203 lines
7.9 KiB
Groff
203 lines
7.9 KiB
Groff
.TH PCRE2STACK 3 "21 November 2014" "PCRE2 10.00"
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.SH NAME
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PCRE2 - Perl-compatible regular expressions (revised API)
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.SH "PCRE2 DISCUSSION OF STACK USAGE"
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.rs
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.sp
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When you call \fBpcre2_match()\fP, it makes use of an internal function called
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\fBmatch()\fP. This calls itself recursively at branch points in the pattern,
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in order to remember the state of the match so that it can back up and try a
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different alternative after a failure. As matching proceeds deeper and deeper
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into the tree of possibilities, the recursion depth increases. The
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\fBmatch()\fP function is also called in other circumstances, for example,
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whenever a parenthesized sub-pattern is entered, and in certain cases of
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repetition.
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.P
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Not all calls of \fBmatch()\fP increase the recursion depth; for an item such
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as a* it may be called several times at the same level, after matching
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different numbers of a's. Furthermore, in a number of cases where the result of
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the recursive call would immediately be passed back as the result of the
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current call (a "tail recursion"), the function is just restarted instead.
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.P
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Each time the internal \fBmatch()\fP function is called recursively, it uses
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memory from the process stack. For certain kinds of pattern and data, very
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large amounts of stack may be needed, despite the recognition of "tail
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recursion". Note that if PCRE2 is compiled with the -fsanitize=address option
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of the GCC compiler, the stack requirements are greatly increased.
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.P
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The above comments apply when \fBpcre2_match()\fP is run in its normal
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interpretive manner. If the compiled pattern was processed by
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\fBpcre2_jit_compile()\fP, and just-in-time compiling was successful, and the
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options passed to \fBpcre2_match()\fP were not incompatible, the matching
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process uses the JIT-compiled code instead of the \fBmatch()\fP function. In
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this case, the memory requirements are handled entirely differently. See the
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.\" HREF
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\fBpcre2jit\fP
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.\"
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documentation for details.
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.P
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The \fBpcre2_dfa_match()\fP function operates in a different way to
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\fBpcre2_match()\fP, and uses recursion only when there is a regular expression
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recursion or subroutine call in the pattern. This includes the processing of
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assertion and "once-only" subpatterns, which are handled like subroutine calls.
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Normally, these are never very deep, and the limit on the complexity of
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\fBpcre2_dfa_match()\fP is controlled by the amount of workspace it is given.
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However, it is possible to write patterns with runaway infinite recursions;
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such patterns will cause \fBpcre2_dfa_match()\fP to run out of stack. At
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present, there is no protection against this.
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.P
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The comments that follow do NOT apply to \fBpcre2_dfa_match()\fP; they are
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relevant only for \fBpcre2_match()\fP without the JIT optimization.
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.
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.
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.SS "Reducing \fBpcre2_match()\fP's stack usage"
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.rs
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.sp
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You can often reduce the amount of recursion, and therefore the
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amount of stack used, by modifying the pattern that is being matched. Consider,
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for example, this pattern:
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.sp
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([^<]|<(?!inet))+
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.sp
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It matches from wherever it starts until it encounters "<inet" or the end of
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the data, and is the kind of pattern that might be used when processing an XML
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file. Each iteration of the outer parentheses matches either one character that
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is not "<" or a "<" that is not followed by "inet". However, each time a
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parenthesis is processed, a recursion occurs, so this formulation uses a stack
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frame for each matched character. For a long string, a lot of stack is
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required. Consider now this rewritten pattern, which matches exactly the same
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strings:
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.sp
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([^<]++|<(?!inet))+
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.sp
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This uses very much less stack, because runs of characters that do not contain
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"<" are "swallowed" in one item inside the parentheses. Recursion happens only
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when a "<" character that is not followed by "inet" is encountered (and we
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assume this is relatively rare). A possessive quantifier is used to stop any
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backtracking into the runs of non-"<" characters, but that is not related to
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stack usage.
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.P
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This example shows that one way of avoiding stack problems when matching long
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subject strings is to write repeated parenthesized subpatterns to match more
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than one character whenever possible.
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.
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.
