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 <div class="section" id="module-ctypes">
<h1>16.16. <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> — A foreign function library for Python<a class="headerlink" href="#module-ctypes" title="Permalink to this headline">¶</a></h1>
<hr class="docutils" />
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> is a foreign function library for Python. It provides C compatible
data types, and allows calling functions in DLLs or shared libraries. It can be
used to wrap these libraries in pure Python.
<div class="section" id="ctypes-tutorial">
<h2>16.16.1. ctypes tutorial<a class="headerlink" href="#ctypes-tutorial" title="Permalink to this headline">¶</a></h2>
Note: The code samples in this tutorial use <a class="reference internal" href="doctest.html#module-doctest" title="doctest: Test pieces of code within docstrings."><code class="xref py py-mod docutils literal notranslate">doctest</code></a> to make sure that
they actually work. Since some code samples behave differently under Linux,
Windows, or Mac OS X, they contain doctest directives in comments.
Note: Some code samples reference the ctypes <a class="reference internal" href="#ctypes.c_int" title="ctypes.c_int"><code class="xref py py-class docutils literal notranslate">c_int</code></a> type. On platforms
where <code class="docutils literal notranslate">sizeof(long) == sizeof(int)</code> it is an alias to <a class="reference internal" href="#ctypes.c_long" title="ctypes.c_long"><code class="xref py py-class docutils literal notranslate">c_long</code></a>.
So, you should not be confused if <a class="reference internal" href="#ctypes.c_long" title="ctypes.c_long"><code class="xref py py-class docutils literal notranslate">c_long</code></a> is printed if you would expect
<a class="reference internal" href="#ctypes.c_int" title="ctypes.c_int"><code class="xref py py-class docutils literal notranslate">c_int</code></a> — they are actually the same type.
<div class="section" id="loading-dynamic-link-libraries">
<h3>16.16.1.1. Loading dynamic link libraries<a class="headerlink" href="#loading-dynamic-link-libraries" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> exports the cdll, and on Windows windll and oledll
objects, for loading dynamic link libraries.
You load libraries by accessing them as attributes of these objects. cdll
loads libraries which export functions using the standard <code class="docutils literal notranslate">cdecl</code> calling
convention, while windll libraries call functions using the <code class="docutils literal notranslate">stdcall</code>
calling convention. oledll also uses the <code class="docutils literal notranslate">stdcall</code> calling convention, and
assumes the functions return a Windows <code class="xref c c-type docutils literal notranslate">HRESULT</code> error code. The error
code is used to automatically raise an <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-class docutils literal notranslate">OSError</code></a> exception when the
function call fails.
<div class="versionchanged">
Changed in version 3.3: Windows errors used to raise <a class="reference internal" href="exceptions.html#WindowsError" title="WindowsError"><code class="xref py py-exc docutils literal notranslate">WindowsError</code></a>, which is now an alias
of <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate">OSError</code></a>.
</div>
Here are some examples for Windows. Note that <code class="docutils literal notranslate">msvcrt</code> is the MS standard C
library containing most standard C functions, and uses the cdecl calling
convention:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import *
&gt;&gt;&gt; print(windll.kernel32) 
&lt;WinDLL &#39;kernel32&#39;, handle ... at ...&gt;
&gt;&gt;&gt; print(cdll.msvcrt) 
&lt;CDLL &#39;msvcrt&#39;, handle ... at ...&gt;
&gt;&gt;&gt; libc = cdll.msvcrt 
&gt;&gt;&gt;
</pre></div>
</div>
Windows appends the usual <code class="docutils literal notranslate">.dll</code> file suffix automatically.
<div class="admonition note">
Note
Accessing the standard C library through <code class="docutils literal notranslate">cdll.msvcrt</code> will use an
outdated version of the library that may be incompatible with the one
being used by Python. Where possible, use native Python functionality,
or else import and use the <code class="docutils literal notranslate">msvcrt</code> module.
</div>
On Linux, it is required to specify the filename including the extension to
load a library, so attribute access can not be used to load libraries. Either the
<code class="xref py py-meth docutils literal notranslate">LoadLibrary()</code> method of the dll loaders should be used, or you should load
the library by creating an instance of CDLL by calling the constructor:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; cdll.LoadLibrary(&quot;libc.so.6&quot;) 
&lt;CDLL &#39;libc.so.6&#39;, handle ... at ...&gt;
&gt;&gt;&gt; libc = CDLL(&quot;libc.so.6&quot;) 
&gt;&gt;&gt; libc 
&lt;CDLL &#39;libc.so.6&#39;, handle ... at ...&gt;
&gt;&gt;&gt;
</pre></div>
</div>
</div>
<div class="section" id="accessing-functions-from-loaded-dlls">
<h3>16.16.1.2. Accessing functions from loaded dlls<a class="headerlink" href="#accessing-functions-from-loaded-dlls" title="Permalink to this headline">¶</a></h3>
Functions are accessed as attributes of dll objects:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import *
&gt;&gt;&gt; libc.printf
&lt;_FuncPtr object at 0x...&gt;
&gt;&gt;&gt; print(windll.kernel32.GetModuleHandleA) 
&lt;_FuncPtr object at 0x...&gt;
&gt;&gt;&gt; print(windll.kernel32.MyOwnFunction) 
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
 File &quot;ctypes.py&quot;, line 239, in __getattr__
 func = _StdcallFuncPtr(name, self)
AttributeError: function &#39;MyOwnFunction&#39; not found
&gt;&gt;&gt;
</pre></div>
</div>
Note that win32 system dlls like <code class="docutils literal notranslate">kernel32</code> and <code class="docutils literal notranslate">user32</code> often export ANSI
as well as UNICODE versions of a function. The UNICODE version is exported with
an <code class="docutils literal notranslate">W</code> appended to the name, while the ANSI version is exported with an <code class="docutils literal notranslate">A</code>
appended to the name. The win32 <code class="docutils literal notranslate">GetModuleHandle</code> function, which returns a
module handle for a given module name, has the following C prototype, and a
macro is used to expose one of them as <code class="docutils literal notranslate">GetModuleHandle</code> depending on whether
UNICODE is defined or not:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>/* ANSI version */
HMODULE GetModuleHandleA(LPCSTR lpModuleName);
/* UNICODE version */
HMODULE GetModuleHandleW(LPCWSTR lpModuleName);
</pre></div>
</div>
windll does not try to select one of them by magic, you must access the
version you need by specifying <code class="docutils literal notranslate">GetModuleHandleA</code> or <code class="docutils literal notranslate">GetModuleHandleW</code>
explicitly, and then call it with bytes or string objects respectively.
Sometimes, dlls export functions with names which aren’t valid Python
identifiers, like <code class="docutils literal notranslate">&quot;??2&#64;YAPAXI&#64;Z&quot;</code>. In this case you have to use
<a class="reference internal" href="functions.html#getattr" title="getattr"><code class="xref py py-func docutils literal notranslate">getattr()</code></a> to retrieve the function:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; getattr(cdll.msvcrt, &quot;??2@YAPAXI@Z&quot;) 
&lt;_FuncPtr object at 0x...&gt;
&gt;&gt;&gt;
</pre></div>
</div>
On Windows, some dlls export functions not by name but by ordinal. These
functions can be accessed by indexing the dll object with the ordinal number:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; cdll.kernel32[1] 
&lt;_FuncPtr object at 0x...&gt;
&gt;&gt;&gt; cdll.kernel32[0] 
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
 File &quot;ctypes.py&quot;, line 310, in __getitem__
 func = _StdcallFuncPtr(name, self)
AttributeError: function ordinal 0 not found
&gt;&gt;&gt;
</pre></div>
</div>
</div>
<div class="section" id="calling-functions">
<h3>16.16.1.3. Calling functions<a class="headerlink" href="#calling-functions" title="Permalink to this headline">¶</a></h3>
You can call these functions like any other Python callable. This example uses
the <code class="docutils literal notranslate">time()</code> function, which returns system time in seconds since the Unix
epoch, and the <code class="docutils literal notranslate">GetModuleHandleA()</code> function, which returns a win32 module
handle.
This example calls both functions with a NULL pointer (<code class="docutils literal notranslate">None</code> should be used
as the NULL pointer):
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; print(libc.time(None)) 
1150640792
&gt;&gt;&gt; print(hex(windll.kernel32.GetModuleHandleA(None))) 
0x1d000000
&gt;&gt;&gt;
</pre></div>
</div>
<div class="admonition note">
Note
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> may raise a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> after calling the function, if
it detects that an invalid number of arguments were passed. This behavior
should not be relied upon. It is deprecated in 3.6.2, and will be removed
in 3.7.
</div>
<a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> is raised when you call an <code class="docutils literal notranslate">stdcall</code> function with the
<code class="docutils literal notranslate">cdecl</code> calling convention, or vice versa:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; cdll.kernel32.GetModuleHandleA(None) 
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
ValueError: Procedure probably called with not enough arguments (4 bytes missing)
&gt;&gt;&gt;

