CBOR-XS/XS.pm

=head1 NAME

CBOR::XS - Concise Binary Object Representation (CBOR, RFC7049)

=encoding utf-8

=head1 SYNOPSIS

 use CBOR::XS;

 $binary_cbor_data = encode_cbor $perl_value;
 $perl_value       = decode_cbor $binary_cbor_data;

 # OO-interface

 $coder = CBOR::XS->new;
 $binary_cbor_data = $coder->encode ($perl_value);
 $perl_value       = $coder->decode ($binary_cbor_data);

 # prefix decoding

 my $many_cbor_strings = ...;
 while (length $many_cbor_strings) {
    my ($data, $length) = $cbor->decode_prefix ($many_cbor_strings);
    # data was decoded
    substr $many_cbor_strings, 0, $length, ""; # remove decoded cbor string
 }

=head1 DESCRIPTION

WARNING! THIS IS A PRE-ALPHA RELEASE! IT WILL CRASH, CORRUPT YOUR DATA
AND EAT YOUR CHILDREN! (Actually, apart from being untested and a bit
feature-limited, it might already be useful).

This module converts Perl data structures to the Concise Binary Object
Representation (CBOR) and vice versa. CBOR is a fast binary serialisation
format that aims to use a superset of the JSON data model, i.e. when you
can represent something in JSON, you should be able to represent it in
CBOR.

This makes it a faster and more compact binary alternative to JSON, with
the added ability of supporting serialising of perl objects.

The primary goal of this module is to be I<correct> and the secondary goal
is to be I<fast>. To reach the latter goal it was written in C.

See MAPPING, below, on how CBOR::XS maps perl values to CBOR values and
vice versa.

=cut

package CBOR::XS;

use common::sense;

our $VERSION = 0.03;
our @ISA = qw(Exporter);

our @EXPORT = qw(encode_cbor decode_cbor);

use Exporter;
use XSLoader;

use Types::Serialiser;

our $MAGIC = "\xd9\xd9\xf7";

=head1 FUNCTIONAL INTERFACE

The following convenience methods are provided by this module. They are
exported by default:

=over 4

=item $cbor_data = encode_cbor $perl_scalar

Converts the given Perl data structure to CBOR representation. Croaks on
error.

=item $perl_scalar = decode_cbor $cbor_data

The opposite of C<encode_cbor>: expects a valid CBOR string to parse,
returning the resulting perl scalar. Croaks on error.

=back


=head1 OBJECT-ORIENTED INTERFACE

The object oriented interface lets you configure your own encoding or
decoding style, within the limits of supported formats.

=over 4

=item $cbor = new CBOR::XS

Creates a new CBOR::XS object that can be used to de/encode CBOR
strings. All boolean flags described below are by default I<disabled>.

The mutators for flags all return the CBOR object again and thus calls can
be chained:

#TODO
   my $cbor = CBOR::XS->new->encode ({a => [1,2]});

=item $cbor = $cbor->max_depth ([$maximum_nesting_depth])

=item $max_depth = $cbor->get_max_depth

Sets the maximum nesting level (default C<512>) accepted while encoding
or decoding. If a higher nesting level is detected in CBOR data or a Perl
data structure, then the encoder and decoder will stop and croak at that
point.

Nesting level is defined by number of hash- or arrayrefs that the encoder
needs to traverse to reach a given point or the number of C<{> or C<[>
characters without their matching closing parenthesis crossed to reach a
given character in a string.

Setting the maximum depth to one disallows any nesting, so that ensures
that the object is only a single hash/object or array.

If no argument is given, the highest possible setting will be used, which
is rarely useful.

Note that nesting is implemented by recursion in C. The default value has
been chosen to be as large as typical operating systems allow without
crashing.

See SECURITY CONSIDERATIONS, below, for more info on why this is useful.

=item $cbor = $cbor->max_size ([$maximum_string_size])

=item $max_size = $cbor->get_max_size

Set the maximum length a CBOR string may have (in bytes) where decoding
is being attempted. The default is C<0>, meaning no limit. When C<decode>
is called on a string that is longer then this many bytes, it will not
attempt to decode the string but throw an exception. This setting has no
effect on C<encode> (yet).