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.SS "Compiling PCRE2 to use heap instead of stack for \fBpcre2_match()\fP"
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.rs
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.sp
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In environments where stack memory is constrained, you might want to compile
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PCRE2 to use heap memory instead of stack for remembering back-up points when
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\fBpcre2_match()\fP is running. This makes it run more slowly, however. Details
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of how to do this are given in the
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.\" HREF
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\fBpcre2build\fP
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.\"
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documentation. When built in this way, instead of using the stack, PCRE2
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gets memory for remembering backup points from the heap. By default, the memory
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is obtained by calling the system \fBmalloc()\fP function, but you can arrange
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to supply your own memory management function. For details, see the section
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entitled
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.\" HTML <a href="pcre2api.html#matchcontext">
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.\" </a>
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"The match context"
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.\"
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in the
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.\" HREF
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\fBpcre2api\fP
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.\"
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documentation. Since the block sizes are always the same, it may be possible to
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implement customized a memory handler that is more efficient than the standard
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function. The memory blocks obtained for this purpose are retained and re-used
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if possible while \fBpcre2_match()\fP is running. They are all freed just
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before it exits.
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.
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.
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.SS "Limiting \fBpcre2_match()\fP's stack usage"
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.rs
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.sp
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You can set limits on the number of times the internal \fBmatch()\fP function
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is called, both in total and recursively. If a limit is exceeded,
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\fBpcre2_match()\fP returns an error code. Setting suitable limits should
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prevent it from running out of stack. The default values of the limits are very
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large, and unlikely ever to operate. They can be changed when PCRE2 is built,
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and they can also be set when \fBpcre2_match()\fP is called. For details of
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these interfaces, see the
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.\" HREF
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\fBpcre2build\fP
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.\"
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documentation and the section entitled
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.\" HTML <a href="pcre2api.html#matchcontext">
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.\" </a>
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"The match context"
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.\"
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in the
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.\" HREF
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\fBpcre2api\fP
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.\"
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documentation.
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.P
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As a very rough rule of thumb, you should reckon on about 500 bytes per
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recursion. Thus, if you want to limit your stack usage to 8Mb, you should set
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the limit at 16000 recursions. A 64Mb stack, on the other hand, can support
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around 128000 recursions.
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.P
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The \fBpcre2test\fP test program has a modifier called "find_limits" which, if
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applied to a subject line, causes it to find the smallest limits that allow a a
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pattern to match. This is done by calling \fBpcre2_match()\fP repeatedly with
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different limits.
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.
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.
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.SS "Changing stack size in Unix-like systems"
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.rs
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.sp
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In Unix-like environments, there is not often a problem with the stack unless
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very long strings are involved, though the default limit on stack size varies
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from system to system. Values from 8Mb to 64Mb are common. You can find your
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default limit by running the command:
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.sp
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ulimit -s
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.sp
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Unfortunately, the effect of running out of stack is often SIGSEGV, though
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sometimes a more explicit error message is given. You can normally increase the
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limit on stack size by code such as this:
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.sp
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struct rlimit rlim;
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getrlimit(RLIMIT_STACK, &rlim);
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rlim.rlim_cur = 100*1024*1024;
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setrlimit(RLIMIT_STACK, &rlim);
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.sp
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This reads the current limits (soft and hard) using \fBgetrlimit()\fP, then
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attempts to increase the soft limit to 100Mb using \fBsetrlimit()\fP. You must
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do this before calling \fBpcre2_match()\fP.
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.
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.
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.SS "Changing stack size in Mac OS X"
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.rs
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.sp
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Using \fBsetrlimit()\fP, as described above, should also work on Mac OS X. It
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is also possible to set a stack size when linking a program. There is a
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discussion about stack sizes in Mac OS X at this web site:
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.\" HTML <a href="http://developer.apple.com/qa/qa2005/qa1419.html">
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.\" </a>
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http://developer.apple.com/qa/qa2005/qa1419.html.
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.\"
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.
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.
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.SH AUTHOR
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.rs
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.sp
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.nf
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Philip Hazel
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University Computing Service
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Cambridge, England.
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.fi
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.
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.
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.SH REVISION
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.rs
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.sp
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.nf
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Last updated: 21 November 2014
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Copyright (c) 1997-2014 University of Cambridge.
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.fi
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