&gt;&gt;&gt; windll.msvcrt.printf(b&quot;spam&quot;) 
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
ValueError: Procedure probably called with too many arguments (4 bytes in excess)
&gt;&gt;&gt;
</pre></div>
</div>
To find out the correct calling convention you have to look into the C header
file or the documentation for the function you want to call.
On Windows, <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> uses win32 structured exception handling to prevent
crashes from general protection faults when functions are called with invalid
argument values:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; windll.kernel32.GetModuleHandleA(32) 
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
OSError: exception: access violation reading 0x00000020
&gt;&gt;&gt;
</pre></div>
</div>
There are, however, enough ways to crash Python with <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a>, so you
should be careful anyway. The <a class="reference internal" href="faulthandler.html#module-faulthandler" title="faulthandler: Dump the Python traceback."><code class="xref py py-mod docutils literal notranslate">faulthandler</code></a> module can be helpful in
debugging crashes (e.g. from segmentation faults produced by erroneous C library
calls).
<code class="docutils literal notranslate">None</code>, integers, bytes objects and (unicode) strings are the only native
Python objects that can directly be used as parameters in these function calls.
<code class="docutils literal notranslate">None</code> is passed as a C <code class="docutils literal notranslate">NULL</code> pointer, bytes objects and strings are passed
as pointer to the memory block that contains their data (<code class="xref c c-type docutils literal notranslate">char *</code> or
<code class="xref c c-type docutils literal notranslate">wchar_t *</code>). Python integers are passed as the platforms default C
<code class="xref c c-type docutils literal notranslate">int</code> type, their value is masked to fit into the C type.
Before we move on calling functions with other parameter types, we have to learn
more about <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> data types.
</div>
<div class="section" id="fundamental-data-types">
<h3>16.16.1.4. Fundamental data types<a class="headerlink" href="#fundamental-data-types" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> defines a number of primitive C compatible data types:
<table border="1" class="docutils">
<colgroup>
<col width="24%" />
<col width="46%" />
<col width="30%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">ctypes type</th>
<th class="head">C type</th>
<th class="head">Python type</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><a class="reference internal" href="#ctypes.c_bool" title="ctypes.c_bool"><code class="xref py py-class docutils literal notranslate">c_bool</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">_Bool</code></td>
<td>bool (1)</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#ctypes.c_char" title="ctypes.c_char"><code class="xref py py-class docutils literal notranslate">c_char</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">char</code></td>
<td>1-character bytes object</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#ctypes.c_wchar" title="ctypes.c_wchar"><code class="xref py py-class docutils literal notranslate">c_wchar</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">wchar_t</code></td>
<td>1-character string</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#ctypes.c_byte" title="ctypes.c_byte"><code class="xref py py-class docutils literal notranslate">c_byte</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">char</code></td>
<td>int</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#ctypes.c_ubyte" title="ctypes.c_ubyte"><code class="xref py py-class docutils literal notranslate">c_ubyte</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">unsigned char</code></td>
<td>int</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#ctypes.c_short" title="ctypes.c_short"><code class="xref py py-class docutils literal notranslate">c_short</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">short</code></td>
<td>int</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#ctypes.c_ushort" title="ctypes.c_ushort"><code class="xref py py-class docutils literal notranslate">c_ushort</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">unsigned short</code></td>
<td>int</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#ctypes.c_int" title="ctypes.c_int"><code class="xref py py-class docutils literal notranslate">c_int</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">int</code></td>
<td>int</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#ctypes.c_uint" title="ctypes.c_uint"><code class="xref py py-class docutils literal notranslate">c_uint</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">unsigned int</code></td>
<td>int</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#ctypes.c_long" title="ctypes.c_long"><code class="xref py py-class docutils literal notranslate">c_long</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">long</code></td>
<td>int</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#ctypes.c_ulong" title="ctypes.c_ulong"><code class="xref py py-class docutils literal notranslate">c_ulong</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">unsigned long</code></td>
<td>int</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#ctypes.c_longlong" title="ctypes.c_longlong"><code class="xref py py-class docutils literal notranslate">c_longlong</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">__int64</code> or <code class="xref c c-type docutils literal notranslate">long long</code></td>
<td>int</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#ctypes.c_ulonglong" title="ctypes.c_ulonglong"><code class="xref py py-class docutils literal notranslate">c_ulonglong</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">unsigned __int64</code> or
<code class="xref c c-type docutils literal notranslate">unsigned long long</code></td>
<td>int</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#ctypes.c_size_t" title="ctypes.c_size_t"><code class="xref py py-class docutils literal notranslate">c_size_t</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">size_t</code></td>
<td>int</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#ctypes.c_ssize_t" title="ctypes.c_ssize_t"><code class="xref py py-class docutils literal notranslate">c_ssize_t</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">ssize_t</code> or
<code class="xref c c-type docutils literal notranslate">Py_ssize_t</code></td>
<td>int</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#ctypes.c_float" title="ctypes.c_float"><code class="xref py py-class docutils literal notranslate">c_float</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">float</code></td>
<td>float</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#ctypes.c_double" title="ctypes.c_double"><code class="xref py py-class docutils literal notranslate">c_double</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">double</code></td>
<td>float</td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#ctypes.c_longdouble" title="ctypes.c_longdouble"><code class="xref py py-class docutils literal notranslate">c_longdouble</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">long double</code></td>
<td>float</td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#ctypes.c_char_p" title="ctypes.c_char_p"><code class="xref py py-class docutils literal notranslate">c_char_p</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">char *</code> (NUL terminated)</td>
<td>bytes object or <code class="docutils literal notranslate">None</code></td>
</tr>
<tr class="row-odd"><td><a class="reference internal" href="#ctypes.c_wchar_p" title="ctypes.c_wchar_p"><code class="xref py py-class docutils literal notranslate">c_wchar_p</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">wchar_t *</code> (NUL terminated)</td>
<td>string or <code class="docutils literal notranslate">None</code></td>
</tr>
<tr class="row-even"><td><a class="reference internal" href="#ctypes.c_void_p" title="ctypes.c_void_p"><code class="xref py py-class docutils literal notranslate">c_void_p</code></a></td>
<td><code class="xref c c-type docutils literal notranslate">void *</code></td>
<td>int or <code class="docutils literal notranslate">None</code></td>
</tr>
</tbody>
</table>
<ol class="arabic simple">
<li>The constructor accepts any object with a truth value.</li>
</ol>
All these types can be created by calling them with an optional initializer of
the correct type and value:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; c_int()
c_long(0)
&gt;&gt;&gt; c_wchar_p(&quot;Hello, World&quot;)
c_wchar_p(140018365411392)
&gt;&gt;&gt; c_ushort(-3)
c_ushort(65533)
&gt;&gt;&gt;
</pre></div>
</div>
Since these types are mutable, their value can also be changed afterwards:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; i = c_int(42)
&gt;&gt;&gt; print(i)
c_long(42)
&gt;&gt;&gt; print(i.value)
42
&gt;&gt;&gt; i.value = -99
&gt;&gt;&gt; print(i.value)
-99
&gt;&gt;&gt;
</pre></div>
</div>
Assigning a new value to instances of the pointer types <a class="reference internal" href="#ctypes.c_char_p" title="ctypes.c_char_p"><code class="xref py py-class docutils literal notranslate">c_char_p</code></a>,
<a class="reference internal" href="#ctypes.c_wchar_p" title="ctypes.c_wchar_p"><code class="xref py py-class docutils literal notranslate">c_wchar_p</code></a>, and <a class="reference internal" href="#ctypes.c_void_p" title="ctypes.c_void_p"><code class="xref py py-class docutils literal notranslate">c_void_p</code></a> changes the memory location they
point to, not the contents of the memory block (of course not, because Python
bytes objects are immutable):
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; s = &quot;Hello, World&quot;
&gt;&gt;&gt; c_s = c_wchar_p(s)
&gt;&gt;&gt; print(c_s)
c_wchar_p(139966785747344)
&gt;&gt;&gt; print(c_s.value)
Hello World
&gt;&gt;&gt; c_s.value = &quot;Hi, there&quot;
&gt;&gt;&gt; print(c_s) # the memory location has changed
c_wchar_p(139966783348904)
&gt;&gt;&gt; print(c_s.value)
Hi, there
&gt;&gt;&gt; print(s) # first object is unchanged
Hello, World
&gt;&gt;&gt;
</pre></div>
</div>
You should be careful, however, not to pass them to functions expecting pointers
to mutable memory. If you need mutable memory blocks, ctypes has a
<a class="reference internal" href="#ctypes.create_string_buffer" title="ctypes.create_string_buffer"><code class="xref py py-func docutils literal notranslate">create_string_buffer()</code></a> function which creates these in various ways. The
current memory block contents can be accessed (or changed) with the <code class="docutils literal notranslate">raw</code>
property; if you want to access it as NUL terminated string, use the <code class="docutils literal notranslate">value</code>
property:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import *
&gt;&gt;&gt; p = create_string_buffer(3) # create a 3 byte buffer, initialized to NUL bytes
&gt;&gt;&gt; print(sizeof(p), repr(p.raw))
3 b&#39;\x00\x00\x00&#39;
&gt;&gt;&gt; p = create_string_buffer(b&quot;Hello&quot;) # create a buffer containing a NUL terminated string
&gt;&gt;&gt; print(sizeof(p), repr(p.raw))
6 b&#39;Hello\x00&#39;
&gt;&gt;&gt; print(repr(p.value))
b&#39;Hello&#39;
&gt;&gt;&gt; p = create_string_buffer(b&quot;Hello&quot;, 10) # create a 10 byte buffer
&gt;&gt;&gt; print(sizeof(p), repr(p.raw))
10 b&#39;Hello\x00\x00\x00\x00\x00&#39;
&gt;&gt;&gt; p.value = b&quot;Hi&quot;
&gt;&gt;&gt; print(sizeof(p), repr(p.raw))
10 b&#39;Hi\x00lo\x00\x00\x00\x00\x00&#39;
&gt;&gt;&gt;
</pre></div>
</div>
The <a class="reference internal" href="#ctypes.create_string_buffer" title="ctypes.create_string_buffer"><code class="xref py py-func docutils literal notranslate">create_string_buffer()</code></a> function replaces the <code class="xref py py-func docutils literal notranslate">c_buffer()</code> function
(which is still available as an alias), as well as the <code class="xref py py-func docutils literal notranslate">c_string()</code> function
from earlier ctypes releases. To create a mutable memory block containing
unicode characters of the C type <code class="xref c c-type docutils literal notranslate">wchar_t</code> use the
<a class="reference internal" href="#ctypes.create_unicode_buffer" title="ctypes.create_unicode_buffer"><code class="xref py py-func docutils literal notranslate">create_unicode_buffer()</code></a> function.
</div>
<div class="section" id="calling-functions-continued">
<h3>16.16.1.5. Calling functions, continued<a class="headerlink" href="#calling-functions-continued" title="Permalink to this headline">¶</a></h3>
Note that printf prints to the real standard output channel, not to
<a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate">sys.stdout</code></a>, so these examples will only work at the console prompt, not
from within IDLE or PythonWin:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; printf = libc.printf
&gt;&gt;&gt; printf(b&quot;Hello, %s\n&quot;, b&quot;World!&quot;)
Hello, World!
14
&gt;&gt;&gt; printf(b&quot;Hello, %S\n&quot;, &quot;World!&quot;)
Hello, World!
14
&gt;&gt;&gt; printf(b&quot;%d bottles of beer\n&quot;, 42)
42 bottles of beer
19
&gt;&gt;&gt; printf(b&quot;%f bottles of beer\n&quot;, 42.5)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
ArgumentError: argument 2: exceptions.TypeError: Don&#39;t know how to convert parameter 2
&gt;&gt;&gt;
</pre></div>
</div>
As has been mentioned before, all Python types except integers, strings, and
bytes objects have to be wrapped in their corresponding <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> type, so
that they can be converted to the required C data type:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; printf(b&quot;An int %d, a double %f\n&quot;, 1234, c_double(3.14))
An int 1234, a double 3.140000
31
&gt;&gt;&gt;
</pre></div>
</div>
</div>
<div class="section" id="calling-functions-with-your-own-custom-data-types">
<h3>16.16.1.6. Calling functions with your own custom data types<a class="headerlink" href="#calling-functions-with-your-own-custom-data-types" title="Permalink to this headline">¶</a></h3>
You can also customize <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> argument conversion to allow instances of
your own classes be used as function arguments. <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> looks for an
<code class="xref py py-attr docutils literal notranslate">_as_parameter_</code> attribute and uses this as the function argument. Of
course, it must be one of integer, string, or bytes:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class Bottles:
... def __init__(self, number):
... self._as_parameter_ = number
...
&gt;&gt;&gt; bottles = Bottles(42)
&gt;&gt;&gt; printf(b&quot;%d bottles of beer\n&quot;, bottles)
42 bottles of beer
19
&gt;&gt;&gt;
</pre></div>
</div>
If you don’t want to store the instance’s data in the <code class="xref py py-attr docutils literal notranslate">_as_parameter_</code>
instance variable, you could define a <a class="reference internal" href="functions.html#property" title="property"><code class="xref py py-class docutils literal notranslate">property</code></a> which makes the
attribute available on request.
</div>
<div class="section" id="specifying-the-required-argument-types-function-prototypes">
<h3>16.16.1.7. Specifying the required argument types (function prototypes)<a class="headerlink" href="#specifying-the-required-argument-types-function-prototypes" title="Permalink to this headline">¶</a></h3>
It is possible to specify the required argument types of functions exported from
DLLs by setting the <code class="xref py py-attr docutils literal notranslate">argtypes</code> attribute.
<code class="xref py py-attr docutils literal notranslate">argtypes</code> must be a sequence of C data types (the <code class="docutils literal notranslate">printf</code> function is
probably not a good example here, because it takes a variable number and
different types of parameters depending on the format string, on the other hand
this is quite handy to experiment with this feature):
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
&gt;&gt;&gt; printf(b&quot;String &#39;%s&#39;, Int %d, Double %f\n&quot;, b&quot;Hi&quot;, 10, 2.2)
String &#39;Hi&#39;, Int 10, Double 2.200000
37
&gt;&gt;&gt;
</pre></div>
</div>
Specifying a format protects against incompatible argument types (just as a
prototype for a C function), and tries to convert the arguments to valid types:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; printf(b&quot;%d %d %d&quot;, 1, 2, 3)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
ArgumentError: argument 2: exceptions.TypeError: wrong type
&gt;&gt;&gt; printf(b&quot;%s %d %f\n&quot;, b&quot;X&quot;, 2, 3)
X 2 3.000000
13
&gt;&gt;&gt;
</pre></div>
</div>
If you have defined your own classes which you pass to function calls, you have
to implement a <code class="xref py py-meth docutils literal notranslate">from_param()</code> class method for them to be able to use them
in the <code class="xref py py-attr docutils literal notranslate">argtypes</code> sequence. The <code class="xref py py-meth docutils literal notranslate">from_param()</code> class method receives
the Python object passed to the function call, it should do a typecheck or
whatever is needed to make sure this object is acceptable, and then return the
object itself, its <code class="xref py py-attr docutils literal notranslate">_as_parameter_</code> attribute, or whatever you want to
pass as the C function argument in this case. Again, the result should be an
integer, string, bytes, a <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> instance, or an object with an
<code class="xref py py-attr docutils literal notranslate">_as_parameter_</code> attribute.
</div>
<div class="section" id="return-types">
<h3>16.16.1.8. Return types<a class="headerlink" href="#return-types" title="Permalink to this headline">¶</a></h3>
By default functions are assumed to return the C <code class="xref c c-type docutils literal notranslate">int</code> type. Other
return types can be specified by setting the <code class="xref py py-attr docutils literal notranslate">restype</code> attribute of the
function object.
Here is a more advanced example, it uses the <code class="docutils literal notranslate">strchr</code> function, which expects
a string pointer and a char, and returns a pointer to a string:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; strchr = libc.strchr
&gt;&gt;&gt; strchr(b&quot;abcdef&quot;, ord(&quot;d&quot;)) 
8059983
&gt;&gt;&gt; strchr.restype = c_char_p # c_char_p is a pointer to a string
&gt;&gt;&gt; strchr(b&quot;abcdef&quot;, ord(&quot;d&quot;))
b&#39;def&#39;
&gt;&gt;&gt; print(strchr(b&quot;abcdef&quot;, ord(&quot;x&quot;)))
None
&gt;&gt;&gt;
</pre></div>
</div>
If you want to avoid the <code class="docutils literal notranslate">ord(&quot;x&quot;)</code> calls above, you can set the
<code class="xref py py-attr docutils literal notranslate">argtypes</code> attribute, and the second argument will be converted from a
single character Python bytes object into a C char:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; strchr.restype = c_char_p
&gt;&gt;&gt; strchr.argtypes = [c_char_p, c_char]
&gt;&gt;&gt; strchr(b&quot;abcdef&quot;, b&quot;d&quot;)
&#39;def&#39;
&gt;&gt;&gt; strchr(b&quot;abcdef&quot;, b&quot;def&quot;)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
ArgumentError: argument 2: exceptions.TypeError: one character string expected
&gt;&gt;&gt; print(strchr(b&quot;abcdef&quot;, b&quot;x&quot;))
None
&gt;&gt;&gt; strchr(b&quot;abcdef&quot;, b&quot;d&quot;)
&#39;def&#39;
&gt;&gt;&gt;
</pre></div>
</div>
You can also use a callable Python object (a function or a class for example) as
the <code class="xref py py-attr docutils literal notranslate">restype</code> attribute, if the foreign function returns an integer. The
callable will be called with the integer the C function returns, and the
result of this call will be used as the result of your function call. This is
useful to check for error return values and automatically raise an exception:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; GetModuleHandle = windll.kernel32.GetModuleHandleA 
&gt;&gt;&gt; def ValidHandle(value):
... if value == 0:
... raise WinError()
... return value
...
&gt;&gt;&gt;
&gt;&gt;&gt; GetModuleHandle.restype = ValidHandle 
&gt;&gt;&gt; GetModuleHandle(None) 
486539264
&gt;&gt;&gt; GetModuleHandle(&quot;something silly&quot;) 
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
 File &quot;&lt;stdin&gt;&quot;, line 3, in ValidHandle
OSError: [Errno 126] The specified module could not be found.
&gt;&gt;&gt;
</pre></div>
</div>
<code class="docutils literal notranslate">WinError</code> is a function which will call Windows <code class="docutils literal notranslate">FormatMessage()</code> api to
get the string representation of an error code, and returns an exception.
<code class="docutils literal notranslate">WinError</code> takes an optional error code parameter, if no one is used, it calls
<a class="reference internal" href="#ctypes.GetLastError" title="ctypes.GetLastError"><code class="xref py py-func docutils literal notranslate">GetLastError()</code></a> to retrieve it.
Please note that a much more powerful error checking mechanism is available
through the <code class="xref py py-attr docutils literal notranslate">errcheck</code> attribute; see the reference manual for details.
</div>
<div class="section" id="passing-pointers-or-passing-parameters-by-reference">
<h3>16.16.1.9. Passing pointers (or: passing parameters by reference)<a class="headerlink" href="#passing-pointers-or-passing-parameters-by-reference" title="Permalink to this headline">¶</a></h3>
Sometimes a C api function expects a pointer to a data type as parameter,
probably to write into the corresponding location, or if the data is too large
to be passed by value. This is also known as passing parameters by reference.
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> exports the <a class="reference internal" href="#ctypes.byref" title="ctypes.byref"><code class="xref py py-func docutils literal notranslate">byref()</code></a> function which is used to pass parameters
by reference. The same effect can be achieved with the <a class="reference internal" href="#ctypes.pointer" title="ctypes.pointer"><code class="xref py py-func docutils literal notranslate">pointer()</code></a> function,
although <a class="reference internal" href="#ctypes.pointer" title="ctypes.pointer"><code class="xref py py-func docutils literal notranslate">pointer()</code></a> does a lot more work since it constructs a real pointer
object, so it is faster to use <a class="reference internal" href="#ctypes.byref" title="ctypes.byref"><code class="xref py py-func docutils literal notranslate">byref()</code></a> if you don’t need the pointer
object in Python itself:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; i = c_int()
&gt;&gt;&gt; f = c_float()
&gt;&gt;&gt; s = create_string_buffer(b&#39;\000&#39; * 32)
&gt;&gt;&gt; print(i.value, f.value, repr(s.value))
0 0.0 b&#39;&#39;
&gt;&gt;&gt; libc.sscanf(b&quot;1 3.14 Hello&quot;, b&quot;%d %f %s&quot;,
... byref(i), byref(f), s)
3
&gt;&gt;&gt; print(i.value, f.value, repr(s.value))
1 3.1400001049 b&#39;Hello&#39;
&gt;&gt;&gt;
</pre></div>
</div>
</div>
<div class="section" id="structures-and-unions">
<h3>16.16.1.10. Structures and unions<a class="headerlink" href="#structures-and-unions" title="Permalink to this headline">¶</a></h3>
Structures and unions must derive from the <a class="reference internal" href="#ctypes.Structure" title="ctypes.Structure"><code class="xref py py-class docutils literal notranslate">Structure</code></a> and <a class="reference internal" href="#ctypes.Union" title="ctypes.Union"><code class="xref py py-class docutils literal notranslate">Union</code></a>
base classes which are defined in the <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> module. Each subclass must
define a <code class="xref py py-attr docutils literal notranslate">_fields_</code> attribute. <code class="xref py py-attr docutils literal notranslate">_fields_</code> must be a list of
2-tuples, containing a field name and a field type.
The field type must be a <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> type like <a class="reference internal" href="#ctypes.c_int" title="ctypes.c_int"><code class="xref py py-class docutils literal notranslate">c_int</code></a>, or any other
derived <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> type: structure, union, array, pointer.
Here is a simple example of a POINT structure, which contains two integers named
x and y, and also shows how to initialize a structure in the constructor:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import *
&gt;&gt;&gt; class POINT(Structure):
... _fields_ = [(&quot;x&quot;, c_int),
... (&quot;y&quot;, c_int)]
...
&gt;&gt;&gt; point = POINT(10, 20)
&gt;&gt;&gt; print(point.x, point.y)
10 20
&gt;&gt;&gt; point = POINT(y=5)
&gt;&gt;&gt; print(point.x, point.y)
0 5
&gt;&gt;&gt; POINT(1, 2, 3)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
ValueError: too many initializers
&gt;&gt;&gt;
</pre></div>
</div>
You can, however, build much more complicated structures. A structure can
itself contain other structures by using a structure as a field type.
Here is a RECT structure which contains two POINTs named upperleft and
lowerright:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class RECT(Structure):
... _fields_ = [(&quot;upperleft&quot;, POINT),
... (&quot;lowerright&quot;, POINT)]
...
&gt;&gt;&gt; rc = RECT(point)
&gt;&gt;&gt; print(rc.upperleft.x, rc.upperleft.y)
0 5
&gt;&gt;&gt; print(rc.lowerright.x, rc.lowerright.y)
0 0
&gt;&gt;&gt;
</pre></div>
</div>
Nested structures can also be initialized in the constructor in several ways:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; r = RECT(POINT(1, 2), POINT(3, 4))
&gt;&gt;&gt; r = RECT((1, 2), (3, 4))
</pre></div>
</div>
Field <a class="reference internal" href="../glossary.html#term-descriptor">descriptor</a>s can be retrieved from the class, they are useful
for debugging because they can provide useful information:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; print(POINT.x)
&lt;Field type=c_long, ofs=0, size=4&gt;
&gt;&gt;&gt; print(POINT.y)
&lt;Field type=c_long, ofs=4, size=4&gt;
&gt;&gt;&gt;
</pre></div>
</div>
<div class="admonition warning" id="ctypes-structureunion-alignment-byte-order">
Warning
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> does not support passing unions or structures with bit-fields
to functions by value. While this may work on 32-bit x86, it’s not
guaranteed by the library to work in the general case. Unions and
structures with bit-fields should always be passed to functions by pointer.
</div>
</div>
<div class="section" id="structure-union-alignment-and-byte-order">
<h3>16.16.1.11. Structure/union alignment and byte order<a class="headerlink" href="#structure-union-alignment-and-byte-order" title="Permalink to this headline">¶</a></h3>
By default, Structure and Union fields are aligned in the same way the C
compiler does it. It is possible to override this behavior be specifying a
<code class="xref py py-attr docutils literal notranslate">_pack_</code> class attribute in the subclass definition. This must be set to a
positive integer and specifies the maximum alignment for the fields. This is
what <code class="docutils literal notranslate">#pragma pack(n)</code> also does in MSVC.
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> uses the native byte order for Structures and Unions. To build
structures with non-native byte order, you can use one of the
<a class="reference internal" href="#ctypes.BigEndianStructure" title="ctypes.BigEndianStructure"><code class="xref py py-class docutils literal notranslate">BigEndianStructure</code></a>, <a class="reference internal" href="#ctypes.LittleEndianStructure" title="ctypes.LittleEndianStructure"><code class="xref py py-class docutils literal notranslate">LittleEndianStructure</code></a>,
<code class="xref py py-class docutils literal notranslate">BigEndianUnion</code>, and <code class="xref py py-class docutils literal notranslate">LittleEndianUnion</code> base classes. These
classes cannot contain pointer fields.
</div>
<div class="section" id="bit-fields-in-structures-and-unions">
<h3>16.16.1.12. Bit fields in structures and unions<a class="headerlink" href="#bit-fields-in-structures-and-unions" title="Permalink to this headline">¶</a></h3>
It is possible to create structures and unions containing bit fields. Bit fields
are only possible for integer fields, the bit width is specified as the third
item in the <code class="xref py py-attr docutils literal notranslate">_fields_</code> tuples:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class Int(Structure):
... _fields_ = [(&quot;first_16&quot;, c_int, 16),
... (&quot;second_16&quot;, c_int, 16)]
...
&gt;&gt;&gt; print(Int.first_16)
&lt;Field type=c_long, ofs=0:0, bits=16&gt;
&gt;&gt;&gt; print(Int.second_16)
&lt;Field type=c_long, ofs=0:16, bits=16&gt;
&gt;&gt;&gt;
</pre></div>
</div>
</div>
<div class="section" id="arrays">
<h3>16.16.1.13. Arrays<a class="headerlink" href="#arrays" title="Permalink to this headline">¶</a></h3>
Arrays are sequences, containing a fixed number of instances of the same type.
The recommended way to create array types is by multiplying a data type with a
positive integer:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>TenPointsArrayType = POINT * 10
</pre></div>
</div>
Here is an example of a somewhat artificial data type, a structure containing 4
POINTs among other stuff:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import *
&gt;&gt;&gt; class POINT(Structure):
... _fields_ = (&quot;x&quot;, c_int), (&quot;y&quot;, c_int)
...
&gt;&gt;&gt; class MyStruct(Structure):
... _fields_ = [(&quot;a&quot;, c_int),
... (&quot;b&quot;, c_float),
... (&quot;point_array&quot;, POINT * 4)]
&gt;&gt;&gt;
&gt;&gt;&gt; print(len(MyStruct().point_array))
4
&gt;&gt;&gt;
</pre></div>
</div>
Instances are created in the usual way, by calling the class:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>arr = TenPointsArrayType()
for pt in arr:
 print(pt.x, pt.y)
</pre></div>
</div>
The above code print a series of <code class="docutils literal notranslate">0 0</code> lines, because the array contents is
initialized to zeros.
Initializers of the correct type can also be specified:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import *
&gt;&gt;&gt; TenIntegers = c_int * 10
&gt;&gt;&gt; ii = TenIntegers(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
&gt;&gt;&gt; print(ii)
&lt;c_long_Array_10 object at 0x...&gt;
&gt;&gt;&gt; for i in ii: print(i, end=&quot; &quot;)
...
1 2 3 4 5 6 7 8 9 10
&gt;&gt;&gt;
</pre></div>
</div>
</div>
<div class="section" id="pointers">
<h3>16.16.1.14. Pointers<a class="headerlink" href="#pointers" title="Permalink to this headline">¶</a></h3>
Pointer instances are created by calling the <a class="reference internal" href="#ctypes.pointer" title="ctypes.pointer"><code class="xref py py-func docutils literal notranslate">pointer()</code></a> function on a
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> type:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import *
&gt;&gt;&gt; i = c_int(42)
&gt;&gt;&gt; pi = pointer(i)
&gt;&gt;&gt;
</pre></div>
</div>
Pointer instances have a <a class="reference internal" href="#ctypes._Pointer.contents" title="ctypes._Pointer.contents"><code class="xref py py-attr docutils literal notranslate">contents</code></a> attribute which
returns the object to which the pointer points, the <code class="docutils literal notranslate">i</code> object above:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; pi.contents
c_long(42)
&gt;&gt;&gt;
</pre></div>
</div>
Note that <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> does not have OOR (original object return), it constructs a
new, equivalent object each time you retrieve an attribute:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; pi.contents is i
False
&gt;&gt;&gt; pi.contents is pi.contents
False
&gt;&gt;&gt;
</pre></div>
</div>
Assigning another <a class="reference internal" href="#ctypes.c_int" title="ctypes.c_int"><code class="xref py py-class docutils literal notranslate">c_int</code></a> instance to the pointer’s contents attribute
would cause the pointer to point to the memory location where this is stored:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; i = c_int(99)
&gt;&gt;&gt; pi.contents = i
&gt;&gt;&gt; pi.contents
c_long(99)
&gt;&gt;&gt;
</pre></div>
</div>
Pointer instances can also be indexed with integers:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; pi[0]
99
&gt;&gt;&gt;
</pre></div>
</div>
Assigning to an integer index changes the pointed to value:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; print(i)
c_long(99)
&gt;&gt;&gt; pi[0] = 22
&gt;&gt;&gt; print(i)
c_long(22)
&gt;&gt;&gt;
</pre></div>
</div>
It is also possible to use indexes different from 0, but you must know what
you’re doing, just as in C: You can access or change arbitrary memory locations.
Generally you only use this feature if you receive a pointer from a C function,
and you know that the pointer actually points to an array instead of a single
item.
Behind the scenes, the <a class="reference internal" href="#ctypes.pointer" title="ctypes.pointer"><code class="xref py py-func docutils literal notranslate">pointer()</code></a> function does more than simply create
pointer instances, it has to create pointer types first. This is done with the
<a class="reference internal" href="#ctypes.POINTER" title="ctypes.POINTER"><code class="xref py py-func docutils literal notranslate">POINTER()</code></a> function, which accepts any <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> type, and returns a
new type:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; PI = POINTER(c_int)
&gt;&gt;&gt; PI
&lt;class &#39;ctypes.LP_c_long&#39;&gt;
&gt;&gt;&gt; PI(42)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
TypeError: expected c_long instead of int
&gt;&gt;&gt; PI(c_int(42))
&lt;ctypes.LP_c_long object at 0x...&gt;
&gt;&gt;&gt;
</pre></div>
</div>
Calling the pointer type without an argument creates a <code class="docutils literal notranslate">NULL</code> pointer.
<code class="docutils literal notranslate">NULL</code> pointers have a <code class="docutils literal notranslate">False</code> boolean value:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; null_ptr = POINTER(c_int)()
&gt;&gt;&gt; print(bool(null_ptr))
False
&gt;&gt;&gt;
</pre></div>
</div>
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> checks for <code class="docutils literal notranslate">NULL</code> when dereferencing pointers (but dereferencing
invalid non-<code class="docutils literal notranslate">NULL</code> pointers would crash Python):
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; null_ptr[0]
Traceback (most recent call last):
 ....
ValueError: NULL pointer access
&gt;&gt;&gt;

