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Introduction to Library Functions PCRESTACK(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE DISCUSSION OF STACK USAGE
When you call pcre_exec(), it makes use of an internal func-
tion 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. Furth-
ermore, in a number of cases where the result of the recur-
sive 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 dif-
ferent 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 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
" 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:
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([^<]++|<(?!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
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. 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'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 recur-
sions. 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
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Introduction to Library Functions PCRESTACK(3)
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 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: 09 July 2008
Copyright (c) 1997-2008 University of Cambridge.
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
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_______________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE|
|____________________|__________________|_
| Availability | library/pcre |
|____________________|__________________|_
| Interface Stability| Uncommitted |
|____________________|_________________|
NOTES
Source for PCRE is available on http://opensolaris.org.
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