Annotation of embedaddon/pcre/doc/pcrestack.3, revision 1.1.1.1
1.1 misho 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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