&gt;&gt;&gt; null_ptr[0] = 1234
Traceback (most recent call last):
 ....
ValueError: NULL pointer access
&gt;&gt;&gt;
</pre></div>
</div>
</div>
<div class="section" id="type-conversions">
<h3>16.16.1.15. Type conversions<a class="headerlink" href="#type-conversions" title="Permalink to this headline">¶</a></h3>
Usually, ctypes does strict type checking. This means, if you have
<code class="docutils literal notranslate">POINTER(c_int)</code> in the <code class="xref py py-attr docutils literal notranslate">argtypes</code> list of a function or as the type of
a member field in a structure definition, only instances of exactly the same
type are accepted. There are some exceptions to this rule, where ctypes accepts
other objects. For example, you can pass compatible array instances instead of
pointer types. So, for <code class="docutils literal notranslate">POINTER(c_int)</code>, ctypes accepts an array of c_int:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class Bar(Structure):
... _fields_ = [(&quot;count&quot;, c_int), (&quot;values&quot;, POINTER(c_int))]
...
&gt;&gt;&gt; bar = Bar()
&gt;&gt;&gt; bar.values = (c_int * 3)(1, 2, 3)
&gt;&gt;&gt; bar.count = 3
&gt;&gt;&gt; for i in range(bar.count):
... print(bar.values[i])
...
1
2
3
&gt;&gt;&gt;
</pre></div>
</div>
In addition, if a function argument is explicitly declared to be a pointer type
(such as <code class="docutils literal notranslate">POINTER(c_int)</code>) in <code class="xref py py-attr docutils literal notranslate">argtypes</code>, an object of the pointed
type (<code class="docutils literal notranslate">c_int</code> in this case) can be passed to the function. ctypes will apply
the required <a class="reference internal" href="#ctypes.byref" title="ctypes.byref"><code class="xref py py-func docutils literal notranslate">byref()</code></a> conversion in this case automatically.
To set a POINTER type field to <code class="docutils literal notranslate">NULL</code>, you can assign <code class="docutils literal notranslate">None</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; bar.values = None
&gt;&gt;&gt;
</pre></div>
</div>
Sometimes you have instances of incompatible types. In C, you can cast one type
into another type. <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> provides a <a class="reference internal" href="#ctypes.cast" title="ctypes.cast"><code class="xref py py-func docutils literal notranslate">cast()</code></a> function which can be
used in the same way. The <code class="docutils literal notranslate">Bar</code> structure defined above accepts
<code class="docutils literal notranslate">POINTER(c_int)</code> pointers or <a class="reference internal" href="#ctypes.c_int" title="ctypes.c_int"><code class="xref py py-class docutils literal notranslate">c_int</code></a> arrays for its <code class="docutils literal notranslate">values</code> field,
but not instances of other types:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; bar.values = (c_byte * 4)()
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
TypeError: incompatible types, c_byte_Array_4 instance instead of LP_c_long instance
&gt;&gt;&gt;
</pre></div>
</div>
For these cases, the <a class="reference internal" href="#ctypes.cast" title="ctypes.cast"><code class="xref py py-func docutils literal notranslate">cast()</code></a> function is handy.
The <a class="reference internal" href="#ctypes.cast" title="ctypes.cast"><code class="xref py py-func docutils literal notranslate">cast()</code></a> function can be used to cast a ctypes instance into a pointer
to a different ctypes data type. <a class="reference internal" href="#ctypes.cast" title="ctypes.cast"><code class="xref py py-func docutils literal notranslate">cast()</code></a> takes two parameters, a ctypes
object that is or can be converted to a pointer of some kind, and a ctypes
pointer type. It returns an instance of the second argument, which references
the same memory block as the first argument:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; a = (c_byte * 4)()
&gt;&gt;&gt; cast(a, POINTER(c_int))
&lt;ctypes.LP_c_long object at ...&gt;
&gt;&gt;&gt;
</pre></div>
</div>
So, <a class="reference internal" href="#ctypes.cast" title="ctypes.cast"><code class="xref py py-func docutils literal notranslate">cast()</code></a> can be used to assign to the <code class="docutils literal notranslate">values</code> field of <code class="docutils literal notranslate">Bar</code> the
structure:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; bar = Bar()
&gt;&gt;&gt; bar.values = cast((c_byte * 4)(), POINTER(c_int))
&gt;&gt;&gt; print(bar.values[0])
0
&gt;&gt;&gt;
</pre></div>
</div>
</div>
<div class="section" id="incomplete-types">
<h3>16.16.1.16. Incomplete Types<a class="headerlink" href="#incomplete-types" title="Permalink to this headline">¶</a></h3>
Incomplete Types are structures, unions or arrays whose members are not yet
specified. In C, they are specified by forward declarations, which are defined
later:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>struct cell; /* forward declaration */