If no argument is given, the limit check will be deactivated (same as when
C<0> is specified).

See SECURITY CONSIDERATIONS, below, for more info on why this is useful.

=item $cbor_data = $cbor->encode ($perl_scalar)

Converts the given Perl data structure (a scalar value) to its CBOR
representation.

=item $perl_scalar = $cbor->decode ($cbor_data)

The opposite of C<encode>: expects CBOR data and tries to parse it,
returning the resulting simple scalar or reference. Croaks on error.

=item ($perl_scalar, $octets) = $cbor->decode_prefix ($cbor_data)

This works like the C<decode> method, but instead of raising an exception
when there is trailing garbage after the CBOR string, it will silently
stop parsing there and return the number of characters consumed so far.

This is useful if your CBOR texts are not delimited by an outer protocol
and you need to know where the first CBOR string ends amd the next one
starts.

   CBOR::XS->new->decode_prefix ("......")
   => ("...", 3)

=back


=head1 MAPPING

This section describes how CBOR::XS maps Perl values to CBOR values and
vice versa. These mappings are designed to "do the right thing" in most
circumstances automatically, preserving round-tripping characteristics
(what you put in comes out as something equivalent).

For the more enlightened: note that in the following descriptions,
lowercase I<perl> refers to the Perl interpreter, while uppercase I<Perl>
refers to the abstract Perl language itself.


=head2 CBOR -> PERL

=over 4

=item integers

CBOR integers become (numeric) perl scalars. On perls without 64 bit
support, 64 bit integers will be truncated or otherwise corrupted.

=item byte strings

Byte strings will become octet strings in Perl (the byte values 0..255
will simply become characters of the same value in Perl).

=item UTF-8 strings

UTF-8 strings in CBOR will be decoded, i.e. the UTF-8 octets will be
decoded into proper Unicode code points. At the moment, the validity of
the UTF-8 octets will not be validated - corrupt input will result in
corrupted Perl strings.

=item arrays, maps

CBOR arrays and CBOR maps will be converted into references to a Perl
array or hash, respectively. The keys of the map will be stringified
during this process.

=item null

CBOR null becomes C<undef> in Perl.

=item true, false, undefined

These CBOR values become C<Types:Serialiser::true>,
C<Types:Serialiser::false> and C<Types::Serialiser::error>,
respectively. They are overloaded to act almost exactly like the numbers
C<1> and C<0> (for true and false) or to throw an exception on access (for
error). See the L<Types::Serialiser> manpage for details.

=item CBOR tag 256 (perl object)

The tag value C<256> (TODO: pending iana registration) will be used to
deserialise a Perl object.

TODO For this to work, the class must be loaded and must have a
C<FROM_CBOR> method. The decoder will then call the C<FROM_CBOR> method
with the constructor arguments provided by the C<TO_CBOR> method (see
below).

The C<FROM_CBOR> method must return a single value that will then be used
as the deserialised value.

=item CBOR tag 55799 (magic header)

The tag 55799 is ignored (this tag implements the magic header).

=item other CBOR tags

Tagged items consists of a numeric tag and another CBOR value. Tags not
handled internally are currently converted into a L<CBOR::XS::Tagged>
object, which is simply a blessed array reference consisting of the
numeric tag value followed by the (decoded) CBOR value.

In the future, support for user-supplied conversions might get added.

=item anything else

Anything else (e.g. unsupported simple values) will raise a decoding
error.

=back


=head2 PERL -> CBOR

The mapping from Perl to CBOR is slightly more difficult, as Perl is a
truly typeless language, so we can only guess which CBOR type is meant by
a Perl value.

=over 4

=item hash references

Perl hash references become CBOR maps. As there is no inherent ordering in
hash keys (or CBOR maps), they will usually be encoded in a pseudo-random
order.

