path: root/lib/erl_interface/doc/src/erl_eterm.xml

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<!DOCTYPE cref SYSTEM "cref.dtd">

<cref>
  <header>
    <copyright>
      <year>1996</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>erl_eterm</title>
    <prepared>Torbj&ouml;rn T&ouml;rnkvist</prepared>
    <responsible>Torbj&ouml;rn T&ouml;rnkvist</responsible>
    <docno></docno>
    <approved>Bjarne D&auml;cker</approved>
    <checked>Torbj&ouml;rn T&ouml;rnkvist</checked>
    <date>1998-07-03</date>
    <rev>A</rev>
    <file>erl_eterm.xml</file>
  </header>
  <lib>erl_eterm</lib>
  <libsummary>Functions for Erlang term construction.</libsummary>
  <description>
    <p>This module provides functions for creating and manipulating
      Erlang terms.</p>

    <p>An Erlang term is represented by a C structure of type
      <c>ETERM</c>. 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.</p>

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

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

    <p>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 <c>ETERM*</c> containing the wrong type. For example,
      passing a tuple to <c>ERL_ATOM_PTR()</c> likely results in garbage.</p>

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

  <funcs>
    <func>
      <name><ret>ETERM *</ret><nametext>erl_cons(head, tail)</nametext></name>
      <fsummary>Prepend a term to the head of a list.</fsummary>
      <type>
        <v>ETERM *head;</v>
        <v>ETERM *tail;</v>
      </type>
      <desc>
        <p>Concatenates two Erlang terms, prepending <c>head</c>
          onto <c>tail</c> and thereby creating a
          <c>cons</c> cell.
          To make a proper list, <c>tail</c> is always to be a list
          or an empty list. Notice that <c>NULL</c> is not a valid list.</p>
        <list type="bulleted">
          <item><c>head</c> is the new term to be added.</item>
          <item><c>tail</c> is the existing list to which
            <c>head</c> is concatenated.</item>
        </list>
        <p>The function returns a new list.</p>
        <p><c>ERL_CONS_HEAD(list)</c> and
          <c>ERL_CONS_TAIL(list)</c>
          can be used to retrieve the head and tail components
          from the list. <c>erl_hd(list)</c> and
          <c>erl_tl(list)</c> do
          the same thing, but check that the argument really is a list.</p>
        <p><em>Example:</em></p>
        <code type="none"><![CDATA[
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);
        ]]></code>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_copy_term(term)</nametext></name>
      <fsummary>Create a copy of an Erlang term.</fsummary>
      <type>
        <v>ETERM *term;</v>
      </type>
      <desc>
        <p>Creates and returns a copy of the Erlang term
          <c>term</c>.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_element(position, tuple)</nametext></name>
      <fsummary>Extract an element from an Erlang tuple.</fsummary>
      <type>
        <v>int position;</v>
        <v>ETERM *tuple;</v>
      </type>
      <desc>
        <p>Extracts a specified element from an Erlang tuple.</p>
        <list type="bulleted">
          <item><c>position</c> specifies which element to retrieve
            from <c>tuple</c>. The elements are numbered starting
            from 1.</item>
          <item><c>tuple</c> is an Erlang term containing at least
            <c>position</c> elements.</item>
        </list>
        <p>Returns a new Erlang term corresponding to the requested element, or
          <c>NULL</c> if <c>position</c> was greater
          than the arity of <c>tuple</c>.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_hd(list)</nametext></name>
      <fsummary>Extract the first element from a list.</fsummary>
      <type>
        <v>ETERM *list;</v>
      </type>
      <desc>
        <p>Extracts the first element from a list.</p>
        <p><c>list</c> is an Erlang term containing a list.</p>
        <p>Returns an Erlang term corresponding to the head
          head element in the list, or a <c>NULL</c> pointer if
          <c>list</c> was not a list.</p>
      </desc>
    </func>

    <func>
      <name><ret>void</ret><nametext>erl_init(NULL, 0)</nametext></name>
      <fsummary>Initialization routine.</fsummary>
      <type>
        <v>void *NULL;</v>
        <v>int 0;</v>
      </type>
      <desc>
        <p>This function must be called before any of the others in the
          <c>Erl_Interface</c> library to initialize the
          library functions. The arguments must be specified as
          <c>erl_init(NULL,0)</c>.</p>
      </desc>
    </func>