struct cell {
 char *name;
 struct cell *next;
};
</pre></div>
</div>
The straightforward translation into ctypes code would be this, but it does not
work:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; class cell(Structure):
... _fields_ = [(&quot;name&quot;, c_char_p),
... (&quot;next&quot;, POINTER(cell))]
...
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
 File &quot;&lt;stdin&gt;&quot;, line 2, in cell
NameError: name &#39;cell&#39; is not defined
&gt;&gt;&gt;
</pre></div>
</div>
because the new <code class="docutils literal notranslate">class cell</code> is not available in the class statement itself.
In <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a>, we can define the <code class="docutils literal notranslate">cell</code> class and set the <code class="xref py py-attr docutils literal notranslate">_fields_</code>
attribute later, after the class statement:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import *
&gt;&gt;&gt; class cell(Structure):
... pass
...
&gt;&gt;&gt; cell._fields_ = [(&quot;name&quot;, c_char_p),
... (&quot;next&quot;, POINTER(cell))]
&gt;&gt;&gt;
</pre></div>
</div>
Lets try it. We create two instances of <code class="docutils literal notranslate">cell</code>, and let them point to each
other, and finally follow the pointer chain a few times:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; c1 = cell()
&gt;&gt;&gt; c1.name = &quot;foo&quot;
&gt;&gt;&gt; c2 = cell()
&gt;&gt;&gt; c2.name = &quot;bar&quot;
&gt;&gt;&gt; c1.next = pointer(c2)
&gt;&gt;&gt; c2.next = pointer(c1)
&gt;&gt;&gt; p = c1
&gt;&gt;&gt; for i in range(8):
... print(p.name, end=&quot; &quot;)
... p = p.next[0]
...
foo bar foo bar foo bar foo bar
&gt;&gt;&gt;
</pre></div>
</div>
</div>
<div class="section" id="callback-functions">
<h3>16.16.1.17. Callback functions<a class="headerlink" href="#callback-functions" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> allows creating C callable function pointers from Python callables.
These are sometimes called callback functions.
First, you must create a class for the callback function. The class knows the
calling convention, the return type, and the number and types of arguments this
function will receive.
The <a class="reference internal" href="#ctypes.CFUNCTYPE" title="ctypes.CFUNCTYPE"><code class="xref py py-func docutils literal notranslate">CFUNCTYPE()</code></a> factory function creates types for callback functions
using the <code class="docutils literal notranslate">cdecl</code> calling convention. On Windows, the <a class="reference internal" href="#ctypes.WINFUNCTYPE" title="ctypes.WINFUNCTYPE"><code class="xref py py-func docutils literal notranslate">WINFUNCTYPE()</code></a>
factory function creates types for callback functions using the <code class="docutils literal notranslate">stdcall</code>
calling convention.
Both of these factory functions are called with the result type as first
argument, and the callback functions expected argument types as the remaining
arguments.
I will present an example here which uses the standard C library’s
<code class="xref c c-func docutils literal notranslate">qsort()</code> function, that is used to sort items with the help of a callback
function. <code class="xref c c-func docutils literal notranslate">qsort()</code> will be used to sort an array of integers:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; IntArray5 = c_int * 5
&gt;&gt;&gt; ia = IntArray5(5, 1, 7, 33, 99)
&gt;&gt;&gt; qsort = libc.qsort
&gt;&gt;&gt; qsort.restype = None
&gt;&gt;&gt;
</pre></div>
</div>
<code class="xref py py-func docutils literal notranslate">qsort()</code> must be called with a pointer to the data to sort, the number of
items in the data array, the size of one item, and a pointer to the comparison
function, the callback. The callback will then be called with two pointers to
items, and it must return a negative integer if the first item is smaller than
the second, a zero if they are equal, and a positive integer otherwise.
So our callback function receives pointers to integers, and must return an
integer. First we create the <code class="docutils literal notranslate">type</code> for the callback function:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; CMPFUNC = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
&gt;&gt;&gt;
</pre></div>
</div>
To get started, here is a simple callback that shows the values it gets
passed:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def py_cmp_func(a, b):
... print(&quot;py_cmp_func&quot;, a[0], b[0])
... return 0
...
&gt;&gt;&gt; cmp_func = CMPFUNC(py_cmp_func)
&gt;&gt;&gt;
</pre></div>
</div>
The result:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; qsort(ia, len(ia), sizeof(c_int), cmp_func) 
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 5 7
py_cmp_func 1 7
&gt;&gt;&gt;
</pre></div>
</div>
Now we can actually compare the two items and return a useful result:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def py_cmp_func(a, b):
... print(&quot;py_cmp_func&quot;, a[0], b[0])
... return a[0] - b[0]
...
&gt;&gt;&gt;
&gt;&gt;&gt; qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func)) 
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 1 7
py_cmp_func 5 7
&gt;&gt;&gt;
</pre></div>
</div>
As we can easily check, our array is sorted now:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; for i in ia: print(i, end=&quot; &quot;)
...
1 5 7 33 99
&gt;&gt;&gt;
</pre></div>
</div>
The function factories can be used as decorator factories, so we may as well
write:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; @CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
... def py_cmp_func(a, b):
... print(&quot;py_cmp_func&quot;, a[0], b[0])
... return a[0] - b[0]
...
&gt;&gt;&gt; qsort(ia, len(ia), sizeof(c_int), py_cmp_func)
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 1 7
py_cmp_func 5 7
&gt;&gt;&gt;
</pre></div>
</div>
<div class="admonition note">
Note
Make sure you keep references to <a class="reference internal" href="#ctypes.CFUNCTYPE" title="ctypes.CFUNCTYPE"><code class="xref py py-func docutils literal notranslate">CFUNCTYPE()</code></a> objects as long as they
are used from C code. <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> doesn’t, and if you don’t, they may be
garbage collected, crashing your program when a callback is made.
Also, note that if the callback function is called in a thread created
outside of Python’s control (e.g. by the foreign code that calls the
callback), ctypes creates a new dummy Python thread on every invocation. This
behavior is correct for most purposes, but it means that values stored with
<a class="reference internal" href="threading.html#threading.local" title="threading.local"><code class="xref py py-class docutils literal notranslate">threading.local</code></a> will not survive across different callbacks, even when
those calls are made from the same C thread.
</div>
</div>
<div class="section" id="accessing-values-exported-from-dlls">
<h3>16.16.1.18. Accessing values exported from dlls<a class="headerlink" href="#accessing-values-exported-from-dlls" title="Permalink to this headline">¶</a></h3>
Some shared libraries not only export functions, they also export variables. An
example in the Python library itself is the <code class="xref c c-data docutils literal notranslate">Py_OptimizeFlag</code>, an integer
set to 0, 1, or 2, depending on the <a class="reference internal" href="../using/cmdline.html#cmdoption-o"><code class="xref std std-option docutils literal notranslate">-O</code></a> or <a class="reference internal" href="../using/cmdline.html#cmdoption-oo"><code class="xref std std-option docutils literal notranslate">-OO</code></a> flag given on
startup.
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> can access values like this with the <code class="xref py py-meth docutils literal notranslate">in_dll()</code> class methods of
the type. pythonapi is a predefined symbol giving access to the Python C
api:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; opt_flag = c_int.in_dll(pythonapi, &quot;Py_OptimizeFlag&quot;)
&gt;&gt;&gt; print(opt_flag)
c_long(0)
&gt;&gt;&gt;
</pre></div>
</div>
If the interpreter would have been started with <a class="reference internal" href="../using/cmdline.html#cmdoption-o"><code class="xref std std-option docutils literal notranslate">-O</code></a>, the sample would
have printed <code class="docutils literal notranslate">c_long(1)</code>, or <code class="docutils literal notranslate">c_long(2)</code> if <a class="reference internal" href="../using/cmdline.html#cmdoption-oo"><code class="xref std std-option docutils literal notranslate">-OO</code></a> would have been
specified.
An extended example which also demonstrates the use of pointers accesses the
<a class="reference internal" href="../c-api/import.html#c.PyImport_FrozenModules" title="PyImport_FrozenModules"><code class="xref c c-data docutils literal notranslate">PyImport_FrozenModules</code></a> pointer exported by Python.
Quoting the docs for that value:
<blockquote>
<div>This pointer is initialized to point to an array of <code class="xref c c-type docutils literal notranslate">struct _frozen</code>
records, terminated by one whose members are all NULL or zero. When a frozen
module is imported, it is searched in this table. Third-party code could play
tricks with this to provide a dynamically created collection of frozen modules.</div></blockquote>
So manipulating this pointer could even prove useful. To restrict the example
size, we show only how this table can be read with <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import *
&gt;&gt;&gt;
&gt;&gt;&gt; class struct_frozen(Structure):
... _fields_ = [(&quot;name&quot;, c_char_p),
... (&quot;code&quot;, POINTER(c_ubyte)),
... (&quot;size&quot;, c_int)]
...
&gt;&gt;&gt;
</pre></div>
</div>
We have defined the <code class="xref c c-type docutils literal notranslate">struct _frozen</code> data type, so we can get the pointer
to the table:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; FrozenTable = POINTER(struct_frozen)
&gt;&gt;&gt; table = FrozenTable.in_dll(pythonapi, &quot;PyImport_FrozenModules&quot;)
&gt;&gt;&gt;
</pre></div>
</div>
Since <code class="docutils literal notranslate">table</code> is a <code class="docutils literal notranslate">pointer</code> to the array of <code class="docutils literal notranslate">struct_frozen</code> records, we
can iterate over it, but we just have to make sure that our loop terminates,
because pointers have no size. Sooner or later it would probably crash with an
access violation or whatever, so it’s better to break out of the loop when we
hit the NULL entry:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; for item in table:
... if item.name is None:
... break
... print(item.name.decode(&quot;ascii&quot;), item.size)
...
_frozen_importlib 31764
_frozen_importlib_external 41499
__hello__ 161
__phello__ -161
__phello__.spam 161
&gt;&gt;&gt;
</pre></div>
</div>
The fact that standard Python has a frozen module and a frozen package
(indicated by the negative size member) is not well known, it is only used for
testing. Try it out with <code class="docutils literal notranslate">import __hello__</code> for example.
</div>
<div class="section" id="surprises">
<h3>16.16.1.19. Surprises<a class="headerlink" href="#surprises" title="Permalink to this headline">¶</a></h3>
There are some edges in <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> where you might expect something other
than what actually happens.
Consider the following example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import *
&gt;&gt;&gt; class POINT(Structure):
... _fields_ = (&quot;x&quot;, c_int), (&quot;y&quot;, c_int)
...
&gt;&gt;&gt; class RECT(Structure):
... _fields_ = (&quot;a&quot;, POINT), (&quot;b&quot;, POINT)
...
&gt;&gt;&gt; p1 = POINT(1, 2)
&gt;&gt;&gt; p2 = POINT(3, 4)
&gt;&gt;&gt; rc = RECT(p1, p2)
&gt;&gt;&gt; print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
1 2 3 4
&gt;&gt;&gt; # now swap the two points
&gt;&gt;&gt; rc.a, rc.b = rc.b, rc.a
&gt;&gt;&gt; print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
3 4 3 4
&gt;&gt;&gt;
</pre></div>
</div>
Hm. We certainly expected the last statement to print <code class="docutils literal notranslate">3 4 1 2</code>. What
happened? Here are the steps of the <code class="docutils literal notranslate">rc.a, rc.b = rc.b, rc.a</code> line above:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; temp0, temp1 = rc.b, rc.a
&gt;&gt;&gt; rc.a = temp0
&gt;&gt;&gt; rc.b = temp1
&gt;&gt;&gt;
</pre></div>
</div>
Note that <code class="docutils literal notranslate">temp0</code> and <code class="docutils literal notranslate">temp1</code> are objects still using the internal buffer of
the <code class="docutils literal notranslate">rc</code> object above. So executing <code class="docutils literal notranslate">rc.a = temp0</code> copies the buffer
contents of <code class="docutils literal notranslate">temp0</code> into <code class="docutils literal notranslate">rc</code> ‘s buffer. This, in turn, changes the
contents of <code class="docutils literal notranslate">temp1</code>. So, the last assignment <code class="docutils literal notranslate">rc.b = temp1</code>, doesn’t have
the expected effect.
Keep in mind that retrieving sub-objects from Structure, Unions, and Arrays
doesn’t copy the sub-object, instead it retrieves a wrapper object accessing
the root-object’s underlying buffer.
Another example that may behave different from what one would expect is this:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; s = c_char_p()
&gt;&gt;&gt; s.value = &quot;abc def ghi&quot;
&gt;&gt;&gt; s.value
&#39;abc def ghi&#39;
&gt;&gt;&gt; s.value is s.value
False
&gt;&gt;&gt;
</pre></div>
</div>
Why is it printing <code class="docutils literal notranslate">False</code>? ctypes instances are objects containing a memory
block plus some <a class="reference internal" href="../glossary.html#term-descriptor">descriptor</a>s accessing the contents of the memory.
Storing a Python object in the memory block does not store the object itself,
instead the <code class="docutils literal notranslate">contents</code> of the object is stored. Accessing the contents again
constructs a new Python object each time!
</div>
<div class="section" id="variable-sized-data-types">
<h3>16.16.1.20. Variable-sized data types<a class="headerlink" href="#variable-sized-data-types" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> provides some support for variable-sized arrays and structures.
The <a class="reference internal" href="#ctypes.resize" title="ctypes.resize"><code class="xref py py-func docutils literal notranslate">resize()</code></a> function can be used to resize the memory buffer of an
existing ctypes object. The function takes the object as first argument, and
the requested size in bytes as the second argument. The memory block cannot be
made smaller than the natural memory block specified by the objects type, a
<a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> is raised if this is tried:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; short_array = (c_short * 4)()
&gt;&gt;&gt; print(sizeof(short_array))
8
&gt;&gt;&gt; resize(short_array, 4)
Traceback (most recent call last):
 ...
ValueError: minimum size is 8
&gt;&gt;&gt; resize(short_array, 32)
&gt;&gt;&gt; sizeof(short_array)
32
&gt;&gt;&gt; sizeof(type(short_array))
8
&gt;&gt;&gt;
</pre></div>
</div>
This is nice and fine, but how would one access the additional elements
contained in this array? Since the type still only knows about 4 elements, we
get errors accessing other elements:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; short_array[:]
[0, 0, 0, 0]
&gt;&gt;&gt; short_array[7]
Traceback (most recent call last):
 ...
IndexError: invalid index
&gt;&gt;&gt;
</pre></div>
</div>
Another way to use variable-sized data types with <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> is to use the
dynamic nature of Python, and (re-)define the data type after the required size
is already known, on a case by case basis.
</div>
</div>
<div class="section" id="ctypes-reference">
<h2>16.16.2. ctypes reference<a class="headerlink" href="#ctypes-reference" title="Permalink to this headline">¶</a></h2>
<div class="section" id="finding-shared-libraries">
<h3>16.16.2.1. Finding shared libraries<a class="headerlink" href="#finding-shared-libraries" title="Permalink to this headline">¶</a></h3>
When programming in a compiled language, shared libraries are accessed when
compiling/linking a program, and when the program is run.
The purpose of the <code class="xref py py-func docutils literal notranslate">find_library()</code> function is to locate a library in a way
similar to what the compiler or runtime loader does (on platforms with several
versions of a shared library the most recent should be loaded), while the ctypes
library loaders act like when a program is run, and call the runtime loader
directly.
The <code class="xref py py-mod docutils literal notranslate">ctypes.util</code> module provides a function which can help to determine
the library to load.
<dl class="data">
<dt>
<code class="descclassname">ctypes.util.</code><code class="descname">find_library</code>(name)</dt>
<dd>Try to find a library and return a pathname. name is the library name without
any prefix like lib, suffix like <code class="docutils literal notranslate">.so</code>, <code class="docutils literal notranslate">.dylib</code> or version number (this
is the form used for the posix linker option <code class="xref std std-option docutils literal notranslate">-l</code>). If no library can
be found, returns <code class="docutils literal notranslate">None</code>.
</dd></dl>

