use heap memory instead of stack for remembering back-up points. This
73 makes it run a lot more slowly, however. Details of how to do this are given in
74 the
75 .\" HREF
76 \fBpcrebuild\fP
77 .\"
78 documentation. When built in this way, instead of using the stack, PCRE obtains
79 and frees memory by calling the functions that are pointed to by the
80 \fBpcre_stack_malloc\fP and \fBpcre_stack_free\fP variables. By default, these
81 point to \fBmalloc()\fP and \fBfree()\fP, but you can replace the pointers to
82 cause PCRE to use your own functions. Since the block sizes are always the
83 same, and are always freed in reverse order, it may be possible to implement
84 customized memory handlers that are more efficient than the standard functions.
85 .
86 .SS "Limiting PCRE's stack usage"
87 .rs
88 .sp
89 PCRE has an internal counter that can be used to limit the depth of recursion,
90 and thus cause \fBpcre_exec()\fP to give an error code before it runs out of
91 stack. By default, the limit is very large, and unlikely ever to operate. It
92 can be changed when PCRE is built, and it can also be set when
93 \fBpcre_exec()\fP is called. For details of these interfaces, see the
94 .\" HREF
95 \fBpcrebuild\fP
96 .\"
97 and
98 .\" HREF
99 \fBpcreapi\fP
100 .\"
101 documentation.
102 .P
103 As a very rough rule of thumb, you should reckon on about 500 bytes per
104 recursion. Thus, if you want to limit your stack usage to 8Mb, you
105 should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can
106 support around 128000 recursions. The \fBpcretest\fP test program has a command
107 line option (\fB-S\fP) that can be used to increase the size of its stack.
108 .
109 .SS "Changing stack size in Unix-like systems"
110 .rs
111 .sp
112 In Unix-like environments, there is not often a problem with the stack unless
113 very long strings are involved, though the default limit on stack size varies
114 from system to system. Values from 8Mb to 64Mb are common. You can find your
115 default limit by running the command:
116 .sp
117 ulimit -s
118 .sp
119 Unfortunately, the effect of running out of stack is often SIGSEGV, though
120 sometimes a more explicit error message is given. You can normally increase the
121 limit on stack size by code such as this:
122 .sp
123 struct rlimit rlim;
124 getrlimit(RLIMIT_STACK, &rlim);
125 rlim.rlim_cur = 100*1024*1024;
126 setrlimit(RLIMIT_STACK, &rlim);
127 .sp
128 This reads the current limits (soft and hard) using \fBgetrlimit()\fP, then
129 attempts to increase the soft limit to 100Mb using \fBsetrlimit()\fP. You must
130 do this before calling \fBpcre_exec()\fP.
131 .
132 .SS "Changing stack size in Mac OS X"
133 .rs
134 .sp
135 Using \fBsetrlimit()\fP, as described above, should also work on Mac OS X. It
136 is also possible to set a stack size when linking a program. There is a
137 discussion about stack sizes in Mac OS X at this web site:
138 .\" HTML <a href="http://developer.apple.com/qa/qa2005/qa1419.html">
139 .\" </a>
140 http://developer.apple.com/qa/qa2005/qa1419.html.
141 .\"
142 .
143 .
145 .rs
146 .sp
147 .nf
148 Philip Hazel
149 University Computing Service
150 Cambridge CB2 3QH, England.
151 .fi
152 .
153 .
155 .rs
156 .sp
157 .nf
158 Last updated: 09 July 2008
159 Copyright (c) 1997-2008 University of Cambridge.
160 .fi


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