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Source file src/encoding/gob/doc.go

Documentation: encoding/gob

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  /*
     6  Package gob manages streams of gobs - binary values exchanged between an
     7  Encoder (transmitter) and a Decoder (receiver). A typical use is transporting
     8  arguments and results of remote procedure calls (RPCs) such as those provided by
     9  package "net/rpc".
    10  
    11  The implementation compiles a custom codec for each data type in the stream and
    12  is most efficient when a single Encoder is used to transmit a stream of values,
    13  amortizing the cost of compilation.
    14  
    15  Basics
    16  
    17  A stream of gobs is self-describing. Each data item in the stream is preceded by
    18  a specification of its type, expressed in terms of a small set of predefined
    19  types. Pointers are not transmitted, but the things they point to are
    20  transmitted; that is, the values are flattened. Nil pointers are not permitted,
    21  as they have no value. Recursive types work fine, but
    22  recursive values (data with cycles) are problematic. This may change.
    23  
    24  To use gobs, create an Encoder and present it with a series of data items as
    25  values or addresses that can be dereferenced to values. The Encoder makes sure
    26  all type information is sent before it is needed. At the receive side, a
    27  Decoder retrieves values from the encoded stream and unpacks them into local
    28  variables.
    29  
    30  Types and Values
    31  
    32  The source and destination values/types need not correspond exactly. For structs,
    33  fields (identified by name) that are in the source but absent from the receiving
    34  variable will be ignored. Fields that are in the receiving variable but missing
    35  from the transmitted type or value will be ignored in the destination. If a field
    36  with the same name is present in both, their types must be compatible. Both the
    37  receiver and transmitter will do all necessary indirection and dereferencing to
    38  convert between gobs and actual Go values. For instance, a gob type that is
    39  schematically,
    40  
    41  	struct { A, B int }
    42  
    43  can be sent from or received into any of these Go types:
    44  
    45  	struct { A, B int }	// the same
    46  	*struct { A, B int }	// extra indirection of the struct
    47  	struct { *A, **B int }	// extra indirection of the fields
    48  	struct { A, B int64 }	// different concrete value type; see below
    49  
    50  It may also be received into any of these:
    51  
    52  	struct { A, B int }	// the same
    53  	struct { B, A int }	// ordering doesn't matter; matching is by name
    54  	struct { A, B, C int }	// extra field (C) ignored
    55  	struct { B int }	// missing field (A) ignored; data will be dropped
    56  	struct { B, C int }	// missing field (A) ignored; extra field (C) ignored.
    57  
    58  Attempting to receive into these types will draw a decode error:
    59  
    60  	struct { A int; B uint }	// change of signedness for B
    61  	struct { A int; B float }	// change of type for B
    62  	struct { }			// no field names in common
    63  	struct { C, D int }		// no field names in common
    64  
    65  Integers are transmitted two ways: arbitrary precision signed integers or
    66  arbitrary precision unsigned integers. There is no int8, int16 etc.
    67  discrimination in the gob format; there are only signed and unsigned integers. As
    68  described below, the transmitter sends the value in a variable-length encoding;
    69  the receiver accepts the value and stores it in the destination variable.
    70  Floating-point numbers are always sent using IEEE-754 64-bit precision (see
    71  below).
    72  
    73  Signed integers may be received into any signed integer variable: int, int16, etc.;
    74  unsigned integers may be received into any unsigned integer variable; and floating
    75  point values may be received into any floating point variable. However,
    76  the destination variable must be able to represent the value or the decode
    77  operation will fail.
    78  
    79  Structs, arrays and slices are also supported. Structs encode and decode only
    80  exported fields. Strings and arrays of bytes are supported with a special,
    81  efficient representation (see below). When a slice is decoded, if the existing
    82  slice has capacity the slice will be extended in place; if not, a new array is
    83  allocated. Regardless, the length of the resulting slice reports the number of
    84  elements decoded.
    85  
    86  In general, if allocation is required, the decoder will allocate memory. If not,
    87  it will update the destination variables with values read from the stream. It does
    88  not initialize them first, so if the destination is a compound value such as a
    89  map, struct, or slice, the decoded values will be merged elementwise into the
    90  existing variables.
    91  
    92  Functions and channels will not be sent in a gob. Attempting to encode such a value
    93  at the top level will fail. A struct field of chan or func type is treated exactly
    94  like an unexported field and is ignored.
    95  
    96  Gob can encode a value of any type implementing the GobEncoder or
    97  encoding.BinaryMarshaler interfaces by calling the corresponding method,
    98  in that order of preference.
