# erl_eterm

### C LIBRARY

erl_eterm

### C LIBRARY SUMMARY

Functions for Erlang Term Construction

### DESCRIPTION

This module contains 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, because it may be changed in future releases to provide faster and more compact term storage. Instead, applications should us the macros and functions provided.

The following macros each take a single ETERM pointer as an argument. They return a non-zero value if the test is true, and 0 otherwise:

`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() will likely result in garbage.

`char *ERL_ATOM_PTR(t)`
  A string representing atom `t`.
`int ERL_ATOM_SIZE(t)`
  The length (in characters) 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)`
  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)`
  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;`

This function concatenates two Erlang terms, prepending `head` onto `tail` and thereby creating a `cons` cell. To make a proper list, `tail` should always be a list or an empty list. Note that NULL is not a valid list.

`head` is the new term to be added.

`tail` is the existing list to which `head` will be 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)` will do the same thing, but check that the argument really is a list.

For 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;`

This function creates and returns a copy of the Erlang term `term`.

`ETERM *erl_element(position, tuple)`

Types:

`int position;`
`ETERM *tuple;`

This function 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.

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

`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 in order to initialize the library functions. The arguments must be specified as `erl_init(NULL,0)`.

`ETERM *erl_hd(list)`

Types:

`ETERM *list;`

Extracts the first element from a list.

`list` is an Erlang term containing a list.

The function returns an Erlang term corresponding to the head element in the list, or a NULL pointer if `list` was not a list.

`ETERM *erl_iolist_to_binary(term)`

Types:

`ETERM *list;`

This function converts an IO list to a binary term.

`list` is an Erlang term containing a list.

This function an Erlang binary term, or NULL if `list` was not an IO list.

Informally, an IO list is a deep list of characters and binaries which can be sent to an Erlang port. In BNF, an IO 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;`

This function converts an IO list to a '\0' terminated C string.

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

This function returns a pointer to a dynamically allocated buffer containing a string. If `list` is not an IO list, or if `list` contains the integer 0, NULL is returned. It is the caller's responsibility free the allocated buffer with `erl_free()`.

Refer to `erl_iolist_to_binary()` for the definition of an IO list.

`int erl_iolist_length(list)`

Types:

`ETERM *list;`

Returns the length of an IO list.

`list` is an Erlang term containing an IO list.

The function returns the length of `list`, or -1 if `list` is not an IO list.

Refer to `erl_iolist_to_binary()` for the definition of an IO list.

`int erl_length(list)`

Types:

`ETERM *list;`

Determines the length of a proper list.

`list` is an Erlang term containing 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:

`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. Note that it is the callers responsibility to make sure that `string` contains a valid name for an atom.

`ERL_ATOM_PTR(atom)` can be used to retrieve the atom name (as a string). Note that the string is not 0-terminated in the atom. `ERL_ATOM_SIZE(atom)`returns the length of the atom name.

`ETERM *erl_mk_binary(bptr, size)`

Types:

`char *bptr;`
`int size;`

This function produces an Erlang binary object from a buffer containing a sequence of bytes.

`bptr` is a pointer to a buffer containg data to be converted.

`size` indicates the length of `bptr`.

The function 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()`

This function creates and returns an empty Erlang list. Note 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;`

This function creates a list from a sequence of bytes.

`string` is a buffer containing a sequence of bytes. The buffer does not need to be zero-terminated.

`len` is the length of `string`.

The function 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.

The function returns an Erlang float object with the value specified in `f`.

`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.

The function returns an Erlang integer object with the value specified in `n`.

`ERL_INT_VALUE(t)` can be used to retrieve the value 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_pid(node, number, serial, creation)`

Types:

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

This function creates an Erlang process identifier. 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. Note though, that these are limited in precision, so only the low 15, 3 and 2 bits of these numbers are actually used.

The function 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;`

This function creates an Erlang port identifier.

`node` is the name of the C node.

`number` and `creation` are arbitrary numbers. Note though, that these are limited in precision, so only the low 18 and 2 bits of these numbers are actually used.

The function 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;`

This function creates an old Erlang reference, with only 18 bits - use `erl_mk_long_ref` instead.

`node` is the name of the C node.

`number` should be chosen uniquely for each reference created for a given C node.

`creation` is an arbitrary number.

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

The function returns an Erlang reference object.

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

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

Types:

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

This function 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 should be chosen uniquely for each reference created for a given C node.

`creation` is an arbitrary number.

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

The function returns an Erlang reference object.

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

`ETERM *erl_mk_string(string)`

Types:

`char *string;`

This function creates a list from a zero terminated string.

`string` is the zero-terminated sequence of characters (i.e. a C string) from which the list will be created.

The function 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 the `erl_size` function (which checks the type of the checked term and works for a binary as well as for a tuple), or the `ERL_TUPLE_SIZE(tuple)` returns the arity of a tuple. `erl_size()` will do the same thing, but it checks that the argument really 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.

The function 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;`

This function creates an unbound Erlang variable. The variable can later be bound through pattern matching or assignment.

`name` specifies a name for the variable.

The function returns an Erlang variable object with the name `name`.

`int erl_print_term(stream, term)`

Types:

`FILE *stream;`
`ETERM *term;`

This function prints the specified Erlang term to the given output stream.

`stream` indicates where the function should send its output.

`term` is the Erlang term to print.

The function returns the number of characters written, or a negative value if there was an error.

`int erl_size(term)`

Types:

`ETERM *term;`

Returns 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.

The function 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.

The function 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;`

This function 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 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` was found in `term`, or if `term` is not a valid Erlang term, NULL is returned.

### AUTHORS

T.Törnkvist - support@erlang.ericsson.se
Gordon Beaton - support@erlang.ericsson.se

erl_interface 3.4
Copyright © 1991-2003 Ericsson Utvecklings AB