| .TH PCRE2STACK 3 "21 November 2014" "PCRE2 10.00" |
| .SH NAME |
| PCRE2 - Perl-compatible regular expressions (revised API) |
| .SH "PCRE2 DISCUSSION OF STACK USAGE" |
| .rs |
| .sp |
| When you call \fBpcre2_match()\fP, it makes use of an internal function called |
| \fBmatch()\fP. 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 |
| \fBmatch()\fP function is also called in other circumstances, for example, |
| whenever a parenthesized sub-pattern is entered, and in certain cases of |
| repetition. |
| .P |
| Not all calls of \fBmatch()\fP 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 |
| Each time the internal \fBmatch()\fP 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 |
| The above comments apply when \fBpcre2_match()\fP is run in its normal |
| interpretive manner. If the compiled pattern was processed by |
| \fBpcre2_jit_compile()\fP, and just-in-time compiling was successful, and the |
| options passed to \fBpcre2_match()\fP were not incompatible, the matching |
| process uses the JIT-compiled code instead of the \fBmatch()\fP function. In |
| this case, the memory requirements are handled entirely differently. See the |
| .\" HREF |
| \fBpcre2jit\fP |
| .\" |
| documentation for details. |
| .P |
| The \fBpcre2_dfa_match()\fP function operates in a different way to |
| \fBpcre2_match()\fP, 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 |
| \fBpcre2_dfa_match()\fP 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 \fBpcre2_dfa_match()\fP to run out of stack. At |
| present, there is no protection against this. |
| .P |
| The comments that follow do NOT apply to \fBpcre2_dfa_match()\fP; they are |
| relevant only for \fBpcre2_match()\fP without the JIT optimization. |
| . |
| . |
| .SS "Reducing \fBpcre2_match()\fP's stack usage" |
| .rs |
| .sp |
| 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: |
| .sp |
| ([^<]|<(?!inet))+ |
| .sp |
| 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: |
| .sp |
| ([^<]++|<(?!inet))+ |
| .sp |
| 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 |
| 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. |
| . |
| . |
| .SS "Compiling PCRE2 to use heap instead of stack for \fBpcre2_match()\fP" |
| .rs |
| .sp |
| 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 |
| \fBpcre2_match()\fP is running. This makes it run more slowly, however. Details |
| of how to do this are given in the |
| .\" HREF |
| \fBpcre2build\fP |
| .\" |
| 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 \fBmalloc()\fP function, but you can arrange |
| to supply your own memory management function. For details, see the section |
| entitled |
| .\" HTML <a href="pcre2api.html#matchcontext"> |
| .\" </a> |
| "The match context" |
| .\" |
| in the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| 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 \fBpcre2_match()\fP is running. They are all freed just |
| before it exits. |
| . |
| . |
| .SS "Limiting \fBpcre2_match()\fP's stack usage" |
| .rs |
| .sp |
| You can set limits on the number of times the internal \fBmatch()\fP function |
| is called, both in total and recursively. If a limit is exceeded, |
| \fBpcre2_match()\fP 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 \fBpcre2_match()\fP is called. For details of |
| these interfaces, see the |
| .\" HREF |
| \fBpcre2build\fP |
| .\" |
| documentation and the section entitled |
| .\" HTML <a href="pcre2api.html#matchcontext"> |
| .\" </a> |
| "The match context" |
| .\" |
| in the |
| .\" HREF |
| \fBpcre2api\fP |
| .\" |
| documentation. |
| .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 |
| The \fBpcre2test\fP 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 \fBpcre2_match()\fP repeatedly with |
| different limits. |
| . |
| . |
| .SS "Changing stack size in Unix-like systems" |
| .rs |
| .sp |
| 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: |
| .sp |
| ulimit -s |
| .sp |
| 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: |
| .sp |
| struct rlimit rlim; |
| getrlimit(RLIMIT_STACK, &rlim); |
| rlim.rlim_cur = 100*1024*1024; |
| setrlimit(RLIMIT_STACK, &rlim); |
| .sp |
| This reads the current limits (soft and hard) using \fBgetrlimit()\fP, then |
| attempts to increase the soft limit to 100Mb using \fBsetrlimit()\fP. You must |
| do this before calling \fBpcre2_match()\fP. |
| . |
| . |
| .SS "Changing stack size in Mac OS X" |
| .rs |
| .sp |
| Using \fBsetrlimit()\fP, 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: |
| .\" HTML <a href="http://developer.apple.com/qa/qa2005/qa1419.html"> |
| .\" </a> |
| http://developer.apple.com/qa/qa2005/qa1419.html. |
| .\" |
| . |
| . |
| .SH AUTHOR |
| .rs |
| .sp |
| .nf |
| Philip Hazel |
| University Computing Service |
| Cambridge, England. |
| .fi |
| . |
| . |
| .SH REVISION |
| .rs |
| .sp |
| .nf |
| Last updated: 21 November 2014 |
| Copyright (c) 1997-2014 University of Cambridge. |
| .fi |