pcrestack(3) — Linux manual page


PCRESTACK(3)            Library Functions Manual            PCRESTACK(3)

NAME         top

       PCRE - Perl-compatible regular expressions


       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

   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:


       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:


       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

         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

         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         top

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

REVISION         top

       Last updated: 24 June 2012
       Copyright (c) 1997-2012 University of Cambridge.

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular
       Expressions) project.  Information about the project can be found
       at ⟨http://www.pcre.org/⟩.  If you have a bug report for this
       manual page, see
       ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This page was
       obtained from the tarball pcre-8.45.tar.gz fetched from
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       2021-08-27.  If you discover any rendering problems in this HTML
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PCRE 8.30                     24 June 2012                  PCRESTACK(3)

Pages that refer to this page: pcreapi(3)