--- /dev/null
+<html>
+<head>
+<title>pcrestack specification</title>
+</head>
+<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
+<h1>pcrestack man page</h1>
+<p>
+Return to the <a href="index.html">PCRE index page</a>.
+</p>
+<p>
+This page is part of the PCRE HTML documentation. It was generated automatically
+from the original man page. If there is any nonsense in it, please consult the
+man page, in case the conversion went wrong.
+<br>
+<br><b>
+PCRE DISCUSSION OF STACK USAGE
+</b><br>
+<P>
+When you call <b>pcre_exec()</b>, it makes use of an internal function called
+<b>match()</b>. This calls itself recursively at branch points in the pattern,
+in order to remember the state of the match so that it can back up and try a
+different alternative if the first one fails. As matching proceeds deeper and
+deeper into the tree of possibilities, the recursion depth increases.
+</P>
+<P>
+Not all calls of <b>match()</b> increase the recursion depth; for an item such
+as a* it may be called several times at the same level, after matching
+different numbers of a's. Furthermore, in a number of cases where the result of
+the recursive call would immediately be passed back as the result of the
+current call (a "tail recursion"), the function is just restarted instead.
+</P>
+<P>
+The <b>pcre_dfa_exec()</b> function operates in an entirely different way, and
+hardly uses recursion at all. The limit on its complexity is the amount of
+workspace it is given. The comments that follow do NOT apply to
+<b>pcre_dfa_exec()</b>; they are relevant only for <b>pcre_exec()</b>.
+</P>
+<P>
+You can set limits on the number of times that <b>match()</b> is called, both in
+total and recursively. If the limit is exceeded, an error occurs. For details,
+see the
+<a href="pcreapi.html#extradata">section on extra data for <b>pcre_exec()</b></a>
+in the
+<a href="pcreapi.html"><b>pcreapi</b></a>
+documentation.
+</P>
+<P>
+Each time that <b>match()</b> is actually called recursively, it uses memory
+from the process stack. For certain kinds of pattern and data, very large
+amounts of stack may be needed, despite the recognition of "tail recursion".
+You can often reduce the amount of recursion, and therefore the amount of stack
+used, by modifying the pattern that is being matched. Consider, for example,
+this pattern:
+<pre>
+ ([^<]|<(?!inet))+
+</pre>
+It matches from wherever it starts until it encounters "<inet" or the end of
+the data, and is the kind of pattern that might be used when processing an XML
+file. Each iteration of the outer parentheses matches either one character that
+is not "<" or a "<" that is not followed by "inet". However, each time a
+parenthesis is processed, a recursion occurs, so this formulation uses a stack
+frame for each matched character. For a long string, a lot of stack is
+required. Consider now this rewritten pattern, which matches exactly the same
+strings:
+<pre>
+ ([^<]++|<(?!inet))+
+</pre>
+This uses very much less stack, because runs of characters that do not contain
+"<" are "swallowed" in one item inside the parentheses. Recursion happens only
+when a "<" character that is not followed by "inet" is encountered (and we
+assume this is relatively rare). A possessive quantifier is used to stop any
+backtracking into the runs of non-"<" characters, but that is not related to
+stack usage.
+</P>
+<P>
+This example shows that one way of avoiding stack problems when matching long
+subject strings is to write repeated parenthesized subpatterns to match more
+than one character whenever possible.
+</P>
+<br><b>
+Compiling PCRE to use heap instead of stack
+</b><br>
+<P>
+In environments where stack memory is constrained, you might want to compile
+PCRE to use heap memory instead of stack for remembering back-up points. This
+makes it run a lot more slowly, however. Details of how to do this are given in
+the
+<a href="pcrebuild.html"><b>pcrebuild</b></a>
+documentation. When built in this way, instead of using the stack, PCRE obtains
+and frees memory by calling the functions that are pointed to by the
+<b>pcre_stack_malloc</b> and <b>pcre_stack_free</b> variables. By default, these
+point to <b>malloc()</b> and <b>free()</b>, but you can replace the pointers to
+cause PCRE to use your own functions. Since the block sizes are always the
+same, and are always freed in reverse order, it may be possible to implement
+customized memory handlers that are more efficient than the standard functions.
+</P>
+<br><b>
+Limiting PCRE's stack usage
+</b><br>
+<P>
+PCRE has an internal counter that can be used to limit the depth of recursion,
+and thus cause <b>pcre_exec()</b> to give an error code before it runs out of
+stack. By default, the limit is very large, and unlikely ever to operate. It
+can be changed when PCRE is built, and it can also be set when
+<b>pcre_exec()</b> is called. For details of these interfaces, see the
+<a href="pcrebuild.html"><b>pcrebuild</b></a>
+and
+<a href="pcreapi.html"><b>pcreapi</b></a>
+documentation.
+</P>
+<P>
+As a very rough rule of thumb, you should reckon on about 500 bytes per
+recursion. Thus, if you want to limit your stack usage to 8Mb, you
+should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can
+support around 128000 recursions. The <b>pcretest</b> test program has a command
+line option (<b>-S</b>) that can be used to increase the size of its stack.
+</P>
+<br><b>
+Changing stack size in Unix-like systems
+</b><br>
+<P>
+In Unix-like environments, there is not often a problem with the stack unless
+very long strings are involved, though the default limit on stack size varies
+from system to system. Values from 8Mb to 64Mb are common. You can find your
+default limit by running the command:
+<pre>
+ ulimit -s
+</pre>
+Unfortunately, the effect of running out of stack is often SIGSEGV, though
+sometimes a more explicit error message is given. You can normally increase the
+limit on stack size by code such as this:
+<pre>
+ struct rlimit rlim;
+ getrlimit(RLIMIT_STACK, &rlim);
+ rlim.rlim_cur = 100*1024*1024;
+ setrlimit(RLIMIT_STACK, &rlim);
+</pre>
+This reads the current limits (soft and hard) using <b>getrlimit()</b>, then
+attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must
+do this before calling <b>pcre_exec()</b>.
+</P>
+<br><b>
+Changing stack size in Mac OS X
+</b><br>
+<P>
+Using <b>setrlimit()</b>, as described above, should also work on Mac OS X. It
+is also possible to set a stack size when linking a program. There is a
+discussion about stack sizes in Mac OS X at this web site:
+<a href="http://developer.apple.com/qa/qa2005/qa1419.html">http://developer.apple.com/qa/qa2005/qa1419.html.</a>
+</P>
+<br><b>
+AUTHOR
+</b><br>
+<P>
+Philip Hazel
+<br>
+University Computing Service
+<br>
+Cambridge CB2 3QH, England.
+<br>
+</P>
+<br><b>
+REVISION
+</b><br>
+<P>
+Last updated: 09 July 2008
+<br>
+Copyright © 1997-2008 University of Cambridge.
+<br>
+<p>
+Return to the <a href="index.html">PCRE index page</a>.
+</p>
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