2007
2023
Ericsson AB, All Rights Reserved
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
The Initial Developer of the Original Code is Ericsson AB.
External Term Format
Kenneth
2007-09-21
PA1
erl_ext_dist.xml
Introduction
The external term format is mainly used in the distribution
mechanism of Erlang.
As Erlang has a fixed number of types, there is no need for a
programmer to define a specification for the external format used
within some application.
All Erlang terms have an external representation and the interpretation
of the different terms is application-specific.
In Erlang the BIF
erlang:term_to_binary/1,2 is used to convert a
term into the external format.
To convert binary data encoding to a term, the BIF
erlang:binary_to_term/1 is used.
The distribution does this implicitly when sending messages across
node boundaries.
The overall format of the term format is as follows:
1 |
1 |
N |
131 |
Tag |
Data |
Term Format
When messages are
passed between
connected nodes and a
distribution
header is used, the first byte containing the version
number (131) is omitted from the terms that follow the distribution
header. This is because the version number is implied by the version
number in the distribution header.
The compressed term format is as follows:
1 |
1 |
4 |
N |
131 |
80 |
UncompressedSize |
Zlib-compressedData |
Compressed Term Format
Uncompressed size (unsigned 32-bit integer in big-endian byte order)
is the size of the data before it was compressed.
The compressed data has the following format when it has been expanded:
1 |
Uncompressed Size |
Tag |
Data |
Compressed Data Format when Expanded
Encoding atoms
As from ERTS 9.0 (OTP 20), atoms may contain any Unicode characters.
Atoms sent over node distribution are always encoded in UTF-8 using
either ATOM_UTF8_EXT,
SMALL_ATOM_UTF8_EXT
or ATOM_CACHE_REF.
Atoms encoded with
erlang:term_to_binary/1,2 or
erlang:term_to_iovec/1,2 are by default still using the
old deprecated Latin-1 format
ATOM_EXT
for atoms that only contain Latin-1 characters (Unicode code points
0-255). Atoms with higher code points will be encoded in UTF-8 using
either ATOM_UTF8_EXT or
SMALL_ATOM_UTF8_EXT.
The maximum number of allowed characters in an atom is 255. In the
UTF-8 case, each character can need 4 bytes to be encoded.
Distribution Header
The distribution header is sent by the erlang distribution to
carry metadata about the coming
control message
and potential payload. It is primarily used to handle the atom cache
in the Erlang distribution. Since OTP-22 it is also used to fragment
large distribution messages into multiple smaller fragments.
For more information about how the distribution uses the distribution header,
see the documentation of the
protocol between
connected nodes in the
distribution protocol
documentation.
Any ATOM_CACHE_REF
entries with corresponding AtomCacheReferenceIndex in terms
encoded on the external format following a distribution header refer
to the atom cache references made in the distribution header. The range
is 0 <= AtomCacheReferenceIndex < 255, that is, at most 255
different atom cache references from the following terms can be made.
Normal Distribution Header
The non-fragmented distribution header format is as follows:
1 |
1 |
1 |
NumberOfAtomCacheRefs/2+1 | 0 |
N | 0 |
131 |
68 |
NumberOfAtomCacheRefs |
Flags |
AtomCacheRefs |
Normal Distribution Header Format
Flags consist of NumberOfAtomCacheRefs/2+1 bytes,
unless NumberOfAtomCacheRefs is 0. If
NumberOfAtomCacheRefs is 0, Flags and
AtomCacheRefs are omitted. Each atom cache reference has
a half byte flag field. Flags corresponding to a specific
AtomCacheReferenceIndex are located in flag byte number
AtomCacheReferenceIndex/2. Flag byte 0 is the first byte
after the NumberOfAtomCacheRefs byte. Flags for an even
AtomCacheReferenceIndex are located in the least significant
half byte and flags for an odd AtomCacheReferenceIndex are
located in the most significant half byte.
The flag field of an atom cache reference has the following
format:
1 bit |
3 bits |
NewCacheEntryFlag |
SegmentIndex |
The most significant bit is the NewCacheEntryFlag. If set,
the corresponding cache reference is new. The three least
significant bits are the SegmentIndex of the corresponding
atom cache entry. An atom cache consists of 8 segments, each of size
256, that is, an atom cache can contain 2048 entries.
