php-doc-en/reference/filesystem/functions/fseek.xml

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<refentry xml:id="function.fseek" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>fseek</refname>
  <refpurpose>Seeks on a file pointer</refpurpose>
 </refnamediv>
 
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>int</type><methodname>fseek</methodname>
   <methodparam><type>resource</type><parameter>stream</parameter></methodparam>
   <methodparam><type>int</type><parameter>offset</parameter></methodparam>
   <methodparam choice="opt"><type>int</type><parameter>whence</parameter><initializer><constant>SEEK_SET</constant></initializer></methodparam>
  </methodsynopsis>
  <para>
   Sets the file position indicator for the file referenced by
   <parameter>stream</parameter>. The new position, measured in bytes
   from the beginning of the file, is obtained by adding
   <parameter>offset</parameter> to the position specified by
   <parameter>whence</parameter>.
  </para>
  <para>
   In general, it is allowed to seek past the end-of-file; if data is then
   written, reads in any unwritten region between the end-of-file and the
   sought position will yield bytes with value 0. However, certain streams
   may not support this behavior, especially when they have an underlying
   fixed size storage.
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>stream</parameter></term>
     <listitem>
      &fs.file.pointer;
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>offset</parameter></term>
     <listitem>
      <para>
       The offset.
      </para>
      <para>
       To move to a position before the end-of-file, you need to pass
       a negative value in <parameter>offset</parameter> and
       set <parameter>whence</parameter>
       to <constant>SEEK_END</constant>.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>whence</parameter></term>
     <listitem>
      <para>
       <parameter>whence</parameter> values are:
       <simplelist>
        <member><constant>SEEK_SET</constant> - Set position equal to <parameter>offset</parameter> bytes.</member>
        <member><constant>SEEK_CUR</constant> - Set position to current location plus <parameter>offset</parameter>.</member>
        <member><constant>SEEK_END</constant> - Set position to end-of-file plus <parameter>offset</parameter>.</member>
       </simplelist>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   Upon success, returns 0; otherwise, returns -1.
  </para>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>fseek</function> example</title>
    <programlisting role="php">
<![CDATA[
<?php

$fp = fopen('somefile.txt', 'r');

// read some data
$data = fgets($fp, 4096);

// move back to the beginning of the file
// same as rewind($fp);
fseek($fp, 0);

?>
]]>
    </programlisting>
   </example>
  </para>
 </refsect1>

 <refsect1 role="notes">
  &reftitle.notes;
  <note>
   <para>
    If you have opened the file in append (<literal>a</literal> or
    <literal>a+</literal>) mode, any data you write to the file will always be
    appended, regardless of the file position, and the result of calling
    <function>fseek</function> will be undefined.
   </para>
  </note>
  <note>
   <para>
    Not all streams support seeking. For those that do not support seeking,
    forward seeking from the current position is accomplished by reading
    and discarding data; other forms of seeking will fail.
   </para>
  </note>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>ftell</function></member>
    <member><function>rewind</function></member>
   </simplelist>
  </para>
 </refsect1>

</refentry>
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