The exact functionality is system dependent.
On Linux, <code class="xref py py-func docutils literal notranslate">find_library()</code> tries to run external programs
(<code class="docutils literal notranslate">/sbin/ldconfig</code>, <code class="docutils literal notranslate">gcc</code>, <code class="docutils literal notranslate">objdump</code> and <code class="docutils literal notranslate">ld</code>) to find the library file.
It returns the filename of the library file.
<div class="versionchanged">
Changed in version 3.6: On Linux, the value of the environment variable <code class="docutils literal notranslate">LD_LIBRARY_PATH</code> is used
when searching for libraries, if a library cannot be found by any other means.
</div>
Here are some examples:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes.util import find_library
&gt;&gt;&gt; find_library(&quot;m&quot;)
&#39;libm.so.6&#39;
&gt;&gt;&gt; find_library(&quot;c&quot;)
&#39;libc.so.6&#39;
&gt;&gt;&gt; find_library(&quot;bz2&quot;)
&#39;libbz2.so.1.0&#39;
&gt;&gt;&gt;
</pre></div>
</div>
On OS X, <code class="xref py py-func docutils literal notranslate">find_library()</code> tries several predefined naming schemes and paths
to locate the library, and returns a full pathname if successful:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes.util import find_library
&gt;&gt;&gt; find_library(&quot;c&quot;)
&#39;/usr/lib/libc.dylib&#39;
&gt;&gt;&gt; find_library(&quot;m&quot;)
&#39;/usr/lib/libm.dylib&#39;
&gt;&gt;&gt; find_library(&quot;bz2&quot;)
&#39;/usr/lib/libbz2.dylib&#39;
&gt;&gt;&gt; find_library(&quot;AGL&quot;)
&#39;/System/Library/Frameworks/AGL.framework/AGL&#39;
&gt;&gt;&gt;
</pre></div>
</div>
On Windows, <code class="xref py py-func docutils literal notranslate">find_library()</code> searches along the system search path, and
returns the full pathname, but since there is no predefined naming scheme a call
like <code class="docutils literal notranslate">find_library(&quot;c&quot;)</code> will fail and return <code class="docutils literal notranslate">None</code>.
If wrapping a shared library with <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a>, it may be better to determine
the shared library name at development time, and hardcode that into the wrapper
module instead of using <code class="xref py py-func docutils literal notranslate">find_library()</code> to locate the library at runtime.
</div>
<div class="section" id="loading-shared-libraries">
<h3>16.16.2.2. Loading shared libraries<a class="headerlink" href="#loading-shared-libraries" title="Permalink to this headline">¶</a></h3>
There are several ways to load shared libraries into the Python process. One
way is to instantiate one of the following classes:
<dl class="class">
<dt id="ctypes.CDLL">
class <code class="descclassname">ctypes.</code><code class="descname">CDLL</code>(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)<a class="headerlink" href="#ctypes.CDLL" title="Permalink to this definition">¶</a></dt>
<dd>Instances of this class represent loaded shared libraries. Functions in these
libraries use the standard C calling convention, and are assumed to return
<code class="xref c c-type docutils literal notranslate">int</code>.
</dd></dl>

<dl class="class">
<dt id="ctypes.OleDLL">
class <code class="descclassname">ctypes.</code><code class="descname">OleDLL</code>(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)<a class="headerlink" href="#ctypes.OleDLL" title="Permalink to this definition">¶</a></dt>
<dd>Windows only: Instances of this class represent loaded shared libraries,
functions in these libraries use the <code class="docutils literal notranslate">stdcall</code> calling convention, and are
assumed to return the windows specific <a class="reference internal" href="#ctypes.HRESULT" title="ctypes.HRESULT"><code class="xref py py-class docutils literal notranslate">HRESULT</code></a> code. <a class="reference internal" href="#ctypes.HRESULT" title="ctypes.HRESULT"><code class="xref py py-class docutils literal notranslate">HRESULT</code></a>
values contain information specifying whether the function call failed or
succeeded, together with additional error code. If the return value signals a
failure, an <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-class docutils literal notranslate">OSError</code></a> is automatically raised.
<div class="versionchanged">
Changed in version 3.3: <a class="reference internal" href="exceptions.html#WindowsError" title="WindowsError"><code class="xref py py-exc docutils literal notranslate">WindowsError</code></a> used to be raised.
</div>
</dd></dl>

<dl class="class">
<dt id="ctypes.WinDLL">
class <code class="descclassname">ctypes.</code><code class="descname">WinDLL</code>(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)<a class="headerlink" href="#ctypes.WinDLL" title="Permalink to this definition">¶</a></dt>
<dd>Windows only: Instances of this class represent loaded shared libraries,
functions in these libraries use the <code class="docutils literal notranslate">stdcall</code> calling convention, and are
assumed to return <code class="xref c c-type docutils literal notranslate">int</code> by default.
On Windows CE only the standard calling convention is used, for convenience the
<a class="reference internal" href="#ctypes.WinDLL" title="ctypes.WinDLL"><code class="xref py py-class docutils literal notranslate">WinDLL</code></a> and <a class="reference internal" href="#ctypes.OleDLL" title="ctypes.OleDLL"><code class="xref py py-class docutils literal notranslate">OleDLL</code></a> use the standard calling convention on this
platform.
</dd></dl>

The Python <a class="reference internal" href="../glossary.html#term-global-interpreter-lock">global interpreter lock</a> is released before calling any
function exported by these libraries, and reacquired afterwards.
<dl class="class">
<dt id="ctypes.PyDLL">
class <code class="descclassname">ctypes.</code><code class="descname">PyDLL</code>(name, mode=DEFAULT_MODE, handle=None)<a class="headerlink" href="#ctypes.PyDLL" title="Permalink to this definition">¶</a></dt>
<dd>Instances of this class behave like <a class="reference internal" href="#ctypes.CDLL" title="ctypes.CDLL"><code class="xref py py-class docutils literal notranslate">CDLL</code></a> instances, except that the
Python GIL is not released during the function call, and after the function
execution the Python error flag is checked. If the error flag is set, a Python
exception is raised.
Thus, this is only useful to call Python C api functions directly.
</dd></dl>

All these classes can be instantiated by calling them with at least one
argument, the pathname of the shared library. If you have an existing handle to
an already loaded shared library, it can be passed as the <code class="docutils literal notranslate">handle</code> named
parameter, otherwise the underlying platforms <code class="docutils literal notranslate">dlopen</code> or <code class="docutils literal notranslate">LoadLibrary</code>
function is used to load the library into the process, and to get a handle to
it.
The mode parameter can be used to specify how the library is loaded. For
details, consult the dlopen(3) manpage. On Windows, mode is
ignored. On posix systems, RTLD_NOW is always added, and is not
configurable.
The use_errno parameter, when set to true, enables a ctypes mechanism that
allows accessing the system <a class="reference internal" href="errno.html#module-errno" title="errno: Standard errno system symbols."><code class="xref py py-data docutils literal notranslate">errno</code></a> error number in a safe way.
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> maintains a thread-local copy of the systems <a class="reference internal" href="errno.html#module-errno" title="errno: Standard errno system symbols."><code class="xref py py-data docutils literal notranslate">errno</code></a>
variable; if you call foreign functions created with <code class="docutils literal notranslate">use_errno=True</code> then the
<a class="reference internal" href="errno.html#module-errno" title="errno: Standard errno system symbols."><code class="xref py py-data docutils literal notranslate">errno</code></a> value before the function call is swapped with the ctypes private
copy, the same happens immediately after the function call.
The function <a class="reference internal" href="#ctypes.get_errno" title="ctypes.get_errno"><code class="xref py py-func docutils literal notranslate">ctypes.get_errno()</code></a> returns the value of the ctypes private
copy, and the function <a class="reference internal" href="#ctypes.set_errno" title="ctypes.set_errno"><code class="xref py py-func docutils literal notranslate">ctypes.set_errno()</code></a> changes the ctypes private copy
to a new value and returns the former value.
The use_last_error parameter, when set to true, enables the same mechanism for
the Windows error code which is managed by the <a class="reference internal" href="#ctypes.GetLastError" title="ctypes.GetLastError"><code class="xref py py-func docutils literal notranslate">GetLastError()</code></a> and
<code class="xref py py-func docutils literal notranslate">SetLastError()</code> Windows API functions; <a class="reference internal" href="#ctypes.get_last_error" title="ctypes.get_last_error"><code class="xref py py-func docutils literal notranslate">ctypes.get_last_error()</code></a> and
<a class="reference internal" href="#ctypes.set_last_error" title="ctypes.set_last_error"><code class="xref py py-func docutils literal notranslate">ctypes.set_last_error()</code></a> are used to request and change the ctypes private
copy of the windows error code.
<dl class="data">
<dt>
<code class="descclassname">ctypes.</code><code class="descname">RTLD_GLOBAL</code></dt>
<dd>Flag to use as mode parameter. On platforms where this flag is not available,
it is defined as the integer zero.
</dd></dl>

<dl class="data">
<dt>
<code class="descclassname">ctypes.</code><code class="descname">RTLD_LOCAL</code></dt>
<dd>Flag to use as mode parameter. On platforms where this is not available, it
is the same as RTLD_GLOBAL.
</dd></dl>

<dl class="data">
<dt>
<code class="descclassname">ctypes.</code><code class="descname">DEFAULT_MODE</code></dt>
<dd>The default mode which is used to load shared libraries. On OSX 10.3, this is
RTLD_GLOBAL, otherwise it is the same as RTLD_LOCAL.
</dd></dl>

Instances of these classes have no public methods. Functions exported by the
shared library can be accessed as attributes or by index. Please note that
accessing the function through an attribute caches the result and therefore
accessing it repeatedly returns the same object each time. On the other hand,
accessing it through an index returns a new object each time:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; libc.time == libc.time
True
&gt;&gt;&gt; libc[&#39;time&#39;] == libc[&#39;time&#39;]
False
</pre></div>
</div>
The following public attributes are available, their name starts with an
underscore to not clash with exported function names:
<dl class="attribute">
<dt id="ctypes.PyDLL._handle">
<code class="descclassname">PyDLL.</code><code class="descname">_handle</code><a class="headerlink" href="#ctypes.PyDLL._handle" title="Permalink to this definition">¶</a></dt>
<dd>The system handle used to access the library.
</dd></dl>

<dl class="attribute">
<dt id="ctypes.PyDLL._name">
<code class="descclassname">PyDLL.</code><code class="descname">_name</code><a class="headerlink" href="#ctypes.PyDLL._name" title="Permalink to this definition">¶</a></dt>
<dd>The name of the library passed in the constructor.
</dd></dl>

