Provided by: libpcre3-dev_8.31-2ubuntu2.3_amd64 bug

NAME

       PCRE - Perl-compatible regular expressions

PCRE JUST-IN-TIME COMPILER SUPPORT


       Just-in-time  compiling  is  a  heavyweight optimization that can greatly speed up pattern
       matching. However, it comes at the cost of extra processing before the match is performed.
       Therefore,  it is of most benefit when the same pattern is going to be matched many times.
       This does not necessarily mean many calls of a matching function; if the  pattern  is  not
       anchored, matching attempts may take place many times at various positions in the subject,
       even for a single call.  Therefore, if the subject string is very long, it may  still  pay
       to use JIT for one-off matches.

       JIT  support  applies  only to the traditional Perl-compatible matching function.  It does
       not apply when the DFA matching function is being used. The  code  for  this  support  was
       written by Zoltan Herczeg.

8-BIT and 16-BIT SUPPORT


       JIT  support  is  available  for  both  the  8-bit and 16-bit PCRE libraries. To keep this
       documentation simple, only the 8-bit interface is described in what follows.  If  you  are
       using  the  16-bit  library,  substitute  the  16-bit functions and 16-bit structures (for
       example, pcre16_jit_stack instead of pcre_jit_stack).

AVAILABILITY OF JIT SUPPORT


       JIT support is an optional feature  of  PCRE.  The  "configure"  option  --enable-jit  (or
       equivalent  CMake  option)  must  be  set  when  PCRE is built if you want to use JIT. The
       support is limited to the following hardware platforms:

         ARM v5, v7, and Thumb2
         Intel x86 32-bit and 64-bit
         MIPS 32-bit
         Power PC 32-bit and 64-bit

       If --enable-jit is set on an unsupported platform, compilation fails.

       A program that is linked with PCRE 8.20 or later can tell if JIT support is  available  by
       calling  pcre_config()  with  the  PCRE_CONFIG_JIT  option.  The  result  is 1 when JIT is
       available, and 0 otherwise. However, a simple program does not need to check this in order
       to  use  JIT.  The API is implemented in a way that falls back to the interpretive code if
       JIT is not available.

       If your program may sometimes be linked with versions of PCRE that are  older  than  8.20,
       but  you  want  to use JIT when it is available, you can test the values of PCRE_MAJOR and
       PCRE_MINOR, or the existence of a JIT macro  such  as  PCRE_CONFIG_JIT,  for  compile-time
       control of your code.

SIMPLE USE OF JIT


       You have to do two things to make use of the JIT support in the simplest way:

         (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for
             each compiled pattern, and pass the resulting pcre_extra block to
             pcre_exec().

         (2) Use pcre_free_study() to free the pcre_extra block when it is
             no longer needed, instead of just freeing it yourself. This
             ensures that any JIT data is also freed.

       For a program that may be linked with pre-8.20 versions of PCRE, you can insert

         #ifndef PCRE_STUDY_JIT_COMPILE
         #define PCRE_STUDY_JIT_COMPILE 0
         #endif

       so  that no option is passed to pcre_study(), and then use something like this to free the
       study data:

         #ifdef PCRE_CONFIG_JIT
             pcre_free_study(study_ptr);
         #else
             pcre_free(study_ptr);
         #endif

       PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for complete matches. If
       you  want  to run partial matches using the PCRE_PARTIAL_HARD or PCRE_PARTIAL_SOFT options
       of pcre_exec(), you should set one or both of the following options  in  addition  to,  or
       instead of, PCRE_STUDY_JIT_COMPILE when you call pcre_study():

         PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
         PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE

       The  JIT  compiler generates different optimized code for each of the three modes (normal,
       soft partial, hard partial). When pcre_exec() is called, the appropriate code is run if it
       is available. Otherwise, the pattern is matched using interpretive code.

       In  some  circumstances  you may need to call additional functions. These are described in
       the section entitled "Controlling the JIT stack" below.

       If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are ignored, and no JIT  data
       is  created. Otherwise, the compiled pattern is passed to the JIT compiler, which turns it
       into machine code that executes much  faster  than  the  normal  interpretive  code.  When
       pcre_exec()  is  passed  a  pcre_extra  block  containing  a  pointer  to  JIT code of the
       appropriate mode (normal or hard/soft partial), it obeys that code instead of running  the
       interpreter. The result is identical, but the compiled JIT code runs much faster.

       There  are  some  pcre_exec()  options that are not supported for JIT execution. There are
       also some pattern items that JIT cannot handle. Details are given below.  In  both  cases,
       execution  automatically  falls back to the interpretive code. If you want to know whether
       JIT was actually used for a particular match,  you  should  arrange  for  a  JIT  callback
       function  to  be  set  up as described in the section entitled "Controlling the JIT stack"
       below, even if you do not need to supply a non-default JIT stack. Such a callback function
       is  called whenever JIT code is about to be obeyed. If the execution options are not right
       for JIT execution, the callback function is not obeyed.