Currently, tied hashes will use the indefinite-length format, while normal
hashes will use the fixed-length format.

=item array references

Perl array references become fixed-length CBOR arrays.

=item other references

Other unblessed references are generally not allowed and will cause an
exception to be thrown, except for references to the integers C<0> and
C<1>, which get turned into false and true in CBOR.

=item CBOR::XS::Tagged objects

Objects of this type must be arrays consisting of a single C<[tag, value]>
pair. The (numerical) tag will be encoded as a CBOR tag, the value will be
encoded as appropriate for the value.

=item Types::Serialiser::true, Types::Serialiser::false, Types::Serialiser::error

These special values become CBOR true, CBOR false and CBOR undefined
values, respectively. You can also use C<\1>, C<\0> and C<\undef> directly
if you want.

=item blessed objects

Other blessed objects currently need to have a C<TO_CBOR> method. It
will be called on every object that is being serialised, and must return
something that can be encoded in CBOR.

=item simple scalars

TODO
Simple Perl scalars (any scalar that is not a reference) are the most
difficult objects to encode: CBOR::XS will encode undefined scalars as
CBOR null values, scalars that have last been used in a string context
before encoding as CBOR strings, and anything else as number value:

   # dump as number
   encode_cbor [2]                      # yields [2]
   encode_cbor [-3.0e17]                # yields [-3e+17]
   my $value = 5; encode_cbor [$value]  # yields [5]

   # used as string, so dump as string
   print $value;
   encode_cbor [$value]                 # yields ["5"]

   # undef becomes null
   encode_cbor [undef]                  # yields [null]

You can force the type to be a CBOR string by stringifying it:

   my $x = 3.1; # some variable containing a number
   "$x";        # stringified
   $x .= "";    # another, more awkward way to stringify
   print $x;    # perl does it for you, too, quite often

You can force the type to be a CBOR number by numifying it:

   my $x = "3"; # some variable containing a string
   $x += 0;     # numify it, ensuring it will be dumped as a number
   $x *= 1;     # same thing, the choice is yours.

You can not currently force the type in other, less obscure, ways. Tell me
if you need this capability (but don't forget to explain why it's needed
:).

Perl values that seem to be integers generally use the shortest possible
representation. Floating-point values will use either the IEEE single
format if possible without loss of precision, otherwise the IEEE double
format will be used. Perls that use formats other than IEEE double to
represent numerical values are supported, but might suffer loss of
precision.

=back


=head2 MAGIC HEADER

There is no way to distinguish CBOR from other formats
programmatically. To make it easier to distinguish CBOR from other
formats, the CBOR specification has a special "magic string" that can be
prepended to any CBOR string without changing it's meaning.

This string is available as C<$CBOR::XS::MAGIC>. This module does not
prepend this string tot he CBOR data it generates, but it will ignroe it
if present, so users can prepend this string as a "file type" indicator as
required.


=head2 CBOR and JSON

CBOR is supposed to implement a superset of the JSON data model, and is,
with some coercion, able to represent all JSON texts (something that other
"binary JSON" formats such as BSON generally do not support).

CBOR implements some extra hints and support for JSON interoperability,
and the spec offers further guidance for conversion between CBOR and
JSON. None of this is currently implemented in CBOR, and the guidelines
in the spec do not result in correct round-tripping of data. If JSON
interoperability is improved in the future, then the goal will be to
ensure that decoded JSON data will round-trip encoding and decoding to
CBOR intact.


=head1 SECURITY CONSIDERATIONS

When you are using CBOR in a protocol, talking to untrusted potentially
hostile creatures requires relatively few measures.

First of all, your CBOR decoder should be secure, that is, should not have
any buffer overflows. Obviously, this module should ensure that and I am
trying hard on making that true, but you never know.

Second, you need to avoid resource-starving attacks. That means you should
limit the size of CBOR data you accept, or make sure then when your
resources run out, that's just fine (e.g. by using a separate process that
can crash safely). The size of a CBOR string in octets is usually a good
indication of the size of the resources required to decode it into a Perl
structure. While CBOR::XS can check the size of the CBOR text, it might be
too late when you already have it in memory, so you might want to check
the size before you accept the string.