    <func>
      <name><ret>int</ret><nametext>erl_iolist_length(list)</nametext></name>
      <fsummary>Return the length of an I/O list.</fsummary>
      <type>
        <v>ETERM *list;</v>
      </type>
      <desc>
        <p>Returns the length of an I/O list.</p>
        <p><c>list</c> is an Erlang term containing an I/O list.</p>
        <p>Returns the length of <c>list</c>, or
          <c>-1</c> if <c>list</c> is not an I/O list.</p>
        <p>For the definition of an I/O list, see
          <seealso marker="#erl_iolist_to_binary">
          <c>erl_iolist_to_binary</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_iolist_to_binary(term)</nametext></name>
      <fsummary>Convert an I/O list to a binary.</fsummary>
      <type>
        <v>ETERM *list;</v>
      </type>
      <desc>
        <p>Converts an I/O list to a binary term.</p>
        <p><c>list</c> is an Erlang term containing a list.</p>
        <p>Returns an Erlang binary term, or <c>NULL</c> if
          <c>list</c> was not an I/O list.</p>
        <p>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:</p>
        <code type="none"><![CDATA[
iolist ::= []
        |   Binary
        |   [iohead | iolist]
        ;
iohead ::= Binary
        |   Byte (integer in the range [0..255])
        |   iolist
        ;
        ]]></code>
      </desc>
    </func>

    <func>
      <name><ret>char *</ret><nametext>erl_iolist_to_string(list)</nametext></name>
      <fsummary>Convert an I/O list to a <c>NULL</c>-terminated string.</fsummary>
      <type>
        <v>ETERM *list;</v>
      </type>
      <desc>
          <p>Converts an I/O list to a <c>NULL</c>-terminated C string.</p>
        <p><c>list</c> 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.</p>
        <p>Returns a pointer to a dynamically allocated
          buffer containing a string. If <c>list</c> is not an I/O
          list, or if <c>list</c> contains the integer 0,
          <c>NULL</c> is returned. It
          is the caller's responsibility to free the allocated buffer
          with <c>erl_free()</c>.</p>
        <p>For the definition of an I/O list, see
          <seealso marker="#erl_iolist_to_binary">
          <c>erl_iolist_to_binary</c></seealso>.</p>
      </desc>
    </func>

