Erl_Interface

Reference Manual

Version 3.13.1

Table of Contents

erl_eterm

C Library

erl_eterm

Library Summary

Functions for Erlang term construction.

Description

Note

The support for VxWorks is deprecated as of OTP 22, and will be removed in OTP 23.

Note

The old legacy erl_interface library (functions with prefix erl_) is deprecated as of OTP 22, and will be removed in OTP 23. This does not apply to the ei library. Reasonably new gcc compilers will issue deprecation warnings. In order to disable these warnings, define the macro EI_NO_DEPR_WARN.

This module provides functions for creating and manipulating Erlang terms.

An Erlang term is represented by a C structure of type ETERM. Applications should not reference any fields in this structure directly, as it can be changed in future releases to provide faster and more compact term storage. Instead, applications should use the macros and functions provided.

Each of the following macros takes a single ETERM pointer as an argument. The macros return a non-zero value if the test is true, otherwise 0.

ERL_IS_INTEGER(t)
True if t is an integer.
ERL_IS_UNSIGNED_INTEGER(t)
True if t is an integer.
ERL_IS_FLOAT(t)
True if t is a floating point number.
ERL_IS_ATOM(t)
True if t is an atom.
ERL_IS_PID(t)
True if t is a pid (process identifier).
ERL_IS_PORT(t)
True if t is a port.
ERL_IS_REF(t)
True if t is a reference.
ERL_IS_TUPLE(t)
True if t is a tuple.
ERL_IS_BINARY(t)
True if t is a binary.
ERL_IS_LIST(t)
True if t is a list with zero or more elements.
ERL_IS_EMPTY_LIST(t)
True if t is an empty list.
ERL_IS_CONS(t)
True if t is a list with at least one element.

The following macros can be used for retrieving parts of Erlang terms. None of these do any type checking. Results are undefined if you pass an ETERM* containing the wrong type. For example, passing a tuple to ERL_ATOM_PTR() likely results in garbage.

char *ERL_ATOM_PTR(t)
char *ERL_ATOM_PTR_UTF8(t)
A string representing atom t.
int ERL_ATOM_SIZE(t)
int ERL_ATOM_SIZE_UTF8(t)
The length (in bytes) of atom t.
void *ERL_BIN_PTR(t)
A pointer to the contents of t.
int ERL_BIN_SIZE(t)
The length (in bytes) of binary object t.
int ERL_INT_VALUE(t)
The integer of t.
unsigned int ERL_INT_UVALUE(t)
The unsigned integer value of t.
double ERL_FLOAT_VALUE(t)
The floating point value of t.
ETERM *ERL_PID_NODE(t)
ETERM *ERL_PID_NODE_UTF8(t)
The node in pid t.
int ERL_PID_NUMBER(t)
The sequence number in pid t.
int ERL_PID_SERIAL(t)
The serial number in pid t.
int ERL_PID_CREATION(t)
The creation number in pid t.
int ERL_PORT_NUMBER(t)
The sequence number in port t.
int ERL_PORT_CREATION(t)
The creation number in port t.
ETERM *ERL_PORT_NODE(t)
ETERM *ERL_PORT_NODE_UTF8(t)
The node in port t.
int ERL_REF_NUMBER(t)
The first part of the reference number in ref t. Use only for compatibility.
int ERL_REF_NUMBERS(t)
Pointer to the array of reference numbers in ref t.
int ERL_REF_LEN(t)
The number of used reference numbers in ref t.
int ERL_REF_CREATION(t)
The creation number in ref t.
int ERL_TUPLE_SIZE(t)
The number of elements in tuple t.
ETERM *ERL_CONS_HEAD(t)
The head element of list t.
ETERM *ERL_CONS_TAIL(t)
A list representing the tail elements of list t.

Exports

ETERM *erl_cons(
head,
tail)

Types

ETERM *head;
ETERM *tail;

Concatenates two Erlang terms, prepending head onto tail and thereby creating a cons cell. To make a proper list, tail is always to be a list or an empty list. Notice that NULL is not a valid list.

