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diff --git a/plugins/Pcre16/docs/doc/pcrestack.3 b/plugins/Pcre16/docs/doc/pcrestack.3 new file mode 100644 index 0000000000..798f0bca63 --- /dev/null +++ b/plugins/Pcre16/docs/doc/pcrestack.3 @@ -0,0 +1,215 @@ +.TH PCRESTACK 3 "24 June 2012" "PCRE 8.30" +.SH NAME +PCRE - Perl-compatible regular expressions +.SH "PCRE DISCUSSION OF STACK USAGE" +.rs +.sp +When you call \fBpcre[16|32]_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[16|32]_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[16|32]_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[16|32]_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[16|32]_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[16|32]_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[16|32]_dfa_exec()\fP; they are +relevant only for \fBpcre[16|32]_exec()\fP without the JIT optimization. +. +. +.SS "Reducing \fBpcre[16|32]_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[16|32]_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[16|32]_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[16|32]_stack_malloc\fP and \fBpcre[16|32]_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[16|32]_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[16|32]_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[16|32]_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[16|32]_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[16|32]_exec()\fP repeatedly with different +limits. +. +. +.SS "Obtaining an estimate of stack usage" +.rs +.sp +The actual amount of stack used per recursion can vary quite a lot, depending +on the compiler that was used to build PCRE and the optimization or debugging +options that were set for it. The rule of thumb value of 500 bytes mentioned +above may be larger or smaller than what is actually needed. A better +approximation can be obtained by running this command: +.sp + pcretest -m -C +.sp +The \fB-C\fP option causes \fBpcretest\fP to output information about the +options with which PCRE was compiled. When \fB-m\fP is also given (before +\fB-C\fP), information about stack use is given in a line like this: +.sp + Match recursion uses stack: approximate frame size = 640 bytes +.sp +The value is approximate because some recursions need a bit more (up to perhaps +16 more bytes). +.P +If the above command is given when PCRE is compiled to use the heap instead of +the stack for recursion, the value that is output is the size of each block +that is obtained from the heap. +. +. +.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[16|32]_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: 24 June 2012 +Copyright (c) 1997-2012 University of Cambridge. +.fi |