trunk/doc/pcrejit.3

.TH PCREJIT 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PCRE JUST-IN-TIME COMPILER SUPPORT"
.rs
.sp
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
\fPpcre_exec()\fP; if the pattern is not anchored, matching attempts may take
place many times at various positions in the subject, even for a single call to
\fBpcre_exec()\fP. If the subject string is very long, it may still pay to use
JIT for one-off matches.
.P
JIT support applies only to the traditional matching function,
\fBpcre_exec()\fP. It does not apply when \fBpcre_dfa_exec()\fP is being used.
The code for this support was written by Zoltan Herczeg.
.
.
.SH "AVAILABILITY OF JIT SUPPORT"
.rs
.sp
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:
.sp
  ARM v5, v7, and Thumb2
  Intel x86 32-bit and 64-bit
  MIPS 32-bit
  Power PC 32-bit and 64-bit (experimental)
.sp
The Power PC support is designated as experimental because it has not been
fully tested. If --enable-jit is set on an unsupported platform, compilation
fails.
.P
A program can tell if JIT support is available by calling \fBpcre_config()\fP
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 ordinary PCRE
code if JIT is not available.
.
.
.SH "SIMPLE USE OF JIT"
.rs
.sp
You have to do two things to make use of the JIT support in the simplest way:
.sp
  (1) Call \fBpcre_study()\fP with the PCRE_STUDY_JIT_COMPILE option for
      each compiled pattern, and pass the resulting \fBpcre_extra\fP block to
      \fBpcre_exec()\fP.
.sp
  (2) Use \fBpcre_free_study()\fP to free the \fBpcre_extra\fP block when it is
      no longer needed instead of just freeing it yourself. This
      ensures that any JIT data is also freed.
.sp
In some circumstances you may need to call additional functions. These are
described in the section entitled
.\" HTML <a href="#stackcontrol">
.\" </a>
"Controlling the JIT stack"
.\"
below.
.P
If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and no JIT
data is set up. 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 \fBpcre_exec()\fP is passed a \fBpcre_extra\fP block
containing a pointer to JIT code, it obeys that instead of the normal code. The
result is identical, but the code runs much faster.
.P
There are some \fBpcre_exec()\fP 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.
.P
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
\fBpcre_fullinfo()\fP 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, or the
JIT compiler was not able to handle the pattern.
.P
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.
.
.
.SH "UNSUPPORTED OPTIONS AND PATTERN ITEMS"
.rs
.sp
The only \fBpcre_exec()\fP options that are supported for JIT execution are
PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and
PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
supported.
.P
The unsupported pattern items are:
.sp
  \eC            match a single byte; not supported in UTF-8 mode
  (?Cn)          callouts
  (*COMMIT)      )
  (*MARK)        )
  (*PRUNE)       ) the backtracking control verbs
  (*SKIP)        )
  (*THEN)        )
.sp
Support for some of these may be added in future.
.
.
.SH "RETURN VALUES FROM JIT EXECUTION"
.rs
.sp
When a pattern is matched using JIT execution, the return values are the same
as those given by the interpretive \fBpcre_exec()\fP 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
.\" HTML <a href="#stackcontrol">
.\" </a>
"Controlling the JIT stack"
.\"
below for a discussion of JIT stack usage. For compatibility with the
interpretive \fBpcre_exec()\fP code, no more than two-thirds of the
\fIovector\fP argument is used for passing back captured substrings.
.P
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.
.
.
.SH "SAVING AND RESTORING COMPILED PATTERNS"
.rs
.sp
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
.\" HREF
\fBpcreprecompile\fP
.\"
documentation. It should be possible to run \fBpcre_study()\fP 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.
.
.
.\" HTML <a name="stackcontrol"></a>
.SH "CONTROLLING THE JIT STACK"
.rs
.sp
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
.\" HTML <a href="#stackcontrol">
.\" </a>
"JIT stack FAQ"
.\"
below. 
.P
The \fBpcre_jit_stack_alloc()\fP 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 \fBpcre_jit_stack\fP, or NULL if there is an error. The
\fBpcre_jit_stack_free()\fP 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.)
.P
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.
.P
The \fBpcre_assign_jit_stack()\fP function specifies which stack JIT code
should use. Its arguments are as follows:
.sp
  pcre_extra         *extra
  pcre_jit_callback  callback
  void               *data
.sp
The \fIextra\fP argument must be the result of studying a pattern with
PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the other two
options:
.sp
  (1) If \fIcallback\fP is NULL and \fIdata\fP is NULL, an internal 32K block
      on the machine stack is used.