Third, CBOR::XS recurses using the C stack when decoding objects and
arrays. The C stack is a limited resource: for instance, on my amd64
machine with 8MB of stack size I can decode around 180k nested arrays but
only 14k nested CBOR objects (due to perl itself recursing deeply on croak
to free the temporary). If that is exceeded, the program crashes. To be
conservative, the default nesting limit is set to 512. If your process
has a smaller stack, you should adjust this setting accordingly with the
C<max_depth> method.

Something else could bomb you, too, that I forgot to think of. In that
case, you get to keep the pieces. I am always open for hints, though...

Also keep in mind that CBOR::XS might leak contents of your Perl data
structures in its error messages, so when you serialise sensitive
information you might want to make sure that exceptions thrown by CBOR::XS
will not end up in front of untrusted eyes.

=head1 CBOR IMPLEMENTATION NOTES

This section contains some random implementation notes. They do not
describe guaranteed behaviour, but merely behaviour as-is implemented
right now.

64 bit integers are only properly decoded when Perl was built with 64 bit
support.

Strings and arrays are encoded with a definite length. Hashes as well,
unless they are tied (or otherwise magical).

Only the double data type is supported for NV data types - when Perl uses
long double to represent floating point values, they might not be encoded
properly. Half precision types are accepted, but not encoded.

Strict mode and canonical mode are not implemented.


=head1 THREADS

This module is I<not> guaranteed to be thread safe and there are no
plans to change this until Perl gets thread support (as opposed to the
horribly slow so-called "threads" which are simply slow and bloated
process simulations - use fork, it's I<much> faster, cheaper, better).

(It might actually work, but you have been warned).


=head1 BUGS

While the goal of this module is to be correct, that unfortunately does
not mean it's bug-free, only that I think its design is bug-free. If you
keep reporting bugs they will be fixed swiftly, though.

Please refrain from using rt.cpan.org or any other bug reporting
service. I put the contact address into my modules for a reason.

=cut

XSLoader::load "CBOR::XS", $VERSION;

=head1 SEE ALSO

The L<JSON> and L<JSON::XS> modules that do similar, but human-readable,
serialisation.

The L<Types::Serialiser> module provides the data model for true, false
and error values.

