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    1 <html>
    2 <head>
    3 <title>pcrestack specification</title>
    4 </head>
    5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
    6 <h1>pcrestack man page</h1>
    7 <p>
    8 Return to the <a href="index.html">PCRE index page</a>.
    9 </p>
   10 <p>
   11 This page is part of the PCRE HTML documentation. It was generated automatically
   12 from the original man page. If there is any nonsense in it, please consult the
   13 man page, in case the conversion went wrong.
   14 <br>
   15 <br><b>
   16 PCRE DISCUSSION OF STACK USAGE
   17 </b><br>
   18 <P>
   19 When you call <b>pcre[16|32]_exec()</b>, it makes use of an internal function
   20 called <b>match()</b>. This calls itself recursively at branch points in the
   21 pattern, in order to remember the state of the match so that it can back up and
   22 try a different alternative if the first one fails. As matching proceeds deeper
   23 and deeper into the tree of possibilities, the recursion depth increases. The
   24 <b>match()</b> function is also called in other circumstances, for example,
   25 whenever a parenthesized sub-pattern is entered, and in certain cases of
   26 repetition.
   27 </P>
   28 <P>
   29 Not all calls of <b>match()</b> increase the recursion depth; for an item such
   30 as a* it may be called several times at the same level, after matching
   31 different numbers of a's. Furthermore, in a number of cases where the result of
   32 the recursive call would immediately be passed back as the result of the
   33 current call (a "tail recursion"), the function is just restarted instead.
   34 </P>
   35 <P>
   36 The above comments apply when <b>pcre[16|32]_exec()</b> is run in its normal
   37 interpretive manner. If the pattern was studied with the
   38 PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling was successful, and
   39 the options passed to <b>pcre[16|32]_exec()</b> were not incompatible, the matching
   40 process uses the JIT-compiled code instead of the <b>match()</b> function. In
   41 this case, the memory requirements are handled entirely differently. See the
   42 <a href="pcrejit.html"><b>pcrejit</b></a>
   43 documentation for details.
   44 </P>
   45 <P>
   46 The <b>pcre[16|32]_dfa_exec()</b> function operates in an entirely different way,
   47 and uses recursion only when there is a regular expression recursion or
   48 subroutine call in the pattern. This includes the processing of assertion and
   49 "once-only" subpatterns, which are handled like subroutine calls. Normally,
   50 these are never very deep, and the limit on the complexity of
   51 <b>pcre[16|32]_dfa_exec()</b> is controlled by the amount of workspace it is given.
   52 However, it is possible to write patterns with runaway infinite recursions;
   53 such patterns will cause <b>pcre[16|32]_dfa_exec()</b> to run out of stack. At
   54 present, there is no protection against this.
   55 </P>
   56 <P>
   57 The comments that follow do NOT apply to <b>pcre[16|32]_dfa_exec()</b>; they are
   58 relevant only for <b>pcre[16|32]_exec()</b> without the JIT optimization.
   59 </P>
   60 <br><b>
   61 Reducing <b>pcre[16|32]_exec()</b>'s stack usage
   62 </b><br>
   63 <P>
   64 Each time that <b>match()</b> is actually called recursively, it uses memory
   65 from the process stack. For certain kinds of pattern and data, very large
   66 amounts of stack may be needed, despite the recognition of "tail recursion".
   67 You can often reduce the amount of recursion, and therefore the amount of stack
   68 used, by modifying the pattern that is being matched. Consider, for example,
   69 this pattern:
   70 <pre>
   71   ([^&#60;]|&#60;(?!inet))+
   72 </pre>
   73 It matches from wherever it starts until it encounters "&#60;inet" or the end of
   74 the data, and is the kind of pattern that might be used when processing an XML
   75 file. Each iteration of the outer parentheses matches either one character that
   76 is not "&#60;" or a "&#60;" that is not followed by "inet". However, each time a
   77 parenthesis is processed, a recursion occurs, so this formulation uses a stack
   78 frame for each matched character. For a long string, a lot of stack is
   79 required. Consider now this rewritten pattern, which matches exactly the same
   80 strings:
   81 <pre>
   82   ([^&#60;]++|&#60;(?!inet))+
   83 </pre>
   84 This uses very much less stack, because runs of characters that do not contain
   85 "&#60;" are "swallowed" in one item inside the parentheses. Recursion happens only
   86 when a "&#60;" character that is not followed by "inet" is encountered (and we
   87 assume this is relatively rare). A possessive quantifier is used to stop any
   88 backtracking into the runs of non-"&#60;" characters, but that is not related to
   89 stack usage.
   90 </P>
   91 <P>
   92 This example shows that one way of avoiding stack problems when matching long
   93 subject strings is to write repeated parenthesized subpatterns to match more
   94 than one character whenever possible.
   95 </P>
   96 <br><b>
   97 Compiling PCRE to use heap instead of stack for <b>pcre[16|32]_exec()</b>
   98 </b><br>
   99 <P>
  100 In environments where stack memory is constrained, you might want to compile
  101 PCRE to use heap memory instead of stack for remembering back-up points when
  102 <b>pcre[16|32]_exec()</b> is running. This makes it run a lot more slowly, however.