  • head is the new term to be added.
  • tail is the existing list to which head is concatenated.

The function returns a new list.

ERL_CONS_HEAD(list) and ERL_CONS_TAIL(list) can be used to retrieve the head and tail components from the list. erl_hd(list) and erl_tl(list) do the same thing, but check that the argument really is a list.

Example:

ETERM *list,*anAtom,*anInt;
anAtom = erl_mk_atom("madonna");
anInt  = erl_mk_int(21);
list   = erl_mk_empty_list();
list   = erl_cons(anAtom, list);
list   = erl_cons(anInt, list);
 ... /* do some work */
erl_free_compound(list);

ETERM *erl_copy_term(
term)

Types

ETERM *term;

Creates and returns a copy of the Erlang term term.

ETERM *erl_element(
position,
tuple)

Types

int position;
ETERM *tuple;

Extracts a specified element from an Erlang tuple.

  • position specifies which element to retrieve from tuple. The elements are numbered starting from 1.
  • tuple is an Erlang term containing at least position elements.

Returns a new Erlang term corresponding to the requested element, or NULL if position was greater than the arity of tuple.

ETERM *erl_hd(
list)

Types

ETERM *list;

Extracts the first element from a list.

list is an Erlang term containing a list.

Returns an Erlang term corresponding to the head head element in the list, or a NULL pointer if list was not a list.

void erl_init(
NULL,
0)

Types

void *NULL;
int 0;

This function must be called before any of the others in the Erl_Interface library to initialize the library functions. The arguments must be specified as erl_init(NULL,0).

int erl_iolist_length(
list)

Types

ETERM *list;

Returns the length of an I/O list.

list is an Erlang term containing an I/O list.

Returns the length of list, or -1 if list is not an I/O list.

For the definition of an I/O list, see erl_iolist_to_binary.

ETERM *erl_iolist_to_binary(
term)

Types

ETERM *list;

Converts an I/O list to a binary term.

list is an Erlang term containing a list.

Returns an Erlang binary term, or NULL if list was not an I/O list.

Informally, an I/O list is a deep list of characters and binaries that can be sent to an Erlang port. In BNF, an I/O list is formally defined as follows:

iolist ::= []
        |   Binary
        |   [iohead | iolist]
        ;
iohead ::= Binary
        |   Byte (integer in the range [0..255])
        |   iolist
        ;

char *erl_iolist_to_string(
list)

Types

ETERM *list;

Converts an I/O list to a NULL-terminated C string.

list is an Erlang term containing an I/O list. The I/O list must not contain the integer 0, as C strings may not contain this value except as a terminating marker.

Returns a pointer to a dynamically allocated buffer containing a string. If list is not an I/O list, or if list contains the integer 0, NULL is returned. It is the caller's responsibility to free the allocated buffer with erl_free().

For the definition of an I/O list, see erl_iolist_to_binary.

int erl_length(
list)

Types

ETERM *list;

Determines the length of a proper list.

list is an Erlang term containing a proper list. In a proper list, all tails except the last point to another list cell, and the last tail points to an empty list.

Returns -1 if list is not a proper list.

ETERM *erl_mk_atom(
string)

Types

const char *string;

Creates an atom.

string is the sequence of characters that will be used to create the atom.

Returns an Erlang term containing an atom. Notice that it is the caller's responsibility to ensure that string contains a valid name for an atom.

ERL_ATOM_PTR(atom) and ERL_ATOM_PTR_UTF8(atom) can be used to retrieve the atom name (as a NULL-terminated string). ERL_ATOM_SIZE(atom) and ERL_ATOM_SIZE_UTF8(atom) return the length of the atom name.

Note

The UTF-8 variants were introduced in Erlang/OTP R16 and the string returned by ERL_ATOM_PTR(atom) was not NULL-terminated on older releases.

