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1 | NAME |
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2 | CBOR::XS - Concise Binary Object Representation (CBOR, RFC7049) |
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3 | |
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4 | SYNOPSIS |
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5 | use CBOR::XS; |
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6 | |
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7 | $binary_cbor_data = encode_cbor $perl_value; |
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8 | $perl_value = decode_cbor $binary_cbor_data; |
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9 | |
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10 | # OO-interface |
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11 | |
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12 | $coder = CBOR::XS->new; |
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13 | $binary_cbor_data = $coder->encode ($perl_value); |
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14 | $perl_value = $coder->decode ($binary_cbor_data); |
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15 | |
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16 | # prefix decoding |
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17 | |
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18 | my $many_cbor_strings = ...; |
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19 | while (length $many_cbor_strings) { |
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20 | my ($data, $length) = $cbor->decode_prefix ($many_cbor_strings); |
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21 | # data was decoded |
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22 | substr $many_cbor_strings, 0, $length, ""; # remove decoded cbor string |
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23 | } |
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24 | |
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25 | DESCRIPTION |
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26 | WARNING! THIS IS A PRE-ALPHA RELEASE! IT WILL CRASH, CORRUPT YOUR DATA |
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27 | AND EAT YOUR CHILDREN! (Actually, apart from being untested and a bit |
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28 | feature-limited, it might already be useful). |
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29 | |
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30 | This module converts Perl data structures to the Concise Binary Object |
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31 | Representation (CBOR) and vice versa. CBOR is a fast binary |
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32 | serialisation format that aims to use a superset of the JSON data model, |
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33 | i.e. when you can represent something in JSON, you should be able to |
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34 | represent it in CBOR. |
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35 | |
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36 | This makes it a faster and more compact binary alternative to JSON, with |
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37 | the added ability of supporting serialising of perl objects. |
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38 | |
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39 | The primary goal of this module is to be *correct* and the secondary |
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40 | goal is to be *fast*. To reach the latter goal it was written in C. |
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41 | |
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42 | See MAPPING, below, on how CBOR::XS maps perl values to CBOR values and |
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43 | vice versa. |
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44 | |
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45 | FUNCTIONAL INTERFACE |
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46 | The following convenience methods are provided by this module. They are |
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47 | exported by default: |
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48 | |
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49 | $cbor_data = encode_cbor $perl_scalar |
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50 | Converts the given Perl data structure to CBOR representation. |
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51 | Croaks on error. |
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52 | |
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53 | $perl_scalar = decode_cbor $cbor_data |
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54 | The opposite of "encode_cbor": expects a valid CBOR string to parse, |
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55 | returning the resulting perl scalar. Croaks on error. |
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56 | |
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57 | OBJECT-ORIENTED INTERFACE |
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58 | The object oriented interface lets you configure your own encoding or |
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59 | decoding style, within the limits of supported formats. |
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60 | |
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61 | $cbor = new CBOR::XS |
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62 | Creates a new CBOR::XS object that can be used to de/encode CBOR |
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63 | strings. All boolean flags described below are by default |
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64 | *disabled*. |
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65 | |
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66 | The mutators for flags all return the CBOR object again and thus |
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67 | calls can be chained: |
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68 | |
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69 | #TODO my $cbor = CBOR::XS->new->encode ({a => [1,2]}); |
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70 | |
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71 | $cbor = $cbor->max_depth ([$maximum_nesting_depth]) |
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72 | $max_depth = $cbor->get_max_depth |
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73 | Sets the maximum nesting level (default 512) accepted while encoding |
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74 | or decoding. If a higher nesting level is detected in CBOR data or a |
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75 | Perl data structure, then the encoder and decoder will stop and |
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76 | croak at that point. |
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77 | |
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78 | Nesting level is defined by number of hash- or arrayrefs that the |
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79 | encoder needs to traverse to reach a given point or the number of |
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80 | "{" or "[" characters without their matching closing parenthesis |
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81 | crossed to reach a given character in a string. |
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82 | |
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83 | Setting the maximum depth to one disallows any nesting, so that |
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84 | ensures that the object is only a single hash/object or array. |
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85 | |
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86 | If no argument is given, the highest possible setting will be used, |
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87 | which is rarely useful. |
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88 | |
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89 | Note that nesting is implemented by recursion in C. The default |
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90 | value has been chosen to be as large as typical operating systems |
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91 | allow without crashing. |
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92 | |
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93 | See SECURITY CONSIDERATIONS, below, for more info on why this is |
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94 | useful. |
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95 | |
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96 | $cbor = $cbor->max_size ([$maximum_string_size]) |
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97 | $max_size = $cbor->get_max_size |
