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.
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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>980703</date>
    <rev>A</rev>
    <file>erl_marshal.sgml</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>Compares encoded byte sequences</fsummary>
      <type>
        <v>unsigned char *bufp1,*bufp2;</v>
      </type>
      <desc>
        <p>This function compares two encoded terms.
          </p>
        <p><c><![CDATA[bufp1]]></c> is a buffer containing an encoded Erlang
          term term1.
          </p>
        <p><c><![CDATA[bufp2]]></c> is a buffer containing an encoded Erlang
          term term2.
          </p>
        <p>The function returns 0 if the terms are equal, -1 if term1
          is less than term2, or 1 if term2 is less than term1. 
          </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>Converts a term from Erlang external format</fsummary>
      <type>
        <v>unsigned char *bufp;</v>
        <v>unsigned char **bufpp;</v>
      </type>
      <desc>
        <p><c><![CDATA[erl_decode()]]></c> and <c><![CDATA[erl_decode_buf()]]></c> decode
          the contents of a buffer and return the corresponding
          Erlang term.  <c><![CDATA[erl_decode_buf()]]></c> provides a simple
          mechanism for dealing with several encoded terms stored
          consecutively in the buffer.</p>
        <p><c><![CDATA[bufp]]></c> is a pointer to a buffer containing one or
          more encoded Erlang terms.
          </p>
        <p><c><![CDATA[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><![CDATA[erl_decode_buf()]]></c>, 
          <c><![CDATA[bufpp]]></c> will be updated so that it points to the next
          encoded term.
          </p>
        <p><c><![CDATA[erl_decode()]]></c> returns an Erlang term
          corresponding to the contents of <c><![CDATA[bufp]]></c> on success, or
          NULL on failure. <c><![CDATA[erl_decode_buf()]]></c> returns an Erlang
          term corresponding to the first of the consecutive terms in
          <c><![CDATA[bufpp]]></c> and moves <c><![CDATA[bufpp]]></c> forward to point to the
          next term in the buffer. On failure, each of the functions
          returns NULL.
          </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>Converts 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><![CDATA[erl_encode()]]></c> and  <c><![CDATA[erl_encode_buf()]]></c> encode 
          Erlang terms into external format for storage or transmission.
          <c><![CDATA[erl_encode_buf()]]></c> provides a simple mechanism for
          encoding several terms consecutively in the same
          buffer.
          </p>
        <p><c>term</c> is an Erlang term to be encoded.
          </p>
        <p><c>bufp</c> is a pointer to a buffer containing one or
          more encoded Erlang terms.
          </p>
        <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><![CDATA[erl_encode_buf()]]></c>, 
          <c>bufpp</c> will be updated so that it points to the
          position for the next encoded term.
          </p>
        <p>
	  These functions returns the number of bytes written to buffer
	  if successful, otherwise returns 0.  
	</p>
        <p>Note that no bounds checking is done on the buffer. It is
          the caller's responsibility to make sure 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><![CDATA[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. Note that this information is
          implementation specific, and may change in future versions.
          If you are unsure, use <c><![CDATA[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><![CDATA[Tuples]]></c></tag>
          <item>need 5 bytes, plus the space for each element.</item>
          <tag><c><![CDATA[Lists]]></c></tag>
          <item>need 5 bytes, plus the space for each element, and 1
           additional byte for the empty list at the end.</item>
          <tag><c><![CDATA[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><![CDATA[Integers]]></c></tag>
          <item>need 5 bytes.</item>
          <tag><c><![CDATA[Characters]]></c></tag>
          <item>(integers &lt; 256) need 2 bytes.</item>
          <tag><c><![CDATA[Floating point numbers]]></c></tag>
          <item>need 32 bytes.</item>
          <tag><c><![CDATA[Pids]]></c></tag>
          <item>need 10 bytes, plus the space for the node name, which
           is an atom.</item>
          <tag><c><![CDATA[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 will be the result calculated
          from the information above, plus 1 additional byte for a 
          version identifier.
          </p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>erl_ext_size(bufp)</nametext></name>
      <fsummary>Counts elements in encoded term</fsummary>
      <type>
        <v>unsigned char *bufp;</v>
      </type>
      <desc>
        <p>This function 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>Determines type of an encoded byte sequence</fsummary>
      <type>
        <v>unsigned char *bufp;</v>
      </type>
      <desc>
        <p>This function identifies and returns the type of Erlang term encoded
          in a buffer. It will skip a trailing <em>magic</em> identifier.
          Returns <c><![CDATA[0]]></c> if the type can't be determined or one of</p>
        <list type="bulleted">
          <item>
            <p>ERL_INTEGER</p>
          </item>
          <item>
            <p>ERL_ATOM</p>
          </item>
          <item>
            <p>ERL_PID <c><![CDATA[/* Erlang process identifier */]]></c></p>
          </item>
          <item>
            <p>ERL_PORT</p>
          </item>
          <item>
            <p>ERL_REF <c><![CDATA[/* Erlang reference */]]></c></p>
          </item>
          <item>
            <p>ERL_EMPTY_LIST</p>
          </item>
          <item>
            <p>ERL_LIST</p>
          </item>
          <item>
            <p>ERL_TUPLE</p>
          </item>
          <item>
            <p>ERL_FLOAT</p>
          </item>
          <item>
            <p>ERL_BINARY</p>
          </item>
          <item>
            <p>ERL_FUNCTION</p>
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name><ret>unsigned char *</ret><nametext>erl_peek_ext(bufp, pos)</nametext></name>
      <fsummary>Steps 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, in order to directly access a
          later term.
          </p>
        <p><c><![CDATA[bufp]]></c> is a pointer to a buffer containing one or
          more encoded Erlang terms.
          </p>
        <p><c><![CDATA[pos]]></c> indicates how many terms to step over in the
          buffer. 
          </p>
        <p>The function returns a pointer to a sub-term that can be
          used in a subsequent call to <c><![CDATA[erl_decode()]]></c> in order to retrieve
          the term at that position. If there is no term, or <c><![CDATA[pos]]></c>
          would exceed the size of the terms in the buffer, NULL is returned.
          </p>
      </desc>
    </func>
    <func>
      <name><ret>int</ret><nametext>erl_term_len(t)</nametext></name>
      <fsummary>Determines encoded size of term</fsummary>
      <type>
        <v>ETERM *t;</v>
      </type>
      <desc>
        <p>This function determines the buffer space that would be
          needed by <c><![CDATA[t]]></c> if it were encoded into Erlang external
          format by <c><![CDATA[erl_encode()]]></c>.
          </p>
        <p>The size in bytes is returned.
          </p>
      </desc>
    </func>
  </funcs>
</cref>