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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <!-- Created by GNU Texinfo 6.8, https://www.gnu.org/software/texinfo/ --> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <!-- This manual is for libffi, a portable foreign function interface library. Copyright (C) 2008-2019, 2021, 2022 Anthony Green and Red Hat, Inc. Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. 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IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. --> <title>Arrays Unions Enums (libffi: the portable foreign function interface library)</title> <meta name="description" content="Arrays Unions Enums (libffi: the portable foreign function interface library)"> <meta name="keywords" content="Arrays Unions Enums (libffi: the portable foreign function interface library)"> <meta name="resource-type" content="document"> <meta name="distribution" content="global"> <meta name="Generator" content="makeinfo"> <meta name="viewport" content="width=device-width,initial-scale=1"> <link href="index.html" rel="start" title="Top"> <link href="Index.html" rel="index" title="Index"> <link href="Types.html" rel="up" title="Types"> <link href="Type-Example.html" rel="next" title="Type Example"> <link href="Size-and-Alignment.html" rel="prev" title="Size and Alignment"> <style type="text/css"> <!-- a.copiable-anchor {visibility: hidden; text-decoration: none; line-height: 0em} a.summary-letter {text-decoration: none} blockquote.indentedblock {margin-right: 0em} div.display {margin-left: 3.2em} div.example {margin-left: 3.2em} kbd {font-style: oblique} pre.display {font-family: inherit} pre.format {font-family: inherit} pre.menu-comment {font-family: serif} pre.menu-preformatted {font-family: serif} span.nolinebreak {white-space: nowrap} span.roman {font-family: initial; font-weight: normal} span.sansserif {font-family: sans-serif; font-weight: normal} span:hover a.copiable-anchor {visibility: visible} ul.no-bullet {list-style: none} --> </style> </head> <body lang="en"> <div class="subsection" id="Arrays-Unions-Enums"> <div class="header"> <p> Next: <a href="Type-Example.html" accesskey="n" rel="next">Type Example</a>, Previous: <a href="Size-and-Alignment.html" accesskey="p" rel="prev">Size and Alignment</a>, Up: <a href="Types.html" accesskey="u" rel="up">Types</a> [<a href="Index.html" title="Index" rel="index">Index</a>]</p> </div> <hr> <span id="Arrays_002c-Unions_002c-and-Enumerations"></span><h4 class="subsection">2.3.4 Arrays, Unions, and Enumerations</h4> <ul class="section-toc"> <li><a href="#Arrays" accesskey="1">Arrays</a></li> <li><a href="#Unions" accesskey="2">Unions</a></li> <li><a href="#Enumerations" accesskey="3">Enumerations</a></li> </ul> <div class="subsubsection" id="Arrays"> <h4 class="subsubsection">2.3.4.1 Arrays</h4> <p><code>libffi</code> does not have direct support for arrays or unions. However, they can be emulated using structures. </p> <p>To emulate an array, simply create an <code>ffi_type</code> using <code>FFI_TYPE_STRUCT</code> with as many members as there are elements in the array. </p> <div class="example"> <pre class="example">ffi_type array_type; ffi_type **elements int i; elements = malloc ((n + 1) * sizeof (ffi_type *)); for (i = 0; i < n; ++i) elements[i] = array_element_type; elements[n] = NULL; array_type.size = array_type.alignment = 0; array_type.type = FFI_TYPE_STRUCT; array_type.elements = elements; </pre></div> <p>Note that arrays cannot be passed or returned by value in C – structure types created like this should only be used to refer to members of real <code>FFI_TYPE_STRUCT</code> objects. </p> <p>However, a phony array type like this will not cause any errors from <code>libffi</code> if you use it as an argument or return type. This may be confusing. </p> </div> <div class="subsubsection" id="Unions"> <h4 class="subsubsection">2.3.4.2 Unions</h4> <p>A union can also be emulated using <code>FFI_TYPE_STRUCT</code>. In this case, however, you must make sure that the size and alignment match the real requirements of the union. </p> <p>One simple way to do this is to ensue that each element type is laid out. Then, give the new structure type a single element; the size of the largest element; and the largest alignment seen as well. </p> <p>This example uses the <code>ffi_prep_cif</code> trick to ensure that each element type is laid out. </p> <div class="example"> <pre class="example">ffi_abi desired_abi; ffi_type union_type; ffi_type **union_elements; int i; ffi_type element_types[2]; element_types[1] = NULL; union_type.size = union_type.alignment = 0; union_type.type = FFI_TYPE_STRUCT; union_type.elements = element_types; for (i = 0; union_elements[i]; ++i) { ffi_cif cif; if (ffi_prep_cif (&cif, desired_abi, 0, union_elements[i], NULL) == FFI_OK) { if (union_elements[i]->size > union_type.size) { union_type.size = union_elements[i]; size = union_elements[i]->size; } if (union_elements[i]->alignment > union_type.alignment) union_type.alignment = union_elements[i]->alignment; } } </pre></div> </div> <div class="subsubsection" id="Enumerations"> <h4 class="subsubsection">2.3.4.3 Enumerations</h4> <p><code>libffi</code> does not have any special support for C <code>enum</code>s. Although any given <code>enum</code> is implemented using a specific underlying integral type, exactly which type will be used cannot be determined by <code>libffi</code> – it may depend on the values in the enumeration or on compiler flags such as <samp>-fshort-enums</samp>. See <a data-manual="gcc" href="https://gcc.gnu.org/onlinedocs/gcc/Structures-unions-enumerations-and-bit_002dfields-implementation.html#Structures-unions-enumerations-and-bit_002dfields-implementation">(gcc)Structures unions enumerations and bit-fields implementation</a>, for more information about how GCC handles enumerations. </p> </div> </div> <hr> <div class="header"> <p> Next: <a href="Type-Example.html">Type Example</a>, Previous: <a href="Size-and-Alignment.html">Size and Alignment</a>, Up: <a href="Types.html">Types</a> [<a href="Index.html" title="Index" rel="index">Index</a>]</p> </div> </body> </html>