-When you call pcre_exec(), it makes use of an internal function called -match(). 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 +When you call pcre[16|32]_exec(), it makes use of an internal function +called match(). 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 match() function is also called in other circumstances, for example, whenever a parenthesized sub-pattern is entered, and in certain cases of repetition. @@ -33,32 +33,32 @@ the recursive call would immediately be passed back as current call (a "tail recursion"), the function is just restarted instead.
-The above comments apply when pcre_exec() is run in its normal +The above comments apply when pcre[16|32]_exec() 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 pcre_exec() were not incompatible, the matching +the options passed to pcre[16|32]_exec() were not incompatible, the matching process uses the JIT-compiled code instead of the match() function. In this case, the memory requirements are handled entirely differently. See the pcrejit documentation for details.
-The pcre_dfa_exec() 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 pcre_dfa_exec() 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 -pcre_dfa_exec() to run out of stack. At present, there is no protection -against this. +The pcre[16|32]_dfa_exec() 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 +pcre[16|32]_dfa_exec() 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 pcre[16|32]_dfa_exec() to run out of stack. At +present, there is no protection against this.
-The comments that follow do NOT apply to pcre_dfa_exec(); they are -relevant only for pcre_exec() without the JIT optimization. +The comments that follow do NOT apply to pcre[16|32]_dfa_exec(); they are +relevant only for pcre[16|32]_exec() without the JIT optimization.
Each time that match() is actually called recursively, it uses memory @@ -94,54 +94,81 @@ subject strings is to write repeated parenthesized sub than one character whenever possible.
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 -pcre_exec() is running. This makes it run a lot more slowly, however. +pcre[16|32]_exec() is running. This makes it run a lot more slowly, however. Details of how to do this are given in the pcrebuild 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 -pcre_stack_malloc and pcre_stack_free variables. By default, these -point to malloc() and free(), 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. +pcre[16|32]_stack_malloc and pcre[16|32]_stack_free variables. By +default, these point to malloc() and free(), 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.
You can set limits on the number of times that match() is called, both in -total and recursively. If a limit is exceeded, pcre_exec() returns an +total and recursively. If a limit is exceeded, pcre[16|32]_exec() 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 -pcre_exec() is called. For details of these interfaces, see the +pcre[16|32]_exec() is called. For details of these interfaces, see the pcrebuild documentation and the -section on extra data for pcre_exec() +section on extra data for pcre[16|32]_exec() in the pcreapi documentation.
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. +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.
In Unix-like environments, the pcretest test program has a command line option (-S) that can be used to increase the size of its stack. As long as the stack is large enough, another option (-M) can be used to find the smallest limits that allow a particular pattern to match a given subject -string. This is done by calling pcre_exec() repeatedly with different +string. This is done by calling pcre[16|32]_exec() repeatedly with different limits.
+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: +
+ pcretest -m -C ++The -C option causes pcretest to output information about the +options with which PCRE was compiled. When -m is also given (before +-C), information about stack use is given in a line like this: +
+ Match recursion uses stack: approximate frame size = 640 bytes ++The value is approximate because some recursions need a bit more (up to perhaps +16 more bytes). + +
+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. +
+@@ -163,7 +190,7 @@ limit on stack size by code such as this: This reads the current limits (soft and hard) using getrlimit(), then attempts to increase the soft limit to 100Mb using setrlimit(). You must -do this before calling pcre_exec(). +do this before calling pcre[16|32]_exec().
-Last updated: 26 August 2011
+Last updated: 24 June 2012
-Copyright © 1997-2011 University of Cambridge.
+Copyright © 1997-2012 University of Cambridge.
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