ETERM *erl_mk_binary(
bptr,
size)

Types

char *bptr;
int size;

Produces an Erlang binary object from a buffer containing a sequence of bytes.

  • bptr is a pointer to a buffer containing data to be converted.
  • size indicates the length of bptr.

Returns an Erlang binary object.

ERL_BIN_PTR(bin) retrieves a pointer to the binary data. ERL_BIN_SIZE(bin) retrieves the size.

ETERM *erl_mk_empty_list()

Creates and returns an empty Erlang list. Notice that NULL is not used to represent an empty list; Use this function instead.

ETERM *erl_mk_estring(
string,
len)

Types

char *string;
int len;

Creates a list from a sequence of bytes.

  • string is a buffer containing a sequence of bytes. The buffer does not need to be NULL-terminated.
  • len is the length of string.

Returns an Erlang list object corresponding to the character sequence in string.

ETERM *erl_mk_float(
f)

Types

double f;

Creates an Erlang float.

f is a value to be converted to an Erlang float.

Returns an Erlang float object with the value specified in f or NULL if f is not finite.

ERL_FLOAT_VALUE(t) can be used to retrieve the value from an Erlang float.

ETERM *erl_mk_int(
n)

Types

int n;

Creates an Erlang integer.

n is a value to be converted to an Erlang integer.

Returns an Erlang integer object with the value specified in n.

ERL_INT_VALUE(t) can be used to retrieve the value from an Erlang integer.

ETERM *erl_mk_list(
array,
arrsize)

Types

ETERM **array;
int arrsize;

Creates an Erlang list from an array of Erlang terms, such that each element in the list corresponds to one element in the array.

  • array is an array of Erlang terms.
  • arrsize is the number of elements in array.

The function creates an Erlang list object, whose length arrsize and whose elements are taken from the terms in array.

ETERM *erl_mk_long_ref(
node,
n1,
n2,
n3,
creation)

Types

const char *node;
unsigned int n1, n2, n3;
unsigned int creation;

Creates an Erlang reference, with 82 bits.

  • node is the name of the C-node.
  • n1, n2, and n3 can be seen as one big number n1*2^64+n2*2^32+n3, which is to be chosen uniquely for each reference created for a given C-node.
  • creation is an arbitrary number.

Notice that n3 and creation are limited in precision, so only the low 18 and 2 bits of these numbers are used.

Returns an Erlang reference object.

ERL_REF_NODE(ref), ERL_REF_NUMBERS(ref), ERL_REF_LEN(ref), and ERL_REF_CREATION(ref) can be used to retrieve the values used to create the reference.

ETERM *erl_mk_pid(
node,
number,
serial,
creation)

Types

const char *node;
unsigned int number;
unsigned int serial;
unsigned int creation;

Creates an Erlang process identifier (pid). The resulting pid can be used by Erlang processes wishing to communicate with the C-node.

  • node is the name of the C-node.
  • number, serial, and creation are arbitrary numbers. Notice that these are limited in precision, so only the low 15, 3, and 2 bits of these numbers are used.

Returns an Erlang pid object.

ERL_PID_NODE(pid), ERL_PID_NUMBER(pid), ERL_PID_SERIAL(pid), and ERL_PID_CREATION(pid) can be used to retrieve the four values used to create the pid.

ETERM *erl_mk_port(
node,
number,
creation)

Types

const char *node;
unsigned int number;
unsigned int creation;

Creates an Erlang port identifier.

  • node is the name of the C-node.
  • number and creation are arbitrary numbers. Notice that these are limited in precision, so only the low 18 and 2 bits of these numbers are used.

Returns an Erlang port object.

ERL_PORT_NODE(port), ERL_PORT_NUMBER(port), and ERL_PORT_CREATION can be used to retrieve the three values used to create the port.

ETERM *erl_mk_ref(
node,
number,
creation)

Types

const char *node;
unsigned int number;
unsigned int creation;

Creates an old Erlang reference, with only 18 bits - use erl_mk_long_ref instead.