.sp
  (2) If \fIcallback\fP is NULL and \fIdata\fP is not NULL, \fIdata\fP must be
      a valid JIT stack, the result of calling \fBpcre_jit_stack_alloc()\fP.
.sp
  (3) If \fIcallback\fP not NULL, it must point to a function that is called
      with \fIdata\fP as an argument at the start of matching, in order to
      set up a JIT stack. If the result is NULL, the internal 32K stack
      is used; otherwise the return value must be a valid JIT stack,
      the result of calling \fBpcre_jit_stack_alloc()\fP.
.sp
You may safely assign the same JIT stack to more than one pattern, as long as
they are all matched sequentially in the same thread. In a multithread
application, each thread must use its own JIT stack.
.P
Strictly speaking, even more is allowed. You can assign the same 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.
.P
This is a suggestion for how a typical multithreaded program might operate:
.sp
  During thread initalization
    thread_local_var = pcre_jit_stack_alloc(...)
.sp
  During thread exit
    pcre_jit_stack_free(thread_local_var)
.sp
  Use a one-line callback function
    return thread_local_var
.sp
All the functions described in this section do nothing if JIT is not available,
and \fBpcre_assign_jit_stack()\fP does nothing unless the \fBextra\fP argument
is non-NULL and points to a \fBpcre_extra\fP block that is the result of a
successful study with PCRE_STUDY_JIT_COMPILE.
.
.
.\" HTML <a name="stackfaq"></a>
.SH "JIT STACK FAQ"
.rs
.sp
(1) Why do we need JIT stacks?
.sp
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.
.P
(2) Why don't we simply allocate blocks of memory with \fBmalloc()\fP? 
.sp
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.
.P
(3) Who "owns" a JIT stack? 
.sp
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
\fBpcre_exec()\fP, (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.
.P
(4) When should a JIT stack be freed?
.sp
You can free a JIT stack at any time, as long as it will not be used by
\fBpcre_exec()\fP 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 \fIdo not\fP call
\fBpcre_exec()\fP with a pattern pointing to an already freed stack, as that
will cause SEGFAULT. (Also, do not free a stack currently used by
\fBpcre_exec()\fP 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.
.P
(5) Should I allocate/free a stack every time before/after calling
\fBpcre_exec()\fP?
.sp
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.
.P
(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? 
.sp
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.
.P
(7) This is too much of a headache. Isn't there any better solution for JIT
stack handling? 
.sp
No, thanks to Windows. If POSIX threads were used everywhere, we could throw
out this complicated API.
.
.
.SH "EXAMPLE CODE"
.rs
.sp
This is a single-threaded example that specifies a JIT stack without using a
callback.
.sp
  int rc;
  int ovector[30];
  pcre *re;
  pcre_extra *extra;
  pcre_jit_stack *jit_stack;
.sp
  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);
.sp
.
.
.SH "SEE ALSO"
.rs
.sp
\fBpcreapi\fP(3)
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel (FAQ by Zoltan Herczeg)
University Computing Service
Cambridge CB2 3QH, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 22 November 2011
Copyright (c) 1997-2011 University of Cambridge.
.fi
1	.TH PCREJIT 3
2	.SH NAME
3	PCRE - Perl-compatible regular expressions
4	.SH "PCRE JUST-IN-TIME COMPILER SUPPORT"
5	.rs
6	.sp
7	Just-in-time compiling is a heavyweight optimization that can greatly speed up
8	pattern matching. However, it comes at the cost of extra processing before the
9	match is performed. Therefore, it is of most benefit when the same pattern is
10	going to be matched many times. This does not necessarily mean many calls of
11	\fPpcre_exec()\fP; if the pattern is not anchored, matching attempts may take
12	place many times at various positions in the subject, even for a single call to
13	\fBpcre_exec()\fP. If the subject string is very long, it may still pay to use
14	JIT for one-off matches.
15	.P
16	JIT support applies only to the traditional matching function,
17	\fBpcre_exec()\fP. It does not apply when \fBpcre_dfa_exec()\fP is being used.
18	The code for this support was written by Zoltan Herczeg.
19	.
20	.
