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169 lines
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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>Venerable Port Interfaces (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Venerable-Port-Interfaces">
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Next: <a href="Using-Ports-from-C.html" accesskey="n" rel="next">Using Ports from C</a>, Previous: <a href="Port-Types.html" accesskey="p" rel="prev">Types of Port</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="Venerable-Port-Interfaces-1"><span>6.12.11 Venerable Port Interfaces<a class="copiable-link" href="#Venerable-Port-Interfaces-1"> ¶</a></span></h4>
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<p>Over the 25 years or so that Guile has been around, its port system has
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evolved, adding many useful features. At the same time there have been
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four major Scheme standards released in those 25 years, which also
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evolve the common Scheme understanding of what a port interface should
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be. Alas, it would be too much to ask for all of these evolutionary
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branches to be consistent. Some of Guile’s original interfaces don’t
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mesh with the later Scheme standards, and yet Guile can’t just drop old
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interfaces. Sadly as well, the R6RS and R7RS standards both part from a
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base of R5RS, but end up in different and somewhat incompatible designs.
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</p>
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<p>Guile’s approach is to pick a set of port primitives that make sense
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together. We document that set of primitives, design our internal
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interfaces around them, and recommend them to users. As the R6RS I/O
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system is the most capable standard that Scheme has yet produced in this
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domain, we mostly recommend that; <code class="code">(ice-9 binary-ports)</code> and
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<code class="code">(ice-9 textual-ports)</code> are wholly modeled on <code class="code">(rnrs io
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ports)</code>. Guile does not wholly copy R6RS, however; See <a class="xref" href="R6RS-Incompatibilities.html">Incompatibilities with the R6RS</a>.
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</p>
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<p>At the same time, we have many venerable port interfaces, lore handed
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down to us from our hacker ancestors. Most of these interfaces even
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predate the expectation that Scheme should have modules, so they are
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present in the default environment. In Guile we support them as well
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and we have no plans to remove them, but again we don’t recommend them
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for new users.
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</p>
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<a class="index-entry-id" id="index-char_002dready_003f-1"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-char_002dready_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">char-ready?</strong> <var class="def-var-arguments">[port]</var><a class="copiable-link" href="#index-char_002dready_003f"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if a character is ready on input <var class="var">port</var>
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and return <code class="code">#f</code> otherwise. If <code class="code">char-ready?</code> returns
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<code class="code">#t</code> then the next <code class="code">read-char</code> operation on
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<var class="var">port</var> is guaranteed not to hang. If <var class="var">port</var> is a file
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port at end of file then <code class="code">char-ready?</code> returns <code class="code">#t</code>.
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</p>
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<p><code class="code">char-ready?</code> exists to make it possible for a
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program to accept characters from interactive ports without
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getting stuck waiting for input. Any input editors associated
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with such ports must make sure that characters whose existence
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has been asserted by <code class="code">char-ready?</code> cannot be rubbed out.
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If <code class="code">char-ready?</code> were to return <code class="code">#f</code> at end of file,
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a port at end of file would be indistinguishable from an
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interactive port that has no ready characters.
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</p>
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<p>Note that <code class="code">char-ready?</code> only works reliably for terminals and
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sockets with one-byte encodings. Under the hood it will return
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<code class="code">#t</code> if the port has any input buffered, or if the file descriptor
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that backs the port polls as readable, indicating that Guile can fetch
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more bytes from the kernel. However being able to fetch one byte
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doesn’t mean that a full character is available; See <a class="xref" href="Encoding.html">Encoding</a>. Also,
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on many systems it’s possible for a file descriptor to poll as readable,
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but then block when it comes time to read bytes. Note also that on
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Linux kernels, all file ports backed by files always poll as readable.
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For non-file ports, this procedure always returns <code class="code">#t</code>, except for
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soft ports, which have a <code class="code">char-ready?</code> handler. See <a class="xref" href="Soft-Ports.html">Soft Ports</a>.
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</p>
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<p>In short, this is a legacy procedure whose semantics are hard to
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provide. However it is a useful check to see if any input is buffered.
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See <a class="xref" href="Non_002dBlocking-I_002fO.html">Non-Blocking I/O</a>.
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</p></dd></dl>
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<a class="index-entry-id" id="index-read_002dchar-3"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-read_002dchar"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">read-char</strong> <var class="def-var-arguments">[port]</var><a class="copiable-link" href="#index-read_002dchar"> ¶</a></span></dt>
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<dd><p>The same as <code class="code">get-char</code>, except that <var class="var">port</var> defaults to the
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current input port. See <a class="xref" href="Textual-I_002fO.html">Textual I/O</a>.
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</p></dd></dl>
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<a class="index-entry-id" id="index-peek_002dchar-3"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-peek_002dchar"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">peek-char</strong> <var class="def-var-arguments">[port]</var><a class="copiable-link" href="#index-peek_002dchar"> ¶</a></span></dt>
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<dd><p>The same as <code class="code">lookahead-char</code>, except that <var class="var">port</var> defaults to
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the current input port. See <a class="xref" href="Textual-I_002fO.html">Textual I/O</a>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-unread_002dchar"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">unread-char</strong> <var class="def-var-arguments">cobj [port]</var><a class="copiable-link" href="#index-unread_002dchar"> ¶</a></span></dt>
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<dd><p>The same as <code class="code">unget-char</code>, except that <var class="var">port</var> defaults to the
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current input port, and the arguments are swapped. See <a class="xref" href="Textual-I_002fO.html">Textual I/O</a>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-unread_002dstring"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">unread-string</strong> <var class="def-var-arguments">str [port]</var><a class="copiable-link" href="#index-unread_002dstring"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005funread_005fstring"><span class="category-def">C Function: </span><span><strong class="def-name">scm_unread_string</strong> <var class="def-var-arguments">(str, port)</var><a class="copiable-link" href="#index-scm_005funread_005fstring"> ¶</a></span></dt>
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<dd><p>The same as <code class="code">unget-string</code>, except that <var class="var">port</var> defaults to the
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current input port, and the arguments are swapped. See <a class="xref" href="Textual-I_002fO.html">Textual I/O</a>.
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</p></dd></dl>
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<a class="index-entry-id" id="index-newline-3"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-newline"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">newline</strong> <var class="def-var-arguments">[port]</var><a class="copiable-link" href="#index-newline"> ¶</a></span></dt>
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<dd><p>Send a newline to <var class="var">port</var>. If <var class="var">port</var> is omitted, send to the
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current output port. Equivalent to <code class="code">(put-char port #\newline)</code>.
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</p></dd></dl>
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<a class="index-entry-id" id="index-write_002dchar-3"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-write_002dchar"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">write-char</strong> <var class="def-var-arguments">chr [port]</var><a class="copiable-link" href="#index-write_002dchar"> ¶</a></span></dt>
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<dd><p>The same as <code class="code">put-char</code>, except that <var class="var">port</var> defaults to the
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current input port, and the arguments are swapped. See <a class="xref" href="Textual-I_002fO.html">Textual I/O</a>.
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</p></dd></dl>
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</div>
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<hr>
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<p>
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Next: <a href="Using-Ports-from-C.html">Using Ports from C</a>, Previous: <a href="Port-Types.html">Types of Port</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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