Another half byte flag field is located along with flag fields for
atom cache references. When NumberOfAtomCacheRefs is even,
this half byte is the least significant half byte of the byte that
follows the atom cache references. When NumberOfAtomCacheRefs
is odd, this half byte is the most significant half byte of the last
byte of the atom cache references (on the wire, it will appear before
the last cache reference). It has the following format:
3 bits |
1 bit |
CurrentlyUnused |
LongAtoms |
The least significant bit in that half byte is flag LongAtoms.
If it is set, 2 bytes are used for atom lengths instead of
1 byte in the distribution header.
After the Flags field follow the AtomCacheRefs. The
first AtomCacheRef is the one corresponding to
AtomCacheReferenceIndex 0. Higher indices follow
in sequence up to index NumberOfAtomCacheRefs - 1.
If the NewCacheEntryFlag for the next AtomCacheRef has
been set, a NewAtomCacheRef on the following format follows:
1 |
1 | 2 |
Length |
InternalSegmentIndex |
Length |
AtomText |
InternalSegmentIndex together with the SegmentIndex
completely identify the location of an atom cache entry in the
atom cache. Length is the number of bytes that AtomText
consists of. Length is a 2 byte big-endian integer
if flag LongAtoms has been set, otherwise a 1 byte
integer. When distribution flag
DFLAG_UTF8_ATOMS
has been exchanged between both nodes in the
distribution handshake,
characters in AtomText are encoded in UTF-8, otherwise
in Latin-1. The following CachedAtomRefs with the same
SegmentIndex and InternalSegmentIndex as this
NewAtomCacheRef refer to this atom until a new
NewAtomCacheRef with the same SegmentIndex
and InternalSegmentIndex appear.
For more information on encoding of atoms, see the
section on UTF-8 encoded atoms
above.
If the NewCacheEntryFlag for the next AtomCacheRef
has not been set, a CachedAtomRef on the following format
follows:
InternalSegmentIndex together with the SegmentIndex
identify the location of the atom cache entry in the atom cache.
The atom corresponding to this CachedAtomRef is the
latest NewAtomCacheRef preceding this CachedAtomRef
in another previously passed distribution header.
Distribution Header for fragmented messages
Messages sent between Erlang nodes can sometimes be
quite large. Since OTP-22 it is possible to split large messages
into smaller fragments in order to allow smaller messages to be interleaved
between larges messages. It is only the message part of each
distributed message
that may be split using fragmentation. Therefore it is recommended to use the
PAYLOAD control messages introduced in OTP-22.
Fragmented distribution messages are only used if the receiving node
signals that it supports them via the
DFLAG_FRAGMENTS
distribution flag.
A process must complete the sending of a fragmented message before it
can start sending any other message on the same distribution channel.
The start of a sequence of fragmented messages looks like this:
1 |
1 |
8 |
8 |
1 |
NumberOfAtomCacheRefs/2+1 | 0 |
N | 0 |
131 |
69 |
SequenceId |
FragmentId |
NumberOfAtomCacheRefs |
Flags |
AtomCacheRefs |
Starting Fragmented Distribution Header Format
The continuation of a sequence of fragmented messages looks like this:
1 |
1 |
8 |
8 |
131 |
70 |
SequenceId |
FragmentId |
Continuing Fragmented Distribution Header Format
The starting distribution header is very similar to a non-fragmented distribution
header. The atom cache works the same as for normal distribution header and
is the same for the entire sequence. The additional fields added are the
sequence id and fragment id.
Sequence ID
-
The sequence id is used to uniquely identify a fragmented message sent
from one process to another on the same distributed connection. This is used
to identify which sequence a fragment is a part of as the same process can
be in the process of receiving multiple sequences at the same time.
As one process can only be sending one fragmented message at once,
it can be convenient to use the local PID as the sequence id.
Fragments ID
-
The Fragment ID is used to number the fragments in a sequence.
