doc/pcre2serialize.3

.TH PCRE2SERIALIZE 3 "27 June 2018" "PCRE2 10.32"
.SH NAME
PCRE2 - Perl-compatible regular expressions (revised API)
.SH "SAVING AND RE-USING PRECOMPILED PCRE2 PATTERNS"
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.B int32_t pcre2_serialize_decode(pcre2_code **\fIcodes\fP,
.B "  int32_t \fInumber_of_codes\fP, const uint8_t *\fIbytes\fP,"
.B "  pcre2_general_context *\fIgcontext\fP);"
.sp
.B int32_t pcre2_serialize_encode(const pcre2_code **\fIcodes\fP,
.B "  int32_t \fInumber_of_codes\fP, uint8_t **\fIserialized_bytes\fP,"
.B "  PCRE2_SIZE *\fIserialized_size\fP, pcre2_general_context *\fIgcontext\fP);"
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.B void pcre2_serialize_free(uint8_t *\fIbytes\fP);
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.B int32_t pcre2_serialize_get_number_of_codes(const uint8_t *\fIbytes\fP);
.fi
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If you are running an application that uses a large number of regular
expression patterns, it may be useful to store them in a precompiled form
instead of having to compile them every time the application is run. However,
if you are using the just-in-time optimization feature, it is not possible to
save and reload the JIT data, because it is position-dependent. The host on
which the patterns are reloaded must be running the same version of PCRE2, with
the same code unit width, and must also have the same endianness, pointer width
and PCRE2_SIZE type. For example, patterns compiled on a 32-bit system using
PCRE2's 16-bit library cannot be reloaded on a 64-bit system, nor can they be
reloaded using the 8-bit library.
.P
Note that "serialization" in PCRE2 does not convert compiled patterns to an
abstract format like Java or .NET serialization. The serialized output is
really just a bytecode dump, which is why it can only be reloaded in the same
environment as the one that created it. Hence the restrictions mentioned above.
Applications that are not statically linked with a fixed version of PCRE2 must
be prepared to recompile patterns from their sources, in order to be immune to
PCRE2 upgrades.
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.SH "SECURITY CONCERNS"
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The facility for saving and restoring compiled patterns is intended for use
within individual applications. As such, the data supplied to
\fBpcre2_serialize_decode()\fP is expected to be trusted data, not data from
arbitrary external sources. There is only some simple consistency checking, not
complete validation of what is being re-loaded. Corrupted data may cause
undefined results. For example, if the length field of a pattern in the
serialized data is corrupted, the deserializing code may read beyond the end of
the byte stream that is passed to it.
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.SH "SAVING COMPILED PATTERNS"
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Before compiled patterns can be saved they must be serialized, which in PCRE2
means converting the pattern to a stream of bytes. A single byte stream may
contain any number of compiled patterns, but they must all use the same
character tables. A single copy of the tables is included in the byte stream
(its size is 1088 bytes). For more details of character tables, see the
.\" HTML <a href="pcre2api.html#localesupport">
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.\" HREF
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documentation.
.P
The function \fBpcre2_serialize_encode()\fP creates a serialized byte stream
from a list of compiled patterns. Its first two arguments specify the list,
being a pointer to a vector of pointers to compiled patterns, and the length of
the vector. The third and fourth arguments point to variables which are set to
point to the created byte stream and its length, respectively. The final
argument is a pointer to a general context, which can be used to specify custom
memory mangagement functions. If this argument is NULL, \fBmalloc()\fP is used
to obtain memory for the byte stream. The yield of the function is the number
of serialized patterns, or one of the following negative error codes:
.sp
  PCRE2_ERROR_BADDATA      the number of patterns is zero or less
  PCRE2_ERROR_BADMAGIC     mismatch of id bytes in one of the patterns
  PCRE2_ERROR_MEMORY       memory allocation failed
  PCRE2_ERROR_MIXEDTABLES  the patterns do not all use the same tables
  PCRE2_ERROR_NULL         the 1st, 3rd, or 4th argument is NULL
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PCRE2_ERROR_BADMAGIC means either that a pattern's code has been corrupted, or
that a slot in the vector does not point to a compiled pattern.
