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<html lang="en">
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<title>The Basics - libffi</title>
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<a name="The-Basics"></a>
<p>
Next:&nbsp;<a rel="next" accesskey="n" href="Simple-Example.html#Simple-Example">Simple Example</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="Using-libffi.html#Using-libffi">Using libffi</a>
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<h3 class="section">2.1 The Basics</h3>

<p>&lsquo;<samp><span class="samp">Libffi</span></samp>&rsquo; assumes that you have a pointer to the function you wish
to call and that you know the number and types of arguments to pass
it, as well as the return type of the function.

   <p>The first thing you must do is create an <code>ffi_cif</code> object that
matches the signature of the function you wish to call.  This is a
separate step because it is common to make multiple calls using a
single <code>ffi_cif</code>.  The <dfn>cif</dfn> in <code>ffi_cif</code> stands for
Call InterFace.  To prepare a call interface object, use the function
<code>ffi_prep_cif</code>. 
<a name="index-cif-6"></a>
<a name="index-ffi_005fprep_005fcif-7"></a>

<div class="defun">
&mdash; Function: <b>ffi_status</b><var> ffi_prep_cif </var>(<var>ffi_cif *cif, ffi_abi abi, unsigned int nargs, ffi_type *rtype, ffi_type **argtypes</var>)<var><a name="index-ffi_005fstatus-8"></a></var><br>
<blockquote><p>This initializes <var>cif</var> according to the given parameters.

        <p><var>abi</var> is the ABI to use; normally <code>FFI_DEFAULT_ABI</code> is what
you want.  <a href="Multiple-ABIs.html#Multiple-ABIs">Multiple ABIs</a> for more information.

        <p><var>nargs</var> is the number of arguments that this function accepts.

        <p><var>rtype</var> is a pointer to an <code>ffi_type</code> structure that
describes the return type of the function.  See <a href="Types.html#Types">Types</a>.

        <p><var>argtypes</var> is a vector of <code>ffi_type</code> pointers. 
<var>argtypes</var> must have <var>nargs</var> elements.  If <var>nargs</var> is 0,
this argument is ignored.

        <p><code>ffi_prep_cif</code> returns a <code>libffi</code> status code, of type
<code>ffi_status</code>.  This will be either <code>FFI_OK</code> if everything
worked properly; <code>FFI_BAD_TYPEDEF</code> if one of the <code>ffi_type</code>
objects is incorrect; or <code>FFI_BAD_ABI</code> if the <var>abi</var> parameter
is invalid. 
</p></blockquote></div>

   <p>If the function being called is variadic (varargs) then <code>ffi_prep_cif_var</code>
must be used instead of <code>ffi_prep_cif</code>.

   <p><a name="index-ffi_005fprep_005fcif_005fvar-9"></a>

<div class="defun">
&mdash; Function: <b>ffi_status</b><var> ffi_prep_cif_var </var>(<var>ffi_cif *cif, ffi_abi abi, unsigned int nfixedargs, unsigned int ntotalargs, ffi_type *rtype, ffi_type **argtypes</var>)<var><a name="index-ffi_005fstatus-10"></a></var><br>
<blockquote><p>This initializes <var>cif</var> according to the given parameters for
a call to a variadic function.  In general it's operation is the
same as for <code>ffi_prep_cif</code> except that:

        <p><var>nfixedargs</var> is the number of fixed arguments, prior to any
variadic arguments.  It must be greater than zero.

        <p><var>ntotalargs</var> the total number of arguments, including variadic
and fixed arguments.

        <p>Note that, different cif's must be prepped for calls to the same
function when different numbers of arguments are passed.

        <p>Also note that a call to <code>ffi_prep_cif_var</code> with <var>nfixedargs</var>=<var>nototalargs</var>
is NOT equivalent to a call to <code>ffi_prep_cif</code>.

        </blockquote></div>

   <p>To call a function using an initialized <code>ffi_cif</code>, use the
<code>ffi_call</code> function:

   <p><a name="index-ffi_005fcall-11"></a>

<div class="defun">
&mdash; Function: <b>void</b><var> ffi_call </var>(<var>ffi_cif *cif, void *fn, void *rvalue, void **avalues</var>)<var><a name="index-void-12"></a></var><br>
<blockquote><p>This calls the function <var>fn</var> according to the description given in
<var>cif</var>.  <var>cif</var> must have already been prepared using
<code>ffi_prep_cif</code>.

        <p><var>rvalue</var> is a pointer to a chunk of memory that will hold the
result of the function call.  This must be large enough to hold the
result and must be suitably aligned; it is the caller's responsibility
to ensure this.  If <var>cif</var> declares that the function returns
<code>void</code> (using <code>ffi_type_void</code>), then <var>rvalue</var> is
ignored.  If <var>rvalue</var> is &lsquo;<samp><span class="samp">NULL</span></samp>&rsquo;, then the return value is
discarded.

        <p><var>avalues</var> is a vector of <code>void *</code> pointers that point to the
memory locations holding the argument values for a call.  If <var>cif</var>
declares that the function has no arguments (i.e., <var>nargs</var> was 0),
then <var>avalues</var> is ignored.  Note that argument values may be
modified by the callee (for instance, structs passed by value); the
burden of copying pass-by-value arguments is placed on the caller. 
</p></blockquote></div>

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