Shared libraries can also be loaded by using one of the prefabricated objects,
which are instances of the <a class="reference internal" href="#ctypes.LibraryLoader" title="ctypes.LibraryLoader"><code class="xref py py-class docutils literal notranslate">LibraryLoader</code></a> class, either by calling the
<code class="xref py py-meth docutils literal notranslate">LoadLibrary()</code> method, or by retrieving the library as attribute of the
loader instance.
<dl class="class">
<dt id="ctypes.LibraryLoader">
class <code class="descclassname">ctypes.</code><code class="descname">LibraryLoader</code>(dlltype)<a class="headerlink" href="#ctypes.LibraryLoader" title="Permalink to this definition">¶</a></dt>
<dd>Class which loads shared libraries. dlltype should be one of the
<a class="reference internal" href="#ctypes.CDLL" title="ctypes.CDLL"><code class="xref py py-class docutils literal notranslate">CDLL</code></a>, <a class="reference internal" href="#ctypes.PyDLL" title="ctypes.PyDLL"><code class="xref py py-class docutils literal notranslate">PyDLL</code></a>, <a class="reference internal" href="#ctypes.WinDLL" title="ctypes.WinDLL"><code class="xref py py-class docutils literal notranslate">WinDLL</code></a>, or <a class="reference internal" href="#ctypes.OleDLL" title="ctypes.OleDLL"><code class="xref py py-class docutils literal notranslate">OleDLL</code></a> types.
<a class="reference internal" href="../reference/datamodel.html#object.__getattr__" title="object.__getattr__"><code class="xref py py-meth docutils literal notranslate">__getattr__()</code></a> has special behavior: It allows loading a shared library by
accessing it as attribute of a library loader instance. The result is cached,
so repeated attribute accesses return the same library each time.
<dl class="method">
<dt id="ctypes.LibraryLoader.LoadLibrary">
<code class="descname">LoadLibrary</code>(name)<a class="headerlink" href="#ctypes.LibraryLoader.LoadLibrary" title="Permalink to this definition">¶</a></dt>
<dd>Load a shared library into the process and return it. This method always
returns a new instance of the library.
</dd></dl>

</dd></dl>

These prefabricated library loaders are available:
<dl class="data">
<dt>
<code class="descclassname">ctypes.</code><code class="descname">cdll</code></dt>
<dd>Creates <a class="reference internal" href="#ctypes.CDLL" title="ctypes.CDLL"><code class="xref py py-class docutils literal notranslate">CDLL</code></a> instances.
</dd></dl>

<dl class="data">
<dt>
<code class="descclassname">ctypes.</code><code class="descname">windll</code></dt>
<dd>Windows only: Creates <a class="reference internal" href="#ctypes.WinDLL" title="ctypes.WinDLL"><code class="xref py py-class docutils literal notranslate">WinDLL</code></a> instances.
</dd></dl>

<dl class="data">
<dt>
<code class="descclassname">ctypes.</code><code class="descname">oledll</code></dt>
<dd>Windows only: Creates <a class="reference internal" href="#ctypes.OleDLL" title="ctypes.OleDLL"><code class="xref py py-class docutils literal notranslate">OleDLL</code></a> instances.
</dd></dl>

<dl class="data">
<dt>
<code class="descclassname">ctypes.</code><code class="descname">pydll</code></dt>
<dd>Creates <a class="reference internal" href="#ctypes.PyDLL" title="ctypes.PyDLL"><code class="xref py py-class docutils literal notranslate">PyDLL</code></a> instances.
</dd></dl>

For accessing the C Python api directly, a ready-to-use Python shared library
object is available:
<dl class="data">
<dt>
<code class="descclassname">ctypes.</code><code class="descname">pythonapi</code></dt>
<dd>An instance of <a class="reference internal" href="#ctypes.PyDLL" title="ctypes.PyDLL"><code class="xref py py-class docutils literal notranslate">PyDLL</code></a> that exposes Python C API functions as
attributes. Note that all these functions are assumed to return C
<code class="xref c c-type docutils literal notranslate">int</code>, which is of course not always the truth, so you have to assign
the correct <code class="xref py py-attr docutils literal notranslate">restype</code> attribute to use these functions.
</dd></dl>

</div>
<div class="section" id="foreign-functions">
<h3>16.16.2.3. Foreign functions<a class="headerlink" href="#foreign-functions" title="Permalink to this headline">¶</a></h3>
As explained in the previous section, foreign functions can be accessed as
attributes of loaded shared libraries. The function objects created in this way
by default accept any number of arguments, accept any ctypes data instances as
arguments, and return the default result type specified by the library loader.
They are instances of a private class:
<dl class="class">
<dt id="ctypes._FuncPtr">
class <code class="descclassname">ctypes.</code><code class="descname">_FuncPtr</code><a class="headerlink" href="#ctypes._FuncPtr" title="Permalink to this definition">¶</a></dt>
<dd>Base class for C callable foreign functions.
Instances of foreign functions are also C compatible data types; they
represent C function pointers.
This behavior can be customized by assigning to special attributes of the
foreign function object.
<dl class="attribute">
<dt id="ctypes._FuncPtr.restype">
<code class="descname">restype</code><a class="headerlink" href="#ctypes._FuncPtr.restype" title="Permalink to this definition">¶</a></dt>
<dd>Assign a ctypes type to specify the result type of the foreign function.
Use <code class="docutils literal notranslate">None</code> for <code class="xref c c-type docutils literal notranslate">void</code>, a function not returning anything.
It is possible to assign a callable Python object that is not a ctypes
type, in this case the function is assumed to return a C <code class="xref c c-type docutils literal notranslate">int</code>, and
the callable will be called with this integer, allowing further
processing or error checking. Using this is deprecated, for more flexible
post processing or error checking use a ctypes data type as
<a class="reference internal" href="#ctypes._FuncPtr.restype" title="ctypes._FuncPtr.restype"><code class="xref py py-attr docutils literal notranslate">restype</code></a> and assign a callable to the <a class="reference internal" href="#ctypes._FuncPtr.errcheck" title="ctypes._FuncPtr.errcheck"><code class="xref py py-attr docutils literal notranslate">errcheck</code></a> attribute.
</dd></dl>

<dl class="attribute">
<dt id="ctypes._FuncPtr.argtypes">
<code class="descname">argtypes</code><a class="headerlink" href="#ctypes._FuncPtr.argtypes" title="Permalink to this definition">¶</a></dt>
<dd>Assign a tuple of ctypes types to specify the argument types that the
function accepts. Functions using the <code class="docutils literal notranslate">stdcall</code> calling convention can
only be called with the same number of arguments as the length of this
tuple; functions using the C calling convention accept additional,
unspecified arguments as well.
When a foreign function is called, each actual argument is passed to the
<code class="xref py py-meth docutils literal notranslate">from_param()</code> class method of the items in the <a class="reference internal" href="#ctypes._FuncPtr.argtypes" title="ctypes._FuncPtr.argtypes"><code class="xref py py-attr docutils literal notranslate">argtypes</code></a>
tuple, this method allows adapting the actual argument to an object that
the foreign function accepts. For example, a <a class="reference internal" href="#ctypes.c_char_p" title="ctypes.c_char_p"><code class="xref py py-class docutils literal notranslate">c_char_p</code></a> item in
the <a class="reference internal" href="#ctypes._FuncPtr.argtypes" title="ctypes._FuncPtr.argtypes"><code class="xref py py-attr docutils literal notranslate">argtypes</code></a> tuple will convert a string passed as argument into
a bytes object using ctypes conversion rules.
New: It is now possible to put items in argtypes which are not ctypes
types, but each item must have a <code class="xref py py-meth docutils literal notranslate">from_param()</code> method which returns a
value usable as argument (integer, string, ctypes instance). This allows
defining adapters that can adapt custom objects as function parameters.
</dd></dl>

<dl class="attribute">
<dt id="ctypes._FuncPtr.errcheck">
<code class="descname">errcheck</code><a class="headerlink" href="#ctypes._FuncPtr.errcheck" title="Permalink to this definition">¶</a></dt>
<dd>Assign a Python function or another callable to this attribute. The
callable will be called with three or more arguments:
<dl class="function">
<dt>
<code class="descname">callable</code>(result, func, arguments)</dt>
<dd>result is what the foreign function returns, as specified by the
<code class="xref py py-attr docutils literal notranslate">restype</code> attribute.
func is the foreign function object itself, this allows reusing the
same callable object to check or post process the results of several
functions.
arguments is a tuple containing the parameters originally passed to
the function call, this allows specializing the behavior on the
arguments used.
</dd></dl>

The object that this function returns will be returned from the
foreign function call, but it can also check the result value
and raise an exception if the foreign function call failed.
</dd></dl>

</dd></dl>

<dl class="exception">
<dt id="ctypes.ArgumentError">
exception <code class="descclassname">ctypes.</code><code class="descname">ArgumentError</code><a class="headerlink" href="#ctypes.ArgumentError" title="Permalink to this definition">¶</a></dt>
<dd>This exception is raised when a foreign function call cannot convert one of the
passed arguments.
</dd></dl>

</div>
<div class="section" id="function-prototypes">
<h3>16.16.2.4. Function prototypes<a class="headerlink" href="#function-prototypes" title="Permalink to this headline">¶</a></h3>
Foreign functions can also be created by instantiating function prototypes.
Function prototypes are similar to function prototypes in C; they describe a
function (return type, argument types, calling convention) without defining an
implementation. The factory functions must be called with the desired result
type and the argument types of the function, and can be used as decorator
factories, and as such, be applied to functions through the <code class="docutils literal notranslate">&#64;wrapper</code> syntax.
See <a class="reference internal" href="#ctypes-callback-functions">Callback functions</a> for examples.
<dl class="function">
<dt id="ctypes.CFUNCTYPE">
<code class="descclassname">ctypes.</code><code class="descname">CFUNCTYPE</code>(restype, *argtypes, use_errno=False, use_last_error=False)<a class="headerlink" href="#ctypes.CFUNCTYPE" title="Permalink to this definition">¶</a></dt>
<dd>The returned function prototype creates functions that use the standard C
calling convention. The function will release the GIL during the call. If
use_errno is set to true, the ctypes private copy of the system
<a class="reference internal" href="errno.html#module-errno" title="errno: Standard errno system symbols."><code class="xref py py-data docutils literal notranslate">errno</code></a> variable is exchanged with the real <a class="reference internal" href="errno.html#module-errno" title="errno: Standard errno system symbols."><code class="xref py py-data docutils literal notranslate">errno</code></a> value before
and after the call; use_last_error does the same for the Windows error
code.
</dd></dl>

<dl class="function">
<dt id="ctypes.WINFUNCTYPE">
<code class="descclassname">ctypes.</code><code class="descname">WINFUNCTYPE</code>(restype, *argtypes, use_errno=False, use_last_error=False)<a class="headerlink" href="#ctypes.WINFUNCTYPE" title="Permalink to this definition">¶</a></dt>
<dd>Windows only: The returned function prototype creates functions that use the
<code class="docutils literal notranslate">stdcall</code> calling convention, except on Windows CE where
<a class="reference internal" href="#ctypes.WINFUNCTYPE" title="ctypes.WINFUNCTYPE"><code class="xref py py-func docutils literal notranslate">WINFUNCTYPE()</code></a> is the same as <a class="reference internal" href="#ctypes.CFUNCTYPE" title="ctypes.CFUNCTYPE"><code class="xref py py-func docutils literal notranslate">CFUNCTYPE()</code></a>. The function will
release the GIL during the call. use_errno and use_last_error have the
same meaning as above.
</dd></dl>

<dl class="function">
<dt id="ctypes.PYFUNCTYPE">
<code class="descclassname">ctypes.</code><code class="descname">PYFUNCTYPE</code>(restype, *argtypes)<a class="headerlink" href="#ctypes.PYFUNCTYPE" title="Permalink to this definition">¶</a></dt>
<dd>The returned function prototype creates functions that use the Python calling
convention. The function will not release the GIL during the call.
</dd></dl>

Function prototypes created by these factory functions can be instantiated in
different ways, depending on the type and number of the parameters in the call:
<blockquote>
<div><dl class="function">
<dt>
<code class="descname">prototype</code>(address)</dt>
<dd>Returns a foreign function at the specified address which must be an integer.
</dd></dl>

<dl class="function">
<dt>
<code class="descname">prototype</code>(callable)</dt>
<dd>Create a C callable function (a callback function) from a Python callable.
</dd></dl>

<dl class="function">
<dt>
<code class="descname">prototype</code>(func_spec[, paramflags])</dt>
<dd>Returns a foreign function exported by a shared library. func_spec must
be a 2-tuple <code class="docutils literal notranslate">(name_or_ordinal, library)</code>. The first item is the name of
the exported function as string, or the ordinal of the exported function
as small integer. The second item is the shared library instance.
</dd></dl>

<dl class="function">
<dt>
<code class="descname">prototype</code>(vtbl_index, name[, paramflags[, iid]])</dt>
<dd>Returns a foreign function that will call a COM method. vtbl_index is
the index into the virtual function table, a small non-negative
integer. name is name of the COM method. iid is an optional pointer to
the interface identifier which is used in extended error reporting.
COM methods use a special calling convention: They require a pointer to
the COM interface as first argument, in addition to those parameters that
are specified in the <code class="xref py py-attr docutils literal notranslate">argtypes</code> tuple.
</dd></dl>