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98 | Set the maximum length a CBOR string may have (in bytes) where |
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99 | decoding is being attempted. The default is 0, meaning no limit. |
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100 | When "decode" is called on a string that is longer then this many |
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101 | bytes, it will not attempt to decode the string but throw an |
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102 | exception. This setting has no effect on "encode" (yet). |
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103 | |
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104 | If no argument is given, the limit check will be deactivated (same |
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105 | as when 0 is specified). |
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106 | |
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107 | See SECURITY CONSIDERATIONS, below, for more info on why this is |
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108 | useful. |
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109 | |
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110 | $cbor_data = $cbor->encode ($perl_scalar) |
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111 | Converts the given Perl data structure (a scalar value) to its CBOR |
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112 | representation. |
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113 | |
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114 | $perl_scalar = $cbor->decode ($cbor_data) |
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115 | The opposite of "encode": expects CBOR data and tries to parse it, |
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116 | returning the resulting simple scalar or reference. Croaks on error. |
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117 | |
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118 | ($perl_scalar, $octets) = $cbor->decode_prefix ($cbor_data) |
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119 | This works like the "decode" method, but instead of raising an |
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120 | exception when there is trailing garbage after the CBOR string, it |
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121 | will silently stop parsing there and return the number of characters |
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122 | consumed so far. |
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123 | |
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124 | This is useful if your CBOR texts are not delimited by an outer |
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125 | protocol and you need to know where the first CBOR string ends amd |
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126 | the next one starts. |
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127 | |
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128 | CBOR::XS->new->decode_prefix ("......") |
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129 | => ("...", 3) |
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130 | |
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131 | MAPPING |
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132 | This section describes how CBOR::XS maps Perl values to CBOR values and |
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133 | vice versa. These mappings are designed to "do the right thing" in most |
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134 | circumstances automatically, preserving round-tripping characteristics |
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135 | (what you put in comes out as something equivalent). |
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136 | |
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137 | For the more enlightened: note that in the following descriptions, |
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138 | lowercase *perl* refers to the Perl interpreter, while uppercase *Perl* |
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139 | refers to the abstract Perl language itself. |
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140 | |
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141 | CBOR -> PERL |
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142 | integers |
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143 | CBOR integers become (numeric) perl scalars. On perls without 64 bit |
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144 | support, 64 bit integers will be truncated or otherwise corrupted. |
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145 | |
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146 | byte strings |
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147 | Byte strings will become octet strings in Perl (the byte values |
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148 | 0..255 will simply become characters of the same value in Perl). |
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149 | |
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150 | UTF-8 strings |
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151 | UTF-8 strings in CBOR will be decoded, i.e. the UTF-8 octets will be |
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152 | decoded into proper Unicode code points. At the moment, the validity |
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153 | of the UTF-8 octets will not be validated - corrupt input will |
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154 | result in corrupted Perl strings. |
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155 | |
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156 | arrays, maps |
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157 | CBOR arrays and CBOR maps will be converted into references to a |
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158 | Perl array or hash, respectively. The keys of the map will be |
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159 | stringified during this process. |
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160 | |
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161 | null |
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162 | CBOR null becomes "undef" in Perl. |
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163 | |
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164 | true, false, undefined |
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165 | These CBOR values become "Types:Serialiser::true", |
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166 | "Types:Serialiser::false" and "Types::Serialiser::error", |
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167 | respectively. They are overloaded to act almost exactly like the |
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168 | numbers 1 and 0 (for true and false) or to throw an exception on |
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169 | access (for error). See the Types::Serialiser manpage for details. |
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170 | |
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171 | CBOR tag 256 (perl object) |
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172 | The tag value 256 (TODO: pending iana registration) will be used to |
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173 | deserialise a Perl object serialised with "FREEZE". See "OBJECT |
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174 | SERIALISATION", below, for details. |
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175 | |
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176 | CBOR tag 55799 (magic header) |
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177 | The tag 55799 is ignored (this tag implements the magic header). |
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178 | |
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179 | other CBOR tags |
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180 | Tagged items consists of a numeric tag and another CBOR value. Tags |
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181 | not handled internally are currently converted into a |
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182 | CBOR::XS::Tagged object, which is simply a blessed array reference |
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183 | consisting of the numeric tag value followed by the (decoded) CBOR |
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184 | value. |
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185 | |
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186 | In the future, support for user-supplied conversions might get |
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187 | added. |
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188 | |
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189 | anything else |
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190 | Anything else (e.g. unsupported simple values) will raise a decoding |
