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Linux 2.6 - man page for pcrestack (linux section 3)

PCRESTACK(3)									     PCRESTACK(3)

       PCRE - Perl-compatible regular expressions


       When you call pcre_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

       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.  Further-
       more,  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  pcre_dfa_exec()  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
       pcre_dfa_exec()	is controlled by the amount of workspace it is given. However, it is pos-
       sible to write patterns	with  runaway  infinite  recursions;  such  patterns  will  cause
       pcre_dfa_exec() to run out of stack. At present, there is no protection against this.

       The  comments  that  follow  do	NOT  apply to pcre_dfa_exec(); they are relevant only for

   Reducing pcre_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  recur-
       sion,  and  therefore  the  amount  of  stack used, by modifying the pattern that is being
       matched. Consider, for example, this pattern:


       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


       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 "<" charac-
       ter 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-"<" charac-
       ters, but that is not related to stack usage.

       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.

   Compiling PCRE to use heap instead of stack for pcre_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_exec() is running.
       This makes it run a lot more slowly, however.  Details of how to do this are given in  the
       pcrebuild  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_stack_malloc and
       pcre_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_exec()'s stack usage

       You can set limits on the number of times that match() is called, both in total and recur-
       sively. If a limit is exceeded, pcre_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_exec() is called. For details of these interfaces, see the pcrebuild
       documentation and the section on extra data for pcre_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  recur-
       sions. 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 pat-
       tern to match a given subject string. This is done by calling pcre_exec() repeatedly  with
       different limits.

   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 com-

	 ulimit -s

       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:

	 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_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.


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


       Last updated: 03 January 2010
       Copyright (c) 1997-2010 University of Cambridge.


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