The optional paramflags parameter creates foreign function wrappers with much
more functionality than the features described above.
paramflags must be a tuple of the same length as <code class="xref py py-attr docutils literal notranslate">argtypes</code>.
Each item in this tuple contains further information about a parameter, it must
be a tuple containing one, two, or three items.
The first item is an integer containing a combination of direction
flags for the parameter:
<blockquote>
<div><dl class="docutils">
<dt>1</dt>
<dd>Specifies an input parameter to the function.</dd>
<dt>2</dt>
<dd>Output parameter. The foreign function fills in a value.</dd>
<dt>4</dt>
<dd>Input parameter which defaults to the integer zero.</dd>
</dl>
</div></blockquote>
The optional second item is the parameter name as string. If this is specified,
the foreign function can be called with named parameters.
The optional third item is the default value for this parameter.
</div></blockquote>
This example demonstrates how to wrap the Windows <code class="docutils literal notranslate">MessageBoxW</code> function so
that it supports default parameters and named arguments. The C declaration from
the windows header file is this:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>WINUSERAPI int WINAPI
MessageBoxW(
 HWND hWnd,
 LPCWSTR lpText,
 LPCWSTR lpCaption,
 UINT uType);
</pre></div>
</div>
Here is the wrapping with <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import c_int, WINFUNCTYPE, windll
&gt;&gt;&gt; from ctypes.wintypes import HWND, LPCWSTR, UINT
&gt;&gt;&gt; prototype = WINFUNCTYPE(c_int, HWND, LPCWSTR, LPCWSTR, UINT)
&gt;&gt;&gt; paramflags = (1, &quot;hwnd&quot;, 0), (1, &quot;text&quot;, &quot;Hi&quot;), (1, &quot;caption&quot;, &quot;Hello from ctypes&quot;), (1, &quot;flags&quot;, 0)
&gt;&gt;&gt; MessageBox = prototype((&quot;MessageBoxW&quot;, windll.user32), paramflags)
</pre></div>
</div>
The <code class="docutils literal notranslate">MessageBox</code> foreign function can now be called in these ways:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; MessageBox()
&gt;&gt;&gt; MessageBox(text=&quot;Spam, spam, spam&quot;)
&gt;&gt;&gt; MessageBox(flags=2, text=&quot;foo bar&quot;)
</pre></div>
</div>
A second example demonstrates output parameters. The win32 <code class="docutils literal notranslate">GetWindowRect</code>
function retrieves the dimensions of a specified window by copying them into
<code class="docutils literal notranslate">RECT</code> structure that the caller has to supply. Here is the C declaration:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>WINUSERAPI BOOL WINAPI
GetWindowRect(
 HWND hWnd,
 LPRECT lpRect);
</pre></div>
</div>
Here is the wrapping with <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import POINTER, WINFUNCTYPE, windll, WinError
&gt;&gt;&gt; from ctypes.wintypes import BOOL, HWND, RECT
&gt;&gt;&gt; prototype = WINFUNCTYPE(BOOL, HWND, POINTER(RECT))
&gt;&gt;&gt; paramflags = (1, &quot;hwnd&quot;), (2, &quot;lprect&quot;)
&gt;&gt;&gt; GetWindowRect = prototype((&quot;GetWindowRect&quot;, windll.user32), paramflags)
&gt;&gt;&gt;
</pre></div>
</div>
Functions with output parameters will automatically return the output parameter
value if there is a single one, or a tuple containing the output parameter
values when there are more than one, so the GetWindowRect function now returns a
RECT instance, when called.
Output parameters can be combined with the <code class="xref py py-attr docutils literal notranslate">errcheck</code> protocol to do
further output processing and error checking. The win32 <code class="docutils literal notranslate">GetWindowRect</code> api
function returns a <code class="docutils literal notranslate">BOOL</code> to signal success or failure, so this function could
do the error checking, and raises an exception when the api call failed:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def errcheck(result, func, args):
... if not result:
... raise WinError()
... return args
...
&gt;&gt;&gt; GetWindowRect.errcheck = errcheck
&gt;&gt;&gt;
</pre></div>
</div>
If the <code class="xref py py-attr docutils literal notranslate">errcheck</code> function returns the argument tuple it receives
unchanged, <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> continues the normal processing it does on the output
parameters. If you want to return a tuple of window coordinates instead of a
<code class="docutils literal notranslate">RECT</code> instance, you can retrieve the fields in the function and return them
instead, the normal processing will no longer take place:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def errcheck(result, func, args):
... if not result:
... raise WinError()
... rc = args[1]
... return rc.left, rc.top, rc.bottom, rc.right
...
&gt;&gt;&gt; GetWindowRect.errcheck = errcheck
&gt;&gt;&gt;
</pre></div>
</div>
</div>
<div class="section" id="utility-functions">
<h3>16.16.2.5. Utility functions<a class="headerlink" href="#utility-functions" title="Permalink to this headline">¶</a></h3>
<dl class="function">
<dt id="ctypes.addressof">
<code class="descclassname">ctypes.</code><code class="descname">addressof</code>(obj)<a class="headerlink" href="#ctypes.addressof" title="Permalink to this definition">¶</a></dt>
<dd>Returns the address of the memory buffer as integer. obj must be an
instance of a ctypes type.
</dd></dl>

<dl class="function">
<dt id="ctypes.alignment">
<code class="descclassname">ctypes.</code><code class="descname">alignment</code>(obj_or_type)<a class="headerlink" href="#ctypes.alignment" title="Permalink to this definition">¶</a></dt>
<dd>Returns the alignment requirements of a ctypes type. obj_or_type must be a
ctypes type or instance.
</dd></dl>

<dl class="function">
<dt id="ctypes.byref">
<code class="descclassname">ctypes.</code><code class="descname">byref</code>(obj[, offset])<a class="headerlink" href="#ctypes.byref" title="Permalink to this definition">¶</a></dt>
<dd>Returns a light-weight pointer to obj, which must be an instance of a
ctypes type. offset defaults to zero, and must be an integer that will be
added to the internal pointer value.
<code class="docutils literal notranslate">byref(obj, offset)</code> corresponds to this C code:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>(((char *)&amp;obj) + offset)
</pre></div>
</div>
The returned object can only be used as a foreign function call parameter.
It behaves similar to <code class="docutils literal notranslate">pointer(obj)</code>, but the construction is a lot faster.
</dd></dl>

<dl class="function">
<dt id="ctypes.cast">
<code class="descclassname">ctypes.</code><code class="descname">cast</code>(obj, type)<a class="headerlink" href="#ctypes.cast" title="Permalink to this definition">¶</a></dt>
<dd>This function is similar to the cast operator in C. It returns a new instance
of type which points to the same memory block as obj. type must be a
pointer type, and obj must be an object that can be interpreted as a
pointer.
</dd></dl>

<dl class="function">
<dt id="ctypes.create_string_buffer">
<code class="descclassname">ctypes.</code><code class="descname">create_string_buffer</code>(init_or_size, size=None)<a class="headerlink" href="#ctypes.create_string_buffer" title="Permalink to this definition">¶</a></dt>
<dd>This function creates a mutable character buffer. The returned object is a
ctypes array of <a class="reference internal" href="#ctypes.c_char" title="ctypes.c_char"><code class="xref py py-class docutils literal notranslate">c_char</code></a>.
init_or_size must be an integer which specifies the size of the array, or a
bytes object which will be used to initialize the array items.
If a bytes object is specified as first argument, the buffer is made one item
larger than its length so that the last element in the array is a NUL
termination character. An integer can be passed as second argument which allows
specifying the size of the array if the length of the bytes should not be used.
</dd></dl>

<dl class="function">
<dt id="ctypes.create_unicode_buffer">
<code class="descclassname">ctypes.</code><code class="descname">create_unicode_buffer</code>(init_or_size, size=None)<a class="headerlink" href="#ctypes.create_unicode_buffer" title="Permalink to this definition">¶</a></dt>
<dd>This function creates a mutable unicode character buffer. The returned object is
a ctypes array of <a class="reference internal" href="#ctypes.c_wchar" title="ctypes.c_wchar"><code class="xref py py-class docutils literal notranslate">c_wchar</code></a>.
init_or_size must be an integer which specifies the size of the array, or a
string which will be used to initialize the array items.
If a string is specified as first argument, the buffer is made one item
larger than the length of the string so that the last element in the array is a
NUL termination character. An integer can be passed as second argument which
allows specifying the size of the array if the length of the string should not
be used.
</dd></dl>

<dl class="function">
<dt id="ctypes.DllCanUnloadNow">
<code class="descclassname">ctypes.</code><code class="descname">DllCanUnloadNow</code>()<a class="headerlink" href="#ctypes.DllCanUnloadNow" title="Permalink to this definition">¶</a></dt>
<dd>Windows only: This function is a hook which allows implementing in-process
COM servers with ctypes. It is called from the DllCanUnloadNow function that
the _ctypes extension dll exports.
</dd></dl>

<dl class="function">
<dt id="ctypes.DllGetClassObject">
<code class="descclassname">ctypes.</code><code class="descname">DllGetClassObject</code>()<a class="headerlink" href="#ctypes.DllGetClassObject" title="Permalink to this definition">¶</a></dt>
<dd>Windows only: This function is a hook which allows implementing in-process
COM servers with ctypes. It is called from the DllGetClassObject function
that the <code class="docutils literal notranslate">_ctypes</code> extension dll exports.
</dd></dl>

<dl class="function">
<dt id="ctypes.util.find_library">
<code class="descclassname">ctypes.util.</code><code class="descname">find_library</code>(name)<a class="headerlink" href="#ctypes.util.find_library" title="Permalink to this definition">¶</a></dt>
<dd>Try to find a library and return a pathname. name is the library name
without any prefix like <code class="docutils literal notranslate">lib</code>, suffix like <code class="docutils literal notranslate">.so</code>, <code class="docutils literal notranslate">.dylib</code> or version
number (this is the form used for the posix linker option <code class="xref std std-option docutils literal notranslate">-l</code>). If
no library can be found, returns <code class="docutils literal notranslate">None</code>.
The exact functionality is system dependent.
</dd></dl>

<dl class="function">
<dt id="ctypes.util.find_msvcrt">
<code class="descclassname">ctypes.util.</code><code class="descname">find_msvcrt</code>()<a class="headerlink" href="#ctypes.util.find_msvcrt" title="Permalink to this definition">¶</a></dt>
<dd>Windows only: return the filename of the VC runtime library used by Python,
and by the extension modules. If the name of the library cannot be
determined, <code class="docutils literal notranslate">None</code> is returned.
If you need to free memory, for example, allocated by an extension module
with a call to the <code class="docutils literal notranslate">free(void *)</code>, it is important that you use the
function in the same library that allocated the memory.
</dd></dl>

<dl class="function">
<dt id="ctypes.FormatError">
<code class="descclassname">ctypes.</code><code class="descname">FormatError</code>([code])<a class="headerlink" href="#ctypes.FormatError" title="Permalink to this definition">¶</a></dt>
<dd>Windows only: Returns a textual description of the error code code. If no
error code is specified, the last error code is used by calling the Windows
api function GetLastError.
</dd></dl>

<dl class="function">
<dt id="ctypes.GetLastError">
<code class="descclassname">ctypes.</code><code class="descname">GetLastError</code>()<a class="headerlink" href="#ctypes.GetLastError" title="Permalink to this definition">¶</a></dt>
<dd>Windows only: Returns the last error code set by Windows in the calling thread.
This function calls the Windows <cite>GetLastError()</cite> function directly,
it does not return the ctypes-private copy of the error code.
</dd></dl>

<dl class="function">
<dt id="ctypes.get_errno">
<code class="descclassname">ctypes.</code><code class="descname">get_errno</code>()<a class="headerlink" href="#ctypes.get_errno" title="Permalink to this definition">¶</a></dt>
<dd>Returns the current value of the ctypes-private copy of the system
<a class="reference internal" href="errno.html#module-errno" title="errno: Standard errno system symbols."><code class="xref py py-data docutils literal notranslate">errno</code></a> variable in the calling thread.
</dd></dl>

<dl class="function">
<dt id="ctypes.get_last_error">
<code class="descclassname">ctypes.</code><code class="descname">get_last_error</code>()<a class="headerlink" href="#ctypes.get_last_error" title="Permalink to this definition">¶</a></dt>
<dd>Windows only: returns the current value of the ctypes-private copy of the system
<code class="xref py py-data docutils literal notranslate">LastError</code> variable in the calling thread.
</dd></dl>

<dl class="function">
<dt id="ctypes.memmove">
<code class="descclassname">ctypes.</code><code class="descname">memmove</code>(dst, src, count)<a class="headerlink" href="#ctypes.memmove" title="Permalink to this definition">¶</a></dt>
<dd>Same as the standard C memmove library function: copies count bytes from
src to dst. dst and src must be integers or ctypes instances that can
be converted to pointers.
</dd></dl>

<dl class="function">
<dt id="ctypes.memset">
<code class="descclassname">ctypes.</code><code class="descname">memset</code>(dst, c, count)<a class="headerlink" href="#ctypes.memset" title="Permalink to this definition">¶</a></dt>
<dd>Same as the standard C memset library function: fills the memory block at
address dst with count bytes of value c. dst must be an integer
specifying an address, or a ctypes instance.
</dd></dl>

<dl class="function">
<dt id="ctypes.POINTER">
<code class="descclassname">ctypes.</code><code class="descname">POINTER</code>(type)<a class="headerlink" href="#ctypes.POINTER" title="Permalink to this definition">¶</a></dt>
<dd>This factory function creates and returns a new ctypes pointer type. Pointer
types are cached and reused internally, so calling this function repeatedly is
cheap. type must be a ctypes type.
</dd></dl>

<dl class="function">
<dt id="ctypes.pointer">
<code class="descclassname">ctypes.</code><code class="descname">pointer</code>(obj)<a class="headerlink" href="#ctypes.pointer" title="Permalink to this definition">¶</a></dt>
<dd>This function creates a new pointer instance, pointing to obj. The returned
object is of the type <code class="docutils literal notranslate">POINTER(type(obj))</code>.
Note: If you just want to pass a pointer to an object to a foreign function
call, you should use <code class="docutils literal notranslate">byref(obj)</code> which is much faster.
</dd></dl>

<dl class="function">
<dt id="ctypes.resize">
<code class="descclassname">ctypes.</code><code class="descname">resize</code>(obj, size)<a class="headerlink" href="#ctypes.resize" title="Permalink to this definition">¶</a></dt>
<dd>This function resizes the internal memory buffer of obj, which must be an
instance of a ctypes type. It is not possible to make the buffer smaller
than the native size of the objects type, as given by <code class="docutils literal notranslate">sizeof(type(obj))</code>,
but it is possible to enlarge the buffer.
</dd></dl>

<dl class="function">
<dt id="ctypes.set_errno">
<code class="descclassname">ctypes.</code><code class="descname">set_errno</code>(value)<a class="headerlink" href="#ctypes.set_errno" title="Permalink to this definition">¶</a></dt>
<dd>Set the current value of the ctypes-private copy of the system <a class="reference internal" href="errno.html#module-errno" title="errno: Standard errno system symbols."><code class="xref py py-data docutils literal notranslate">errno</code></a>
variable in the calling thread to value and return the previous value.
</dd></dl>

<dl class="function">
<dt id="ctypes.set_last_error">
<code class="descclassname">ctypes.</code><code class="descname">set_last_error</code>(value)<a class="headerlink" href="#ctypes.set_last_error" title="Permalink to this definition">¶</a></dt>
<dd>Windows only: set the current value of the ctypes-private copy of the system
<code class="xref py py-data docutils literal notranslate">LastError</code> variable in the calling thread to value and return the
previous value.
</dd></dl>

<dl class="function">
<dt id="ctypes.sizeof">
<code class="descclassname">ctypes.</code><code class="descname">sizeof</code>(obj_or_type)<a class="headerlink" href="#ctypes.sizeof" title="Permalink to this definition">¶</a></dt>
<dd>Returns the size in bytes of a ctypes type or instance memory buffer.
Does the same as the C <code class="docutils literal notranslate">sizeof</code> operator.
</dd></dl>

<dl class="function">
<dt id="ctypes.string_at">
<code class="descclassname">ctypes.</code><code class="descname">string_at</code>(address, size=-1)<a class="headerlink" href="#ctypes.string_at" title="Permalink to this definition">¶</a></dt>
<dd>This function returns the C string starting at memory address address as a bytes
object. If size is specified, it is used as size, otherwise the string is assumed
to be zero-terminated.
</dd></dl>