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191 | error. |
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192 | |
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193 | PERL -> CBOR |
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194 | The mapping from Perl to CBOR is slightly more difficult, as Perl is a |
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195 | truly typeless language, so we can only guess which CBOR type is meant |
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196 | by a Perl value. |
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197 | |
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198 | hash references |
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199 | Perl hash references become CBOR maps. As there is no inherent |
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200 | ordering in hash keys (or CBOR maps), they will usually be encoded |
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201 | in a pseudo-random order. |
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202 | |
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203 | Currently, tied hashes will use the indefinite-length format, while |
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204 | normal hashes will use the fixed-length format. |
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205 | |
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206 | array references |
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207 | Perl array references become fixed-length CBOR arrays. |
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208 | |
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209 | other references |
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210 | Other unblessed references are generally not allowed and will cause |
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211 | an exception to be thrown, except for references to the integers 0 |
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212 | and 1, which get turned into false and true in CBOR. |
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213 | |
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214 | CBOR::XS::Tagged objects |
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215 | Objects of this type must be arrays consisting of a single "[tag, |
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216 | value]" pair. The (numerical) tag will be encoded as a CBOR tag, the |
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217 | value will be encoded as appropriate for the value. |
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218 | |
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219 | Types::Serialiser::true, Types::Serialiser::false, |
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220 | Types::Serialiser::error |
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221 | These special values become CBOR true, CBOR false and CBOR undefined |
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222 | values, respectively. You can also use "\1", "\0" and "\undef" |
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223 | directly if you want. |
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224 | |
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225 | other blessed objects |
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226 | Other blessed objects are serialised via "TO_CBOR" or "FREEZE". See |
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227 | "OBJECT SERIALISATION", below, for details. |
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228 | |
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229 | simple scalars |
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230 | TODO Simple Perl scalars (any scalar that is not a reference) are |
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231 | the most difficult objects to encode: CBOR::XS will encode undefined |
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232 | scalars as CBOR null values, scalars that have last been used in a |
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233 | string context before encoding as CBOR strings, and anything else as |
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234 | number value: |
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235 | |
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236 | # dump as number |
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237 | encode_cbor [2] # yields [2] |
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238 | encode_cbor [-3.0e17] # yields [-3e+17] |
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239 | my $value = 5; encode_cbor [$value] # yields [5] |
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240 | |
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241 | # used as string, so dump as string |
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242 | print $value; |
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243 | encode_cbor [$value] # yields ["5"] |
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244 | |
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245 | # undef becomes null |
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246 | encode_cbor [undef] # yields [null] |
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247 | |
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248 | You can force the type to be a CBOR string by stringifying it: |
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249 | |
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250 | my $x = 3.1; # some variable containing a number |
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251 | "$x"; # stringified |
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252 | $x .= ""; # another, more awkward way to stringify |
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253 | print $x; # perl does it for you, too, quite often |
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254 | |
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255 | You can force the type to be a CBOR number by numifying it: |
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256 | |
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257 | my $x = "3"; # some variable containing a string |
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258 | $x += 0; # numify it, ensuring it will be dumped as a number |
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259 | $x *= 1; # same thing, the choice is yours. |
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260 | |
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261 | You can not currently force the type in other, less obscure, ways. |
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262 | Tell me if you need this capability (but don't forget to explain why |
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263 | it's needed :). |
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264 | |
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265 | Perl values that seem to be integers generally use the shortest |
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266 | possible representation. Floating-point values will use either the |
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267 | IEEE single format if possible without loss of precision, otherwise |
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268 | the IEEE double format will be used. Perls that use formats other |
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269 | than IEEE double to represent numerical values are supported, but |
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270 | might suffer loss of precision. |
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271 | |
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272 | OBJECT SERIALISATION |
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273 | This module knows two way to serialise a Perl object: The CBOR-specific |
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274 | way, and the generic way. |
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275 | |
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276 | Whenever the encoder encounters a Perl object that it cnanot serialise |
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277 | directly (most of them), it will first look up the "TO_CBOR" method on |
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278 | it. |
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279 | |
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280 | If it has a "TO_CBOR" method, it will call it with the object as only |
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281 | argument, and expects exactly one return value, which it will then |
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282 | substitute and encode it in the place of the object. |
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283 | |
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284 | Otherwise, it will look up the "FREEZE" method. If it exists, it will |
