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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <!-- This manual is for libffi, a portable foreign function interface library. Copyright (C) 2008-2019 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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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> <dt id="index-ffi_005fstatus-2">Function: <strong>ffi_status</strong> <em>ffi_get_struct_offsets (ffi_abi abi, ffi_type *struct_type, size_t *offsets)</em></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> <hr> <div class="header"> <p> Next: <a href="Arrays-Unions-Enums.html" accesskey="n" rel="next">Arrays Unions Enums</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> </body> </html>