PCRESTACK(3)              OpenBSD Programmer's 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
       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
       successful, and the options passed to pcre[16|32]_exec() were not
       incompatible, 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
       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[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
       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[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
       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[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
       optimization 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),
       information 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
       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[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)
