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Code Editor : Size-and-Alignment.html
<!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 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. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 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>Size and Alignment (libffi: the portable foreign function interface library)</title> <meta name="description" content="Size and Alignment (libffi: the portable foreign function interface library)"> <meta name="keywords" content="Size and Alignment (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="Arrays-Unions-Enums.html" rel="next" title="Arrays Unions Enums"> <link href="Structures.html" rel="prev" title="Structures"> <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="Size-and-Alignment"> <div class="header"> <p> Next: <a href="Arrays-Unions-Enums.html" accesskey="n" rel="next">Arrays, Unions, and Enumerations</a>, Previous: <a href="Structures.html" accesskey="p" rel="prev">Structures</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="Size-and-Alignment-1"></span><h4 class="subsection">2.3.3 Size and Alignment</h4> <p><code>libffi</code> will set the <code>size</code> and <code>alignment</code> fields of an <code>ffi_type</code> object for you. It does so using its knowledge of the ABI. </p> <p>You might expect that you can simply read these fields for a type that has been laid out by <code>libffi</code>. However, there are some caveats. </p> <ul> <li> The size or alignment of some of the built-in types may vary depending on the chosen ABI. </li><li> The size and alignment of a new structure type will not be set by <code>libffi</code> until it has been passed to <code>ffi_prep_cif</code> or <code>ffi_get_struct_offsets</code>. </li><li> A structure type cannot be shared across ABIs. Instead each ABI needs its own copy of the structure type. </li></ul> <p>So, before examining these fields, it is safest to pass the <code>ffi_type</code> object to <code>ffi_prep_cif</code> or <code>ffi_get_struct_offsets</code> first. This function will do all the needed setup. </p> <div class="example"> <pre class="example">ffi_type *desired_type; ffi_abi desired_abi; … ffi_cif cif; if (ffi_prep_cif (&cif, desired_abi, 0, desired_type, NULL) == FFI_OK) { size_t size = desired_type->size; unsigned short alignment = desired_type->alignment; } </pre></div> <p><code>libffi</code> also provides a way to get the offsets of the members of a structure. </p> <span id="index-ffi_005fget_005fstruct_005foffsets"></span> <dl class="def"> <dt id="index-ffi_005fstatus-2"><span class="category">Function: </span><span><strong>ffi_status</strong> <em>ffi_get_struct_offsets (ffi_abi abi, ffi_type *struct_type, size_t *offsets)</em><a href='#index-ffi_005fstatus-2' class='copiable-anchor'> ¶</a></span></dt> <dd><p>Compute the offset of each element of the given structure type. <var>abi</var> is the ABI to use; this is needed because in some cases the layout depends on the ABI. </p> <p><var>offsets</var> is an out parameter. The caller is responsible for providing enough space for all the results to be written – one element per element type in <var>struct_type</var>. If <var>offsets</var> is <code>NULL</code>, then the type will be laid out but not otherwise modified. This can be useful for accessing the type’s size or layout, as mentioned above. </p> <p>This function returns <code>FFI_OK</code> on success; <code>FFI_BAD_ABI</code> if <var>abi</var> is invalid; or <code>FFI_BAD_TYPEDEF</code> if <var>struct_type</var> is invalid in some way. Note that only <code>FFI_STRUCT</code> types are valid here. </p></dd></dl> </div> <hr> <div class="header"> <p> Next: <a href="Arrays-Unions-Enums.html">Arrays, Unions, and Enumerations</a>, Previous: <a href="Structures.html">Structures</a>, Up: <a href="Types.html">Types</a> [<a href="Index.html" title="Index" rel="index">Index</a>]</p> </div> </body> </html>
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