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diff --git a/doc/html/pcrestack.html b/doc/html/pcrestack.html new file mode 100644 index 0000000..af6406d --- /dev/null +++ b/doc/html/pcrestack.html @@ -0,0 +1,225 @@ +<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[16|32]_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. The +<b>match()</b> function is also called in other circumstances, for example, +whenever a parenthesized sub-pattern is entered, and in certain cases of +repetition. +</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 above comments apply when <b>pcre[16|32]_exec()</b> is run in its normal +interpretive manner. If the pattern was studied with the +PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling was successful, and +the options passed to <b>pcre[16|32]_exec()</b> were not incompatible, the matching +process uses the JIT-compiled code instead of the <b>match()</b> function. In +this case, the memory requirements are handled entirely differently. See the +<a href="pcrejit.html"><b>pcrejit</b></a> +documentation for details. +</P> +<P> +The <b>pcre[16|32]_dfa_exec()</b> function operates in an entirely different way, +and uses recursion only when there is a regular expression recursion or +subroutine call in the pattern. This includes the processing of assertion and +"once-only" subpatterns, which are handled like subroutine calls. Normally, +these are never very deep, and the limit on the complexity of +<b>pcre[16|32]_dfa_exec()</b> is controlled by the amount of workspace it is given. +However, it is possible to write patterns with runaway infinite recursions; +such patterns will cause <b>pcre[16|32]_dfa_exec()</b> to run out of stack. At +present, there is no protection against this. +</P> +<P> +The comments that follow do NOT apply to <b>pcre[16|32]_dfa_exec()</b>; they are +relevant only for <b>pcre[16|32]_exec()</b> without the JIT optimization. +</P> +<br><b> +Reducing <b>pcre[16|32]_exec()</b>'s stack usage +</b><br> +<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 for <b>pcre[16|32]_exec()</b> +</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 when +<b>pcre[16|32]_exec()</b> is running. 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[16|32]_stack_malloc</b> and <b>pcre[16|32]_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 <b>pcre[16|32]_exec()</b>'s stack usage +</b><br> +<P> +You can set limits on the number of times that <b>match()</b> is called, both in +total and recursively. If a limit is exceeded, <b>pcre[16|32]_exec()</b> returns an +error code. Setting suitable limits should prevent it from running out of +stack. The default values of the limits are very large, and unlikely ever to +operate. They can be changed when PCRE is built, and they can also be set when +<b>pcre[16|32]_exec()</b> is called. For details of these interfaces, see the +<a href="pcrebuild.html"><b>pcrebuild</b></a> +documentation and the +<a href="pcreapi.html#extradata">section on extra data for <b>pcre[16|32]_exec()</b></a> +in the +<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. +</P> +<P> +In Unix-like environments, 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. As long +as the stack is large enough, another option (<b>-M</b>) can be used to find the +smallest limits that allow a particular pattern to match a given subject +string. This is done by calling <b>pcre[16|32]_exec()</b> repeatedly with different +limits. +</P> +<br><b> +Obtaining an estimate of stack usage +</b><br> +<P> +The actual amount of stack used per recursion can vary quite a lot, depending +on the compiler that was used to build PCRE and the optimization or debugging +options that were set for it. The rule of thumb value of 500 bytes mentioned +above may be larger or smaller than what is actually needed. A better +approximation can be obtained by running this command: +<pre> + pcretest -m -C +</pre> +The <b>-C</b> option causes <b>pcretest</b> to output information about the +options with which PCRE was compiled. When <b>-m</b> is also given (before +<b>-C</b>), information about stack use is given in a line like this: +<pre> + Match recursion uses stack: approximate frame size = 640 bytes +</pre> +The value is approximate because some recursions need a bit more (up to perhaps +16 more bytes). +</P> +<P> +If the above command is given when PCRE is compiled to use the heap instead of +the stack for recursion, the value that is output is the size of each block +that is obtained from the heap. +</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[16|32]_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: 24 June 2012 +<br> +Copyright © 1997-2012 University of Cambridge. +<br> +<p> +Return to the <a href="index.html">PCRE index page</a>. +</p> |