| .TH PCRESTACK 3 |
| .SH NAME |
| PCRE - Perl-compatible regular expressions |
| .SH "PCRE DISCUSSION OF STACK USAGE" |
| .rs |
| .sp |
| When you call \fBpcre_exec()\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 if the first one fails. 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 |
| The above comments apply when \fBpcre_exec()\fP 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 \fBpcre_exec()\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 |
| \fBpcrejit\fP |
| .\" |
| documentation for details. |
| .P |
| The \fBpcre_dfa_exec()\fP function operates in an entirely different 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" |
| subpatterns, which are handled like subroutine calls. Normally, these are never |
| very deep, and the limit on the complexity of \fBpcre_dfa_exec()\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 |
| \fBpcre_dfa_exec()\fP to run out of stack. At present, there is no protection |
| against this. |
| .P |
| The comments that follow do NOT apply to \fBpcre_dfa_exec()\fP; they are |
| relevant only for \fBpcre_exec()\fP without the JIT optimization. |
| . |
| . |
| .SS "Reducing \fBpcre_exec()\fP's stack usage" |
| .rs |
| .sp |
| Each time that \fBmatch()\fP 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: |
| .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 PCRE to use heap instead of stack for \fBpcre_exec()\fP" |
| .rs |
| .sp |
| 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 |
| \fBpcre_exec()\fP is running. This makes it run a lot more slowly, however. |
| Details of how to do this are given in the |
| .\" HREF |
| \fBpcrebuild\fP |
| .\" |
| 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 |
| \fBpcre_stack_malloc\fP and \fBpcre_stack_free\fP variables. By default, these |
| point to \fBmalloc()\fP and \fBfree()\fP, 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. |
| . |
| . |
| .SS "Limiting \fBpcre_exec()\fP's stack usage" |
| .rs |
| .sp |
| You can set limits on the number of times that \fBmatch()\fP is called, both in |
| total and recursively. If a limit is exceeded, \fBpcre_exec()\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 PCRE is built, and they can also be set when |
| \fBpcre_exec()\fP is called. For details of these interfaces, see the |
| .\" HREF |
| \fBpcrebuild\fP |
| .\" |
| documentation and the |
| .\" HTML <a href="pcreapi.html#extradata"> |
| .\" </a> |
| section on extra data for \fBpcre_exec()\fP |
| .\" |
| in the |
| .\" HREF |
| \fBpcreapi\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 |
| In Unix-like environments, the \fBpcretest\fP test program has a command line |
| option (\fB-S\fP) that can be used to increase the size of its stack. As long |
| as the stack is large enough, another option (\fB-M\fP) can be used to find the |
| smallest limits that allow a particular pattern to match a given subject |
| string. This is done by calling \fBpcre_exec()\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 \fBpcre_exec()\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 CB2 3QH, England. |
| .fi |
| . |
| . |
| .SH REVISION |
| .rs |
| .sp |
| .nf |
| Last updated: 26 August 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| .fi |