--- CBOR-XS/README 2013/11/28 16:09:04 1.10 +++ CBOR-XS/README 2016/02/08 04:37:12 1.16 @@ -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 +170,24 @@ 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. + + FUTURE DIRECTION: the motivation behind this option is to avoid + *real* cycles - future versions of this module might chose to decode + cyclic data structures using weak references when this option is + off, instead of throwing an error. + + This option does not affect "encode" in any way - shared values and + references will always be encoded 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 @@ -188,6 +207,29 @@ This option does not affect "decode" in any way - string references will always be decoded properly if present. + $cbor = $cbor->validate_utf8 ([$enable]) + $enabled = $cbor->get_validate_utf8 + If $enable is true (or missing), then "decode" will validate that + elements (text strings) containing UTF-8 data in fact contain valid + UTF-8 data (instead of blindly accepting it). This validation + obviously takes extra time during decoding. + + The concept of "valid UTF-8" used is perl's concept, which is a + superset of the official UTF-8. + + If $enable is false (the default), then "decode" will blindly accept + UTF-8 data, marking them as valid UTF-8 in the resulting data + structure regardless of whether thats true or not. + + Perl isn't too happy about corrupted UTF-8 in strings, but should + generally not crash or do similarly evil things. Extensions might be + not so forgiving, so it's recommended to turn on this setting if you + receive untrusted CBOR. + + This option does not affect "encode" in any way - strings that are + supposedly valid UTF-8 will simply be dumped into the resulting CBOR + string without checking whether that is, in fact, true or not. + $cbor = $cbor->filter ([$cb->($tag, $value)]) $cb_or_undef = $cbor->get_filter Sets or replaces the tagged value decoding filter (when $cb is @@ -252,6 +294,64 @@ CBOR::XS->new->decode_prefix ("......") => ("...", 3) + INCREMENTAL PARSING + In some cases, there is the need for incremental parsing of JSON texts. + While this module always has to keep both CBOR text and resulting Perl + data structure in memory at one time, it does allow you to parse a CBOR + stream incrementally, using a similar to using "decode_prefix" to see if + a full CBOR object is available, but is much more efficient. + + It basically works by parsing as much of a CBOR string as possible - if + the CBOR data is not complete yet, the pasrer will remember where it + was, to be able to restart when more data has been accumulated. Once + enough data is available to either decode a complete CBOR value or raise + an error, a real decode will be attempted. + + A typical use case would be a network protocol that consists of sending + and receiving CBOR-encoded messages. The solution that works with CBOR + and about anything else is by prepending a length to every CBOR value, + so the receiver knows how many octets to read. More compact (and + slightly slower) would be to just send CBOR values back-to-back, as + "CBOR::XS" knows where a CBOR value ends, and doesn't need an explicit + length. + + The following methods help with this: + + @decoded = $cbor->incr_parse ($buffer) + This method attempts to decode exactly one CBOR value from the + beginning of the given $buffer. The value is removed from the + $buffer on success. When $buffer doesn't contain a complete value + yet, it returns nothing. Finally, when the $buffer doesn't start + with something that could ever be a valid CBOR value, it raises an + exception, just as "decode" would. In the latter case the decoder + state is undefined and must be reset before being able to parse + further. + + This method modifies the $buffer in place. When no CBOR value can be + decoded, the decoder stores the current string offset. On the next + call, continues decoding at the place where it stopped before. For + this to make sense, the $buffer must begin with the same octets as + on previous unsuccessful calls. + + You can call this method in scalar context, in which case it either + returns a decoded value or "undef". This makes it impossible to + distinguish between CBOR null values (which decode to "undef") and + an unsuccessful decode, which is often acceptable. + + @decoded = $cbor->incr_parse_multiple ($buffer) + Same as "incr_parse", but attempts to decode as many CBOR values as + possible in one go, instead of at most one. Calls to "incr_parse" + and "incr_parse_multiple" can be interleaved. + + $cbor->incr_reset + Resets the incremental decoder. This throws away any saved state, so + that subsequent calls to "incr_parse" or "incr_parse_multiple" start + to parse a new CBOR value from the beginning of the $buffer again. + + This method can be caled at any time, but it *must* be called if you + want to change your $buffer or there was a decoding error and you + want to reuse the $cbor object for future incremental parsings. + 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 @@ -398,10 +498,15 @@ 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 +521,17 @@ 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 @@ -586,12 +696,26 @@ 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, ) + 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 + 256, 25 (stringref-namespace, stringref, ) These tags are automatically decoded when encountered. They are only encoded, however, when "pack_strings" is enabled. @@ -618,6 +742,16 @@ to provide these modules. The decoding usually fails with an exception if the required module cannot be loaded. + 0, 1 (date/time string, seconds since the epoch) + These tags are decoded into Time::Piece objects. The corresponding + "Time::Piece::TO_CBOR" method always encodes into tag 1 values + currently. + + The Time::Piece API is generally surprisingly bad, and fractional + seconds are only accidentally kept intact, so watch out. On the plus + side, the module comes with perl since 5.10, which has to count for + something. + 2, 3 (positive/negative bignum) These tags are decoded into Math::BigInt objects. The corresponding "Math::BigInt::TO_CBOR" method encodes "small" bigints into normal @@ -705,6 +839,14 @@ 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, as all major Perl distributions + are built with 64 bit integer support), 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