    <func>
      <name><ret>int</ret><nametext>erl_length(list)</nametext></name>
      <fsummary>Determine the length of a list.</fsummary>
      <type>
        <v>ETERM *list;</v>
      </type>
      <desc>
        <p>Determines the length of a proper list.</p>
        <p><c>list</c> 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.</p>
        <p>Returns <c>-1</c> if <c>list</c> is not a proper
          list.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_atom(string)</nametext></name>
      <fsummary>Create an atom.</fsummary>
      <type>
        <v>const char *string;</v>
      </type>
      <desc>
        <p>Creates an atom.</p>
        <p><c>string</c> is the sequence of characters that will be
          used to create the atom.</p>
        <p>Returns an Erlang term containing an atom. Notice that it is
          the caller's responsibility to ensure that <c>string</c>
          contains a valid name for an atom.</p>
        <p><c>ERL_ATOM_PTR(atom)</c> and
          <c>ERL_ATOM_PTR_UTF8(atom)</c>
          can be used to retrieve the atom name (as a <c>NULL</c>-terminated string).
          <c>ERL_ATOM_SIZE(atom)</c>
	  and <c>ERL_ATOM_SIZE_UTF8(atom)</c> return the length
          of the atom name.</p>
	<note>
          <p>The UTF-8 variants were introduced in Erlang/OTP R16 and the
	    string returned by <c>ERL_ATOM_PTR(atom)</c> was not
        <c>NULL</c>-terminated on older releases.</p>
	</note>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_binary(bptr, size)</nametext></name>
      <fsummary>Create a binary object.</fsummary>
      <type>
        <v>char *bptr;</v>
        <v>int size;</v>
      </type>
      <desc>
        <p>Produces an Erlang binary object from a
          buffer containing a sequence of bytes.</p>
        <list type="bulleted">
          <item><c>bptr</c> is a pointer to a buffer containing
            data to be converted.</item>
          <item><c>size</c> indicates the length of
            <c>bptr</c>.</item>
        </list>
        <p>Returns an Erlang binary object.</p>
        <p><c>ERL_BIN_PTR(bin)</c> retrieves a pointer to
          the binary data. <c>ERL_BIN_SIZE(bin)</c> retrieves the
          size.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_empty_list()</nametext></name>
      <fsummary>Create an empty Erlang list.</fsummary>
      <desc>
        <p>Creates and returns an empty Erlang list.
          Notice that <c>NULL</c> is not used to represent an empty list;
          Use this function instead.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_estring(string, len)</nametext></name>
      <fsummary>Create an Erlang string.</fsummary>
      <type>
        <v>char *string;</v>
        <v>int len;</v>
      </type>
      <desc>
        <p>Creates a list from a sequence of bytes.</p>
        <list type="bulleted">
          <item><c>string</c> is a buffer containing a sequence of
              bytes. The buffer does not need to be <c>NULL</c>-terminated.</item>
          <item><c>len</c> is the length of
            <c>string</c>.</item>
        </list>
        <p>Returns an Erlang list object corresponding to
          the character sequence in <c>string</c>.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_float(f)</nametext></name>
      <fsummary>Create an Erlang float.</fsummary>
      <type>
        <v>double f;</v>
      </type>
      <desc>
        <p>Creates an Erlang float.</p>
        <p><c>f</c> is a value to be converted to an Erlang
          float.</p>
        <p>Returns an Erlang float object with the value
          specified in <c>f</c> or <c>NULL</c> if
          <c>f</c> is not finite.</p>
        <p><c>ERL_FLOAT_VALUE(t)</c> can be used to retrieve the
          value from an Erlang float.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_int(n)</nametext></name>
      <fsummary>Create an Erlang integer.</fsummary>
      <type>
        <v>int n;</v>
      </type>
      <desc>
        <p>Creates an Erlang integer.</p>
        <p><c>n</c> is a value to be converted to an Erlang
          integer.</p>
        <p>Returns an Erlang integer object with the
          value specified in <c>n</c>.</p>
        <p><c>ERL_INT_VALUE(t)</c> can be used to retrieve the
          value from an Erlang integer.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_list(array, arrsize)</nametext></name>
      <fsummary>Create a list from an array.</fsummary>
      <type>
        <v>ETERM **array;</v>
        <v>int arrsize;</v>
      </type>
      <desc>
        <p>Creates an Erlang list from an array of Erlang terms, such
          that each element in the list corresponds to one element in
          the array.</p>
        <list type="bulleted">
          <item><c>array</c> is an array of Erlang terms.</item>
          <item><c>arrsize</c> is the number of elements in
            <c>array</c>.</item>
        </list>
        <p>The function creates an Erlang list object, whose length
          <c>arrsize</c> and whose elements are taken from the
          terms in <c>array</c>.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_long_ref(node, n1, n2, n3, creation)</nametext></name>
      <fsummary>Create an Erlang reference.</fsummary>
      <type>
        <v>const char *node;</v>
        <v>unsigned int n1, n2, n3;</v>
        <v>unsigned int creation;</v>
      </type>
      <desc>
        <p>Creates an Erlang reference, with 82 bits.</p>
        <list type="bulleted">
          <item><c>node</c> is the name of the C-node.</item>
          <item><c>n1</c>, <c>n2</c>, and
            <c>n3</c> can be seen as one big number
            <c>n1*2^64+n2*2^32+n3</c>, which is to be chosen
            uniquely for each reference created for a given C-node.</item>
          <item><c>creation</c> is an arbitrary number.</item>
        </list>
        <p>Notice that <c>n3</c> and <c>creation</c>
          are limited in precision, so only the low 18 and 2 bits of these
          numbers are used.</p>
        <p>Returns an Erlang reference object.</p>
        <p><c>ERL_REF_NODE(ref)</c>,
