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

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  <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

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      distributed under the License is distributed on an "AS IS" BASIS,
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    </legalnotice>

    <title>erl_marshal</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_marshal.xml</file>
  </header>
  <lib>erl_marshal</lib>
  <libsummary>Encoding and decoding of Erlang terms.</libsummary>
  <description>
    <p>This module contains functions for encoding Erlang terms into
      a sequence of bytes, and for decoding Erlang terms from a
      sequence of bytes.</p>
  </description>

  <funcs>
    <func>
      <name><ret>int</ret><nametext>erl_compare_ext(bufp1, bufp2)</nametext></name>
      <fsummary>Compare encoded byte sequences.</fsummary>
      <type>
        <v>unsigned char *bufp1,*bufp2;</v>
      </type>
      <desc>
        <p>Compares two encoded terms.</p>
        <list type="bulleted">
          <item><c>bufp1</c> is a buffer containing an encoded
            Erlang term term1.</item>
          <item><c>bufp2</c> is a buffer containing an encoded
            Erlang term term2.</item>
        </list>
        <p>Returns <c>0</c> if the terms are equal, <c>-1</c> if
          <c>term1</c> &lt; <c>term2</c>, or <c>1</c> if <c>term2</c> &lt;
          <c>term1</c>.</p>
      </desc>
    </func>

    <func>
      <name><ret>ETERM *</ret><nametext>erl_decode(bufp)</nametext></name>
      <name><ret>ETERM *</ret><nametext>erl_decode_buf(bufpp)</nametext></name>
      <fsummary>Convert a term from Erlang external format.</fsummary>
      <type>
        <v>unsigned char *bufp;</v>
        <v>unsigned char **bufpp;</v>
      </type>
      <desc>
        <p><c>erl_decode()</c> and
          <c>erl_decode_buf()</c> decode
          the contents of a buffer and return the corresponding
          Erlang term. <c>erl_decode_buf()</c> provides a simple
          mechanism for dealing with several encoded terms stored
          consecutively in the buffer.</p>
        <list type="bulleted">
          <item>
            <p><c>bufp</c> is a pointer to a buffer containing one
              or more encoded Erlang terms.</p>
          </item>
          <item>
            <p><c>bufpp</c> is the address of a buffer pointer. The
              buffer contains one or more consecutively encoded Erlang terms.
              Following a successful call to
              <c>erl_decode_buf()</c>, <c>bufpp</c> is
              updated so that it points to the next encoded term.</p>
          </item>
        </list>
        <p><c>erl_decode()</c> returns an Erlang term
          corresponding to the contents of <c>bufp</c> on success,
          otherwise <c>NULL</c>. <c>erl_decode_buf()</c>
          returns an Erlang
          term corresponding to the first of the consecutive terms in
          <c>bufpp</c> and moves <c>bufpp</c> forward
          to point to the
          next term in the buffer. On failure, each of the functions
          return <c>NULL</c>.</p>
      </desc>
    </func>

    <func>
      <name><ret>int</ret><nametext>erl_encode(term, bufp)</nametext></name>
      <name><ret>int</ret><nametext>erl_encode_buf(term, bufpp)</nametext></name>
      <fsummary>Convert a term into Erlang external format.</fsummary>
      <type>
        <v>ETERM *term;</v>
        <v>unsigned char *bufp;</v>
        <v>unsigned char **bufpp;</v>
      </type>
      <desc>
        <p><c>erl_encode()</c> and
          <c>erl_encode_buf()</c> encode
          Erlang terms into external format for storage or transmission.
          <c>erl_encode_buf()</c> provides a simple mechanism for
          encoding several terms consecutively in the same buffer.</p>
        <list type="bulleted">
          <item>
            <p><c>term</c> is an Erlang term to be encoded.</p>
          </item>
          <item>
            <p><c>bufp</c> is a pointer to a buffer containing one or
              more encoded Erlang terms.</p>
          </item>
          <item>
            <p><c>bufpp</c> is a pointer to a pointer to a buffer
              containing one or more consecutively encoded Erlang terms.
              Following a successful call to
              <c>erl_encode_buf()</c>, <c>bufpp</c> is updated so
              that it points to the
              position for the next encoded term.</p>
          </item>
        </list>
        <p>These functions return the number of bytes written to buffer
	  on success, otherwise <c>0</c>.</p>
        <p>Notice that no bounds checking is done on the buffer. It is
          the caller's responsibility to ensure 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 <c>erl_term_len()</c>
          to determine the exact requirements for a given term.</p>
        <p>The following can help you estimate the buffer
          requirements for a term. Notice that this information is
          implementation-specific, and can change in future versions.
          If you are unsure, use <c>erl_term_len()</c>.</p>
        <p>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:</p>
        <taglist>
          <tag><c>Tuples</c></tag>
          <item>Need 5 bytes, plus the space for each element.</item>
          <tag><c>Lists</c></tag>
          <item>Need 5 bytes, plus the space for each element, and 1
            more byte for the empty list at the end.</item>
          <tag><c>Strings and atoms</c></tag>
          <item>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.</item>
          <tag><c>Integers</c></tag>
          <item>Need 5 bytes.</item>
          <tag><c>Characters</c></tag>
          <item>(Integers &lt; 256) need 2 bytes.</item>
          <tag><c>Floating point numbers</c></tag>
          <item>Need 32 bytes.</item>
          <tag><c>Pids</c></tag>
          <item>Need 10 bytes, plus the space for the node name, which
            is an atom.</item>
          <tag><c>Ports and Refs</c></tag>
          <item>Need 6 bytes, plus the space for the node name, which
            is an atom.</item>
        </taglist>
        <p>The total space required is the result calculated
          from the information above, plus 1 more byte for a 
          version identifier.</p>
      </desc>
    </func>