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285 | call it with the object as first argument, and the constant string |
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286 | "CBOR" as the second argument, to distinguish it from other serialisers. |
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287 | |
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288 | The "FREEZE" method can return any number of values (i.e. zero or more). |
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289 | These will be encoded as CBOR perl object, together with the classname. |
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290 | |
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291 | If an object supports neither "TO_CBOR" nor "FREEZE", encoding will fail |
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292 | with an error. |
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293 | |
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294 | Objects encoded via "TO_CBOR" cannot be automatically decoded, but |
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295 | objects encoded via "FREEZE" can be decoded using the following |
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296 | protocol: |
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297 | |
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298 | When an encoded CBOR perl object is encountered by the decoder, it will |
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299 | look up the "THAW" method, by using the stored classname, and will fail |
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300 | if the method cannot be found. |
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301 | |
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302 | After the lookup it will call the "THAW" method with the stored |
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303 | classname as first argument, the constant string "CBOR" as second |
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304 | argument, and all values returned by "FREEZE" as remaining arguments. |
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305 | |
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306 | EXAMPLES |
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307 | Here is an example "TO_CBOR" method: |
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308 | |
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309 | sub My::Object::TO_CBOR { |
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310 | my ($obj) = @_; |
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311 | |
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312 | ["this is a serialised My::Object object", $obj->{id}] |
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313 | } |
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314 | |
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315 | When a "My::Object" is encoded to CBOR, it will instead encode a simple |
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316 | array with two members: a string, and the "object id". Decoding this |
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317 | CBOR string will yield a normal perl array reference in place of the |
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318 | object. |
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319 | |
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320 | A more useful and practical example would be a serialisation method for |
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321 | the URI module. CBOR has a custom tag value for URIs, namely 32: |
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322 | |
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323 | sub URI::TO_CBOR { |
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324 | my ($self) = @_; |
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325 | my $uri = "$self"; # stringify uri |
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326 | utf8::upgrade $uri; # make sure it will be encoded as UTF-8 string |
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327 | CBOR::XS::tagged 32, "$_[0]" |
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328 | } |
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329 | |
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330 | This will encode URIs as a UTF-8 string with tag 32, which indicates an |
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331 | URI. |
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332 | |
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333 | Decoding such an URI will not (currently) give you an URI object, but |
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334 | instead a CBOR::XS::Tagged object with tag number 32 and the string - |
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335 | exactly what was returned by "TO_CBOR". |
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336 | |
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337 | To serialise an object so it can automatically be deserialised, you need |
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338 | to use "FREEZE" and "THAW". To take the URI module as example, this |
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339 | would be a possible implementation: |
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340 | |
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341 | sub URI::FREEZE { |
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342 | my ($self, $serialiser) = @_; |
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343 | "$self" # encode url string |
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344 | } |
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345 | |
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346 | sub URI::THAW { |
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347 | my ($class, $serialiser, $uri) = @_; |
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348 | |
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349 | $class->new ($uri) |
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350 | } |
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351 | |
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352 | Unlike "TO_CBOR", multiple values can be returned by "FREEZE". For |
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353 | example, a "FREEZE" method that returns "type", "id" and "variant" |
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354 | values would cause an invocation of "THAW" with 5 arguments: |
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355 | |
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356 | sub My::Object::FREEZE { |
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357 | my ($self, $serialiser) = @_; |
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358 | |
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359 | ($self->{type}, $self->{id}, $self->{variant}) |
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360 | } |
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361 | |
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362 | sub My::Object::THAW { |
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363 | my ($class, $serialiser, $type, $id, $variant) = @_; |
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364 | |
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365 | $class-<new (type => $type, id => $id, variant => $variant) |
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366 | } |
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367 | |
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368 | MAGIC HEADER |
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369 | There is no way to distinguish CBOR from other formats programmatically. |
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370 | To make it easier to distinguish CBOR from other formats, the CBOR |
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371 | specification has a special "magic string" that can be prepended to any |
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372 | CBOR string without changing it's meaning. |
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373 | |
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374 | This string is available as $CBOR::XS::MAGIC. This module does not |
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375 | prepend this string tot he CBOR data it generates, but it will ignroe it |
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376 | if present, so users can prepend this string as a "file type" indicator |
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377 | as required. |
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378 | |
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379 | CBOR and JSON |
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380 | CBOR is supposed to implement a superset of the JSON data model, and is, |
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381 | with some coercion, able to represent all JSON texts (something that |
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382 | other "binary JSON" formats such as BSON generally do not support). |