.P
Once a set of patterns has been serialized you can save the data in any
appropriate manner. Here is sample code that compiles two patterns and writes
them to a file. It assumes that the variable \fIfd\fP refers to a file that is
open for output. The error checking that should be present in a real
application has been omitted for simplicity.
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  int errorcode;
  uint8_t *bytes;
  PCRE2_SIZE erroroffset;
  PCRE2_SIZE bytescount;
  pcre2_code *list_of_codes[2];
  list_of_codes[0] = pcre2_compile("first pattern",
    PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL);
  list_of_codes[1] = pcre2_compile("second pattern",
    PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL);
  errorcode = pcre2_serialize_encode(list_of_codes, 2, &bytes,
    &bytescount, NULL);
  errorcode = fwrite(bytes, 1, bytescount, fd);
.sp
Note that the serialized data is binary data that may contain any of the 256
possible byte values. On systems that make a distinction between binary and
non-binary data, be sure that the file is opened for binary output.
.P
Serializing a set of patterns leaves the original data untouched, so they can
still be used for matching. Their memory must eventually be freed in the usual
way by calling \fBpcre2_code_free()\fP. When you have finished with the byte
stream, it too must be freed by calling \fBpcre2_serialize_free()\fP. If this
function is called with a NULL argument, it returns immediately without doing
anything.
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.SH "RE-USING PRECOMPILED PATTERNS"
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In order to re-use a set of saved patterns you must first make the serialized
byte stream available in main memory (for example, by reading from a file). The
management of this memory block is up to the application. You can use the
\fBpcre2_serialize_get_number_of_codes()\fP function to find out how many
compiled patterns are in the serialized data without actually decoding the
patterns:
.sp
  uint8_t *bytes = <serialized data>;
  int32_t number_of_codes = pcre2_serialize_get_number_of_codes(bytes);
.sp
The \fBpcre2_serialize_decode()\fP function reads a byte stream and recreates
the compiled patterns in new memory blocks, setting pointers to them in a
vector. The first two arguments are a pointer to a suitable vector and its
length, and the third argument points to a byte stream. The final argument is a
pointer to a general context, which can be used to specify custom memory
mangagement functions for the decoded patterns. If this argument is NULL,
\fBmalloc()\fP and \fBfree()\fP are used. After deserialization, the byte
stream is no longer needed and can be discarded.
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  pcre2_code *list_of_codes[2];
  uint8_t *bytes = <serialized data>;
  int32_t number_of_codes =
    pcre2_serialize_decode(list_of_codes, 2, bytes, NULL);
.sp
If the vector is not large enough for all the patterns in the byte stream, it
is filled with those that fit, and the remainder are ignored. The yield of the
function is the number of decoded patterns, or one of the following negative
error codes:
.sp
  PCRE2_ERROR_BADDATA    second argument is zero or less
  PCRE2_ERROR_BADMAGIC   mismatch of id bytes in the data
  PCRE2_ERROR_BADMODE    mismatch of code unit size or PCRE2 version
  PCRE2_ERROR_BADSERIALIZEDDATA  other sanity check failure
  PCRE2_ERROR_MEMORY     memory allocation failed
  PCRE2_ERROR_NULL       first or third argument is NULL
.sp
PCRE2_ERROR_BADMAGIC may mean that the data is corrupt, or that it was compiled
on a system with different endianness.
.P
Decoded patterns can be used for matching in the usual way, and must be freed
by calling \fBpcre2_code_free()\fP. However, be aware that there is a potential
race issue if you are using multiple patterns that were decoded from a single
byte stream in a multithreaded application. A single copy of the character
tables is used by all the decoded patterns and a reference count is used to
arrange for its memory to be automatically freed when the last pattern is
freed, but there is no locking on this reference count. Therefore, if you want
to call \fBpcre2_code_free()\fP for these patterns in different threads, you
must arrange your own locking, and ensure that \fBpcre2_code_free()\fP cannot
be called by two threads at the same time.
.P
If a pattern was processed by \fBpcre2_jit_compile()\fP before being
serialized, the JIT data is discarded and so is no longer available after a
save/restore cycle. You can, however, process a restored pattern with
\fBpcre2_jit_compile()\fP if you wish.
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.SH AUTHOR
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Philip Hazel
University Computing Service
Cambridge, England.
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.SH REVISION
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Last updated: 27 June 2018
Copyright (c) 1997-2018 University of Cambridge.
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