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_exec()</b>, it makes use of an internal function called 20 <b>match()</b>. This calls itself recursively at branch points in the pattern, 21 in order to remember the state of the match so that it can back up and try a 22 different alternative if the first one fails. As matching proceeds deeper and 23 deeper into the tree of possibilities, the recursion depth increases. 24 </P> 25 <P> 26 Not all calls of <b>match()</b> increase the recursion depth; for an item such 27 as a* it may be called several times at the same level, after matching 28 different numbers of a's. Furthermore, in a number of cases where the result of 29 the recursive call would immediately be passed back as the result of the 30 current call (a "tail recursion"), the function is just restarted instead. 31 </P> 32 <P> 33 The <b>pcre_dfa_exec()</b> function operates in an entirely different way, and 34 uses recursion only when there is a regular expression recursion or subroutine 35 call in the pattern. This includes the processing of assertion and "once-only" 36 subpatterns, which are handled like subroutine calls. Normally, these are never 37 very deep, and the limit on the complexity of <b>pcre_dfa_exec()</b> is 38 controlled by the amount of workspace it is given. However, it is possible to 39 write patterns with runaway infinite recursions; such patterns will cause 40 <b>pcre_dfa_exec()</b> to run out of stack. At present, there is no protection 41 against this. 42 </P> 43 <P> 44 The comments that follow do NOT apply to <b>pcre_dfa_exec()</b>; they are 45 relevant only for <b>pcre_exec()</b>. 46 </P> 47 <br><b> 48 Reducing <b>pcre_exec()</b>'s stack usage 49 </b><br> 50 <P> 51 Each time that <b>match()</b> is actually called recursively, it uses memory 52 from the process stack. For certain kinds of pattern and data, very large 53 amounts of stack may be needed, despite the recognition of "tail recursion". 54 You can often reduce the amount of recursion, and therefore the amount of stack 55 used, by modifying the pattern that is being matched. Consider, for example, 56 this pattern: 57 <pre> 58 ([^<]|<(?!inet))+ 59 </pre> 60 It matches from wherever it starts until it encounters "<inet" or the end of 61 the data, and is the kind of pattern that might be used when processing an XML 62 file. Each iteration of the outer parentheses matches either one character that 63 is not "<" or a "<" that is not followed by "inet". However, each time a 64 parenthesis is processed, a recursion occurs, so this formulation uses a stack 65 frame for each matched character. For a long string, a lot of stack is 66 required. Consider now this rewritten pattern, which matches exactly the same 67 strings: 68 <pre> 69 ([^<]++|<(?!inet))+ 70 </pre> 71 This uses very much less stack, because runs of characters that do not contain 72 "<" are "swallowed" in one item inside the parentheses. Recursion happens only 73 when a "<" character that is not followed by "inet" is encountered (and we 74 assume this is relatively rare). A possessive quantifier is used to stop any 75 backtracking into the runs of non-"<" characters, but that is not related to 76 stack usage. 77 </P> 78 <P> 79 This example shows that one way of avoiding stack problems when matching long 80 subject strings is to write repeated parenthesized subpatterns to match more 81 than one character whenever possible. 82 </P> 83 <br><b> 84 Compiling PCRE to use heap instead of stack for <b>pcre_exec()</b> 85 </b><br> 86 <P> 87 In environments where stack memory is constrained, you might want to compile 88 PCRE to use heap memory instead of stack for remembering back-up points when 89 <b>pcre_exec()</b> is running. This makes it run a lot more slowly, however. 90 Details of how to do this are given in the 91 <a href="pcrebuild.html"><b>pcrebuild</b></a> 92 documentation. When built in this way, instead of using the stack, PCRE obtains 93 and frees memory by calling the functions that are pointed to by the 94 <b>pcre_stack_malloc</b> and <b>pcre_stack_free</b> variables. By default, these 95 point to <b>malloc()</b> and <b>free()</b>, but you can replace the pointers to 96 cause PCRE to use your own functions. Since the block sizes are always the 97 same, and are always freed in reverse order, it may be possible to implement 98 customized memory handlers that are more efficient than the standard functions. 99 </P> 100 <br><b> 101 Limiting <b>pcre_exec()</b>'s stack usage 102 </b><br> 103 <P> 104 You can set limits on the number of times that <b>match()</b> is called, both in 105 total and recursively. If a limit is exceeded, <b>pcre_exec()</b> returns an 106 error code. Setting suitable limits should prevent it from running out of 107 stack. The default values of the limits are very large, and unlikely ever to 108 operate. They can be changed when PCRE is built, and they can also be set when 109 <b>pcre_exec()</b> is called. For details of these interfaces, see the 110 <a href="pcrebuild.html"><b>pcrebuild</b></a> 111 documentation and the 112 <a href="pcreapi.html#extradata">section on extra data for <b>pcre_exec()</b></a> 113 in the 114 <a href="pcreapi.html"><b>pcreapi</b></a> 115 documentation. 116 </P> 117 <P> 118 As a very rough rule of thumb, you should reckon on about 500 bytes per 119 recursion. Thus, if you want to limit your stack usage to 8Mb, you 120 should set the limit at 16000 recursions. A 64Mb stack, on the other hand, can 121 support around 128000 recursions. 122 </P> 123 <P> 124 In Unix-like environments, the <b>pcretest</b> test program has a command line 125 option (<b>-S</b>) that can be used to increase the size of its stack. As long 126 as the stack is large enough, another option (<b>-M</b>) can be used to find the 127 smallest limits that allow a particular pattern to match a given subject 128 string. This is done by calling <b>pcre_exec()</b> repeatedly with different 129 limits. 130 </P> 131 <br><b> 132 Changing stack size in Unix-like systems 133 </b><br> 134 <P> 135 In Unix-like environments, there is not often a problem with the stack unless 136 very long strings are involved, though the default limit on stack size varies 137 from system to system. Values from 8Mb to 64Mb are common. You can find your 138 default limit by running the command: 139 <pre> 140 ulimit -s 141 </pre> 142 Unfortunately, the effect of running out of stack is often SIGSEGV, though 143 sometimes a more explicit error message is given. You can normally increase the 144 limit on stack size by code such as this: 145 <pre> 146 struct rlimit rlim; 147 getrlimit(RLIMIT_STACK, &rlim); 148 rlim.rlim_cur = 100*1024*1024; 149 setrlimit(RLIMIT_STACK, &rlim); 150 </pre> 151 This reads the current limits (soft and hard) using <b>getrlimit()</b>, then 152 attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must 153 do this before calling <b>pcre_exec()</b>. 154 </P> 155 <br><b> 156 Changing stack size in Mac OS X 157 </b><br> 158 <P> 159 Using <b>setrlimit()</b>, as described above, should also work on Mac OS X. It 160 is also possible to set a stack size when linking a program. There is a 161 discussion about stack sizes in Mac OS X at this web site: 162 <a href="http://developer.apple.com/qa/qa2005/qa1419.html">http://developer.apple.com/qa/qa2005/qa1419.html.</a> 163 </P> 164 <br><b> 165 AUTHOR 166 </b><br> 167 <P> 168 Philip Hazel 169 <br> 170 University Computing Service 171 <br> 172 Cambridge CB2 3QH, England. 173 <br> 174 </P> 175 <br><b> 176 REVISION 177 </b><br> 178 <P> 179 Last updated: 03 January 2010 180 <br> 181 Copyright © 1997-2010 University of Cambridge. 182 <br> 183 <p> 184 Return to the <a href="index.html">PCRE index page</a>. 185 </p> 186