    99  
   100  Gob can decode a value of any type implementing the GobDecoder or
   101  encoding.BinaryUnmarshaler interfaces by calling the corresponding method,
   102  again in that order of preference.
   103  
   104  Encoding Details
   105  
   106  This section documents the encoding, details that are not important for most
   107  users. Details are presented bottom-up.
   108  
   109  An unsigned integer is sent one of two ways. If it is less than 128, it is sent
   110  as a byte with that value. Otherwise it is sent as a minimal-length big-endian
   111  (high byte first) byte stream holding the value, preceded by one byte holding the
   112  byte count, negated. Thus 0 is transmitted as (00), 7 is transmitted as (07) and
   113  256 is transmitted as (FE 01 00).
   114  
   115  A boolean is encoded within an unsigned integer: 0 for false, 1 for true.
   116  
   117  A signed integer, i, is encoded within an unsigned integer, u. Within u, bits 1
   118  upward contain the value; bit 0 says whether they should be complemented upon
   119  receipt. The encode algorithm looks like this:
   120  
   121  	var u uint
   122  	if i < 0 {
   123  		u = (^uint(i) << 1) | 1 // complement i, bit 0 is 1
   124  	} else {
   125  		u = (uint(i) << 1) // do not complement i, bit 0 is 0
   126  	}
   127  	encodeUnsigned(u)
   128  
   129  The low bit is therefore analogous to a sign bit, but making it the complement bit
   130  instead guarantees that the largest negative integer is not a special case. For
   131  example, -129=^128=(^256>>1) encodes as (FE 01 01).
   132  
   133  Floating-point numbers are always sent as a representation of a float64 value.
   134  That value is converted to a uint64 using math.Float64bits. The uint64 is then
   135  byte-reversed and sent as a regular unsigned integer. The byte-reversal means the
   136  exponent and high-precision part of the mantissa go first. Since the low bits are
   137  often zero, this can save encoding bytes. For instance, 17.0 is encoded in only
   138  three bytes (FE 31 40).
   139  
   140  Strings and slices of bytes are sent as an unsigned count followed by that many
   141  uninterpreted bytes of the value.
   142  
   143  All other slices and arrays are sent as an unsigned count followed by that many
   144  elements using the standard gob encoding for their type, recursively.
   145  
   146  Maps are sent as an unsigned count followed by that many key, element
   147  pairs. Empty but non-nil maps are sent, so if the receiver has not allocated
   148  one already, one will always be allocated on receipt unless the transmitted map
   149  is nil and not at the top level.
   150  
   151  In slices and arrays, as well as maps, all elements, even zero-valued elements,
   152  are transmitted, even if all the elements are zero.
   153  
   154  Structs are sent as a sequence of (field number, field value) pairs. The field
   155  value is sent using the standard gob encoding for its type, recursively. If a
   156  field has the zero value for its type (except for arrays; see above), it is omitted
   157  from the transmission. The field number is defined by the type of the encoded
   158  struct: the first field of the encoded type is field 0, the second is field 1,
   159  etc. When encoding a value, the field numbers are delta encoded for efficiency
   160  and the fields are always sent in order of increasing field number; the deltas are
   161  therefore unsigned. The initialization for the delta encoding sets the field
   162  number to -1, so an unsigned integer field 0 with value 7 is transmitted as unsigned
   163  delta = 1, unsigned value = 7 or (01 07). Finally, after all the fields have been
   164  sent a terminating mark denotes the end of the struct. That mark is a delta=0
   165  value, which has representation (00).
   166  
   167  Interface types are not checked for compatibility; all interface types are
   168  treated, for transmission, as members of a single "interface" type, analogous to
   169  int or []byte - in effect they're all treated as interface{}. Interface values
   170  are transmitted as a string identifying the concrete type being sent (a name
   171  that must be pre-defined by calling Register), followed by a byte count of the
   172  length of the following data (so the value can be skipped if it cannot be
   173  stored), followed by the usual encoding of concrete (dynamic) value stored in
   174  the interface value. (A nil interface value is identified by the empty string
   175  and transmits no value.) Upon receipt, the decoder verifies that the unpacked
   176  concrete item satisfies the interface of the receiving variable.
   177  
   178  If a value is passed to Encode and the type is not a struct (or pointer to struct,
   179  etc.), for simplicity of processing it is represented as a struct of one field.
   180  The only visible effect of this is to encode a zero byte after the value, just as
   181  after the last field of an encoded struct, so that the decode algorithm knows when
   182  the top-level value is complete.
   183  
   184  The representation of types is described below. When a type is defined on a given
   185  connection between an Encoder and Decoder, it is assigned a signed integer type
   186  id. When Encoder.Encode(v) is called, it makes sure there is an id assigned for
   187  the type of v and all its elements and then it sends the pair (typeid, encoded-v)
   188  where typeid is the type id of the encoded type of v and encoded-v is the gob
   189  encoding of the value v.
