erl_marshal

C LIBRARY

erl_marshal

LIBRARY SUMMARY

Encoding and Decoding of Erlang terms

DESCRIPTION

This module contains functions for encoding Erlang terms into a sequence of bytes, and for decoding Erlang terms from a sequence of bytes.

EXPORTS

int erl_compare_ext(bufp1, bufp2)

Types:

unsigned char *bufp1,*bufp2;

This function compares two encoded terms.

bufp1 is a buffer containing an encoded Erlang term term1.

bufp2 is a buffer containing an encoded Erlang term term2.

The function returns 0 if the terms are equal, -1 if term1 is less than term2, or 1 if term2 is less than term1.

ETERM *erl_decode(bufp)
ETERM *erl_decode_buf(bufpp)

Types:

unsigned char *bufp;

unsigned char **bufpp;

erl_decode() and erl_decode_buf() decode the contents of a buffer and return the corresponding Erlang term. erl_decode_buf() provides a simple mechanism for dealing with several encoded terms stored consecutively in the buffer.

bufp is a pointer to a buffer containing one or more encoded Erlang terms.

bufpp is the address of a buffer pointer. The buffer contains one or more consecutively encoded Erlang terms. Following a successful call to erl_decode_buf(), bufpp will be updated so that it points to the next encoded term.

erl_decode() returns an Erlang term corresponding to the contents of bufp on success, or NULL on failure. erl_decode_buf() returns an Erlang term corresponding to the first of the consecutive terms in bufpp and moves bufpp forward to point to the next term in the buffer. On failure, each of the functions returns NULL.

int erl_encode(term, bufp)
int erl_encode_buf(term, bufpp)

Types:

ETERM *term;

unsigned char *bufp;

unsigned char **bufpp;

erl_encode() and erl_encode_buf() encode Erlang terms into external format for storage or transmission. erl_encode_buf() provides a simple mechanism for encoding several terms consecutively in the same buffer.

term is an Erlang term to be encoded.

bufp is a pointer to a buffer containing one or more encoded Erlang terms.

bufpp is a pointer to a pointer to a buffer containing one or more consecutively encoded Erlang terms. Following a successful call to erl_encode_buf(), bufpp will be updated so that it points to the position for the next encoded term.

These functions returns the number of bytes written to buffer if successful, otherwise returns 0.

Note that no bounds checking is done on the buffer. It is the caller's responsibility to make sure that the buffer is large enough to hold the encoded terms. You can either use a static buffer that is large enough to hold the terms you expect to need in your program, or use erl_term_len() to determine the exact requirements for a given term.

The following can help you estimate the buffer requirements for a term. Note that this information is implementation specific, and may change in future versions. If you are unsure, use erl_term_len().

Erlang terms are encoded with a 1 byte tag that identifies the type of object, a 2- or 4-byte length field, and then the data itself. Specifically:

Tuples: need 5 bytes, plus the space for each element.
Lists: need 5 bytes, plus the space for each element, and 1 additional byte for the empty list at the end.
Strings and atoms: need 3 bytes, plus 1 byte for each character (the terminating 0 is not encoded). Really long strings (more than 64k characters) are encoded as lists. Atoms cannot contain more than 256 characters.
Integers: need 5 bytes.
Characters: (integers < 256) need 2 bytes.
Floating point numbers: need 32 bytes.
Pids: need 10 bytes, plus the space for the node name, which is an atom.
Ports and Refs: need 6 bytes, plus the space for the node name, which is an atom.

The total space required will be the result calculated from the information above, plus 1 additional byte for a version identifier.

int erl_ext_size(bufp)

Types:

unsigned char *bufp;

This function returns the number of elements in an encoded term.

unsigned char erl_ext_type(bufp)

Types:

unsigned char *bufp;

This function identifies and returns the type of Erlang term encoded in a buffer. It will skip a trailing magic identifier. Returns 0 if the type can't be determined or one of

ERL_INTEGER
ERL_ATOM
ERL_PID /* Erlang process identifier */
ERL_PORT
ERL_REF /* Erlang reference */
ERL_EMPTY_LIST
ERL_LIST
ERL_TUPLE
ERL_FLOAT
ERL_BINARY
ERL_FUNCTION

unsigned char *erl_peek_ext(bufp, pos)

Types:

unsigned char *bufp;

int pos;

This function is used for stepping over one or more encoded terms in a buffer, in order to directly access a later term.

bufp is a pointer to a buffer containing one or more encoded Erlang terms.

pos indicates how many terms to step over in the buffer.

The function returns a pointer to a sub-term that can be used in a subsequent call to erl_decode() in order to retrieve the term at that position. If there is no term, or pos would exceed the size of the terms in the buffer, NULL is returned.

int erl_term_len(t)

Types:

ETERM *t;

This function determines the buffer space that would be needed by t if it were encoded into Erlang external format by erl_encode().

The size in bytes is returned.