179 lines
9.4 KiB
HTML
179 lines
9.4 KiB
HTML
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Foreign Structs (Guile Reference Manual)</title>
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<link href="index.html" rel="start" title="Top">
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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="Foreign-Function-Interface.html" rel="up" title="Foreign Function Interface">
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<link href="More-Foreign-Functions.html" rel="next" title="More Foreign Functions">
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<link href="Void-Pointers-and-Byte-Access.html" rel="prev" title="Void Pointers and Byte Access">
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<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Foreign-Structs">
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<div class="nav-panel">
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<p>
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Next: <a href="More-Foreign-Functions.html" accesskey="n" rel="next">More Foreign Functions</a>, Previous: <a href="Void-Pointers-and-Byte-Access.html" accesskey="p" rel="prev">Void Pointers and Byte Access</a>, Up: <a href="Foreign-Function-Interface.html" accesskey="u" rel="up">Foreign Function Interface</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="Foreign-Structs-1"><span>6.19.7 Foreign Structs<a class="copiable-link" href="#Foreign-Structs-1"> ¶</a></span></h4>
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<p>Finally, one last note on foreign values before moving on to actually
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calling foreign functions. Sometimes you need to deal with C structs,
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which requires interpreting each element of the struct according to the
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its type, offset, and alignment. The <code class="code">(system foreign)</code> module has
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some primitives to support this.
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</p>
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<div class="example">
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<pre class="example-preformatted">(use-modules (system foreign))
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-sizeof"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">sizeof</strong> <var class="def-var-arguments">type</var><a class="copiable-link" href="#index-sizeof"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fsizeof"><span class="category-def">C Function: </span><span><strong class="def-name">scm_sizeof</strong> <var class="def-var-arguments">(type)</var><a class="copiable-link" href="#index-scm_005fsizeof"> ¶</a></span></dt>
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<dd><p>Return the size of <var class="var">type</var>, in bytes.
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</p>
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<p><var class="var">type</var> should be a valid C type, like <code class="code">int</code>.
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Alternately <var class="var">type</var> may be the symbol <code class="code">*</code>, in which
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case the size of a pointer is returned. <var class="var">type</var> may
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also be a list of types, in which case the size of a
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<code class="code">struct</code> with ABI-conventional packing is returned.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-alignof"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">alignof</strong> <var class="def-var-arguments">type</var><a class="copiable-link" href="#index-alignof"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005falignof"><span class="category-def">C Function: </span><span><strong class="def-name">scm_alignof</strong> <var class="def-var-arguments">(type)</var><a class="copiable-link" href="#index-scm_005falignof"> ¶</a></span></dt>
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<dd><p>Return the alignment of <var class="var">type</var>, in bytes.
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</p>
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<p><var class="var">type</var> should be a valid C type, like <code class="code">int</code>.
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Alternately <var class="var">type</var> may be the symbol <code class="code">*</code>, in which
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case the alignment of a pointer is returned. <var class="var">type</var> may
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also be a list of types, in which case the alignment of a
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<code class="code">struct</code> with ABI-conventional packing is returned.
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</p></dd></dl>
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<p>Guile also provides some convenience syntax to efficiently read and
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write C structs to and from bytevectors.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-read_002dc_002dstruct"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">read-c-struct</strong> <var class="def-var-arguments">bv offset <br> ((field type) …) k</var><a class="copiable-link" href="#index-read_002dc_002dstruct"> ¶</a></span></dt>
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<dd><p>Read a C struct with fields of type <var class="var">type</var>... from the bytevector
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<var class="var">bv</var>, at offset <var class="var">offset</var>. Bind the fields to the identifiers
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<var class="var">field</var>..., and return <code class="code">(<var class="var">k</var> <var class="var">field</var> ...)</code>.
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</p>
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<p>Unless cross-compiling, the field types are evaluated at macro-expansion
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time. This allows the resulting bytevector accessors and size/alignment
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computations to be completely inlined.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-write_002dc_002dstruct"><span class="category-def">Scheme Syntax: </span><span><strong class="def-name">write-c-struct</strong> <var class="def-var-arguments">bv offset <br> ((field type) …)</var><a class="copiable-link" href="#index-write_002dc_002dstruct"> ¶</a></span></dt>
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<dd><p>Write a C struct with fields <var class="var">field</var>... of type <var class="var">type</var>... to the bytevector
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<var class="var">bv</var>, at offset <var class="var">offset</var>. Return zero values.
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</p>
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<p>Like <code class="code">write-c-struct</code> above, unless cross-compiling, the field
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types are evaluated at macro-expansion time.
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</p></dd></dl>
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<p>For example, to define a parser and serializer for the equivalent of a
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<code class="code">struct { int64_t a; uint8_t b; }</code>, one might do this:
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</p>
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<div class="example">
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<pre class="example-preformatted">(use-modules (system foreign) (rnrs bytevectors))
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(define-syntax-rule
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(define-serialization (reader writer) (field type) ...)
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(begin
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(define (reader bv offset)
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(read-c-struct bv offset ((field type) ...) values))
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(define (writer bv offset field ...)
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(write-c-struct bv offset ((field type) ...)))))
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(define-serialization (read-struct write-struct)
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(a int64) (b uint8))
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(define bv (make-bytevector (sizeof (list int64 uint8))))
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(write-struct bv 0 300 43)
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(call-with-values (lambda () (read-struct bv 0))
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list)
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⇒ (300 43)
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</pre></div>
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<p>There is also an older interface that is mostly equivalent to
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<code class="code">read-c-struct</code> and <code class="code">write-c-struct</code>, but which uses run-time
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dispatch, and operates on foreign pointers instead of bytevectors.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-parse_002dc_002dstruct"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">parse-c-struct</strong> <var class="def-var-arguments">foreign types</var><a class="copiable-link" href="#index-parse_002dc_002dstruct"> ¶</a></span></dt>
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<dd><p>Parse a foreign pointer to a C struct, returning a list of values.
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</p>
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<p><code class="code">types</code> should be a list of C types.
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</p></dd></dl>
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<p>Our parser and serializer example for <code class="code">struct { int64_t a; uint8_t
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b; }</code> looks more like this:
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</p>
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<div class="example">
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<pre class="example-preformatted">(parse-c-struct (make-c-struct (list int64 uint8)
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(list 300 43))
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(list int64 uint8))
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⇒ (300 43)
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</pre></div>
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<p>As yet, Guile only has convenience routines to support
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conventionally-packed structs. But given the <code class="code">bytevector->pointer</code>
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and <code class="code">pointer->bytevector</code> routines, one can create and parse
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tightly packed structs and unions by hand. See the code for
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<code class="code">(system foreign)</code> for details.
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</p>
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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="More-Foreign-Functions.html">More Foreign Functions</a>, Previous: <a href="Void-Pointers-and-Byte-Access.html">Void Pointers and Byte Access</a>, Up: <a href="Foreign-Function-Interface.html">Foreign Function Interface</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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