=head1 AUTHOR

 Marc Lehmann <schmorp@schmorp.de>
 http://home.schmorp.de/

=cut

1

Revision:	1.6
Committed:	Sun Oct 27 20:40:25 2013 UTC (10 years, 6 months ago) by root
Branch:	MAIN
Changes since 1.5:	+54 -39 lines
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.1	=head1 NAME
2
3			CBOR::XS - Concise Binary Object Representation (CBOR, RFC7049)
4
5			=encoding utf-8
6
7			=head1 SYNOPSIS
8
9			use CBOR::XS;
10
11			$binary_cbor_data = encode_cbor $perl_value;
12			$perl_value = decode_cbor $binary_cbor_data;
13
14			# OO-interface
15
16			$coder = CBOR::XS->new;
17	root	1.6	$binary_cbor_data = $coder->encode ($perl_value);
18			$perl_value = $coder->decode ($binary_cbor_data);
19
20			# prefix decoding
21
22			my $many_cbor_strings = ...;
23			while (length $many_cbor_strings) {
24			my ($data, $length) = $cbor->decode_prefix ($many_cbor_strings);
25			# data was decoded
26			substr $many_cbor_strings, 0, $length, ""; # remove decoded cbor string
27			}
28	root	1.1
29			=head1 DESCRIPTION
30
31	root	1.5	WARNING! THIS IS A PRE-ALPHA RELEASE! IT WILL CRASH, CORRUPT YOUR DATA
32			AND EAT YOUR CHILDREN! (Actually, apart from being untested and a bit
33			feature-limited, it might already be useful).
34
35			This module converts Perl data structures to the Concise Binary Object
36			Representation (CBOR) and vice versa. CBOR is a fast binary serialisation
37			format that aims to use a superset of the JSON data model, i.e. when you
38			can represent something in JSON, you should be able to represent it in
39			CBOR.
40	root	1.1
41	root	1.6	This makes it a faster and more compact binary alternative to JSON, with
42			the added ability of supporting serialising of perl objects.
43	root	1.5
44			The primary goal of this module is to be I<correct> and the secondary goal
45			is to be I<fast>. To reach the latter goal it was written in C.
46	root	1.1
47			See MAPPING, below, on how CBOR::XS maps perl values to CBOR values and
48			vice versa.
49
50			=cut
51
52			package CBOR::XS;
53
54			use common::sense;
55
56	root	1.5	our $VERSION = 0.03;
57	root	1.1	our @ISA = qw(Exporter);
58
59			our @EXPORT = qw(encode_cbor decode_cbor);
60
61			use Exporter;
62			use XSLoader;
63
64	root	1.6	use Types::Serialiser;
65
66	root	1.3	our $MAGIC = "\xd9\xd9\xf7";
67
68	root	1.1	=head1 FUNCTIONAL INTERFACE
69
70			The following convenience methods are provided by this module. They are
71			exported by default:
72
73			=over 4
74
75			=item $cbor_data = encode_cbor $perl_scalar
76
77			Converts the given Perl data structure to CBOR representation. Croaks on
78			error.
79
80			=item $perl_scalar = decode_cbor $cbor_data
81
82			The opposite of C<encode_cbor>: expects a valid CBOR string to parse,
83			returning the resulting perl scalar. Croaks on error.
84
85			=back
86
87
88			=head1 OBJECT-ORIENTED INTERFACE
89
90			The object oriented interface lets you configure your own encoding or
91			decoding style, within the limits of supported formats.
92
93			=over 4
94
95			=item $cbor = new CBOR::XS
96
97			Creates a new CBOR::XS object that can be used to de/encode CBOR
98			strings. All boolean flags described below are by default I<disabled>.
99
100			The mutators for flags all return the CBOR object again and thus calls can
101			be chained:
102
103			#TODO
104			my $cbor = CBOR::XS->new->encode ({a => [1,2]});
105
106			=item $cbor = $cbor->max_depth ([$maximum_nesting_depth])
107
108			=item $max_depth = $cbor->get_max_depth
109
110			Sets the maximum nesting level (default C<512>) accepted while encoding
111			or decoding. If a higher nesting level is detected in CBOR data or a Perl
112			data structure, then the encoder and decoder will stop and croak at that
113			point.
114
115			Nesting level is defined by number of hash- or arrayrefs that the encoder
116			needs to traverse to reach a given point or the number of C<{> or C<[>
117			characters without their matching closing parenthesis crossed to reach a
118			given character in a string.
119
120			Setting the maximum depth to one disallows any nesting, so that ensures
121			that the object is only a single hash/object or array.
122
123			If no argument is given, the highest possible setting will be used, which
124			is rarely useful.
125
126			Note that nesting is implemented by recursion in C. The default value has
127			been chosen to be as large as typical operating systems allow without
128			crashing.
129
130			See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
131
132			=item $cbor = $cbor->max_size ([$maximum_string_size])
133
