208 lines
8.5 KiB
HTML
208 lines
8.5 KiB
HTML
<html>
|
|
<head>
|
|
<title>pcre2stack specification</title>
|
|
</head>
|
|
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
|
|
<h1>pcre2stack man page</h1>
|
|
<p>
|
|
Return to the <a href="index.html">PCRE2 index page</a>.
|
|
</p>
|
|
<p>
|
|
This page is part of the PCRE2 HTML documentation. It was generated
|
|
automatically from the original man page. If there is any nonsense in it,
|
|
please consult the man page, in case the conversion went wrong.
|
|
<br>
|
|
<br><b>
|
|
PCRE2 DISCUSSION OF STACK USAGE
|
|
</b><br>
|
|
<P>
|
|
When you call <b>pcre2_match()</b>, it makes use of an internal function called
|
|
<b>match()</b>. 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 after a failure. As matching proceeds deeper and deeper
|
|
into the tree of possibilities, the recursion depth increases. The
|
|
<b>match()</b> function is also called in other circumstances, for example,
|
|
whenever a parenthesized sub-pattern is entered, and in certain cases of
|
|
repetition.
|
|
</P>
|
|
<P>
|
|
Not all calls of <b>match()</b> 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.
|
|
</P>
|
|
<P>
|
|
Each time the internal <b>match()</b> function is 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". Note that if PCRE2 is compiled with the -fsanitize=address option
|
|
of the GCC compiler, the stack requirements are greatly increased.
|
|
</P>
|
|
<P>
|
|
The above comments apply when <b>pcre2_match()</b> is run in its normal
|
|
interpretive manner. If the compiled pattern was processed by
|
|
<b>pcre2_jit_compile()</b>, and just-in-time compiling was successful, and the
|
|
options passed to <b>pcre2_match()</b> were not incompatible, the matching
|
|
process uses the JIT-compiled code instead of the <b>match()</b> function. In
|
|
this case, the memory requirements are handled entirely differently. See the
|
|
<a href="pcre2jit.html"><b>pcre2jit</b></a>
|
|
documentation for details.
|
|
</P>
|
|
<P>
|
|
The <b>pcre2_dfa_match()</b> function operates in a different way to
|
|
<b>pcre2_match()</b>, 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
|
|
<b>pcre2_dfa_match()</b> 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 <b>pcre2_dfa_match()</b> to run out of stack. At
|
|
present, there is no protection against this.
|
|
</P>
|
|
<P>
|
|
The comments that follow do NOT apply to <b>pcre2_dfa_match()</b>; they are
|
|
relevant only for <b>pcre2_match()</b> without the JIT optimization.
|
|
</P>
|
|
<br><b>
|
|
Reducing <b>pcre2_match()</b>'s stack usage
|
|
</b><br>
|
|
<P>
|
|
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:
|
|
<pre>
|
|
([^<]|<(?!inet))+
|
|
</pre>
|
|
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:
|
|
<pre>
|
|
([^<]++|<(?!inet))+
|
|
</pre>
|
|
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.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<br><b>
|
|
Compiling PCRE2 to use heap instead of stack for <b>pcre2_match()</b>
|
|
</b><br>
|
|
<P>
|
|
In environments where stack memory is constrained, you might want to compile
|
|
PCRE2 to use heap memory instead of stack for remembering back-up points when
|
|
<b>pcre2_match()</b> is running. This makes it run more slowly, however. Details
|
|
of how to do this are given in the
|
|
<a href="pcre2build.html"><b>pcre2build</b></a>
|
|
documentation. When built in this way, instead of using the stack, PCRE2
|
|
gets memory for remembering backup points from the heap. By default, the memory
|
|
is obtained by calling the system <b>malloc()</b> function, but you can arrange
|
|
to supply your own memory management function. For details, see the section
|
|
entitled
|
|
<a href="pcre2api.html#matchcontext">"The match context"</a>
|
|
in the
|
|
<a href="pcre2api.html"><b>pcre2api</b></a>
|
|
documentation. Since the block sizes are always the same, it may be possible to
|
|
implement customized a memory handler that is more efficient than the standard
|
|
function. The memory blocks obtained for this purpose are retained and re-used
|
|
if possible while <b>pcre2_match()</b> is running. They are all freed just
|
|
before it exits.
|
|
</P>
|
|
<br><b>
|
|
Limiting <b>pcre2_match()</b>'s stack usage
|
|
</b><br>
|
|
<P>
|
|
You can set limits on the number of times the internal <b>match()</b> function
|
|
is called, both in total and recursively. If a limit is exceeded,
|
|
<b>pcre2_match()</b> 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 PCRE2 is built,
|
|
and they can also be set when <b>pcre2_match()</b> is called. For details of
|
|
these interfaces, see the
|
|
<a href="pcre2build.html"><b>pcre2build</b></a>
|
|
documentation and the section entitled
|
|
<a href="pcre2api.html#matchcontext">"The match context"</a>
|
|
in the
|
|
<a href="pcre2api.html"><b>pcre2api</b></a>
|
|
documentation.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
The <b>pcre2test</b> test program has a modifier called "find_limits" which, if
|
|
applied to a subject line, causes it to find the smallest limits that allow a a
|
|
pattern to match. This is done by calling <b>pcre2_match()</b> repeatedly with
|
|
different limits.
|
|
</P>
|
|
<br><b>
|
|
Changing stack size in Unix-like systems
|
|
</b><br>
|
|
<P>
|
|
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:
|
|
<pre>
|
|
ulimit -s
|
|
</pre>
|
|
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:
|
|
<pre>
|
|
struct rlimit rlim;
|
|
getrlimit(RLIMIT_STACK, &rlim);
|
|
rlim.rlim_cur = 100*1024*1024;
|
|
setrlimit(RLIMIT_STACK, &rlim);
|
|
</pre>
|
|
This reads the current limits (soft and hard) using <b>getrlimit()</b>, then
|
|
attempts to increase the soft limit to 100Mb using <b>setrlimit()</b>. You must
|
|
do this before calling <b>pcre2_match()</b>.
|
|
</P>
|
|
<br><b>
|
|
Changing stack size in Mac OS X
|
|
</b><br>
|
|
<P>
|
|
Using <b>setrlimit()</b>, 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:
|
|
<a href="http://developer.apple.com/qa/qa2005/qa1419.html">http://developer.apple.com/qa/qa2005/qa1419.html.</a>
|
|
</P>
|
|
<br><b>
|
|
AUTHOR
|
|
</b><br>
|
|
<P>
|
|
Philip Hazel
|
|
<br>
|
|
University Computing Service
|
|
<br>
|
|
Cambridge, England.
|
|
<br>
|
|
</P>
|
|
<br><b>
|
|
REVISION
|
|
</b><br>
|
|
<P>
|
|
Last updated: 21 November 2014
|
|
<br>
|
|
Copyright © 1997-2014 University of Cambridge.
|
|
<br>
|
|
<p>
|
|
Return to the <a href="index.html">PCRE2 index page</a>.
|
|
</p>
|