21	.SH "AVAILABILITY OF JIT SUPPORT"
22	.rs
23	.sp
24	JIT support is an optional feature of PCRE. The "configure" option --enable-jit
25	(or equivalent CMake option) must be set when PCRE is built if you want to use
26	JIT. The support is limited to the following hardware platforms:
27	.sp
28	ARM v5, v7, and Thumb2
29	Intel x86 32-bit and 64-bit
30	MIPS 32-bit
31	Power PC 32-bit and 64-bit (experimental)
32	.sp
33	The Power PC support is designated as experimental because it has not been
34	fully tested. If --enable-jit is set on an unsupported platform, compilation
35	fails.
36	.P
37	A program can tell if JIT support is available by calling \fBpcre_config()\fP
38	with the PCRE_CONFIG_JIT option. The result is 1 when JIT is available, and 0
39	otherwise. However, a simple program does not need to check this in order to
40	use JIT. The API is implemented in a way that falls back to the ordinary PCRE
41	code if JIT is not available.
42	.
43	.
44	.SH "SIMPLE USE OF JIT"
45	.rs
46	.sp
47	You have to do two things to make use of the JIT support in the simplest way:
48	.sp
49	(1) Call \fBpcre_study()\fP with the PCRE_STUDY_JIT_COMPILE option for
50	each compiled pattern, and pass the resulting \fBpcre_extra\fP block to
51	\fBpcre_exec()\fP.
52	.sp
53	(2) Use \fBpcre_free_study()\fP to free the \fBpcre_extra\fP block when it is
54	no longer needed instead of just freeing it yourself. This
55	ensures that any JIT data is also freed.
56	.sp
57	In some circumstances you may need to call additional functions. These are
58	described in the section entitled
59	.\" HTML <a href="#stackcontrol">
60	.\" </a>
61	"Controlling the JIT stack"
62	.\"
63	below.
64	.P
65	If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and no JIT
66	data is set up. Otherwise, the compiled pattern is passed to the JIT compiler,
67	which turns it into machine code that executes much faster than the normal
68	interpretive code. When \fBpcre_exec()\fP is passed a \fBpcre_extra\fP block
69	containing a pointer to JIT code, it obeys that instead of the normal code. The
70	result is identical, but the code runs much faster.
71	.P
72	There are some \fBpcre_exec()\fP options that are not supported for JIT
73	execution. There are also some pattern items that JIT cannot handle. Details
74	are given below. In both cases, execution automatically falls back to the
75	interpretive code.
76	.P
77	If the JIT compiler finds an unsupported item, no JIT data is generated. You
78	can find out if JIT execution is available after studying a pattern by calling
79	\fBpcre_fullinfo()\fP with the PCRE_INFO_JIT option. A result of 1 means that
80	JIT compilation was successful. A result of 0 means that JIT support is not
81	available, or the pattern was not studied with PCRE_STUDY_JIT_COMPILE, or the
82	JIT compiler was not able to handle the pattern.
83	.P
84	Once a pattern has been studied, with or without JIT, it can be used as many
85	times as you like for matching different subject strings.
86	.
87	.
88	.SH "UNSUPPORTED OPTIONS AND PATTERN ITEMS"
89	.rs
90	.sp
91	The only \fBpcre_exec()\fP options that are supported for JIT execution are
92	PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and
93	PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
94	supported.
95	.P
96	The unsupported pattern items are:
97	.sp
98	\eC match a single byte; not supported in UTF-8 mode
99	(?Cn) callouts
100	(*COMMIT) )
101	(*MARK) )
102	(*PRUNE) ) the backtracking control verbs
103	(*SKIP) )
104	(*THEN) )
105	.sp
106	Support for some of these may be added in future.
107	.
108	.
109	.SH "RETURN VALUES FROM JIT EXECUTION"
110	.rs
111	.sp
112	When a pattern is matched using JIT execution, the return values are the same
113	as those given by the interpretive \fBpcre_exec()\fP code, with the addition of
114	one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means that the memory used
115	for the JIT stack was insufficient. See
116	.\" HTML <a href="#stackcontrol">
117	.\" </a>
118	"Controlling the JIT stack"
119	.\"
120	below for a discussion of JIT stack usage. For compatibility with the
121	interpretive \fBpcre_exec()\fP code, no more than two-thirds of the
122	\fIovector\fP argument is used for passing back captured substrings.
123	.P
124	The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if searching a
125	very large pattern tree goes on for too long, as it is in the same circumstance
126	when JIT is not used, but the details of exactly what is counted are not the
127	same. The PCRE_ERROR_RECURSIONLIMIT error code is never returned by JIT
128	execution.