134			=item $max_size = $cbor->get_max_size
135
136			Set the maximum length a CBOR string may have (in bytes) where decoding
137			is being attempted. The default is C<0>, meaning no limit. When C<decode>
138			is called on a string that is longer then this many bytes, it will not
139			attempt to decode the string but throw an exception. This setting has no
140			effect on C<encode> (yet).
141
142			If no argument is given, the limit check will be deactivated (same as when
143			C<0> is specified).
144
145			See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
146
147			=item $cbor_data = $cbor->encode ($perl_scalar)
148
149			Converts the given Perl data structure (a scalar value) to its CBOR
150			representation.
151
152			=item $perl_scalar = $cbor->decode ($cbor_data)
153
154			The opposite of C<encode>: expects CBOR data and tries to parse it,
155			returning the resulting simple scalar or reference. Croaks on error.
156
157			=item ($perl_scalar, $octets) = $cbor->decode_prefix ($cbor_data)
158
159			This works like the C<decode> method, but instead of raising an exception
160			when there is trailing garbage after the CBOR string, it will silently
161			stop parsing there and return the number of characters consumed so far.
162
163			This is useful if your CBOR texts are not delimited by an outer protocol
164			and you need to know where the first CBOR string ends amd the next one
165			starts.
166
167			CBOR::XS->new->decode_prefix ("......")
168			=> ("...", 3)
169
170			=back
171
172
173			=head1 MAPPING
174
175			This section describes how CBOR::XS maps Perl values to CBOR values and
176			vice versa. These mappings are designed to "do the right thing" in most
177			circumstances automatically, preserving round-tripping characteristics
178			(what you put in comes out as something equivalent).
179
180			For the more enlightened: note that in the following descriptions,
181			lowercase I<perl> refers to the Perl interpreter, while uppercase I<Perl>
182			refers to the abstract Perl language itself.
183
184
185			=head2 CBOR -> PERL
186
187			=over 4
188
189	root	1.4	=item integers
190
191			CBOR integers become (numeric) perl scalars. On perls without 64 bit
192			support, 64 bit integers will be truncated or otherwise corrupted.
193
194			=item byte strings
195
196			Byte strings will become octet strings in Perl (the byte values 0..255
197			will simply become characters of the same value in Perl).
198
199			=item UTF-8 strings
200
201			UTF-8 strings in CBOR will be decoded, i.e. the UTF-8 octets will be
202			decoded into proper Unicode code points. At the moment, the validity of
203			the UTF-8 octets will not be validated - corrupt input will result in
204			corrupted Perl strings.
205
206			=item arrays, maps
207
208			CBOR arrays and CBOR maps will be converted into references to a Perl
209			array or hash, respectively. The keys of the map will be stringified
210			during this process.
211
212	root	1.6	=item null
213
214			CBOR null becomes C<undef> in Perl.
215
216			=item true, false, undefined
217	root	1.1
218	root	1.6	These CBOR values become C<Types:Serialiser::true>,
219			C<Types:Serialiser::false> and C<Types::Serialiser::error>,
220	root	1.1	respectively. They are overloaded to act almost exactly like the numbers
221	root	1.6	C<1> and C<0> (for true and false) or to throw an exception on access (for
222			error). See the L<Types::Serialiser> manpage for details.
223
224			=item CBOR tag 256 (perl object)
225
226			The tag value C<256> (TODO: pending iana registration) will be used to
227			deserialise a Perl object.
228
229			TODO For this to work, the class must be loaded and must have a
230			C<FROM_CBOR> method. The decoder will then call the C<FROM_CBOR> method
231			with the constructor arguments provided by the C<TO_CBOR> method (see
232			below).
233
234			The C<FROM_CBOR> method must return a single value that will then be used
235			as the deserialised value.
236	root	1.1
237	root	1.6	=item CBOR tag 55799 (magic header)
238	root	1.4
239	root	1.6	The tag 55799 is ignored (this tag implements the magic header).
240	root	1.1
241	root	1.6	=item other CBOR tags
242	root	1.4
243	root	1.6	Tagged items consists of a numeric tag and another CBOR value. Tags not
244			handled internally are currently converted into a L<CBOR::XS::Tagged>
245			object, which is simply a blessed array reference consisting of the
246			numeric tag value followed by the (decoded) CBOR value.
247	root	1.4
248	root	1.6	In the future, support for user-supplied conversions might get added.
