pcrestack

PCRESTACK(3)               Library Functions Manual               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 en-
       tered, 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 en-
       tirely 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  oc-
       curs,  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, de-
       pending  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 ac-
       tually 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 in-
       stead  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://de-
       veloper.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)