129	.
130	.
131	.SH "SAVING AND RESTORING COMPILED PATTERNS"
132	.rs
133	.sp
134	The code that is generated by the JIT compiler is architecture-specific, and is
135	also position dependent. For those reasons it cannot be saved (in a file or
136	database) and restored later like the bytecode and other data of a compiled
137	pattern. Saving and restoring compiled patterns is not something many people
138	do. More detail about this facility is given in the
139	.\" HREF
140	\fBpcreprecompile\fP
141	.\"
142	documentation. It should be possible to run \fBpcre_study()\fP on a saved and
143	restored pattern, and thereby recreate the JIT data, but because JIT
144	compilation uses significant resources, it is probably not worth doing this;
145	you might as well recompile the original pattern.
146	.
147	.
148	.\" HTML <a name="stackcontrol"></a>
149	.SH "CONTROLLING THE JIT STACK"
150	.rs
151	.sp
152	When the compiled JIT code runs, it needs a block of memory to use as a stack.
153	By default, it uses 32K on the machine stack. However, some large or
154	complicated patterns need more than this. The error PCRE_ERROR_JIT_STACKLIMIT
155	is given when there is not enough stack. Three functions are provided for
156	managing blocks of memory for use as JIT stacks. There is further discussion
157	about the use of JIT stacks in the section entitled
158	.\" HTML <a href="#stackcontrol">
159	.\" </a>
160	"JIT stack FAQ"
161	.\"
162	below.
163	.P
164	The \fBpcre_jit_stack_alloc()\fP function creates a JIT stack. Its arguments
165	are a starting size and a maximum size, and it returns a pointer to an opaque
166	structure of type \fBpcre_jit_stack\fP, or NULL if there is an error. The
167	\fBpcre_jit_stack_free()\fP function can be used to free a stack that is no
168	longer needed. (For the technically minded: the address space is allocated by
169	mmap or VirtualAlloc.)
170	.P
171	JIT uses far less memory for recursion than the interpretive code,
172	and a maximum stack size of 512K to 1M should be more than enough for any
173	pattern.
174	.P
175	The \fBpcre_assign_jit_stack()\fP function specifies which stack JIT code
176	should use. Its arguments are as follows:
177	.sp
178	pcre_extra *extra
179	pcre_jit_callback callback
180	void *data
181	.sp
182	The \fIextra\fP argument must be the result of studying a pattern with
183	PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the other two
184	options:
185	.sp
186	(1) If \fIcallback\fP is NULL and \fIdata\fP is NULL, an internal 32K block
187	on the machine stack is used.
188	.sp
189	(2) If \fIcallback\fP is NULL and \fIdata\fP is not NULL, \fIdata\fP must be
190	a valid JIT stack, the result of calling \fBpcre_jit_stack_alloc()\fP.
191	.sp
192	(3) If \fIcallback\fP not NULL, it must point to a function that is called
193	with \fIdata\fP as an argument at the start of matching, in order to
194	set up a JIT stack. If the result is NULL, the internal 32K stack
195	is used; otherwise the return value must be a valid JIT stack,
196	the result of calling \fBpcre_jit_stack_alloc()\fP.
197	.sp
198	You may safely assign the same JIT stack to more than one pattern, as long as
199	they are all matched sequentially in the same thread. In a multithread
200	application, each thread must use its own JIT stack.
201	.P
202	Strictly speaking, even more is allowed. You can assign the same stack to any
203	number of patterns as long as they are not used for matching by multiple
204	threads at the same time. For example, you can assign the same stack to all
205	compiled patterns, and use a global mutex in the callback to wait until the
206	stack is available for use. However, this is an inefficient solution, and
207	not recommended.
208	.P
209	This is a suggestion for how a typical multithreaded program might operate:
210	.sp
211	During thread initalization
212	thread_local_var = pcre_jit_stack_alloc(...)
213	.sp
214	During thread exit
215	pcre_jit_stack_free(thread_local_var)
216	.sp
217	Use a one-line callback function
218	return thread_local_var
219	.sp
220	All the functions described in this section do nothing if JIT is not available,
221	and \fBpcre_assign_jit_stack()\fP does nothing unless the \fBextra\fP argument
222	is non-NULL and points to a \fBpcre_extra\fP block that is the result of a
223	successful study with PCRE_STUDY_JIT_COMPILE.
224	.
225	.