The id starts at the total number of fragments and then decrements to 1
(which is the final fragment). So if a sequence consists of 3 fragments
the fragment id in the starting header will be 3, and then fragments 2 and 1
are sent.
The fragments must be delivered in the correct order, so if an unordered
distribution carrier is used, they must be ordered before delivered to the
Erlang run-time.
Example:
As an example, let say that we want to send
{call, <0.245.2>, {set_get_state, <<0:1024>>}} to
registered process reg using a fragment size of 128. To send
this message we need a distribution header, atom cache updates,
the control message (which would be {6, <0.245.2>, [], reg} in this case)
and finally the actual message. This would all be encoded into:
131,69,0,0,2,168,0,0,5,83,0,0,0,0,0,0,0,2, %% Header with seq and frag id
5,4,137,9,10,5,236,3,114,101,103,9,4,99,97,108,108, %% Atom cache updates
238,13,115,101,116,95,103,101,116,95,115,116,97,116,101,
104,4,97,6,103,82,0,0,0,0,85,0,0,0,0,2,82,1,82,2, %% Control message
104,3,82,3,103,82,0,0,0,0,245,0,0,0,2,2, %% Actual message using cached atoms
104,2,82,4,109,0,0,0,128,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
131,70,0,0,2,168,0,0,5,83,0,0,0,0,0,0,0,1, %% Cont Header with seq and frag id
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, %% Rest of payload
0,0,0,0
Let us break that apart into its components. First we have the
distribution header tags together with the sequence id and
a fragment id of 2.
131,69, %% Start fragment header
0,0,2,168,0,0,5,83, %% The sequence ID
0,0,0,0,0,0,0,2, %% The fragment ID
Then we have the updates to the atom cache:
5,4,137,9, %% 5 atoms and their flags
10,5, %% The already cached atom ids
236,3,114,101,103, %% The atom 'reg'
9,4,99,97,108,108, %% The atom 'call'
238,13,115,101,116,95,103,101,116,95,115,116,97,116,101, %% The atom 'set_get_state'
The first byte says that we have 5 atoms that are part
of the cache. Then follows three bytes that are the
atom cache ref flags. Each of the flags uses 4 bits so
they are a bit hard to read in decimal byte form. In
binary half-byte form they look like this:
0000, 0100, 1000, 1001, 1001
As the high bit of the first two atoms in the
cache are not set we know that they are already in the cache,
so they do not have to be sent again (this is the node name of the
receiving and sending node). Then follows the atoms that have to be sent,
together with their segment ids.
Then the listing of the atoms comes, starting with 10 and 5
which are the atom refs of the already cached atoms. Then the
new atoms are sent.
When the atom cache is setup correctly the control message is sent.
104,4,97,6,103,82,0,0,0,0,85,0,0,0,0,2,82,1,82,2,
Note that up until here it is not allowed to fragments the message.
The entire atom cache and control message has to be part of the
starting fragment. After the control message the payload of the message
is sent using 128 bytes:
104,3,82,3,103,82,0,0,0,0,245,0,0,0,2,2,
104,2,82,4,109,0,0,0,128,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
Since the payload is larger than 128-bytes it is split into two
fragments. The second fragment does not have any atom cache update
instructions so it is a lot simpler:
131,70,0,0,2,168,0,0,5,83,0,0,0,0,0,0,0,1, %% Continuation dist header 70 with seq and frag id
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, %% remaining payload
0,0,0,0
The fragment size of 128 is only used as an example.
Any fragments size may be used when sending fragmented messages.
ATOM_CACHE_REF
1 |
1 |
82 |
AtomCacheReferenceIndex |
ATOM_CACHE_REF
Refers to the atom with AtomCacheReferenceIndex in the
distribution header.
SMALL_INTEGER_EXT
1 |
1 |
97 |
Int |
SMALL_INTEGER_EXT
Unsigned 8-bit integer.
INTEGER_EXT
Signed 32-bit integer in big-endian format.
FLOAT_EXT
1 |
31 |
99 |
Float string |
FLOAT_EXT
A finite float (i.e. not inf, -inf or NaN) is stored in
string format. The format used in sprintf to format the
float is "%.20e"
(there are more bytes allocated than necessary).