       If the JIT compiler finds an unsupported item, no JIT data is generated. You can find  out
       if JIT execution is available after studying a pattern by calling pcre_fullinfo() with the
       PCRE_INFO_JIT option. A result of 1 means that JIT compilation was successful. A result of
       0  means  that  JIT  support  is  not  available,  or  the  pattern  was  not studied with
       PCRE_STUDY_JIT_COMPILE etc., or the JIT compiler was not able to handle the pattern.

       Once a pattern has been studied, with or without JIT, it can be used as many times as  you
       like for matching different subject strings.

UNSUPPORTED OPTIONS AND PATTERN ITEMS


       The  only pcre_exec() options that are supported for JIT execution are PCRE_NO_UTF8_CHECK,
       PCRE_NO_UTF16_CHECK,  PCRE_NOTBOL,  PCRE_NOTEOL,   PCRE_NOTEMPTY,   PCRE_NOTEMPTY_ATSTART,
       PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT.

       The unsupported pattern items are:

         \C             match a single byte; not supported in UTF-8 mode
         (?Cn)          callouts
         (*PRUNE)       )
         (*SKIP)        ) backtracking control verbs
         (*THEN)        )

       Support for some of these may be added in future.

RETURN VALUES FROM JIT EXECUTION


       When  a  pattern  is  matched using JIT execution, the return values are the same as those
       given by the interpretive pcre_exec() code, with the  addition  of  one  new  error  code:
       PCRE_ERROR_JIT_STACKLIMIT.  This  means  that  the  memory  used  for  the  JIT  stack was
       insufficient. See "Controlling the JIT stack" below for a discussion of JIT  stack  usage.
       For  compatibility  with the interpretive pcre_exec() code, no more than two-thirds of the
       ovector argument is used for passing back captured substrings.

       The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a very large
       pattern tree goes on for too long, as it is in the same circumstance when JIT is not used,
       but the details of exactly what is counted are not the same. The PCRE_ERROR_RECURSIONLIMIT
       error code is never returned by JIT execution.

SAVING AND RESTORING COMPILED PATTERNS


       The  code  that  is  generated  by  the JIT compiler is architecture-specific, and is also
       position dependent. For those reasons it cannot be saved  (in  a  file  or  database)  and
       restored  later  like  the  bytecode  and  other  data  of  a compiled pattern. Saving and
       restoring compiled patterns is not something  many  people  do.  More  detail  about  this
       facility  is  given  in  the  pcreprecompile  documentation.  It should be possible to run
       pcre_study() on a saved and restored pattern, and  thereby  recreate  the  JIT  data,  but
       because  JIT  compilation uses significant resources, it is probably not worth doing this;
       you might as well recompile the original pattern.

CONTROLLING THE JIT STACK


       When the compiled JIT code runs, it needs a block  of  memory  to  use  as  a  stack.   By
       default,  it  uses  32K  on the machine stack. However, some large or complicated patterns
       need more than this. The error PCRE_ERROR_JIT_STACKLIMIT is given when there is not enough
       stack.  Three  functions are provided for managing blocks of memory for use as JIT stacks.
       There is further discussion about the use of JIT stacks in the section entitled "JIT stack
       FAQ" below.

       The pcre_jit_stack_alloc() function creates a JIT stack. Its arguments are a starting size
       and  a  maximum  size,  and  it  returns  a  pointer  to  an  opaque  structure  of   type
       pcre_jit_stack,  or  NULL  if there is an error. The pcre_jit_stack_free() function can be
       used to free a stack that is no longer needed. (For the technically  minded:  the  address
       space is allocated by mmap or VirtualAlloc.)

       JIT  uses  far  less  memory for recursion than the interpretive code, and a maximum stack
       size of 512K to 1M should be more than enough for any pattern.

       The pcre_assign_jit_stack() function specifies  which  stack  JIT  code  should  use.  Its
       arguments are as follows:

         pcre_extra         *extra
         pcre_jit_callback  callback
         void               *data

       The  extra  argument  must be the result of studying a pattern with PCRE_STUDY_JIT_COMPILE
       etc. There are three cases for the values of the other two options:

         (1) If callback is NULL and data is NULL, an internal 32K block
             on the machine stack is used.

         (2) If callback is NULL and data is not NULL, data must be
             a valid JIT stack, the result of calling pcre_jit_stack_alloc().

         (3) If callback is not NULL, it must point to a function that is
             called with data as an argument at the start of matching, in
             order to set up a JIT stack. If the return from the callback
             function is NULL, the internal 32K stack is used; otherwise the
             return value must be a valid JIT stack, the result of calling
             pcre_jit_stack_alloc().

       A callback function is obeyed whenever JIT code is about to be run; it is not obeyed  when
       pcre_exec()  is  called  with  options that are incompatible for JIT execution. A callback
       function can therefore be used to determine whether a match operation was executed by  JIT
       or by the interpreter.

