The overall format of the term format is:

A compressed term looks like this:

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:

As of erts version 5.7.2 the old atom cache protocol was dropped and a new one was introduced. This atom cache protocol introduced the distribution header. Nodes with erts versions earlier than 5.7.2 can still communicate with new nodes, but no distribution header and no atom cache will be used.

The distribution header currently only contains an atom cache reference section, but could in the future contain more information. The distribution header precedes one or more Erlang terms on the external format. For more information see the documentation of the protocol between connected nodes in the distribution protocol documentation.

ATOM_CACHE_REF entries with corresponding AtomCacheReferenceIndex in terms encoded on the external format following a distribution header refers to the atom cache references made in the distribution header. The range is 0 <= AtomCacheReferenceIndex < 255, i.e., at most 255 different atom cache references from the following terms can be made.

The distribution header format is:

Flags consists of NumberOfAtomCacheRefs/2+1 bytes, unless NumberOfAtomCacheRefs is 0. If NumberOfAtomCacheRefs is 0, Flags and AtomCacheRefs are omitted. Each atom cache reference have 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:

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, i.e., an atom cache can contain 2048 entries.

After flag fields for atom cache references, another half byte flag field is located which has the following format:

The least significant bit in that half byte is the LongAtoms flag. If it is set, 2 bytes are used for atom lengths instead of 1 byte in the distribution header. However, the current emulator cannot handle long atoms, so it will currently always be 0.

After the Flags field follow the AtomCacheRefs. The first AtomCacheRef is the one corresponding to AtomCacheReferenceIndex 0. Higher indices follows in sequence up to index NumberOfAtomCacheRefs - 1.

If the NewCacheEntryFlag for the next AtomCacheRef has been set, a NewAtomCacheRef on the following format will follow:

InternalSegmentIndex together with the SegmentIndex completely identify the location of an atom cache entry in the atom cache. Length is number of one byte characters that the atom text consists of. Length is a two byte big endian integer if the LongAtoms flag has been set, otherwise a one byte integer. Subsequent CachedAtomRefs with the same SegmentIndex and InternalSegmentIndex as this NewAtomCacheRef will refer to this atom until a new NewAtomCacheRef with the same SegmentIndex and InternalSegmentIndex appear.

If the NewCacheEntryFlag for the next AtomCacheRef has not been set, a CachedAtomRef on the following format will follow:

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.

Refers to the atom with AtomCacheReferenceIndex in the distribution header.

Unsigned 8 bit integer.

Signed 32 bit integer in big-endian format (i.e. MSB first)

A float 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 .

An atom is stored with a 2 byte unsigned length in big-endian order, followed by Len numbers of 8 bit characters that forms the AtomName. Note: The maximum allowed value for Len is 255.

Encode a reference object (an object generated with make_ref/0). The Node term is an encoded atom, i.e. ATOM_EXT, SMALL_ATOM_EXT or ATOM_CACHE_REF. The ID field contains a big-endian unsigned integer, but should be regarded as uninterpreted data since this field is node specific. Creation is a byte containing a node serial number that makes it possible to separate old (crashed) nodes from a new one.

In ID, only 18 bits are significant; the rest should be 0. In Creation, only 2 bits are significant; the rest should be 0. See NEW_REFERENCE_EXT.

Encode a port object (obtained form open_port/2). The ID is a node specific identifier for a local port. Port operations are not allowed across node boundaries. The Creation works just like in REFERENCE_EXT.

Encode a process identifier object (obtained from spawn/3 or friends). The ID and Creation fields works just like in REFERENCE_EXT, while the Serial field is used to improve safety. In ID, only 15 bits are significant; the rest should be 0.

SMALL_TUPLE_EXT encodes a tuple. The Arity field is an unsigned byte that determines how many element that follows in the Elements section.

Same as SMALL_TUPLE_EXT with the exception that Arity is an unsigned 4 byte integer in big endian format.

The representation for an empty list, i.e. the Erlang syntax [].

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. Since the Length field is an unsigned 2 byte integer (big endian), implementations must make sure that lists longer than 65535 elements are encoded as LIST_EXT.

Length is the number of elements that follows in the Elements section. Tail is the final tail of the list; it is NIL_EXT for a proper list, but may be anything type if the list is improper (for instance [a|b]).

Binaries are generated with bit syntax expression or with list_to_binary/1, term_to_binary/1, or as input from binary ports. The Len length field is an unsigned 4 byte integer (big endian).

Bignums are stored in unary form with a Sign byte that is 0 if the binum is positive and 1 if is negative. The digits are stored with the LSB 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))

Same as SMALL_BIG_EXT with the difference that the length field is an unsigned 4 byte integer.

Node and Creation are as in REFERENCE_EXT.

ID contains a sequence of big-endian unsigned integers (4 bytes each, so N' is a multiple of 4), but should be regarded as uninterpreted data.

N' = 4 * Len.

In the first word (four bytes) of ID, only 18 bits are significant, the rest should be 0. In Creation, only 2 bits are significant, the rest should be 0.

NEW_REFERENCE_EXT was introduced with distribution version 4. In version 4, N' should be at most 12.

See REFERENCE_EXT).

An atom is stored with a 1 byte unsigned length, followed by Len numbers of 8 bit characters that forms the AtomName. Longer atoms can be represented by ATOM_EXT. Note the SMALL_ATOM_EXT was introduced in erts version 5.7.2 and require a small atom distribution flag exchanged in the distribution handshake.

This is the new encoding of internal funs: fun F/A and fun(Arg1,..) -> ... end.

This term is the encoding for external funs: fun M:F/A.

Module and Function are atoms (encoded using ATOM_EXT, SMALL_ATOM_EXT or ATOM_CACHE_REF).

Arity is an integer encoded using SMALL_INTEGER_EXT.

This term represents a bitstring whose length in bits is not a multiple of 8 (created using the bit syntax in R12B and later). The Len field is an unsigned 4 byte integer (big endian). The Bits field is the number of bits that are used in the last byte in the data field, counting from the most significant bit towards the least significant.

A float is stored as 8 bytes in big-endian IEEE format.

This term is used in minor version 1 of the external format.