To unpack the float, use sscanf with format "%lf".
This term is used in minor version 0 of the external format;
it has been superseded by
NEW_FLOAT_EXT.
PORT_EXT
1 |
N |
4 |
1 |
102 |
Node |
ID |
Creation |
PORT_EXT
Same as NEW_PORT_EXT
except the Creation field is only one byte and only two
bits are significant, the rest are to be 0.
NEW_PORT_EXT
1 |
N |
4 |
4 |
89 |
Node |
ID |
Creation |
NEW_PORT_EXT
Same as V4_PORT_EXT
except the ID field is only four bytes. Only 28 bits are
significant; the rest are to be 0.
NEW_PORT_EXT was introduced in OTP 19, but only to be decoded
and echoed back. Not encoded for local ports.
In OTP 23 distribution flag
DFLAG_BIG_CREATION
became mandatory. All ports are now
encoded using NEW_PORT_EXT, even external ports received as PORT_EXT from older nodes.
V4_PORT_EXT
1 |
N |
8 |
4 |
120 |
Node |
ID |
Creation |
V4_PORT_EXT
Encodes a port identifier (obtained from
erlang:open_port/2).
Node is the originating node,
encoded as an atom.
ID is a 64-bit big endian unsigned integer. The Creation
works just like in NEW_PID_EXT.
Port operations are not allowed across node boundaries.
In OTP 26 distribution flag
DFLAG_V4_NC
as well as V4_PORT_EXT became mandatory accepting full
64-bit ports to be decoded and echoed back.
PID_EXT
1 |
N |
4 |
4 |
1 |
103 |
Node |
ID |
Serial |
Creation |
PID_EXT
Same as NEW_PID_EXT
except the Creation field is only one byte and only two
bits are significant, the rest are to be 0.
NEW_PID_EXT
1 |
N |
4 |
4 |
4 |
88 |
Node |
ID |
Serial |
Creation |
NEW_PID_EXT
Encodes an Erlang process identifier object.
Node
The name of the originating node,
encoded as an atom.
ID
A 32-bit big endian unsigned integer.
Serial
A 32-bit big endian unsigned integer.
Creation
A 32-bit big endian unsigned integer. All identifiers
originating from the same node incarnation must have identical Creation
values. This makes it possible to separate identifiers from old
(crashed) nodes from a new one. The value zero is reserved and
must be avoided for normal operations.
NEW_PID_EXT was introduced in OTP 19, but only to be decoded
and echoed back. Not encoded for local processes.
In OTP 23 distribution flag
DFLAG_BIG_CREATION
became mandatory. All pids are now encoded using NEW_PID_EXT,
even external pids received as
PID_EXT from older nodes.
In OTP 26 distribution flag
DFLAG_V4_NC
became mandatory accepting full 64-bit pids to be decoded
and echoed back.
SMALL_TUPLE_EXT
1 |
1 |
N |
104 |
Arity |
Elements |
SMALL_TUPLE_EXT
Encodes a tuple. The Arity
field is an unsigned byte that determines how many elements
that follows in section Elements.
LARGE_TUPLE_EXT
1 |
4 |
N |
105 |
Arity |
Elements |
LARGE_TUPLE_EXT
Same as
SMALL_TUPLE_EXT
except that Arity is an
unsigned 4 byte integer in big-endian format.
MAP_EXT
1 |
4 |
N |
116 |
Arity |
Pairs |
MAP_EXT
Encodes a map. The Arity field is an unsigned
4 byte integer in big-endian format that determines the number of
key-value pairs in the map. Key and value pairs (Ki => Vi)
are encoded in section Pairs in the following order:
K1, V1, K2, V2,..., Kn, Vn.
Duplicate keys are not allowed within the same map.
As from Erlang/OTP 17.0
NIL_EXT
The representation for an empty list, that is, the Erlang syntax
[].
STRING_EXT
1 |
2 |
Len |
107 |
Length |
Characters |
STRING_EXT
String does not have a corresponding Erlang representation,
but is an optimization for sending lists of bytes (integer in
the range 0-255) more efficiently over the distribution.