249	root	1.4
250			=item anything else
251
252			Anything else (e.g. unsupported simple values) will raise a decoding
253			error.
254	root	1.1
255			=back
256
257
258			=head2 PERL -> CBOR
259
260			The mapping from Perl to CBOR is slightly more difficult, as Perl is a
261			truly typeless language, so we can only guess which CBOR type is meant by
262			a Perl value.
263
264			=over 4
265
266			=item hash references
267
268	root	1.4	Perl hash references become CBOR maps. As there is no inherent ordering in
269			hash keys (or CBOR maps), they will usually be encoded in a pseudo-random
270			order.
271
272			Currently, tied hashes will use the indefinite-length format, while normal
273			hashes will use the fixed-length format.
274	root	1.1
275			=item array references
276
277	root	1.4	Perl array references become fixed-length CBOR arrays.
278	root	1.1
279			=item other references
280
281			Other unblessed references are generally not allowed and will cause an
282			exception to be thrown, except for references to the integers C<0> and
283	root	1.4	C<1>, which get turned into false and true in CBOR.
284
285			=item CBOR::XS::Tagged objects
286
287			Objects of this type must be arrays consisting of a single C<[tag, value]>
288			pair. The (numerical) tag will be encoded as a CBOR tag, the value will be
289			encoded as appropriate for the value.
290	root	1.1
291	root	1.6	=item Types::Serialiser::true, Types::Serialiser::false, Types::Serialiser::error
292	root	1.1
293	root	1.6	These special values become CBOR true, CBOR false and CBOR undefined
294			values, respectively. You can also use C<\1>, C<\0> and C<\undef> directly
295			if you want.
296	root	1.1
297			=item blessed objects
298
299	root	1.4	Other blessed objects currently need to have a C<TO_CBOR> method. It
300			will be called on every object that is being serialised, and must return
301			something that can be encoded in CBOR.
302	root	1.1
303			=item simple scalars
304
305			TODO
306			Simple Perl scalars (any scalar that is not a reference) are the most
307			difficult objects to encode: CBOR::XS will encode undefined scalars as
308	root	1.4	CBOR null values, scalars that have last been used in a string context
309	root	1.1	before encoding as CBOR strings, and anything else as number value:
310
311			# dump as number
312			encode_cbor [2] # yields [2]
313			encode_cbor [-3.0e17] # yields [-3e+17]
314			my $value = 5; encode_cbor [$value] # yields [5]
315
316			# used as string, so dump as string
317			print $value;
318			encode_cbor [$value] # yields ["5"]
319
320			# undef becomes null
321			encode_cbor [undef] # yields [null]
322
323			You can force the type to be a CBOR string by stringifying it:
324
325			my $x = 3.1; # some variable containing a number
326			"$x"; # stringified
327			$x .= ""; # another, more awkward way to stringify
328			print $x; # perl does it for you, too, quite often
329
330			You can force the type to be a CBOR number by numifying it:
331
332			my $x = "3"; # some variable containing a string
333			$x += 0; # numify it, ensuring it will be dumped as a number
334			$x *= 1; # same thing, the choice is yours.
335
336			You can not currently force the type in other, less obscure, ways. Tell me
337			if you need this capability (but don't forget to explain why it's needed
338			:).
339
340	root	1.4	Perl values that seem to be integers generally use the shortest possible
341			representation. Floating-point values will use either the IEEE single
342			format if possible without loss of precision, otherwise the IEEE double
343			format will be used. Perls that use formats other than IEEE double to
344			represent numerical values are supported, but might suffer loss of
345			precision.
346	root	1.1
347			=back
348
349
350	root	1.3	=head2 MAGIC HEADER
351
352			There is no way to distinguish CBOR from other formats
353			programmatically. To make it easier to distinguish CBOR from other
354			formats, the CBOR specification has a special "magic string" that can be
355			prepended to any CBOR string without changing it's meaning.
356
357			This string is available as C<$CBOR::XS::MAGIC>. This module does not
358			prepend this string tot he CBOR data it generates, but it will ignroe it
359			if present, so users can prepend this string as a "file type" indicator as
360			required.
361
362
363	root	1.1	=head2 CBOR and JSON
364
365	root	1.4	CBOR is supposed to implement a superset of the JSON data model, and is,
366			with some coercion, able to represent all JSON texts (something that other