          <c>ERL_REF_NUMBERS(ref)</c>,
          <c>ERL_REF_LEN(ref)</c>, and
          <c>ERL_REF_CREATION(ref)</c> can be used to retrieve the
            values used to create the reference.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_pid(node, number, serial, creation)</nametext></name>
      <fsummary>Create a process identifier.</fsummary>
      <type>
        <v>const char *node;</v>
        <v>unsigned int number;</v>
        <v>unsigned int serial;</v>
        <v>unsigned int creation;</v>
      </type>
      <desc>
        <p>Creates an Erlang process identifier (pid). The
          resulting pid can be used by Erlang processes wishing to
          communicate with the C-node.</p>
        <list type="bulleted">
          <item><c>node</c> is the name of the C-node.</item>
          <item><c>number</c>, <c>serial</c>, and
            <c>creation</c> are
            arbitrary numbers. Notice that these are limited in
            precision, so only the low 15, 3, and 2 bits of these numbers
            are used.</item>
        </list>
        <p>Returns an Erlang pid object.</p>
        <p><c>ERL_PID_NODE(pid)</c>,
          <c>ERL_PID_NUMBER(pid)</c>,
          <c>ERL_PID_SERIAL(pid)</c>, and
          <c>ERL_PID_CREATION(pid)</c>
          can be used to retrieve the four values used to create the pid.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_port(node, number, creation)</nametext></name>
      <fsummary>Create a port identifier.</fsummary>
      <type>
        <v>const char *node;</v>
        <v>unsigned int number;</v>
        <v>unsigned int creation;</v>
      </type>
      <desc>
        <p>Creates an Erlang port identifier.</p>
        <list type="bulleted">
          <item><c>node</c> is the name of the C-node.</item>
          <item><c>number</c> and <c>creation</c> are
            arbitrary numbers. Notice that these are limited in
            precision, so only the low 18 and 2 bits of these numbers
            are used.</item>
        </list>
        <p>Returns an Erlang port object.</p>
        <p><c>ERL_PORT_NODE(port)</c>,
          <c>ERL_PORT_NUMBER(port)</c>,
          and <c>ERL_PORT_CREATION</c> can be used to retrieve the
          three values used to create the port.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_ref(node, number, creation)</nametext></name>
      <fsummary>Create an old Erlang reference.</fsummary>
      <type>
        <v>const char *node;</v>
        <v>unsigned int number;</v>
        <v>unsigned int creation;</v>
      </type>
      <desc>
        <p>Creates an old Erlang reference, with
          only 18 bits - use <c>erl_mk_long_ref</c> instead.</p>
        <list type="bulleted">
          <item><c>node</c> is the name of the C-node.</item>
          <item><c>number</c> is to be chosen uniquely for each
            reference created for a given C-node.</item>
          <item><c>creation</c> is an arbitrary number.</item>
        </list>
        <p>Notice that <c>number</c> and <c>creation</c>
          are limited in precision, so only the low 18 and 2 bits of these
          numbers are used.</p>
        <p>Returns an Erlang reference object.</p>
        <p><c>ERL_REF_NODE(ref)</c>,
          <c>ERL_REF_NUMBER(ref)</c>, and
          <c>ERL_REF_CREATION(ref)</c> can be used to retrieve the
          three values used to create the reference.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_string(string)</nametext></name>
      <fsummary>Create a string.</fsummary>
      <type>
        <v>char *string;</v>
      </type>
      <desc>
          <p>Creates a list from a <c>NULL</c>-terminated string.</p>
          <p><c>string</c> is a <c>NULL</c>-terminated sequence of
          characters
          (that is, a C string) from which the list will be created.</p>
        <p>Returns an Erlang list.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_tuple(array, arrsize)</nametext></name>
      <fsummary>Create an Erlang tuple from an array.</fsummary>
      <type>
        <v>ETERM **array;</v>
        <v>int arrsize;</v>
      </type>
      <desc>
        <p>Creates an Erlang tuple from an array of Erlang terms.</p>
        <list type="bulleted">
          <item><c>array</c> is an array of Erlang terms.</item>
          <item><c>arrsize</c> is the number of elements in
            <c>array</c>.</item>
        </list>
        <p>The function creates an Erlang tuple, whose arity is
          <c>size</c> and whose elements are taken from the terms
          in <c>array</c>.</p>
        <p>To retrieve the size of a tuple, either use function
          <c>erl_size</c> (which checks the type of the
          checked term and works for a binary as well as for a tuple) or
          <c>ERL_TUPLE_SIZE(tuple)</c> returns the arity of a tuple.
          <c>erl_size()</c> does the same thing, but it checks
          that the argument is a tuple.
          <c>erl_element(index,tuple)</c> returns the element
          corresponding to a given position in the tuple.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_uint(n)</nametext></name>
      <fsummary>Create an unsigned integer.</fsummary>
      <type>
        <v>unsigned int n;</v>
      </type>
      <desc>
        <p>Creates an Erlang unsigned integer.</p>
        <p><c>n</c> is a value to be converted to an Erlang
          unsigned integer.</p>
        <p>Returns an Erlang unsigned integer object with
          the value specified in <c>n</c>.</p>
        <p><c>ERL_INT_UVALUE(t)</c> can be used to retrieve the
          value from an Erlang unsigned integer.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_mk_var(name)</nametext></name>
      <fsummary>Create an Erlang variable.</fsummary>
      <type>
        <v>char *name;</v>
      </type>
      <desc>
        <p>Creates an unbound Erlang variable. The variable can later be bound
          through pattern matching or assignment.</p>
        <p><c>name</c> specifies a name for the variable.</p>
        <p>Returns an Erlang variable object with the
          name <c>name</c>.</p>
      </desc>
    </func>

