ViewVC logotype

Contents of /code/trunk/doc/pcrestack.3

Parent Directory Parent Directory | Revision Log Revision Log

Revision 1055 - (show annotations)
Tue Oct 16 15:53:30 2012 UTC (8 years, 6 months ago) by chpe
File size: 8901 byte(s)
pcre32: Add 32-bit library

Create libpcre32 that operates on 32-bit characters (UTF-32).

This turned out to be surprisingly simple after the UTF-16 support
was introduced; mostly just extra ifdefs and adjusting and adding
some tests.
1 .TH PCRESTACK 3 "24 June 2012" "PCRE 8.30"
3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 When you call \fBpcre[16|32]_exec()\fP, it makes use of an internal function
8 called \fBmatch()\fP. This calls itself recursively at branch points in the
9 pattern, in order to remember the state of the match so that it can back up and
10 try a different alternative if the first one fails. As matching proceeds deeper
11 and 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[16|32]_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[16|32]_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[16|32]_dfa_exec()\fP function operates in an entirely different way,
34 and uses recursion only when there is a regular expression recursion or
35 subroutine call in the pattern. This includes the processing of assertion and
36 "once-only" subpatterns, which are handled like subroutine calls. Normally,
37 these are never very deep, and the limit on the complexity of
38 \fBpcre[16|32]_dfa_exec()\fP is controlled by the amount of workspace it is given.
39 However, it is possible to write patterns with runaway infinite recursions;
40 such patterns will cause \fBpcre[16|32]_dfa_exec()\fP to run out of stack. At
41 present, there is no protection against this.
42 .P
43 The comments that follow do NOT apply to \fBpcre[16|32]_dfa_exec()\fP; they are
44 relevant only for \fBpcre[16|32]_exec()\fP without the JIT optimization.
45 .
46 .
47 .SS "Reducing \fBpcre[16|32]_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[16|32]_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[16|32]_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[16|32]_stack_malloc\fP and \fBpcre[16|32]_stack_free\fP variables. By
95 default, these point to \fBmalloc()\fP and \fBfree()\fP, but you can replace
96 the pointers to cause PCRE to use your own functions. Since the block sizes are
97 always the same, and are always freed in reverse order, it may be possible to
98 implement customized memory handlers that are more efficient than the standard
99 functions.
100 .
101 .
102 .SS "Limiting \fBpcre[16|32]_exec()\fP's stack usage"
103 .rs
104 .sp
105 You can set limits on the number of times that \fBmatch()\fP is called, both in
106 total and recursively. If a limit is exceeded, \fBpcre[16|32]_exec()\fP returns an
107 error code. Setting suitable limits should prevent it from running out of
108 stack. The default values of the limits are very large, and unlikely ever to
109 operate. They can be changed when PCRE is built, and they can also be set when
110 \fBpcre[16|32]_exec()\fP is called. For details of these interfaces, see the
111 .\" HREF
112 \fBpcrebuild\fP
113 .\"
114 documentation and the
115 .\" HTML <a href="pcreapi.html#extradata">
116 .\" </a>
117 section on extra data for \fBpcre[16|32]_exec()\fP
118 .\"
119 in the
120 .\" HREF
121 \fBpcreapi\fP
122 .\"
123 documentation.
124 .P
125 As a very rough rule of thumb, you should reckon on about 500 bytes per
126 recursion. Thus, if you want to limit your stack usage to 8Mb, you should set
127 the limit at 16000 recursions. A 64Mb stack, on the other hand, can support
128 around 128000 recursions.
129 .P
130 In Unix-like environments, the \fBpcretest\fP test program has a command line
131 option (\fB-S\fP) that can be used to increase the size of its stack. As long
132 as the stack is large enough, another option (\fB-M\fP) can be used to find the
133 smallest limits that allow a particular pattern to match a given subject
134 string. This is done by calling \fBpcre[16|32]_exec()\fP repeatedly with different
135 limits.
136 .
137 .
138 .SS "Obtaining an estimate of stack usage"
139 .rs
140 .sp
141 The actual amount of stack used per recursion can vary quite a lot, depending
142 on the compiler that was used to build PCRE and the optimization or debugging
143 options that were set for it. The rule of thumb value of 500 bytes mentioned
144 above may be larger or smaller than what is actually needed. A better
145 approximation can be obtained by running this command:
146 .sp
147 pcretest -m -C
148 .sp
149 The \fB-C\fP option causes \fBpcretest\fP to output information about the
150 options with which PCRE was compiled. When \fB-m\fP is also given (before
151 \fB-C\fP), information about stack use is given in a line like this:
152 .sp
153 Match recursion uses stack: approximate frame size = 640 bytes
154 .sp
155 The value is approximate because some recursions need a bit more (up to perhaps
156 16 more bytes).
157 .P
158 If the above command is given when PCRE is compiled to use the heap instead of
159 the stack for recursion, the value that is output is the size of each block
160 that is obtained from the heap.
161 .
162 .
163 .SS "Changing stack size in Unix-like systems"
164 .rs
165 .sp
166 In Unix-like environments, there is not often a problem with the stack unless
167 very long strings are involved, though the default limit on stack size varies
168 from system to system. Values from 8Mb to 64Mb are common. You can find your
169 default limit by running the command:
170 .sp
171 ulimit -s
172 .sp
173 Unfortunately, the effect of running out of stack is often SIGSEGV, though
174 sometimes a more explicit error message is given. You can normally increase the
175 limit on stack size by code such as this:
176 .sp
177 struct rlimit rlim;
178 getrlimit(RLIMIT_STACK, &rlim);
179 rlim.rlim_cur = 100*1024*1024;
180 setrlimit(RLIMIT_STACK, &rlim);
181 .sp
182 This reads the current limits (soft and hard) using \fBgetrlimit()\fP, then
183 attempts to increase the soft limit to 100Mb using \fBsetrlimit()\fP. You must
184 do this before calling \fBpcre[16|32]_exec()\fP.
185 .
186 .
187 .SS "Changing stack size in Mac OS X"
188 .rs
189 .sp
190 Using \fBsetrlimit()\fP, as described above, should also work on Mac OS X. It
191 is also possible to set a stack size when linking a program. There is a
192 discussion about stack sizes in Mac OS X at this web site:
193 .\" HTML <a href="http://developer.apple.com/qa/qa2005/qa1419.html">
194 .\" </a>
195 http://developer.apple.com/qa/qa2005/qa1419.html.
196 .\"
197 .
198 .
200 .rs
201 .sp
202 .nf
203 Philip Hazel
204 University Computing Service
205 Cambridge CB2 3QH, England.
206 .fi
207 .
208 .
210 .rs
211 .sp
212 .nf
213 Last updated: 24 June 2012
214 Copyright (c) 1997-2012 University of Cambridge.
215 .fi


Name Value
svn:eol-style native
svn:keywords "Author Date Id Revision Url"

  ViewVC Help
Powered by ViewVC 1.1.5