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383 | |
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384 | CBOR implements some extra hints and support for JSON interoperability, |
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385 | and the spec offers further guidance for conversion between CBOR and |
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386 | JSON. None of this is currently implemented in CBOR, and the guidelines |
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387 | in the spec do not result in correct round-tripping of data. If JSON |
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388 | interoperability is improved in the future, then the goal will be to |
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389 | ensure that decoded JSON data will round-trip encoding and decoding to |
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390 | CBOR intact. |
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391 | |
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392 | SECURITY CONSIDERATIONS |
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393 | When you are using CBOR in a protocol, talking to untrusted potentially |
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394 | hostile creatures requires relatively few measures. |
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395 | |
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396 | First of all, your CBOR decoder should be secure, that is, should not |
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397 | have any buffer overflows. Obviously, this module should ensure that and |
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398 | I am trying hard on making that true, but you never know. |
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399 | |
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400 | Second, you need to avoid resource-starving attacks. That means you |
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401 | should limit the size of CBOR data you accept, or make sure then when |
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402 | your resources run out, that's just fine (e.g. by using a separate |
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403 | process that can crash safely). The size of a CBOR string in octets is |
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404 | usually a good indication of the size of the resources required to |
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405 | decode it into a Perl structure. While CBOR::XS can check the size of |
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406 | the CBOR text, it might be too late when you already have it in memory, |
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407 | so you might want to check the size before you accept the string. |
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408 | |
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409 | Third, CBOR::XS recurses using the C stack when decoding objects and |
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410 | arrays. The C stack is a limited resource: for instance, on my amd64 |
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411 | machine with 8MB of stack size I can decode around 180k nested arrays |
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412 | but only 14k nested CBOR objects (due to perl itself recursing deeply on |
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413 | croak to free the temporary). If that is exceeded, the program crashes. |
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414 | To be conservative, the default nesting limit is set to 512. If your |
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415 | process has a smaller stack, you should adjust this setting accordingly |
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416 | with the "max_depth" method. |
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417 | |
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418 | Something else could bomb you, too, that I forgot to think of. In that |
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419 | case, you get to keep the pieces. I am always open for hints, though... |
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420 | |
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421 | Also keep in mind that CBOR::XS might leak contents of your Perl data |
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422 | structures in its error messages, so when you serialise sensitive |
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423 | information you might want to make sure that exceptions thrown by |
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424 | CBOR::XS will not end up in front of untrusted eyes. |
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425 | |
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426 | CBOR IMPLEMENTATION NOTES |
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427 | This section contains some random implementation notes. They do not |
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428 | describe guaranteed behaviour, but merely behaviour as-is implemented |
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429 | right now. |
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430 | |
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431 | 64 bit integers are only properly decoded when Perl was built with 64 |
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432 | bit support. |
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433 | |
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434 | Strings and arrays are encoded with a definite length. Hashes as well, |
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435 | unless they are tied (or otherwise magical). |
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436 | |
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437 | Only the double data type is supported for NV data types - when Perl |
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438 | uses long double to represent floating point values, they might not be |
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439 | encoded properly. Half precision types are accepted, but not encoded. |
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440 | |
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441 | Strict mode and canonical mode are not implemented. |
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442 | |
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443 | THREADS |
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444 | This module is *not* guaranteed to be thread safe and there are no plans |
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445 | to change this until Perl gets thread support (as opposed to the |
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446 | horribly slow so-called "threads" which are simply slow and bloated |
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447 | process simulations - use fork, it's *much* faster, cheaper, better). |
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448 | |
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449 | (It might actually work, but you have been warned). |
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450 | |
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451 | BUGS |
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452 | While the goal of this module is to be correct, that unfortunately does |
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453 | not mean it's bug-free, only that I think its design is bug-free. If you |
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454 | keep reporting bugs they will be fixed swiftly, though. |
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455 | |
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456 | Please refrain from using rt.cpan.org or any other bug reporting |
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457 | service. I put the contact address into my modules for a reason. |
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458 | |
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459 | SEE ALSO |
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460 | The JSON and JSON::XS modules that do similar, but human-readable, |
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461 | serialisation. |
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462 | |
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463 | The Types::Serialiser module provides the data model for true, false and |
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464 | error values. |
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465 | |
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466 | AUTHOR |
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467 | Marc Lehmann <schmorp@schmorp.de> |
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468 | http://home.schmorp.de/ |
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469 | |