--- CBOR-XS/README 2013/11/28 16:09:04 1.10 +++ CBOR-XS/README 2013/11/30 18:42:27 1.11 @@ -46,8 +46,8 @@ In addition to the core CBOR data format, this module implements a number of extensions, to support cyclic and shared data structures (see - "allow_sharing"), string deduplication (see "pack_strings") and scalar - references (always enabled). + "allow_sharing" and "allow_cycles"), string deduplication (see + "pack_strings") and scalar references (always enabled). The primary goal of this module is to be *correct* and the secondary goal is to be *fast*. To reach the latter goal it was written in C. @@ -143,7 +143,8 @@ This means that such values will only be encoded once, and will not result in a deep cloning of the value on decode, in decoders supporting the value sharing extension. This also makes it possible - to encode cyclic data structures. + to encode cyclic data structures (which need "allow_cycles" to ne + enabled to be decoded by this module). It is recommended to leave it off unless you know your communication partner supports the value sharing extensions to CBOR @@ -153,7 +154,7 @@ Detecting shared values incurs a runtime overhead when values are encoded that have a reference counter large than one, and might unnecessarily increase the encoded size, as potentially shared - values are encode as sharable whether or not they are actually + values are encode as shareable whether or not they are actually shared. At the moment, only targets of references can be shared (e.g. @@ -169,6 +170,19 @@ This option does not affect "decode" in any way - shared values and references will always be decoded properly if present. + $cbor = $cbor->allow_cycles ([$enable]) + $enabled = $cbor->get_allow_cycles + If $enable is true (or missing), then "decode" will happily decode + self-referential (cyclic) data structures. By default these will not + be decoded, as they need manual cleanup to avoid memory leaks, so + code that isn't prepared for this will not leak memory. + + If $enable is false (the default), then "decode" will throw an error + when it encounters a self-referential/cyclic data structure. + + This option does not affect "encode" in any way - shared values and + references will always be decoded properly if present. + $cbor = $cbor->pack_strings ([$enable]) $enabled = $cbor->get_pack_strings If $enable is true (or missing), then "encode" will try not to @@ -398,10 +412,15 @@ might suffer loss of precision. OBJECT SERIALISATION + This module implements both a CBOR-specific and the generic + Types::Serialier object serialisation protocol. The following + subsections explain both methods. + + ENCODING This module knows two way to serialise a Perl object: The CBOR-specific way, and the generic way. - Whenever the encoder encounters a Perl object that it cnanot serialise + Whenever the encoder encounters a Perl object that it cannot serialise directly (most of them), it will first look up the "TO_CBOR" method on it. @@ -416,12 +435,17 @@ The "FREEZE" method can return any number of values (i.e. zero or more). These will be encoded as CBOR perl object, together with the classname. + These methods *MUST NOT* change the data structure that is being + serialised. Failure to comply to this can result in memory corruption - + and worse. + If an object supports neither "TO_CBOR" nor "FREEZE", encoding will fail with an error. - Objects encoded via "TO_CBOR" cannot be automatically decoded, but - objects encoded via "FREEZE" can be decoded using the following - protocol: + DECODING + Objects encoded via "TO_CBOR" cannot (normally) be automatically + decoded, but objects encoded via "FREEZE" can be decoded using the + following protocol: When an encoded CBOR perl object is encountered by the decoder, it will look up the "THAW" method, by using the stored classname, and will fail @@ -586,10 +610,24 @@ objects using the "FREEZE/THAW" methods (the Types::Serialier object serialisation protocol). See "OBJECT SERIALISATION" for details. - 28, 29 (sharable, sharedref, L ) - These tags are automatically decoded when encountered, resulting in - shared values in the decoded object. They are only encoded, however, - when "allow_sharable" is enabled. + 28, 29 (shareable, sharedref, L ) + These tags are automatically decoded when encountered (and they do + not result in a cyclic data structure, see "allow_cycles"), + resulting in shared values in the decoded object. They are only + encoded, however, when "allow_sharing" is enabled. + + Not all shared values can be successfully decoded: values that + reference themselves will *currently* decode as "undef" (this is not + the same as a reference pointing to itself, which will be + represented as a value that contains an indirect reference to itself + - these will be decoded properly). + + Note that considerably more shared value data structures can be + decoded than will be encoded - currently, only values pointed to by + references will be shared, others will not. While non-reference + shared values can be generated in Perl with some effort, they were + considered too unimportant to be supported in the encoder. The + decoder, however, will decode these values as shared values. 256, 25 (stringref-namespace, stringref, L ) @@ -705,6 +743,13 @@ Strict mode and canonical mode are not implemented. +LIMITATIONS ON PERLS WITHOUT 64-BIT INTEGER SUPPORT + On perls that were built without 64 bit integer support (these are rare + nowadays, even on 32 bit architectures), support for any kind of 64 bit + integer in CBOR is very limited - most likely, these 64 bit values will + be truncated, corrupted, or otherwise not decoded correctly. This also + includes string, array and map sizes that are stored as 64 bit integers. + THREADS This module is *not* guaranteed to be thread safe and there are no plans to change this until Perl gets thread support (as opposed to the