    <func>
      <name><ret>int</ret><nametext>erl_print_term(stream, term)</nametext></name>
      <fsummary>Print an Erlang term.</fsummary>
      <type>
        <v>FILE *stream;</v>
        <v>ETERM *term;</v>
      </type>
      <desc>
        <p>Prints the specified Erlang term to the specified output stream.</p>
        <list type="bulleted">
          <item><c>stream</c> indicates where the function is to
            send its output.</item>
          <item><c>term</c> is the Erlang term to print.</item>
        </list>
        <p>Returns the number of characters written on success, otherwise a
          negative value.</p>
      </desc>
    </func>

    <func>
      <name><ret>void</ret><nametext>erl_set_compat_rel(release_number)</nametext></name>
      <fsummary>Set the Erl_Interface library in compatibility mode.</fsummary>
      <type>
        <v>unsigned release_number;</v>
      </type>
      <desc>
        <p>By default, the <c>Erl_Interface</c> library is only
          guaranteed to be compatible with other Erlang/OTP components from the
          same release as the <c>Erl_Interface</c> library itself.
          For example, <c>Erl_Interface</c> from Erlang/OTP R10
          is not compatible
          with an Erlang emulator from Erlang/OTP R9 by default.</p>
        <p>A call to <c>erl_set_compat_rel(release_number)</c> sets
          the <c>Erl_Interface</c> library in compatibility mode of
          release <c>release_number</c>. Valid range of
          <c>release_number</c>
          is [7, current release]. This makes it possible to
          communicate with Erlang/OTP components from earlier releases.</p>
        <note>
          <p>If this function is called, it may only be called once
            directly after the call to function
            <seealso marker="#erl_init">erl_init()</seealso>.</p>
        </note>
        <warning>
          <p>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.</p>
        </warning>
      </desc>
    </func>

    <func>
      <name><ret>int</ret><nametext>erl_size(term)</nametext></name>
      <fsummary>Return the arity of a tuple or binary.</fsummary>
      <type>
        <v>ETERM *term;</v>
      </type>
      <desc>
        <p>Returns either the arity of an Erlang tuple or the
          number of bytes in an Erlang binary object.</p>
        <p><c>term</c> is an Erlang tuple or an Erlang binary
          object.</p>
        <p>Returns the size of <c>term</c> as described
          above, or <c>-1</c> if <c>term</c> is not one of the two
          supported types.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_tl(list)</nametext></name>
      <fsummary>Extract the tail from a list.</fsummary>
      <type>
        <v>ETERM *list;</v>
      </type>
      <desc>
        <p>Extracts the tail from a list.</p>
        <p><c>list</c> is an Erlang term containing a list.</p>
        <p>Returns an Erlang list corresponding to the
          original list minus the first element, or <c>NULL</c> pointer if
          <c>list</c> was not a list.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_var_content(term, name)</nametext></name>
      <fsummary>Extract the content of a variable.</fsummary>
      <type>
        <v>ETERM *term;</v>
        <v>char *name;</v>
      </type>
      <desc>
        <p>Returns the contents of the specified variable in an Erlang term.</p>
        <list type="bulleted">
          <item><c>term</c> is an Erlang term. In order for this
            function to succeed,
            <c>term</c> 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.</item>
          <item><c>name</c> is the name of an Erlang variable.
          </item>
        </list>
        <p>Returns the Erlang object corresponding to the value of
          <c>name</c> in <c>term</c>. If no variable
          with the name <c>name</c> is found in
          <c>term</c>, or if <c>term</c> is
          not a valid Erlang term, <c>NULL</c> is returned.</p>
      </desc>
    </func>
  </funcs>
</cref>