161 lines
6.1 KiB
Groff
161 lines
6.1 KiB
Groff
.TH PCRESTACK 3
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.SH NAME
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PCRE - Perl-compatible regular expressions
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.SH "PCRE DISCUSSION OF STACK USAGE"
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.rs
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.sp
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When you call \fBpcre_exec()\fP, it makes use of an internal function called
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\fBmatch()\fP. This calls itself recursively at branch points in the pattern,
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in order to remember the state of the match so that it can back up and try a
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different alternative if the first one fails. As matching proceeds deeper and
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deeper into the tree of possibilities, the recursion depth increases.
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.P
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Not all calls of \fBmatch()\fP increase the recursion depth; for an item such
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as a* it may be called several times at the same level, after matching
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different numbers of a's. Furthermore, in a number of cases where the result of
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the recursive call would immediately be passed back as the result of the
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current call (a "tail recursion"), the function is just restarted instead.
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.P
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The \fBpcre_dfa_exec()\fP function operates in an entirely different way, and
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hardly uses recursion at all. The limit on its complexity is the amount of
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workspace it is given. The comments that follow do NOT apply to
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\fBpcre_dfa_exec()\fP; they are relevant only for \fBpcre_exec()\fP.
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.P
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You can set limits on the number of times that \fBmatch()\fP is called, both in
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total and recursively. If the limit is exceeded, an error occurs. For details,
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see the
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.\" HTML <a href="pcreapi.html#extradata">
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.\" </a>
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section on extra data for \fBpcre_exec()\fP
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.\"
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in the
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.\" HREF
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\fBpcreapi\fP
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.\"
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documentation.
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.P
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Each time that \fBmatch()\fP is actually called recursively, it uses memory
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from the process stack. For certain kinds of pattern and data, very large
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amounts of stack may be needed, despite the recognition of "tail recursion".
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You can often reduce the amount of recursion, and therefore the amount of stack
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used, by modifying the pattern that is being matched. Consider, for example,
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this pattern:
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.sp
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([^<]|<(?!inet))+
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.sp
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It matches from wherever it starts until it encounters "<inet" or the end of
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the data, and is the kind of pattern that might be used when processing an XML
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file. Each iteration of the outer parentheses matches either one character that
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is not "<" or a "<" that is not followed by "inet". However, each time a
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parenthesis is processed, a recursion occurs, so this formulation uses a stack
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frame for each matched character. For a long string, a lot of stack is
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required. Consider now this rewritten pattern, which matches exactly the same
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strings:
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.sp
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([^<]++|<(?!inet))+
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.sp
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This uses very much less stack, because runs of characters that do not contain
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"<" are "swallowed" in one item inside the parentheses. Recursion happens only
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when a "<" character that is not followed by "inet" is encountered (and we
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assume this is relatively rare). A possessive quantifier is used to stop any
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backtracking into the runs of non-"<" characters, but that is not related to
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stack usage.
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.P
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This example shows that one way of avoiding stack problems when matching long
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subject strings is to write repeated parenthesized subpatterns to match more
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than one character whenever possible.
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.
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.SS "Compiling PCRE to use heap instead of stack"
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.rs
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.sp
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In environments where stack memory is constrained, you might want to compile
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PCRE to use heap memory instead of stack for remembering back-up points. This
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makes it run a lot more slowly, however. Details of how to do this are given in
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the
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.\" HREF
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\fBpcrebuild\fP
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.\"
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documentation. When built in this way, instead of using the stack, PCRE obtains
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and frees memory by calling the functions that are pointed to by the
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\fBpcre_stack_malloc\fP and \fBpcre_stack_free\fP variables. By default, these
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point to \fBmalloc()\fP and \fBfree()\fP, but you can replace the pointers to
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cause PCRE to use your own functions. Since the block sizes are always the
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same, and are always freed in reverse order, it may be possible to implement
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customized memory handlers that are more efficient than the standard functions.
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.
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.SS "Limiting PCRE's stack usage"
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.rs
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.sp
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PCRE has an internal counter that can be used to limit the depth of recursion,
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and thus cause \fBpcre_exec()\fP to give an error code before it runs out of
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stack. By default, the limit is very large, and unlikely ever to operate. It
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can be changed when PCRE is built, and it can also be set when
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\fBpcre_exec()\fP is called. For details of these interfaces, see the
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.\" HREF
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\fBpcrebuild\fP
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.\"
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and
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.\" HREF
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\fBpcreapi\fP
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.\"
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documentation.
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.P
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As a very rough rule of thumb, you should reckon on about 500 bytes per
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recursion. Thus, if you want to limit your stack usage to 8Mb, you
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should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can
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support around 128000 recursions. The \fBpcretest\fP test program has a command
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line option (\fB-S\fP) that can be used to increase the size of its stack.
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.
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.SS "Changing stack size in Unix-like systems"
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.rs
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.sp
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In Unix-like environments, there is not often a problem with the stack unless
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very long strings are involved, though the default limit on stack size varies
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from system to system. Values from 8Mb to 64Mb are common. You can find your
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default limit by running the command:
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.sp
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ulimit -s
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.sp
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Unfortunately, the effect of running out of stack is often SIGSEGV, though
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sometimes a more explicit error message is given. You can normally increase the
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limit on stack size by code such as this:
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.sp
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struct rlimit rlim;
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getrlimit(RLIMIT_STACK, &rlim);
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rlim.rlim_cur = 100*1024*1024;
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setrlimit(RLIMIT_STACK, &rlim);
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.sp
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This reads the current limits (soft and hard) using \fBgetrlimit()\fP, then
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attempts to increase the soft limit to 100Mb using \fBsetrlimit()\fP. You must
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do this before calling \fBpcre_exec()\fP.
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.
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.SS "Changing stack size in Mac OS X"
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.rs
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.sp
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Using \fBsetrlimit()\fP, as described above, should also work on Mac OS X. It
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is also possible to set a stack size when linking a program. There is a
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discussion about stack sizes in Mac OS X at this web site:
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.\" HTML <a href="http://developer.apple.com/qa/qa2005/qa1419.html">
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.\" </a>
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http://developer.apple.com/qa/qa2005/qa1419.html.
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.\"
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.
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.
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.SH AUTHOR
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.rs
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.sp
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.nf
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Philip Hazel
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University Computing Service
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Cambridge CB2 3QH, England.
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.fi
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.
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.
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.SH REVISION
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.rs
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.sp
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.nf
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Last updated: 09 July 2008
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Copyright (c) 1997-2008 University of Cambridge.
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.fi
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