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libraries/spcre/libpcre/pcre/doc/html/pcrestack.html
author Tom Sutcliffe <thomas.sutcliffe@accenture.com>
Tue, 07 Sep 2010 20:26:27 +0100
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<html>
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<head>
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<title>pcrestack specification</title>
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</head>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcrestack man page</h1>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
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<p>
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This page is part of the PCRE HTML documentation. It was generated automatically
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from the original man page. If there is any nonsense in it, please consult the
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man page, in case the conversion went wrong.
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<br>
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<br><b>
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PCRE DISCUSSION OF STACK USAGE
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</b><br>
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<P>
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When you call <b>pcre_exec()</b>, it makes use of an internal function called
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<b>match()</b>. 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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<P>
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Not all calls of <b>match()</b> 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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<P>
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The <b>pcre_dfa_exec()</b> 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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<b>pcre_dfa_exec()</b>; they are relevant only for <b>pcre_exec()</b>.
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</P>
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<P>
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You can set limits on the number of times that <b>match()</b> 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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<a href="pcreapi.html#extradata">section on extra data for <b>pcre_exec()</b></a>
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in the
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<a href="pcreapi.html"><b>pcreapi</b></a>
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documentation.
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</P>
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<P>
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Each time that <b>match()</b> 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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<pre>
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  ([^&#60;]|&#60;(?!inet))+
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</pre>
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It matches from wherever it starts until it encounters "&#60;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 "&#60;" or a "&#60;" 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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<pre>
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  ([^&#60;]++|&#60;(?!inet))+
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</pre>
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This uses very much less stack, because runs of characters that do not contain
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"&#60;" are "swallowed" in one item inside the parentheses. Recursion happens only
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when a "&#60;" 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-"&#60;" characters, but that is not related to
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stack usage.
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</P>
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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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</P>
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<br><b>
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Compiling PCRE to use heap instead of stack
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</b><br>
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<P>
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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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<a href="pcrebuild.html"><b>pcrebuild</b></a>
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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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<b>pcre_stack_malloc</b> and <b>pcre_stack_free</b> variables. By default, these
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point to <b>malloc()</b> and <b>free()</b>, 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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</P>
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<br><b>
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Limiting PCRE's stack usage
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</b><br>
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<P>
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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 <b>pcre_exec()</b> 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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<b>pcre_exec()</b> is called. For details of these interfaces, see the
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<a href="pcrebuild.html"><b>pcrebuild</b></a>
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and
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<a href="pcreapi.html"><b>pcreapi</b></a>
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documentation.
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</P>
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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 <b>pcretest</b> test program has a command
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line option (<b>-S</b>) that can be used to increase the size of its stack.
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</P>
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<br><b>
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Changing stack size in Unix-like systems
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</b><br>
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<P>
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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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<pre>
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  ulimit -s
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</pre>
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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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<pre>
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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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</pre>
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This reads the current limits (soft and hard) using <b>getrlimit()</b>, then
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attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must
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do this before calling <b>pcre_exec()</b>.
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</P>
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<br><b>
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Changing stack size in Mac OS X
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</b><br>
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<P>
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Using <b>setrlimit()</b>, 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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<a href="http://developer.apple.com/qa/qa2005/qa1419.html">http://developer.apple.com/qa/qa2005/qa1419.html.</a>
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</P>
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<br><b>
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AUTHOR
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</b><br>
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<P>
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Philip Hazel
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<br>
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University Computing Service
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<br>
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Cambridge CB2 3QH, England.
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<br>
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</P>
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<br><b>
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REVISION
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</b><br>
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<P>
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Last updated: 09 July 2008
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<br>
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Copyright &copy; 1997-2008 University of Cambridge.
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<br>
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<p>
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Return to the <a href="index.html">PCRE index page</a>.
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</p>
MCL/sf/os/fshell