226	.\" HTML <a name="stackfaq"></a>
227	.SH "JIT STACK FAQ"
228	.rs
229	.sp
230	(1) Why do we need JIT stacks?
231	.sp
232	PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack where
233	the local data of the current node is pushed before checking its child nodes.
234	Allocating real machine stack on some platforms is difficult. For example, the
235	stack chain needs to be updated every time if we extend the stack on PowerPC.
236	Although it is possible, its updating time overhead decreases performance. So
237	we do the recursion in memory.
238	.P
239	(2) Why don't we simply allocate blocks of memory with \fBmalloc()\fP?
240	.sp
241	Modern operating systems have a nice feature: they can reserve an address space
242	instead of allocating memory. We can safely allocate memory pages inside this
243	address space, so the stack could grow without moving memory data (this is
244	important because of pointers). Thus we can allocate 1M address space, and use
245	only a single memory page (usually 4K) if that is enough. However, we can still
246	grow up to 1M anytime if needed.
247	.P
248	(3) Who "owns" a JIT stack?
249	.sp
250	The owner of the stack is the user program, not the JIT studied pattern or
251	anything else. The user program must ensure that if a stack is used by
252	\fBpcre_exec()\fP, (that is, it is assigned to the pattern currently running),
253	that stack must not be used by any other threads (to avoid overwriting the same
254	memory area). The best practice for multithreaded programs is to allocate a
255	stack for each thread, and return this stack through the JIT callback function.
256	.P
257	(4) When should a JIT stack be freed?
258	.sp
259	You can free a JIT stack at any time, as long as it will not be used by
260	\fBpcre_exec()\fP again. When you assign the stack to a pattern, only a pointer
261	is set. There is no reference counting or any other magic. You can free the
262	patterns and stacks in any order, anytime. Just \fIdo not\fP call
263	\fBpcre_exec()\fP with a pattern pointing to an already freed stack, as that
264	will cause SEGFAULT. (Also, do not free a stack currently used by
265	\fBpcre_exec()\fP in another thread). You can also replace the stack for a
266	pattern at any time. You can even free the previous stack before assigning a
267	replacement.
268	.P
269	(5) Should I allocate/free a stack every time before/after calling
270	\fBpcre_exec()\fP?
271	.sp
272	No, because this is too costly in terms of resources. However, you could
273	implement some clever idea which release the stack if it is not used in let's
274	say two minutes. The JIT callback can help to achive this without keeping a
275	list of the currently JIT studied patterns.
276	.P
277	(6) OK, the stack is for long term memory allocation. But what happens if a
278	pattern causes stack overflow with a stack of 1M? Is that 1M kept until the
279	stack is freed?
280	.sp
281	Especially on embedded sytems, it might be a good idea to release
282	memory sometimes without freeing the stack. There is no API for this at the
283	moment. Probably a function call which returns with the currently allocated
284	memory for any stack and another which allows releasing memory (shrinking the
285	stack) would be a good idea if someone needs this.
286	.P
287	(7) This is too much of a headache. Isn't there any better solution for JIT
288	stack handling?
289	.sp
290	No, thanks to Windows. If POSIX threads were used everywhere, we could throw
291	out this complicated API.
292	.
293	.
294	.SH "EXAMPLE CODE"
295	.rs
296	.sp
297	This is a single-threaded example that specifies a JIT stack without using a
298	callback.
299	.sp
300	int rc;
301	int ovector[30];
302	pcre *re;
303	pcre_extra *extra;
304	pcre_jit_stack *jit_stack;
305	.sp
306	re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
307	/* Check for errors */
308	extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
309	jit_stack = pcre_jit_stack_alloc(321024, 5121024);
310	/* Check for error (NULL) */
311	pcre_assign_jit_stack(extra, NULL, jit_stack);
312	rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
313	/* Check results */
314	pcre_free(re);
315	pcre_free_study(extra);
316	pcre_jit_stack_free(jit_stack);
317	.sp
318	.
319	.
320	.SH "SEE ALSO"
321	.rs
322	.sp
323	\fBpcreapi\fP(3)
324	.
325	.
326	.SH AUTHOR
327	.rs
328	.sp
329	.nf
330	Philip Hazel (FAQ by Zoltan Herczeg)
331	University Computing Service
332	Cambridge CB2 3QH, England.
333	.fi
334	.
335	.
336	.SH REVISION
337	.rs
338	.sp
339	.nf
340	Last updated: 22 November 2011
341	Copyright (c) 1997-2011 University of Cambridge.
342	.fi