176 lines
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176 lines
11 KiB
HTML
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<!-- This manual documents Guile version 3.0.10.
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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>Ports (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Input-and-Output.html" rel="up" title="Input and Output">
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<link href="Binary-I_002fO.html" rel="next" title="Binary I/O">
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<body lang="en">
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<div class="subsection-level-extent" id="Ports">
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<div class="nav-panel">
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<p>
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Next: <a href="Binary-I_002fO.html" accesskey="n" rel="next">Binary I/O</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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</div>
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<hr>
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<h4 class="subsection" id="Ports-1"><span>6.12.1 Ports<a class="copiable-link" href="#Ports-1"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-Port"></a>
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<p>Ports are the way that Guile performs input and output. Guile can read
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in characters or bytes from an <em class="dfn">input port</em>, or write them out to an
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<em class="dfn">output port</em>. Some ports support both interfaces.
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</p>
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<p>There are a number of different port types implemented in Guile. File
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ports provide input and output over files, as you might imagine. For
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example, we might display a string to a file like this:
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</p>
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<div class="example">
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<pre class="example-preformatted">(let ((port (open-output-file "foo.txt")))
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(display "Hello, world!\n" port)
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(close-port port))
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</pre></div>
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<p>There are also string ports, for taking input from a string, or
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collecting output to a string; bytevector ports, for doing the same but
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using a bytevector as a source or sink of data; and custom ports, for
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arranging to call Scheme functions to provide input or handle output.
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See <a class="xref" href="Port-Types.html">Types of Port</a>.
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</p>
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<p>Ports should be <em class="dfn">closed</em> when they are not needed by calling
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<code class="code">close-port</code> on them, as in the example above. This will make sure
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that any pending output is successfully written out to disk, in the case
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of a file port, or otherwise to whatever mutable store is backed by the
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port. Any error that occurs while writing out that buffered data would
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also be raised promptly at the <code class="code">close-port</code>, and not later when the
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port is closed by the garbage collector. See <a class="xref" href="Buffering.html">Buffering</a>, for more on
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buffered output.
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</p>
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<p>Closing a port also releases any precious resource the file might have.
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Usually in Scheme a programmer doesn’t have to clean up after their data
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structures (see <a class="pxref" href="Memory-Management.html">Memory Management and Garbage Collection</a>), but most systems have strict
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limits on how many files can be open, both on a per-process and a
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system-wide basis. A program that uses many files should take care not
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to hit those limits. The same applies to similar system resources such
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as pipes and sockets.
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</p>
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<p>Indeed for these reasons the above example is not the most idiomatic way
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to use ports. It is more common to acquire ports via procedures like
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<code class="code">call-with-output-file</code>, which handle the <code class="code">close-port</code>
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automatically:
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</p>
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<div class="example">
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<pre class="example-preformatted">(call-with-output-file "foo.txt"
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(lambda (port)
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(display "Hello, world!\n" port)))
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</pre></div>
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<p>Finally, all ports have associated input and output buffers, as
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appropriate. Buffering is a common strategy to limit the overhead of
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small reads and writes: without buffering, each character fetched from a
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file would involve at least one call into the kernel, and maybe more
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depending on the character and the encoding. Instead, Guile will batch
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reads and writes into internal buffers. However, sometimes you want to
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make output on a port show up immediately. See <a class="xref" href="Buffering.html">Buffering</a>, for more
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on interfaces to control port buffering.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-port_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">port?</strong> <var class="def-var-arguments">x</var><a class="copiable-link" href="#index-port_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fport_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_port_p</strong> <var class="def-var-arguments">(x)</var><a class="copiable-link" href="#index-scm_005fport_005fp"> ¶</a></span></dt>
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<dd><p>Return a boolean indicating whether <var class="var">x</var> is a port.
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Equivalent to <code class="code">(or (input-port? <var class="var">x</var>) (output-port? <var class="var">x</var>))</code>.
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</p></dd></dl>
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<a class="index-entry-id" id="index-input_002dport_003f-3"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-input_002dport_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">input-port?</strong> <var class="def-var-arguments">x</var><a class="copiable-link" href="#index-input_002dport_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005finput_005fport_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_input_port_p</strong> <var class="def-var-arguments">(x)</var><a class="copiable-link" href="#index-scm_005finput_005fport_005fp"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">x</var> is an input port, otherwise return
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<code class="code">#f</code>. Any object satisfying this predicate also satisfies
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<code class="code">port?</code>.
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</p></dd></dl>
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<a class="index-entry-id" id="index-output_002dport_003f-3"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-output_002dport_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">output-port?</strong> <var class="def-var-arguments">x</var><a class="copiable-link" href="#index-output_002dport_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005foutput_005fport_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_output_port_p</strong> <var class="def-var-arguments">(x)</var><a class="copiable-link" href="#index-scm_005foutput_005fport_005fp"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">x</var> is an output port, otherwise return
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<code class="code">#f</code>. Any object satisfying this predicate also satisfies
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<code class="code">port?</code>.
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</p></dd></dl>
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<a class="index-entry-id" id="index-Closing-ports"></a>
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<a class="index-entry-id" id="index-Port_002c-close"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-close_002dport"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">close-port</strong> <var class="def-var-arguments">port</var><a class="copiable-link" href="#index-close_002dport"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fclose_005fport"><span class="category-def">C Function: </span><span><strong class="def-name">scm_close_port</strong> <var class="def-var-arguments">(port)</var><a class="copiable-link" href="#index-scm_005fclose_005fport"> ¶</a></span></dt>
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<dd><p>Close the specified port object. Return <code class="code">#t</code> if it successfully
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closes a port or <code class="code">#f</code> if it was already closed. An exception may
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be raised if an error occurs, for example when flushing buffered output.
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See <a class="xref" href="Buffering.html">Buffering</a>, for more on buffered output. See <a class="xref" href="Ports-and-File-Descriptors.html">close</a>, for a procedure which can close file descriptors.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-port_002dclosed_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">port-closed?</strong> <var class="def-var-arguments">port</var><a class="copiable-link" href="#index-port_002dclosed_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fport_005fclosed_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_port_closed_p</strong> <var class="def-var-arguments">(port)</var><a class="copiable-link" href="#index-scm_005fport_005fclosed_005fp"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">port</var> is closed or <code class="code">#f</code> if it is
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open.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-call_002dwith_002dport"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">call-with-port</strong> <var class="def-var-arguments">port proc</var><a class="copiable-link" href="#index-call_002dwith_002dport"> ¶</a></span></dt>
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<dd><p>Call <var class="var">proc</var>, passing it <var class="var">port</var> and closing <var class="var">port</var> upon exit
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of <var class="var">proc</var>. Return the return values of <var class="var">proc</var>.
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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="Binary-I_002fO.html">Binary I/O</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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</div>
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</body>
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</html>
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