As field Length is an unsigned 2 byte integer
(big-endian), implementations must ensure that lists longer than
65535 elements are encoded as
LIST_EXT.
LIST_EXT
1 |
4 |
|
|
108 |
Length |
Elements |
Tail |
LIST_EXT
Length is the number of elements that follows in section
Elements. Tail is the final tail of the list; it is
NIL_EXT
for a proper list, but can be any type if the list is
improper (for example, [a|b]).
BINARY_EXT
1 |
4 |
Len |
109 |
Len |
Data |
BINARY_EXT
Binaries are generated with bit syntax expression or with
erlang:list_to_binary/1,
erlang:term_to_binary/1,
or as input from binary ports.
The Len length field is an unsigned 4 byte integer
(big-endian).
SMALL_BIG_EXT
1 |
1 |
1 |
n |
110 |
n |
Sign |
d(0) ... d(n-1) |
SMALL_BIG_EXT
Bignums are stored in unary form with a Sign byte,
that is, 0 if the bignum is positive and 1 if it is negative. The
digits are stored with the least significant byte stored first. To
calculate the integer, the following formula can be used:
B = 256
(d0*B^0 + d1*B^1 + d2*B^2 + ... d(N-1)*B^(n-1))
LARGE_BIG_EXT
1 |
4 |
1 |
n |
111 |
n |
Sign |
d(0) ... d(n-1) |
LARGE_BIG_EXT
Same as
SMALL_BIG_EXT
except that the length field is an unsigned 4 byte integer.
REFERENCE_EXT (deprecated)
1 |
N |
4 |
1 |
101 |
Node |
ID |
Creation |
REFERENCE_EXT
The same as
NEW_REFERENCE_EXT except ID is only one word
(Len = 1).
NEW_REFERENCE_EXT
1 |
2 |
N |
1 |
N' |
114 |
Len |
Node |
Creation |
ID ... |
NEW_REFERENCE_EXT
The same as
NEWER_REFERENCE_EXT except:
ID
In the first word (4 bytes) of ID, only 18 bits are
significant, the rest must be 0.
Creation
Only one byte long and only two bits are significant, the rest must be 0.
NEWER_REFERENCE_EXT
1 |
2 |
N |
4 |
N' |
90 |
Len |
Node |
Creation |
ID ... |
NEWER_REFERENCE_EXT
Encodes a reference term generated with
erlang:make_ref/0.
Node
The name of the originating node,
encoded as an atom.
Len
A 16-bit big endian unsigned integer not larger than 5.
ID
A sequence of Len big-endian unsigned integers
(4 bytes each, so N' = 4 * Len),
but is to be regarded as uninterpreted data.
Creation
Works just like in
NEW_PID_EXT.
NEWER_REFERENCE_EXT was introduced in OTP 19, but only to be decoded
and echoed back. Not encoded for local references.
In OTP 23 distribution flag
DFLAG_BIG_CREATION
became mandatory. All references are now encoded using
NEWER_REFERENCE_EXT, even external references received as
NEW_REFERENCE_EXT
from older nodes.
In OTP 26 distribution flag
DFLAG_V4_NC
became mandatory. References now can contain up to 5 ID words.
FUN_EXT (removed)
1 |
4 |
N1 |
N2 |
N3 |
N4 |
N5 |
117 |
NumFree |
Pid |
Module |
Index |
Uniq |
Free vars ... |
FUN_EXT
Not emitted since OTP R8, and not decoded since OTP 23.
NEW_FUN_EXT
1 |
4 |
1 |
16 |
4 |
4 |
N1 |
N2 |
N3 |
N4 |
N5 |
112 |
Size |
Arity |
Uniq |
Index |
NumFree |
Module |
OldIndex |
OldUniq |
Pid |
Free Vars |
NEW_FUN_EXT
This is the encoding of internal funs: fun F/A and
fun(Arg1,..) -> ... end.
Size
-
The total number of bytes, including field Size.
Arity
-
The arity of the function implementing the fun.
