PCRESTACK(3)                                                      PCRESTACK(3)



NAME
       PCRE - Perl-compatible regular expressions

PCRE DISCUSSION OF STACK USAGE

       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 increases.

       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 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 possible 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 pcre_exec().

   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 recursion, and
       therefore 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
       character. 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.
       Recursion 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
       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 recursively. 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 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_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 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
       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.

AUTHOR

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

REVISION

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



                                                                  PCRESTACK(3)
