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154 lines
9.3 KiB
HTML
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Random Access (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Random-Access">
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<p>
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Next: <a href="Line_002fDelimited.html" accesskey="n" rel="next">Line Oriented and Delimited Text</a>, Previous: <a href="Buffering.html" accesskey="p" rel="prev">Buffering</a>, Up: <a href="Input-and-Output.html" accesskey="u" rel="up">Input and Output</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
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<h4 class="subsection" id="Random-Access-1"><span>6.12.7 Random Access<a class="copiable-link" href="#Random-Access-1"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-Random-access_002c-ports"></a>
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<a class="index-entry-id" id="index-Port_002c-random-access"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-seek"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">seek</strong> <var class="def-var-arguments">fd_port offset whence</var><a class="copiable-link" href="#index-seek"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fseek"><span class="category-def">C Function: </span><span><strong class="def-name">scm_seek</strong> <var class="def-var-arguments">(fd_port, offset, whence)</var><a class="copiable-link" href="#index-scm_005fseek"> ¶</a></span></dt>
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<dd><p>Sets the current position of <var class="var">fd_port</var> to the integer
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<var class="var">offset</var>. For a file port, <var class="var">offset</var> is expressed
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as a number of bytes; for other types of ports, such as string
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ports, <var class="var">offset</var> is an abstract representation of the
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position within the port’s data, not necessarily expressed
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as a number of bytes. <var class="var">offset</var> is interpreted according to
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the value of <var class="var">whence</var>.
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</p>
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<p>One of the following variables should be supplied for
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<var class="var">whence</var>:
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</p><dl class="first-defvr first-defvar-alias-first-defvr">
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<dt class="defvr defvar-alias-defvr" id="index-SEEK_005fSET"><span class="category-def">Variable: </span><span><strong class="def-name">SEEK_SET</strong><a class="copiable-link" href="#index-SEEK_005fSET"> ¶</a></span></dt>
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<dd><p>Seek from the beginning of the file.
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</p></dd></dl>
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<dl class="first-defvr first-defvar-alias-first-defvr">
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<dt class="defvr defvar-alias-defvr" id="index-SEEK_005fCUR"><span class="category-def">Variable: </span><span><strong class="def-name">SEEK_CUR</strong><a class="copiable-link" href="#index-SEEK_005fCUR"> ¶</a></span></dt>
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<dd><p>Seek from the current position.
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</p></dd></dl>
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<dl class="first-defvr first-defvar-alias-first-defvr">
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<dt class="defvr defvar-alias-defvr" id="index-SEEK_005fEND"><span class="category-def">Variable: </span><span><strong class="def-name">SEEK_END</strong><a class="copiable-link" href="#index-SEEK_005fEND"> ¶</a></span></dt>
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<dd><p>Seek from the end of the file.
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</p></dd></dl>
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<p>On systems that support it, such as GNU/Linux, the following
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constants can be used for <var class="var">whence</var> to navigate “holes” in
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sparse files:
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</p><dl class="first-defvr first-defvar-alias-first-defvr">
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<dt class="defvr defvar-alias-defvr" id="index-SEEK_005fDATA"><span class="category-def">Variable: </span><span><strong class="def-name">SEEK_DATA</strong><a class="copiable-link" href="#index-SEEK_005fDATA"> ¶</a></span></dt>
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<dd><p>Seek to the next location in the file greater than or equal to
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<var class="var">offset</var> containing data. If <var class="var">offset</var> points to data,
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then the file offset is set to <var class="var">offset</var>.
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</p></dd></dl>
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<dl class="first-defvr first-defvar-alias-first-defvr">
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<dt class="defvr defvar-alias-defvr" id="index-SEEK_005fHOLE"><span class="category-def">Variable: </span><span><strong class="def-name">SEEK_HOLE</strong><a class="copiable-link" href="#index-SEEK_005fHOLE"> ¶</a></span></dt>
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<dd><p>Seek to the next hole in the file greater than or equal to the
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<var class="var">offset</var>. If <var class="var">offset</var> points into the middle of a hole,
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then the file offset is set to <var class="var">offset</var>. If there is no hole
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past <var class="var">offset</var>, then the file offset is adjusted to the end of
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the file—i.e., there is an implicit hole at the end of any file.
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</p></dd></dl>
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<p>If <var class="var">fd_port</var> is a file descriptor, the underlying system call
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is <code class="code">lseek</code> (see <a data-manual="libc" href="https://www.gnu.org/software/libc/manual/html_node/File-Position-Primitive.html#File-Position-Primitive">File Position Primitive</a> in <cite class="cite">The GNU C
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Library Reference Manual</cite>). <var class="var">port</var> may be a string port.
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</p>
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<p>The value returned is the new position in <var class="var">fd_port</var>. This means
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that the current position of a port can be obtained using:
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</p><div class="example lisp">
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<pre class="lisp-preformatted">(seek port 0 SEEK_CUR)
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</pre></div>
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</dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-ftell"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">ftell</strong> <var class="def-var-arguments">fd_port</var><a class="copiable-link" href="#index-ftell"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fftell"><span class="category-def">C Function: </span><span><strong class="def-name">scm_ftell</strong> <var class="def-var-arguments">(fd_port)</var><a class="copiable-link" href="#index-scm_005fftell"> ¶</a></span></dt>
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<dd><p>Return an integer representing the current position of
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<var class="var">fd_port</var>, measured from the beginning. Equivalent to:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(seek port 0 SEEK_CUR)
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</pre></div>
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</dd></dl>
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<a class="index-entry-id" id="index-truncate-1"></a>
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<a class="index-entry-id" id="index-ftruncate"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-truncate_002dfile"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">truncate-file</strong> <var class="def-var-arguments">file [length]</var><a class="copiable-link" href="#index-truncate_002dfile"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005ftruncate_005ffile"><span class="category-def">C Function: </span><span><strong class="def-name">scm_truncate_file</strong> <var class="def-var-arguments">(file, length)</var><a class="copiable-link" href="#index-scm_005ftruncate_005ffile"> ¶</a></span></dt>
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<dd><p>Truncate <var class="var">file</var> to <var class="var">length</var> bytes. <var class="var">file</var> can be a
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filename string, a port object, or an integer file descriptor. The
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return value is unspecified.
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</p>
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<p>For a port or file descriptor <var class="var">length</var> can be omitted, in which
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case the file is truncated at the current position (per <code class="code">ftell</code>
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above).
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</p>
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<p>On most systems a file can be extended by giving a length greater than
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the current size, but this is not mandatory in the POSIX standard.
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</p></dd></dl>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Line_002fDelimited.html">Line Oriented and Delimited Text</a>, Previous: <a href="Buffering.html">Buffering</a>, Up: <a href="Input-and-Output.html">Input and Output</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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