367			"binary JSON" formats such as BSON generally do not support).
368
369			CBOR implements some extra hints and support for JSON interoperability,
370			and the spec offers further guidance for conversion between CBOR and
371			JSON. None of this is currently implemented in CBOR, and the guidelines
372			in the spec do not result in correct round-tripping of data. If JSON
373			interoperability is improved in the future, then the goal will be to
374			ensure that decoded JSON data will round-trip encoding and decoding to
375			CBOR intact.
376	root	1.1
377
378			=head1 SECURITY CONSIDERATIONS
379
380			When you are using CBOR in a protocol, talking to untrusted potentially
381			hostile creatures requires relatively few measures.
382
383			First of all, your CBOR decoder should be secure, that is, should not have
384			any buffer overflows. Obviously, this module should ensure that and I am
385			trying hard on making that true, but you never know.
386
387			Second, you need to avoid resource-starving attacks. That means you should
388			limit the size of CBOR data you accept, or make sure then when your
389			resources run out, that's just fine (e.g. by using a separate process that
390			can crash safely). The size of a CBOR string in octets is usually a good
391			indication of the size of the resources required to decode it into a Perl
392			structure. While CBOR::XS can check the size of the CBOR text, it might be
393			too late when you already have it in memory, so you might want to check
394			the size before you accept the string.
395
396			Third, CBOR::XS recurses using the C stack when decoding objects and
397			arrays. The C stack is a limited resource: for instance, on my amd64
398			machine with 8MB of stack size I can decode around 180k nested arrays but
399			only 14k nested CBOR objects (due to perl itself recursing deeply on croak
400			to free the temporary). If that is exceeded, the program crashes. To be
401			conservative, the default nesting limit is set to 512. If your process
402			has a smaller stack, you should adjust this setting accordingly with the
403			C<max_depth> method.
404
405			Something else could bomb you, too, that I forgot to think of. In that
406			case, you get to keep the pieces. I am always open for hints, though...
407
408			Also keep in mind that CBOR::XS might leak contents of your Perl data
409			structures in its error messages, so when you serialise sensitive
410			information you might want to make sure that exceptions thrown by CBOR::XS
411			will not end up in front of untrusted eyes.
412
413			=head1 CBOR IMPLEMENTATION NOTES
414
415			This section contains some random implementation notes. They do not
416			describe guaranteed behaviour, but merely behaviour as-is implemented
417			right now.
418
419			64 bit integers are only properly decoded when Perl was built with 64 bit
420			support.
421
422			Strings and arrays are encoded with a definite length. Hashes as well,
423			unless they are tied (or otherwise magical).
424
425			Only the double data type is supported for NV data types - when Perl uses
426			long double to represent floating point values, they might not be encoded
427			properly. Half precision types are accepted, but not encoded.
428
429			Strict mode and canonical mode are not implemented.
430
431
432			=head1 THREADS
433
434			This module is I<not> guaranteed to be thread safe and there are no
435			plans to change this until Perl gets thread support (as opposed to the
436			horribly slow so-called "threads" which are simply slow and bloated
437			process simulations - use fork, it's I<much> faster, cheaper, better).
438
439			(It might actually work, but you have been warned).
440
441
442			=head1 BUGS
443
444			While the goal of this module is to be correct, that unfortunately does
445			not mean it's bug-free, only that I think its design is bug-free. If you
446			keep reporting bugs they will be fixed swiftly, though.
447
448			Please refrain from using rt.cpan.org or any other bug reporting
449			service. I put the contact address into my modules for a reason.
450
451			=cut
452
453			XSLoader::load "CBOR::XS", $VERSION;
454
455			=head1 SEE ALSO
456
457			The L<JSON> and L<JSON::XS> modules that do similar, but human-readable,
458			serialisation.
459
460	root	1.6	The L<Types::Serialiser> module provides the data model for true, false
461			and error values.
462
463	root	1.1	=head1 AUTHOR
464
465			Marc Lehmann <schmorp@schmorp.de>
466			http://home.schmorp.de/
467
468			=cut
469
470	root	1.6	1
471