Uniq
-
The 16 bytes MD5 of the significant parts of the Beam file.
Index
-
An index number. Each fun within a module has an unique
index. Index is stored in big-endian byte order.
NumFree
-
The number of free variables.
Module
-
The module that the fun is implemented in,
encoded as an atom.
OldIndex
-
An integer encoded using
SMALL_INTEGER_EXT or
INTEGER_EXT.
Is typically a small index into the module's fun table.
OldUniq
-
An integer encoded using
SMALL_INTEGER_EXT or
INTEGER_EXT.
Uniq is the hash value of the parse tree for the fun.
Pid
-
A process identifier as in
PID_EXT.
Represents the process in which the fun was created.
Free vars
-
NumFree number of terms, each one encoded according
to its type.
EXPORT_EXT
1 |
N1 |
N2 |
N3 |
113 |
Module |
Function |
Arity |
EXPORT_EXT
This term is the encoding for external funs: fun M:F/A.
Module and Function are
encoded as atoms.
Arity is an integer encoded using
SMALL_INTEGER_EXT.
BIT_BINARY_EXT
1 |
4 |
1 |
Len |
77 |
Len |
Bits |
Data |
BIT_BINARY_EXT
This term represents a bitstring whose length in bits does
not have to be a multiple of 8.
The Len field is an unsigned 4 byte integer (big-endian).
The Bits field is the number of bits (1-8) that are used
in the last byte in the data field,
counting from the most significant bit to the least significant.
NEW_FLOAT_EXT
1 |
8 |
70 |
IEEE float |
NEW_FLOAT_EXT
A finite float (i.e. not inf, -inf or NaN) is stored as 8 bytes
in big-endian IEEE format.
This term is used in minor version 1 of the external format.
ATOM_UTF8_EXT
1 |
2 |
Len |
118 |
Len |
AtomName |
ATOM_UTF8_EXT
An atom is stored with a 2 byte unsigned length in big-endian order,
followed by Len bytes containing the AtomName encoded
in UTF-8.
For more information, see the
section on encoding atoms
in the beginning of this page.
SMALL_ATOM_UTF8_EXT
1 |
1 |
Len |
119 |
Len |
AtomName |
SMALL_ATOM_UTF8_EXT
An atom is stored with a 1 byte unsigned length,
followed by Len bytes containing the AtomName encoded
in UTF-8. Longer atoms encoded in UTF-8 can be represented using
ATOM_UTF8_EXT.
For more information, see the
section on encoding atoms
in the beginning of this page.
ATOM_EXT (deprecated)
1 |
2 |
Len |
100 |
Len |
AtomName |
ATOM_EXT
An atom is stored with a 2 byte unsigned length in big-endian order,
followed by Len numbers of 8-bit Latin-1 characters that forms
the AtomName. The maximum allowed value for Len is 255.
SMALL_ATOM_EXT (deprecated)
1 |
1 |
Len |
115 |
Len |
AtomName |
SMALL_ATOM_EXT
An atom is stored with a 1 byte unsigned length,
followed by Len numbers of 8-bit Latin-1 characters that
forms the AtomName.
SMALL_ATOM_EXT was introduced in ERTS 5.7.2 and
require an exchange of distribution flag
DFLAG_SMALL_ATOM_TAGS in the
distribution handshake.
LOCAL_EXT
Marks that this is encoded on an alternative local external term
format intended to only be decoded by a specific local decoder.
The bytes following from here on may contain any unspecified type
of encoding of terms. It is the responsibility of the user to only
attempt to decode terms on the local external term format which has
been produced by a matching encoder.
This tag is used by the Erlang runtime system upon encoding the local
external term format when the
local
option is passed to
term_to_binary/2,
but can be used by other encoders as well providing similar
functionality. The Erlang runtime system adds a hash immediately
following the LOCAL_EXT tag which is verified on decoding in
order to verify that encoder and decoder match which might be a good
practice. This will very likely catch mistakes made by users, but
is not guaranteed to, and is not intended to, prevent decoding of an
intentionally forged encoding on the local external term format.
LOCAL_EXT was introduced in OTP @OTP-18477@.