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pcrestack(3)                                                      pcrestack(3)




NAME

       PCRE - Perl-compatible regular expressions


PCRE DISCUSSION OF STACK USAGE


       When  you call pcre[16|32]_exec(), it makes use of an internal function
       called match(). 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 match() function is also called in other
       circumstances,  for  example,  whenever  a parenthesized sub-pattern is
       entered, and in certain cases of repetition.

       Not all calls of match() 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.

       The  above  comments apply when pcre[16|32]_exec() 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 success-
       ful, and the options passed to pcre[16|32]_exec() were  not  incompati-
       ble,  the  matching  process  uses the JIT-compiled code instead of the
       match() function. In this case, the  memory  requirements  are  handled
       entirely differently. See the pcrejit documentation for details.

       The  pcre[16|32]_dfa_exec()  function operates in an entirely different
       way, and uses recursion only when there is a regular expression  recur-
       sion or subroutine call in the pattern. This includes the processing of
       assertion and "once-only" subpatterns, which are handled  like  subrou-
       tine  calls.  Normally, these are never very deep, and the limit on the
       complexity of pcre[16|32]_dfa_exec() 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
       pcre[16|32]_dfa_exec()  to  run  out  of stack. At present, there is no
       protection against this.

       The comments that follow do NOT apply to  pcre[16|32]_dfa_exec();  they
       are  relevant only for pcre[16|32]_exec() without the JIT optimization.

   Reducing pcre[16|32]_exec()'s stack usage

       Each time that match() 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 there-
       fore the amount of stack used, by modifying the pattern that  is  being
       matched. Consider, for example, this pattern:

         ([^<]|<(?!inet))+

       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 charac-
       ter.  For  a long string, a lot of stack is required. Consider now this
       rewritten pattern, which matches exactly the same strings:

         ([^<]++|<(?!inet))+

       This uses very much less stack, because runs of characters that do  not
       contain  "<" are "swallowed" in one item inside the parentheses. Recur-
       sion 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.

       This  example shows that one way of avoiding stack problems when match-
       ing long subject strings is to write repeated parenthesized subpatterns
       to match more than one character whenever possible.

   Compiling PCRE to use heap instead of stack for pcre[16|32]_exec()

       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 pcre[16|32]_exec() is running. This makes it run a lot
       more slowly, however.  Details of how to do this are given in the pcre-
       build  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 pcre[16|32]_stack_malloc and pcre[16|32]_stack_free
       variables. By default, these point to malloc() and free(), 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.

   Limiting pcre[16|32]_exec()'s stack usage

       You can set limits on the number of times that match() is called,  both
       in  total  and  recursively. If a limit is exceeded, pcre[16|32]_exec()
       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 pcre[16|32]_exec() is called. For details
       of these interfaces, see the pcrebuild documentation and the section on
       extra data for pcre[16|32]_exec() in the pcreapi documentation.

       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.

       In Unix-like environments, the pcretest test program has a command line
       option (-S) that can be used to increase the size of its stack. As long
       as the stack is large enough, another option (-M) can be used  to  find
       the  smallest  limits  that allow a particular pattern to match a given
       subject string. This is done by calling  pcre[16|32]_exec()  repeatedly
       with different limits.

   Obtaining an estimate of stack usage

       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 optimiza-
       tion 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:

         pcretest -m -C

       The -C option causes pcretest to output information about  the  options
       with which PCRE was compiled. When -m is also given (before -C), infor-
       mation about stack use is given in a line like this:

         Match recursion uses stack: approximate frame size = 640 bytes

       The value is approximate because some recursions need a bit more (up to
       perhaps 16 more bytes).

       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.

   Changing stack size in Unix-like systems

       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:

         ulimit -s

       Unfortunately, the effect of running out of  stack  is  often  SIGSEGV,
       though  sometimes  a more explicit error message is given. You can nor-
       mally increase the limit on stack size by code such as this:

         struct rlimit rlim;
         getrlimit(RLIMIT_STACK, &rlim);
         rlim.rlim_cur = 100*1024*1024;
         setrlimit(RLIMIT_STACK, &rlim);

       This reads the current limits (soft and hard) using  getrlimit(),  then
       attempts  to  increase  the  soft limit to 100Mb using setrlimit(). You
       must do this before calling pcre[16|32]_exec().

   Changing stack size in Mac OS X

       Using setrlimit(), 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:
       http://developer.apple.com/qa/qa2005/qa1419.html.


AUTHOR


       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.


REVISION


       Last updated: 24 June 2012
       Copyright (c) 1997-2012 University of Cambridge.



PCRE 8.30                        24 June 2012                     pcrestack(3)

pcre 8.32 - Generated Sun Dec 2 13:21:01 CST 2012