pcrestack(3)
PCRESTACK(3) C LIBRARY FUNCTIONS 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 inter-
nal 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 dif-
ferent 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 recur-
sion 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 func-
tion 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 dif-
ferent 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" subpat-
terns, 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 optimi-
zation.
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.
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PCRESTACK(3) C LIBRARY FUNCTIONS PCRESTACK(3)
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 itera-
tion 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 subpat-
terns 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 run-
ning. 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.
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PCRESTACK(3) C LIBRARY FUNCTIONS PCRESTACK(3)
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 recur-
sions. 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() repeat-
edly 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
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PCRESTACK(3) C LIBRARY FUNCTIONS PCRESTACK(3)
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 link-
ing 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 Last change: 24 June 2012 4
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