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Sun Sep 11 14:31:21 2011 UTC (3 years ago) by ph10
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Final source and document tidies for 8.20-RC1.

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()\fP 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 above comments apply when \fBpcre_exec()\fP is run in its normal
23 interpretive manner. If the pattern was studied with the
24 PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling was successful, and
25 the options passed to \fBpcre_exec()\fP were not incompatible, the matching
26 process uses the JIT-compiled code instead of the \fBmatch()\fP function. In
27 this case, the memory requirements are handled entirely differently. See the
28 .\" HREF
29 \fBpcrejit\fP
30 .\"
31 documentation for details.
32 .P
33 The \fBpcre_dfa_exec()\fP function operates in an entirely different way, and
34 uses recursion only when there is a regular expression recursion or subroutine
35 call in the pattern. This includes the processing of assertion and "once-only"
36 subpatterns, which are handled like subroutine calls. Normally, these are never
37 very deep, and the limit on the complexity of \fBpcre_dfa_exec()\fP is
38 controlled by the amount of workspace it is given. However, it is possible to
39 write patterns with runaway infinite recursions; such patterns will cause
40 \fBpcre_dfa_exec()\fP to run out of stack. At present, there is no protection
41 against this.
42 .P
43 The comments that follow do NOT apply to \fBpcre_dfa_exec()\fP; they are
44 relevant only for \fBpcre_exec()\fP without the JIT optimization.
45 .
46 .
47 .SS "Reducing \fBpcre_exec()\fP's stack usage"
48 .rs
49 .sp
50 Each time that \fBmatch()\fP is actually called recursively, it uses memory
51 from the process stack. For certain kinds of pattern and data, very large
52 amounts of stack may be needed, despite the recognition of "tail recursion".
53 You can often reduce the amount of recursion, and therefore the amount of stack
54 used, by modifying the pattern that is being matched. Consider, for example,
55 this pattern:
56 .sp
57 ([^<]|<(?!inet))+
58 .sp
59 It matches from wherever it starts until it encounters "<inet" or the end of
60 the data, and is the kind of pattern that might be used when processing an XML
61 file. Each iteration of the outer parentheses matches either one character that
62 is not "<" or a "<" that is not followed by "inet". However, each time a
63 parenthesis is processed, a recursion occurs, so this formulation uses a stack
64 frame for each matched character. For a long string, a lot of stack is
65 required. Consider now this rewritten pattern, which matches exactly the same
66 strings:
67 .sp
68 ([^<]++|<(?!inet))+
69 .sp
70 This uses very much less stack, because runs of characters that do not contain
71 "<" are "swallowed" in one item inside the parentheses. Recursion happens only
72 when a "<" character that is not followed by "inet" is encountered (and we
73 assume this is relatively rare). A possessive quantifier is used to stop any
74 backtracking into the runs of non-"<" characters, but that is not related to
75 stack usage.
76 .P
77 This example shows that one way of avoiding stack problems when matching long
78 subject strings is to write repeated parenthesized subpatterns to match more
79 than one character whenever possible.
80 .
81 .
82 .SS "Compiling PCRE to use heap instead of stack for \fBpcre_exec()\fP"
83 .rs
84 .sp
85 In environments where stack memory is constrained, you might want to compile
86 PCRE to use heap memory instead of stack for remembering back-up points when
87 \fBpcre_exec()\fP is running. This makes it run a lot more slowly, however.
88 Details of how to do this are given in the
89 .\" HREF
90 \fBpcrebuild\fP
91 .\"
92 documentation. When built in this way, instead of using the stack, PCRE obtains
93 and frees memory by calling the functions that are pointed to by the
94 \fBpcre_stack_malloc\fP and \fBpcre_stack_free\fP variables. By default, these
95 point to \fBmalloc()\fP and \fBfree()\fP, but you can replace the pointers to
96 cause PCRE to use your own functions. Since the block sizes are always the
97 same, and are always freed in reverse order, it may be possible to implement
98 customized memory handlers that are more efficient than the standard functions.
99 .
100 .
101 .SS "Limiting \fBpcre_exec()\fP's stack usage"
102 .rs
103 .sp
104 You can set limits on the number of times that \fBmatch()\fP is called, both in
105 total and recursively. If a limit is exceeded, \fBpcre_exec()\fP returns an
106 error code. Setting suitable limits should prevent it from running out of
107 stack. The default values of the limits are very large, and unlikely ever to
108 operate. They can be changed when PCRE is built, and they can also be set when
109 \fBpcre_exec()\fP is called. For details of these interfaces, see the
110 .\" HREF
111 \fBpcrebuild\fP
112 .\"
113 documentation and the
114 .\" HTML <a href="pcreapi.html#extradata">
115 .\" </a>
116 section on extra data for \fBpcre_exec()\fP
117 .\"
118 in the
119 .\" HREF
120 \fBpcreapi\fP
121 .\"
122 documentation.
123 .P
124 As a very rough rule of thumb, you should reckon on about 500 bytes per
125 recursion. Thus, if you want to limit your stack usage to 8Mb, you
126 should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can
127 support around 128000 recursions.
128 .P
129 In Unix-like environments, the \fBpcretest\fP test program has a command line
130 option (\fB-S\fP) that can be used to increase the size of its stack. As long
131 as the stack is large enough, another option (\fB-M\fP) can be used to find the
132 smallest limits that allow a particular pattern to match a given subject
133 string. This is done by calling \fBpcre_exec()\fP repeatedly with different
134 limits.
135 .
136 .
137 .SS "Changing stack size in Unix-like systems"
138 .rs
139 .sp
140 In Unix-like environments, there is not often a problem with the stack unless
141 very long strings are involved, though the default limit on stack size varies
142 from system to system. Values from 8Mb to 64Mb are common. You can find your
143 default limit by running the command:
144 .sp
145 ulimit -s
146 .sp
147 Unfortunately, the effect of running out of stack is often SIGSEGV, though
148 sometimes a more explicit error message is given. You can normally increase the
149 limit on stack size by code such as this:
150 .sp
151 struct rlimit rlim;
152 getrlimit(RLIMIT_STACK, &rlim);
153 rlim.rlim_cur = 100*1024*1024;
154 setrlimit(RLIMIT_STACK, &rlim);
155 .sp
156 This reads the current limits (soft and hard) using \fBgetrlimit()\fP, then
157 attempts to increase the soft limit to 100Mb using \fBsetrlimit()\fP. You must
158 do this before calling \fBpcre_exec()\fP.
159 .
160 .
161 .SS "Changing stack size in Mac OS X"
162 .rs
163 .sp
164 Using \fBsetrlimit()\fP, as described above, should also work on Mac OS X. It
165 is also possible to set a stack size when linking a program. There is a
166 discussion about stack sizes in Mac OS X at this web site:
167 .\" HTML <a href="http://developer.apple.com/qa/qa2005/qa1419.html">
168 .\" </a>
169 http://developer.apple.com/qa/qa2005/qa1419.html.
170 .\"
171 .
172 .
173 .SH AUTHOR
174 .rs
175 .sp
176 .nf
177 Philip Hazel
178 University Computing Service
179 Cambridge CB2 3QH, England.
180 .fi
181 .
182 .
183 .SH REVISION
184 .rs
185 .sp
186 .nf
187 Last updated: 26 August 2011
188 Copyright (c) 1997-2011 University of Cambridge.
189 .fi

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