  103 Details of how to do this are given in the
  104 <a href="pcrebuild.html"><b>pcrebuild</b></a>
  105 documentation. When built in this way, instead of using the stack, PCRE obtains
  106 and frees memory by calling the functions that are pointed to by the
  107 <b>pcre[16|32]_stack_malloc</b> and <b>pcre[16|32]_stack_free</b> variables. By
  108 default, these point to <b>malloc()</b> and <b>free()</b>, but you can replace
  109 the pointers to cause PCRE to use your own functions. Since the block sizes are
  110 always the same, and are always freed in reverse order, it may be possible to
  111 implement customized memory handlers that are more efficient than the standard
  112 functions.
  113 </P>
  114 <br><b>
  115 Limiting <b>pcre[16|32]_exec()</b>'s stack usage
  116 </b><br>
  117 <P>
  118 You can set limits on the number of times that <b>match()</b> is called, both in
  119 total and recursively. If a limit is exceeded, <b>pcre[16|32]_exec()</b> returns an
  120 error code. Setting suitable limits should prevent it from running out of
  121 stack. The default values of the limits are very large, and unlikely ever to
  122 operate. They can be changed when PCRE is built, and they can also be set when
  123 <b>pcre[16|32]_exec()</b> is called. For details of these interfaces, see the
  124 <a href="pcrebuild.html"><b>pcrebuild</b></a>
  125 documentation and the
  126 <a href="pcreapi.html#extradata">section on extra data for <b>pcre[16|32]_exec()</b></a>
  127 in the
  128 <a href="pcreapi.html"><b>pcreapi</b></a>
  129 documentation.
  130 </P>
  131 <P>
  132 As a very rough rule of thumb, you should reckon on about 500 bytes per
  133 recursion. Thus, if you want to limit your stack usage to 8Mb, you should set
  134 the limit at 16000 recursions. A 64Mb stack, on the other hand, can support
  135 around 128000 recursions.
  136 </P>
  137 <P>
  138 In Unix-like environments, the <b>pcretest</b> test program has a command line
  139 option (<b>-S</b>) that can be used to increase the size of its stack. As long
  140 as the stack is large enough, another option (<b>-M</b>) can be used to find the
  141 smallest limits that allow a particular pattern to match a given subject
  142 string. This is done by calling <b>pcre[16|32]_exec()</b> repeatedly with different
  143 limits.
  144 </P>
  145 <br><b>
  146 Obtaining an estimate of stack usage
  147 </b><br>
  148 <P>
  149 The actual amount of stack used per recursion can vary quite a lot, depending
  150 on the compiler that was used to build PCRE and the optimization or debugging
  151 options that were set for it. The rule of thumb value of 500 bytes mentioned
  152 above may be larger or smaller than what is actually needed. A better
  153 approximation can be obtained by running this command:
  154 <pre>
  155   pcretest -m -C
  156 </pre>
  157 The <b>-C</b> option causes <b>pcretest</b> to output information about the
  158 options with which PCRE was compiled. When <b>-m</b> is also given (before
  159 <b>-C</b>), information about stack use is given in a line like this:
  160 <pre>
  161   Match recursion uses stack: approximate frame size = 640 bytes
  162 </pre>
  163 The value is approximate because some recursions need a bit more (up to perhaps
  164 16 more bytes).
  165 </P>
  166 <P>
  167 If the above command is given when PCRE is compiled to use the heap instead of
  168 the stack for recursion, the value that is output is the size of each block
  169 that is obtained from the heap.
  170 </P>
  171 <br><b>
  172 Changing stack size in Unix-like systems
  173 </b><br>
  174 <P>
  175 In Unix-like environments, there is not often a problem with the stack unless
  176 very long strings are involved, though the default limit on stack size varies
  177 from system to system. Values from 8Mb to 64Mb are common. You can find your
  178 default limit by running the command:
  179 <pre>
  180   ulimit -s
  181 </pre>
  182 Unfortunately, the effect of running out of stack is often SIGSEGV, though
  183 sometimes a more explicit error message is given. You can normally increase the
  184 limit on stack size by code such as this:
  185 <pre>
  186   struct rlimit rlim;
  187   getrlimit(RLIMIT_STACK, &rlim);
  188   rlim.rlim_cur = 100*1024*1024;
  189   setrlimit(RLIMIT_STACK, &rlim);
  190 </pre>
  191 This reads the current limits (soft and hard) using <b>getrlimit()</b>, then
  192 attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must
  193 do this before calling <b>pcre[16|32]_exec()</b>.
  194 </P>
  195 <br><b>
  196 Changing stack size in Mac OS X
  197 </b><br>
  198 <P>
  199 Using <b>setrlimit()</b>, as described above, should also work on Mac OS X. It
  200 is also possible to set a stack size when linking a program. There is a
  201 discussion about stack sizes in Mac OS X at this web site:
  202 <a href="http://developer.apple.com/qa/qa2005/qa1419.html">http://developer.apple.com/qa/qa2005/qa1419.html.</a>
  203 </P>
  204 <br><b>
  205 AUTHOR
  206 </b><br>
  207 <P>
  208 Philip Hazel
  209 <br>
  210 University Computing Service
  211 <br>
  212 Cambridge CB2 3QH, England.
  213 <br>
  214 </P>
  215 <br><b>
  216 REVISION
  217 </b><br>
  218 <P>
  219 Last updated: 24 June 2012
  220 <br>
  221 Copyright &copy; 1997-2012 University of Cambridge.
  222 <br>
  223 <p>
  224 Return to the <a href="index.html">PCRE index page</a>.
  225 </p>