19962013 Ericsson AB. All Rights Reserved. The contents of this file are subject to the Erlang Public License, Version 1.1, (the "License"); you may not use this file except in compliance with the License. You should have received a copy of the Erlang Public License along with this software. If not, it can be retrieved online at http://www.erlang.org/. Software distributed under the License is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License for the specific language governing rights and limitations under the License. erl_eterm Torbjörn Törnkvist Torbjörn Törnkvist Bjarne Däcker Torbjörn Törnkvist 980703 A erl_eterm.sgml
erl_eterm Functions for Erlang Term Construction

This module contains functions for creating and manipulating Erlang terms.

An Erlang term is represented by a C structure of type . 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:

True if is an integer. True if is an integer. True if is a floating point number. True if is an atom. True if is a Pid (process identifier). True if is a port. True if is a reference. True if is a tuple. True if is a binary. True if is a list with zero or more elements. True if is an empty list. True if 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.

A string representing atom . The length (in bytes) of atom t. A pointer to the contents of The length (in bytes) of binary object . The integer of . The unsigned integer value of . The floating point value of . The Node in pid . The sequence number in pid . The serial number in pid . The creation number in pid . The sequence number in port . The creation number in port . The node in port . The first part of the reference number in ref . Use only for compatibility. Pointer to the array of reference numbers in ref . The number of used reference numbers in ref . The creation number in ref . The number of elements in tuple . The head element of list . A List representing the tail elements of list . ETERM *erl_cons(head, tail) Prepends a term to the head of a list. ETERM *head; ETERM *tail;

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

is the new term to be added.

is the existing list to which will be concatenated.

The function returns a new list.

and can be used to retrieve the head and tail components from the list. and will do the same thing, but check that the argument really is a list.

For example:

 ETERM *erl_copy_term(term) Creates a copy of an Erlang term ETERM *term;

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

 ETERM *erl_element(position, tuple) Extracts an element from an Erlang tuple int position; ETERM *tuple;

This function extracts a specified element from an Erlang tuple.

specifies which element to retrieve from . The elements are numbered starting from 1.

is an Erlang term containing at least elements.

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

 voiderl_init(NULL, 0) Initialization routine void *NULL; int 0;

This function must be called before any of the others in the library in order to initialize the library functions. The arguments must be specified as .

 ETERM *erl_hd(list) Extracts the first element from a list ETERM *list;

Extracts the first element from a 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 was not a list.

 ETERM *erl_iolist_to_binary(term) Converts an IO list to a binary ETERM *list;

This function converts an IO list to a binary term.

is an Erlang term containing a list.

This function an Erlang binary term, or NULL if 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:

char *erl_iolist_to_string(list) Converts an IO list to a zero terminated string ETERM *list;

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

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 is not an IO list, or if contains the integer 0, NULL is returned. It is the caller's responsibility free the allocated buffer with .

Refer to for the definition of an IO list.

interl_iolist_length(list) Return the length of an IO list ETERM *list;

Returns the length of an IO list.

is an Erlang term containing an IO list.

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

Refer to for the definition of an IO list.

interl_length(list) Determines the length of a list ETERM *list;

Determines the length of a proper 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 is not a proper list.

 ETERM *erl_mk_atom(string) Creates an atom const char *string;

Creates an atom.

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 contains a valid name for an atom.

and can be used to retrieve the atom name (as a null terminated string). and returns the length of the atom name.

Note that the UTF8 variants were introduced in Erlang/OTP releases R16 and the string returned by ERL_ATOM_PTR(atom) was not null terminated on older releases.

 ETERM *erl_mk_binary(bptr, size) Creates a binary object char *bptr; int size;

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

is a pointer to a buffer containing data to be converted.

indicates the length of .

The function returns an Erlang binary object.

retrieves a pointer to the binary data. retrieves the size.

ETERM *erl_mk_empty_list() Creates an empty Erlang 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) Creates an Erlang string char *string; int len;

This function creates a list from a sequence of bytes.

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

is the length of .

The function returns an Erlang list object corresponding to the character sequence in .

 ETERM *erl_mk_float(f) Creates an Erlang float double f;

Creates an Erlang float.

is a value to be converted to an Erlang float.

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

can be used to retrieve the value from an Erlang float.

 ETERM *erl_mk_int(n) Creates an Erlang integer int n;

Creates an Erlang integer.

is a value to be converted to an Erlang integer.

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

can be used to retrieve the value value from an Erlang integer.

 ETERM *erl_mk_list(array, arrsize) Creates a list from an array 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.

is an array of Erlang terms.

is the number of elements in .

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

 ETERM *erl_mk_pid(node, number, serial, creation) Creates a process identifier 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.

is the name of the C node.

, and 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.

, , and can be used to retrieve the four values used to create the pid.

 ETERM *erl_mk_port(node, number, creation) Creates a port identifier const char *node; unsigned int number; unsigned int creation;

This function creates an Erlang port identifier.

is the name of the C node.

and 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.

, and can be used to retrieve the three values used to create the port.

ETERM *erl_mk_ref(node, number, creation) Creates an old Erlang reference const char *node; unsigned int number; unsigned int creation;

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

is the name of the C node.

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

is an arbitrary number.

Note that and 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.

, , and to retrieve the three values used to create the reference.

ETERM *erl_mk_long_ref(node, n1, n2, n3, creation) Creates an Erlang reference const char *node; unsigned int n1, n2, n3; unsigned int creation;

This function creates an Erlang reference, with 82 bits.

is the name of the C node.

, and can be seen as one big number which should be chosen uniquely for each reference created for a given C node.

is an arbitrary number.

Note that and 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.

, , and to retrieve the values used to create the reference.

ETERM *erl_mk_string(string) Creates a string char *string;

This function creates a list from a zero terminated 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) Creates an Erlang tuple from an array ETERM **array; int arrsize;

Creates an Erlang tuple from an array of Erlang terms.

is an array of Erlang terms.

is the number of elements in .

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

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

ETERM *erl_mk_uint(n) Creates an unsigned integer unsigned int n;

Creates an Erlang unsigned integer.

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

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

can be used to retrieve the value from an Erlang unsigned integer.

 ETERM *erl_mk_var(name) Creates an Erlang variable char *name;

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

specifies a name for the variable.

The function returns an Erlang variable object with the name .

interl_print_term(stream, term) Prints an Erlang term FILE *stream; ETERM *term;

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

indicates where the function should send its output.

is the Erlang term to print.

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

 voiderl_set_compat_rel(release_number) Set the erl_interface library in compatibility mode unsigned release_number;

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

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

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

You may run into trouble if this feature is used carelessly. Always make sure 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.

 interl_size(term) Return the arity of a tuple or binary ETERM *term;

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

is an Erlang tuple or an Erlang binary object.

The function returns the size of as described above, or -1 if is not one of the two supported types.

ETERM *erl_tl(list) Extracts the tail from a list ETERM *list;

Extracts the tail from a 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 was not a list.

 ETERM *erl_var_content(term, name) Extracts the content of a variable ETERM *term; char *name;

This function returns the contents of the specified variable in an Erlang term.

is an Erlang term. In order for this function to succeed, 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.

is the name of an Erlang variable.

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