    <func>
      <name><ret>int</ret><nametext>erl_ext_size(bufp)</nametext></name>
      <fsummary>Count elements in encoded term.</fsummary>
      <type>
        <v>unsigned char *bufp;</v>
      </type>
      <desc>
        <p>Returns the number of elements in an encoded term.</p>
      </desc>
    </func>

    <func>
      <name><ret>unsigned char</ret><nametext>erl_ext_type(bufp)</nametext></name>
      <fsummary>Determine type of an encoded byte sequence.</fsummary>
      <type>
        <v>unsigned char *bufp;</v>
      </type>
      <desc>
        <p>Identifies and returns the type of Erlang term encoded
          in a buffer. It skips a trailing <em>magic</em> identifier.</p>
        <p>Returns <c>0</c> if the type cannot be determined or
          one of:</p>
        <list type="bulleted">
          <item><c>ERL_INTEGER</c>
          </item>
          <item><c>ERL_ATOM</c>
          </item>
          <item><c>ERL_PID</c> (Erlang process identifier)
          </item>
          <item><c>ERL_PORT</c>
          </item>
          <item><c>ERL_REF</c> (Erlang reference)
          </item>
          <item><c>ERL_EMPTY_LIST</c>
          </item>
          <item><c>ERL_LIST</c>
          </item>
          <item><c>ERL_TUPLE</c>
          </item>
          <item><c>ERL_FLOAT</c>
          </item>
          <item><c>ERL_BINARY</c>
          </item>
          <item><c>ERL_FUNCTION</c>
          </item>
        </list>
      </desc>
    </func>

    <func>
      <name><ret>unsigned char *</ret><nametext>erl_peek_ext(bufp, pos)</nametext></name>
      <fsummary>Step over encoded term.</fsummary>
      <type>
        <v>unsigned char *bufp;</v>
        <v>int pos;</v>
      </type>
      <desc>
        <p>This function is used for stepping over one or more
          encoded terms in a buffer, to directly access later term.</p>
        <list type="bulleted">
          <item><c>bufp</c> is a pointer to a buffer containing one
            or more encoded Erlang terms.</item>
          <item><c>pos</c> indicates how many terms to step over in
            the buffer.</item>
        </list>
        <p>Returns a pointer to a subterm that can be
          used in a later call to <c>erl_decode()</c> to retrieve
          the term at that position. If there is no term, or
          <c>pos</c> would exceed the size of the terms in the
          buffer, <c>NULL</c> is returned.</p>
      </desc>
    </func>

    <func>
      <name><ret>int</ret><nametext>erl_term_len(t)</nametext></name>
      <fsummary>Determine encoded size of term.</fsummary>
      <type>
        <v>ETERM *t;</v>
      </type>
      <desc>
        <p>Determines the buffer space that would be
          needed by <c>t</c> if it were encoded into Erlang external
          format by <c>erl_encode()</c>.</p>
        <p>Returns the size in bytes.</p>
      </desc>
    </func>
  </funcs>
</cref>