   190  
   191  To define a type, the encoder chooses an unused, positive type id and sends the
   192  pair (-type id, encoded-type) where encoded-type is the gob encoding of a wireType
   193  description, constructed from these types:
   194  
   195  	type wireType struct {
   196  		ArrayT  *ArrayType
   197  		SliceT  *SliceType
   198  		StructT *StructType
   199  		MapT    *MapType
   200  	}
   201  	type arrayType struct {
   202  		CommonType
   203  		Elem typeId
   204  		Len  int
   205  	}
   206  	type CommonType struct {
   207  		Name string // the name of the struct type
   208  		Id  int    // the id of the type, repeated so it's inside the type
   209  	}
   210  	type sliceType struct {
   211  		CommonType
   212  		Elem typeId
   213  	}
   214  	type structType struct {
   215  		CommonType
   216  		Field []*fieldType // the fields of the struct.
   217  	}
   218  	type fieldType struct {
   219  		Name string // the name of the field.
   220  		Id   int    // the type id of the field, which must be already defined
   221  	}
   222  	type mapType struct {
   223  		CommonType
   224  		Key  typeId
   225  		Elem typeId
   226  	}
   227  
   228  If there are nested type ids, the types for all inner type ids must be defined
   229  before the top-level type id is used to describe an encoded-v.
   230  
   231  For simplicity in setup, the connection is defined to understand these types a
   232  priori, as well as the basic gob types int, uint, etc. Their ids are:
   233  
   234  	bool        1
   235  	int         2
   236  	uint        3
   237  	float       4
   238  	[]byte      5
   239  	string      6
   240  	complex     7
   241  	interface   8
   242  	// gap for reserved ids.
   243  	WireType    16
   244  	ArrayType   17
   245  	CommonType  18
   246  	SliceType   19
   247  	StructType  20
   248  	FieldType   21
   249  	// 22 is slice of fieldType.
   250  	MapType     23
   251  
   252  Finally, each message created by a call to Encode is preceded by an encoded
   253  unsigned integer count of the number of bytes remaining in the message. After
   254  the initial type name, interface values are wrapped the same way; in effect, the
   255  interface value acts like a recursive invocation of Encode.
   256  
   257  In summary, a gob stream looks like
   258  
   259  	(byteCount (-type id, encoding of a wireType)* (type id, encoding of a value))*
   260  
   261  where * signifies zero or more repetitions and the type id of a value must
   262  be predefined or be defined before the value in the stream.
   263  
   264  Compatibility: Any future changes to the package will endeavor to maintain
   265  compatibility with streams encoded using previous versions. That is, any released
   266  version of this package should be able to decode data written with any previously
   267  released version, subject to issues such as security fixes. See the Go compatibility
   268  document for background: https://golang.org/doc/go1compat
   269  
   270  See "Gobs of data" for a design discussion of the gob wire format:
   271  https://blog.golang.org/gobs-of-data
   272  */
   273  package gob
   274  
   275  /*
   276  Grammar:
   277  
   278  Tokens starting with a lower case letter are terminals; int(n)
   279  and uint(n) represent the signed/unsigned encodings of the value n.
   280  
   281  GobStream:
   282  	DelimitedMessage*
   283  DelimitedMessage:
   284  	uint(lengthOfMessage) Message
   285  Message:
   286  	TypeSequence TypedValue
   287  TypeSequence
   288  	(TypeDefinition DelimitedTypeDefinition*)?
   289  DelimitedTypeDefinition:
   290  	uint(lengthOfTypeDefinition) TypeDefinition
   291  TypedValue:
   292  	int(typeId) Value
   293  TypeDefinition:
   294  	int(-typeId) encodingOfWireType
   295  Value:
   296  	SingletonValue | StructValue
   297  SingletonValue:
   298  	uint(0) FieldValue
   299  FieldValue:
   300  	builtinValue | ArrayValue | MapValue | SliceValue | StructValue | InterfaceValue
   301  InterfaceValue:
   302  	NilInterfaceValue | NonNilInterfaceValue
   303  NilInterfaceValue:
   304  	uint(0)
   305  NonNilInterfaceValue:
   306  	ConcreteTypeName TypeSequence InterfaceContents
   307  ConcreteTypeName:
   308  	uint(lengthOfName) [already read=n] name
   309  InterfaceContents:
   310  	int(concreteTypeId) DelimitedValue
   311  DelimitedValue:
   312  	uint(length) Value
   313  ArrayValue:
   314  	uint(n) FieldValue*n [n elements]
   315  MapValue:
   316  	uint(n) (FieldValue FieldValue)*n  [n (key, value) pairs]
   317  SliceValue:
   318  	uint(n) FieldValue*n [n elements]
   319  StructValue:
   320  	(uint(fieldDelta) FieldValue)*
   321  */
   322  
   323  /*
   324  For implementers and the curious, here is an encoded example. Given
   325  	type Point struct {X, Y int}
   326  and the value
   327  	p := Point{22, 33}
   328  the bytes transmitted that encode p will be:
   329  	1f ff 81 03 01 01 05 50 6f 69 6e 74 01 ff 82 00
   330  	01 02 01 01 58 01 04 00 01 01 59 01 04 00 00 00
   331  	07 ff 82 01 2c 01 42 00
   332  They are determined as follows.
   333  
   334  Since this is the first transmission of type Point, the type descriptor
   335  for Point itself must be sent before the value. This is the first type
   336  we've sent on this Encoder, so it has type id 65 (0 through 64 are
   337  reserved).
   338  
   339  	1f	// This item (a type descriptor) is 31 bytes long.
   340  	ff 81	// The negative of the id for the type we're defining, -65.
   341  		// This is one byte (indicated by FF = -1) followed by
   342  		// ^-65<<1 | 1. The low 1 bit signals to complement the
   343  		// rest upon receipt.
   344  
   345  	// Now we send a type descriptor, which is itself a struct (wireType).
   346  	// The type of wireType itself is known (it's built in, as is the type of
   347  	// all its components), so we just need to send a *value* of type wireType
   348  	// that represents type "Point".
   349  	// Here starts the encoding of that value.
   350  	// Set the field number implicitly to -1; this is done at the beginning
   351  	// of every struct, including nested structs.
   352  	03	// Add 3 to field number; now 2 (wireType.structType; this is a struct).
   353  		// structType starts with an embedded CommonType, which appears
   354  		// as a regular structure here too.
   355  	01	// add 1 to field number (now 0); start of embedded CommonType.
   356  	01	// add 1 to field number (now 0, the name of the type)
   357  	05	// string is (unsigned) 5 bytes long
   358  	50 6f 69 6e 74	// wireType.structType.CommonType.name = "Point"
   359  	01	// add 1 to field number (now 1, the id of the type)
   360  	ff 82	// wireType.structType.CommonType._id = 65
   361  	00	// end of embedded wiretype.structType.CommonType struct
   362  	01	// add 1 to field number (now 1, the field array in wireType.structType)
   363  	02	// There are two fields in the type (len(structType.field))
   364  	01	// Start of first field structure; add 1 to get field number 0: field[0].name
   365  	01	// 1 byte
   366  	58	// structType.field[0].name = "X"
   367  	01	// Add 1 to get field number 1: field[0].id
   368  	04	// structType.field[0].typeId is 2 (signed int).
   369  	00	// End of structType.field[0]; start structType.field[1]; set field number to -1.
   370  	01	// Add 1 to get field number 0: field[1].name
   371  	01	// 1 byte
   372  	59	// structType.field[1].name = "Y"
   373  	01	// Add 1 to get field number 1: field[1].id
   374  	04	// struct.Type.field[1].typeId is 2 (signed int).
   375  	00	// End of structType.field[1]; end of structType.field.
   376  	00	// end of wireType.structType structure
   377  	00	// end of wireType structure
   378  
   379  Now we can send the Point value. Again the field number resets to -1:
   380  
   381  	07	// this value is 7 bytes long
   382  	ff 82	// the type number, 65 (1 byte (-FF) followed by 65<<1)
   383  	01	// add one to field number, yielding field 0
   384  	2c	// encoding of signed "22" (0x2c = 44 = 22<<1); Point.x = 22
   385  	01	// add one to field number, yielding field 1
   386  	42	// encoding of signed "33" (0x42 = 66 = 33<<1); Point.y = 33
   387  	00	// end of structure
   388  
   389  The type encoding is long and fairly intricate but we send it only once.
   390  If p is transmitted a second time, the type is already known so the
   391  output will be just:
   392  
   393  	07 ff 82 01 2c 01 42 00
   394  
   395  A single non-struct value at top level is transmitted like a field with
   396  delta tag 0. For instance, a signed integer with value 3 presented as
   397  the argument to Encode will emit:
   398  
   399  	03 04 00 06
   400  
   401  Which represents:
   402  
   403  	03	// this value is 3 bytes long
   404  	04	// the type number, 2, represents an integer
   405  	00	// tag delta 0
   406  	06	// value 3
   407  
   408  */
   409  

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