  • node is the name of the C-node.
  • number is to be chosen uniquely for each reference created for a given C-node.
  • creation is an arbitrary number.

Notice that number and creation are limited in precision, so only the low 18 and 2 bits of these numbers are used.

Returns an Erlang reference object.

ERL_REF_NODE(ref), ERL_REF_NUMBER(ref), and ERL_REF_CREATION(ref) can be used to retrieve the three values used to create the reference.

ETERM *erl_mk_string(
string)

Types

char *string;

Creates a list from a NULL-terminated string.

string is a NULL-terminated sequence of characters (that is, a C string) from which the list will be created.

Returns an Erlang list.

ETERM *erl_mk_tuple(
array,
arrsize)

Types

ETERM **array;
int arrsize;

Creates an Erlang tuple from an array of Erlang terms.

  • array is an array of Erlang terms.
  • arrsize is the number of elements in array.

The function creates an Erlang tuple, whose arity is size and whose elements are taken from the terms in array.

To retrieve the size of a tuple, either use function erl_size (which checks the type of the checked term and works for a binary as well as for a tuple) or ERL_TUPLE_SIZE(tuple) returns the arity of a tuple. erl_size() does the same thing, but it checks that the argument is a tuple. erl_element(index,tuple) returns the element corresponding to a given position in the tuple.

ETERM *erl_mk_uint(
n)

Types

unsigned int n;

Creates an Erlang unsigned integer.

n is a value to be converted to an Erlang unsigned integer.

Returns an Erlang unsigned integer object with the value specified in n.

ERL_INT_UVALUE(t) can be used to retrieve the value from an Erlang unsigned integer.

ETERM *erl_mk_var(
name)

Types

char *name;

Creates an unbound Erlang variable. The variable can later be bound through pattern matching or assignment.

name specifies a name for the variable.

Returns an Erlang variable object with the name name.

int erl_print_term(
stream,
term)

Types

FILE *stream;
ETERM *term;

Prints the specified Erlang term to the specified output stream.

  • stream indicates where the function is to send its output.
  • term is the Erlang term to print.

Returns the number of characters written on success, otherwise a negative value.

void erl_set_compat_rel(
release_number)

Types

unsigned release_number;

By default, the Erl_Interface library is only guaranteed to be compatible with other Erlang/OTP components from the same release as the Erl_Interface library itself. For example, Erl_Interface from Erlang/OTP R10 is not compatible with an Erlang emulator from Erlang/OTP R9 by default.

A call to erl_set_compat_rel(release_number) sets the Erl_Interface library in compatibility mode of release release_number. Valid range of release_number is [7, current release]. This makes it possible to communicate with Erlang/OTP components from earlier releases.

Note

If this function is called, it may only be called once directly after the call to function erl_init().

Warning

You may run into trouble if this feature is used carelessly. Always ensure that all communicating components are either from the same Erlang/OTP release, or from release X and release Y where all components from release Y are in compatibility mode of release X.

int erl_size(
term)

Types

ETERM *term;

Returns either the arity of an Erlang tuple or the number of bytes in an Erlang binary object.

term is an Erlang tuple or an Erlang binary object.

Returns the size of term as described above, or -1 if term is not one of the two supported types.

ETERM *erl_tl(
list)

Types

ETERM *list;

Extracts the tail from a list.

list is an Erlang term containing a list.

Returns an Erlang list corresponding to the original list minus the first element, or NULL pointer if list was not a list.

ETERM *erl_var_content(
term,
name)

Types

ETERM *term;
char *name;

Returns the contents of the specified variable in an Erlang term.

  • term is an Erlang term. In order for this function to succeed, term must either be an Erlang variable with the specified name, or it must be an Erlang list or tuple containing a variable with the specified name. Other Erlang types cannot contain variables.
  • name is the name of an Erlang variable.

Returns the Erlang object corresponding to the value of name in term. If no variable with the name name is found in term, or if term is not a valid Erlang term, NULL is returned.