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    Command:

    pcrestack

    
    
    

    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  pro-
           ceeds  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
           hardly uses recursion at all. The limit on its complexity is the amount
           of  workspace  it  is  given.  The comments that follow do NOT apply to
           pcre_dfa_exec(); they are relevant only for pcre_exec().
    
           You can set limits on the number of times that match() is called,  both
           in  total  and  recursively. If the limit is exceeded, an error occurs.
           For details, see the section on  extra  data  for  pcre_exec()  in  the
           pcreapi documentation.
    
           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.
           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 mem-
           ory handlers that are more efficient than the standard functions.
    
       Limiting PCRE's stack usage
    
           PCRE has an internal counter that can be used to  limit  the  depth  of
           recursion,  and  thus cause pcre_exec() to give an error code before it
           runs out of stack. By default, the limit is very  large,  and  unlikely
           ever  to operate. It can be changed when PCRE is built, and it can also
           be set when pcre_exec() is called. For details of these interfaces, see
           the pcrebuild and 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. The pcretest  test  program
           has a command line option (-S) that can be used to increase the size of
           its stack.
    
       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_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.
    
    
    

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