       You  may  safely  use  the  same  JIT stack for more than one pattern (either by assigning
       directly or by callback), as long as the patterns are all matched sequentially in the same
       thread.  In a multithread application, if you do not specify a JIT stack, or if you assign
       or pass back NULL from a callback, that is thread-safe, because each thread  has  its  own
       machine  stack.  However,  if you assign or pass back a non-NULL JIT stack, this must be a
       different stack for each thread so that the application is thread-safe.

       Strictly speaking, even more is allowed. You can assign the same  non-NULL  stack  to  any
       number  of  patterns  as long as they are not used for matching by multiple threads at the
       same time. For example, you can assign the same stack to all compiled patterns, and use  a
       global  mutex  in the callback to wait until the stack is available for use. However, this
       is an inefficient solution, and not recommended.

       This is a suggestion for how a multithreaded program that needs to set up non-default  JIT
       stacks might operate:

         During thread initalization
           thread_local_var = pcre_jit_stack_alloc(...)

         During thread exit
           pcre_jit_stack_free(thread_local_var)

         Use a one-line callback function
           return thread_local_var

       All  the  functions  described  in  this  section  do nothing if JIT is not available, and
       pcre_assign_jit_stack() does nothing unless the extra argument is non-NULL and points to a
       pcre_extra block that is the result of a successful study with PCRE_STUDY_JIT_COMPILE etc.

JIT STACK FAQ


       (1) Why do we need JIT stacks?

       PCRE  (and  JIT)  is  a recursive, depth-first engine, so it needs a stack where the local
       data of the current node is pushed before  checking  its  child  nodes.   Allocating  real
       machine  stack  on  some  platforms is difficult. For example, the stack chain needs to be
       updated every time if we extend the stack  on  PowerPC.   Although  it  is  possible,  its
       updating time overhead decreases performance. So we do the recursion in memory.

       (2) Why don't we simply allocate blocks of memory with malloc()?

       Modern operating systems have a nice feature: they can reserve an address space instead of
       allocating memory. We can safely allocate memory pages inside this address space,  so  the
       stack  could grow without moving memory data (this is important because of pointers). Thus
       we can allocate 1M address space, and use only a single memory page (usually 4K)  if  that
       is enough. However, we can still grow up to 1M anytime if needed.

       (3) Who "owns" a JIT stack?

       The  owner of the stack is the user program, not the JIT studied pattern or anything else.
       The user program must ensure that if a stack is used  by  pcre_exec(),  (that  is,  it  is
       assigned  to  the  pattern  currently  running),  that stack must not be used by any other
       threads (to avoid overwriting the same memory area). The best practice  for  multithreaded
       programs  is  to  allocate  a stack for each thread, and return this stack through the JIT
       callback function.

       (4) When should a JIT stack be freed?

       You can free a JIT stack at any time, as long as it will not be used by pcre_exec() again.
       When  you  assign  the  stack  to  a pattern, only a pointer is set. There is no reference
       counting or any other magic. You can free the patterns and stacks in any  order,  anytime.
       Just  do  not  call pcre_exec() with a pattern pointing to an already freed stack, as that
       will cause SEGFAULT. (Also, do not free a stack currently used by pcre_exec()  in  another
       thread).  You  can also replace the stack for a pattern at any time. You can even free the
       previous stack before assigning a replacement.

       (5) Should I allocate/free a stack every time before/after calling pcre_exec()?

       No, because this is too costly in terms of resources. However, you  could  implement  some
       clever  idea  which  release the stack if it is not used in let's say two minutes. The JIT
       callback can help to achive this without keeping a  list  of  the  currently  JIT  studied
       patterns.

       (6) OK, the stack is for long term memory allocation. But what happens if a pattern causes
       stack overflow with a stack of 1M? Is that 1M kept until the stack is freed?

       Especially on embedded sytems, it might be a good idea to release memory sometimes without
       freeing the stack. There is no API for this at the moment.  Probably a function call which
       returns with the currently allocated  memory  for  any  stack  and  another  which  allows
       releasing memory (shrinking the stack) would be a good idea if someone needs this.

       (7)  This  is  too  much  of  a  headache.  Isn't  there any better solution for JIT stack
       handling?

       No, thanks to Windows. If POSIX threads were used everywhere,  we  could  throw  out  this
       complicated API.

EXAMPLE CODE


       This is a single-threaded example that specifies a JIT stack without using a callback.

         int rc;
         int ovector[30];
         pcre *re;
         pcre_extra *extra;
         pcre_jit_stack *jit_stack;

         re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
         /* Check for errors */
         extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
         jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
         /* Check for error (NULL) */
         pcre_assign_jit_stack(extra, NULL, jit_stack);
         rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
         /* Check results */
         pcre_free(re);
         pcre_free_study(extra);
         pcre_jit_stack_free(jit_stack);

SEE ALSO


       pcreapi(3)

AUTHOR


       Philip Hazel (FAQ by Zoltan Herczeg)
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION


       Last updated: 04 May 2012
       Copyright (c) 1997-2012 University of Cambridge.