<dl class="function">
<dt id="ctypes.WinError">
<code class="descclassname">ctypes.</code><code class="descname">WinError</code>(code=None, descr=None)<a class="headerlink" href="#ctypes.WinError" title="Permalink to this definition">¶</a></dt>
<dd>Windows only: this function is probably the worst-named thing in ctypes. It
creates an instance of OSError. If code is not specified,
<code class="docutils literal notranslate">GetLastError</code> is called to determine the error code. If descr is not
specified, <a class="reference internal" href="#ctypes.FormatError" title="ctypes.FormatError"><code class="xref py py-func docutils literal notranslate">FormatError()</code></a> is called to get a textual description of the
error.
<div class="versionchanged">
Changed in version 3.3: An instance of <a class="reference internal" href="exceptions.html#WindowsError" title="WindowsError"><code class="xref py py-exc docutils literal notranslate">WindowsError</code></a> used to be created.
</div>
</dd></dl>

<dl class="function">
<dt id="ctypes.wstring_at">
<code class="descclassname">ctypes.</code><code class="descname">wstring_at</code>(address, size=-1)<a class="headerlink" href="#ctypes.wstring_at" title="Permalink to this definition">¶</a></dt>
<dd>This function returns the wide character string starting at memory address
address as a string. If size is specified, it is used as the number of
characters of the string, otherwise the string is assumed to be
zero-terminated.
</dd></dl>

</div>
<div class="section" id="data-types">
<h3>16.16.2.6. Data types<a class="headerlink" href="#data-types" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="ctypes._CData">
class <code class="descclassname">ctypes.</code><code class="descname">_CData</code><a class="headerlink" href="#ctypes._CData" title="Permalink to this definition">¶</a></dt>
<dd>This non-public class is the common base class of all ctypes data types.
Among other things, all ctypes type instances contain a memory block that
hold C compatible data; the address of the memory block is returned by the
<a class="reference internal" href="#ctypes.addressof" title="ctypes.addressof"><code class="xref py py-func docutils literal notranslate">addressof()</code></a> helper function. Another instance variable is exposed as
<a class="reference internal" href="#ctypes._CData._objects" title="ctypes._CData._objects"><code class="xref py py-attr docutils literal notranslate">_objects</code></a>; this contains other Python objects that need to be kept
alive in case the memory block contains pointers.
Common methods of ctypes data types, these are all class methods (to be
exact, they are methods of the <a class="reference internal" href="../glossary.html#term-metaclass">metaclass</a>):
<dl class="method">
<dt id="ctypes._CData.from_buffer">
<code class="descname">from_buffer</code>(source[, offset])<a class="headerlink" href="#ctypes._CData.from_buffer" title="Permalink to this definition">¶</a></dt>
<dd>This method returns a ctypes instance that shares the buffer of the
source object. The source object must support the writeable buffer
interface. The optional offset parameter specifies an offset into the
source buffer in bytes; the default is zero. If the source buffer is not
large enough a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> is raised.
</dd></dl>

<dl class="method">
<dt id="ctypes._CData.from_buffer_copy">
<code class="descname">from_buffer_copy</code>(source[, offset])<a class="headerlink" href="#ctypes._CData.from_buffer_copy" title="Permalink to this definition">¶</a></dt>
<dd>This method creates a ctypes instance, copying the buffer from the
source object buffer which must be readable. The optional offset
parameter specifies an offset into the source buffer in bytes; the default
is zero. If the source buffer is not large enough a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> is
raised.
</dd></dl>

<dl class="method">
<dt id="ctypes._CData.from_address">
<code class="descname">from_address</code>(address)<a class="headerlink" href="#ctypes._CData.from_address" title="Permalink to this definition">¶</a></dt>
<dd>This method returns a ctypes type instance using the memory specified by
address which must be an integer.
</dd></dl>

<dl class="method">
<dt id="ctypes._CData.from_param">
<code class="descname">from_param</code>(obj)<a class="headerlink" href="#ctypes._CData.from_param" title="Permalink to this definition">¶</a></dt>
<dd>This method adapts obj to a ctypes type. It is called with the actual
object used in a foreign function call when the type is present in the
foreign function’s <code class="xref py py-attr docutils literal notranslate">argtypes</code> tuple; it must return an object that
can be used as a function call parameter.
All ctypes data types have a default implementation of this classmethod
that normally returns obj if that is an instance of the type. Some
types accept other objects as well.
</dd></dl>

<dl class="method">
<dt id="ctypes._CData.in_dll">
<code class="descname">in_dll</code>(library, name)<a class="headerlink" href="#ctypes._CData.in_dll" title="Permalink to this definition">¶</a></dt>
<dd>This method returns a ctypes type instance exported by a shared
library. name is the name of the symbol that exports the data, library
is the loaded shared library.
</dd></dl>

Common instance variables of ctypes data types:
<dl class="attribute">
<dt id="ctypes._CData._b_base_">
<code class="descname">_b_base_</code><a class="headerlink" href="#ctypes._CData._b_base_" title="Permalink to this definition">¶</a></dt>
<dd>Sometimes ctypes data instances do not own the memory block they contain,
instead they share part of the memory block of a base object. The
<a class="reference internal" href="#ctypes._CData._b_base_" title="ctypes._CData._b_base_"><code class="xref py py-attr docutils literal notranslate">_b_base_</code></a> read-only member is the root ctypes object that owns the
memory block.
</dd></dl>

<dl class="attribute">
<dt id="ctypes._CData._b_needsfree_">
<code class="descname">_b_needsfree_</code><a class="headerlink" href="#ctypes._CData._b_needsfree_" title="Permalink to this definition">¶</a></dt>
<dd>This read-only variable is true when the ctypes data instance has
allocated the memory block itself, false otherwise.
</dd></dl>

<dl class="attribute">
<dt id="ctypes._CData._objects">
<code class="descname">_objects</code><a class="headerlink" href="#ctypes._CData._objects" title="Permalink to this definition">¶</a></dt>
<dd>This member is either <code class="docutils literal notranslate">None</code> or a dictionary containing Python objects
that need to be kept alive so that the memory block contents is kept
valid. This object is only exposed for debugging; never modify the
contents of this dictionary.
</dd></dl>

</dd></dl>

</div>
<div class="section" id="ctypes-fundamental-data-types-2">
<h3>16.16.2.7. Fundamental data types<a class="headerlink" href="#ctypes-fundamental-data-types-2" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="ctypes._SimpleCData">
class <code class="descclassname">ctypes.</code><code class="descname">_SimpleCData</code><a class="headerlink" href="#ctypes._SimpleCData" title="Permalink to this definition">¶</a></dt>
<dd>This non-public class is the base class of all fundamental ctypes data
types. It is mentioned here because it contains the common attributes of the
fundamental ctypes data types. <a class="reference internal" href="#ctypes._SimpleCData" title="ctypes._SimpleCData"><code class="xref py py-class docutils literal notranslate">_SimpleCData</code></a> is a subclass of
<a class="reference internal" href="#ctypes._CData" title="ctypes._CData"><code class="xref py py-class docutils literal notranslate">_CData</code></a>, so it inherits their methods and attributes. ctypes data
types that are not and do not contain pointers can now be pickled.
Instances have a single attribute:
<dl class="attribute">
<dt id="ctypes._SimpleCData.value">
<code class="descname">value</code><a class="headerlink" href="#ctypes._SimpleCData.value" title="Permalink to this definition">¶</a></dt>
<dd>This attribute contains the actual value of the instance. For integer and
pointer types, it is an integer, for character types, it is a single
character bytes object or string, for character pointer types it is a
Python bytes object or string.
When the <code class="docutils literal notranslate">value</code> attribute is retrieved from a ctypes instance, usually
a new object is returned each time. <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> does not implement
original object return, always a new object is constructed. The same is
true for all other ctypes object instances.
</dd></dl>

</dd></dl>

Fundamental data types, when returned as foreign function call results, or, for
example, by retrieving structure field members or array items, are transparently
converted to native Python types. In other words, if a foreign function has a
<code class="xref py py-attr docutils literal notranslate">restype</code> of <a class="reference internal" href="#ctypes.c_char_p" title="ctypes.c_char_p"><code class="xref py py-class docutils literal notranslate">c_char_p</code></a>, you will always receive a Python bytes
object, not a <a class="reference internal" href="#ctypes.c_char_p" title="ctypes.c_char_p"><code class="xref py py-class docutils literal notranslate">c_char_p</code></a> instance.
Subclasses of fundamental data types do not inherit this behavior. So, if a
foreign functions <code class="xref py py-attr docutils literal notranslate">restype</code> is a subclass of <a class="reference internal" href="#ctypes.c_void_p" title="ctypes.c_void_p"><code class="xref py py-class docutils literal notranslate">c_void_p</code></a>, you will
receive an instance of this subclass from the function call. Of course, you can
get the value of the pointer by accessing the <code class="docutils literal notranslate">value</code> attribute.
These are the fundamental ctypes data types:
<dl class="class">
<dt id="ctypes.c_byte">
class <code class="descclassname">ctypes.</code><code class="descname">c_byte</code><a class="headerlink" href="#ctypes.c_byte" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">signed char</code> datatype, and interprets the value as
small integer. The constructor accepts an optional integer initializer; no
overflow checking is done.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_char">
class <code class="descclassname">ctypes.</code><code class="descname">c_char</code><a class="headerlink" href="#ctypes.c_char" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">char</code> datatype, and interprets the value as a single
character. The constructor accepts an optional string initializer, the
length of the string must be exactly one character.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_char_p">
class <code class="descclassname">ctypes.</code><code class="descname">c_char_p</code><a class="headerlink" href="#ctypes.c_char_p" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">char *</code> datatype when it points to a zero-terminated
string. For a general character pointer that may also point to binary data,
<code class="docutils literal notranslate">POINTER(c_char)</code> must be used. The constructor accepts an integer
address, or a bytes object.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_double">
class <code class="descclassname">ctypes.</code><code class="descname">c_double</code><a class="headerlink" href="#ctypes.c_double" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">double</code> datatype. The constructor accepts an
optional float initializer.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_longdouble">
class <code class="descclassname">ctypes.</code><code class="descname">c_longdouble</code><a class="headerlink" href="#ctypes.c_longdouble" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">long double</code> datatype. The constructor accepts an
optional float initializer. On platforms where <code class="docutils literal notranslate">sizeof(long double) ==
sizeof(double)</code> it is an alias to <a class="reference internal" href="#ctypes.c_double" title="ctypes.c_double"><code class="xref py py-class docutils literal notranslate">c_double</code></a>.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_float">
class <code class="descclassname">ctypes.</code><code class="descname">c_float</code><a class="headerlink" href="#ctypes.c_float" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">float</code> datatype. The constructor accepts an
optional float initializer.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_int">
class <code class="descclassname">ctypes.</code><code class="descname">c_int</code><a class="headerlink" href="#ctypes.c_int" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">signed int</code> datatype. The constructor accepts an
optional integer initializer; no overflow checking is done. On platforms
where <code class="docutils literal notranslate">sizeof(int) == sizeof(long)</code> it is an alias to <a class="reference internal" href="#ctypes.c_long" title="ctypes.c_long"><code class="xref py py-class docutils literal notranslate">c_long</code></a>.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_int8">
class <code class="descclassname">ctypes.</code><code class="descname">c_int8</code><a class="headerlink" href="#ctypes.c_int8" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C 8-bit <code class="xref c c-type docutils literal notranslate">signed int</code> datatype. Usually an alias for
<a class="reference internal" href="#ctypes.c_byte" title="ctypes.c_byte"><code class="xref py py-class docutils literal notranslate">c_byte</code></a>.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_int16">
class <code class="descclassname">ctypes.</code><code class="descname">c_int16</code><a class="headerlink" href="#ctypes.c_int16" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C 16-bit <code class="xref c c-type docutils literal notranslate">signed int</code> datatype. Usually an alias for
<a class="reference internal" href="#ctypes.c_short" title="ctypes.c_short"><code class="xref py py-class docutils literal notranslate">c_short</code></a>.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_int32">
class <code class="descclassname">ctypes.</code><code class="descname">c_int32</code><a class="headerlink" href="#ctypes.c_int32" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C 32-bit <code class="xref c c-type docutils literal notranslate">signed int</code> datatype. Usually an alias for
<a class="reference internal" href="#ctypes.c_int" title="ctypes.c_int"><code class="xref py py-class docutils literal notranslate">c_int</code></a>.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_int64">
class <code class="descclassname">ctypes.</code><code class="descname">c_int64</code><a class="headerlink" href="#ctypes.c_int64" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C 64-bit <code class="xref c c-type docutils literal notranslate">signed int</code> datatype. Usually an alias for
<a class="reference internal" href="#ctypes.c_longlong" title="ctypes.c_longlong"><code class="xref py py-class docutils literal notranslate">c_longlong</code></a>.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_long">
class <code class="descclassname">ctypes.</code><code class="descname">c_long</code><a class="headerlink" href="#ctypes.c_long" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">signed long</code> datatype. The constructor accepts an
optional integer initializer; no overflow checking is done.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_longlong">
class <code class="descclassname">ctypes.</code><code class="descname">c_longlong</code><a class="headerlink" href="#ctypes.c_longlong" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">signed long long</code> datatype. The constructor accepts
an optional integer initializer; no overflow checking is done.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_short">
class <code class="descclassname">ctypes.</code><code class="descname">c_short</code><a class="headerlink" href="#ctypes.c_short" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">signed short</code> datatype. The constructor accepts an
optional integer initializer; no overflow checking is done.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_size_t">
class <code class="descclassname">ctypes.</code><code class="descname">c_size_t</code><a class="headerlink" href="#ctypes.c_size_t" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">size_t</code> datatype.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_ssize_t">
class <code class="descclassname">ctypes.</code><code class="descname">c_ssize_t</code><a class="headerlink" href="#ctypes.c_ssize_t" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">ssize_t</code> datatype.
<div class="versionadded">
New in version 3.2.
</div>
</dd></dl>

<dl class="class">
<dt id="ctypes.c_ubyte">
class <code class="descclassname">ctypes.</code><code class="descname">c_ubyte</code><a class="headerlink" href="#ctypes.c_ubyte" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">unsigned char</code> datatype, it interprets the value as
small integer. The constructor accepts an optional integer initializer; no
overflow checking is done.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_uint">
class <code class="descclassname">ctypes.</code><code class="descname">c_uint</code><a class="headerlink" href="#ctypes.c_uint" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">unsigned int</code> datatype. The constructor accepts an
optional integer initializer; no overflow checking is done. On platforms
where <code class="docutils literal notranslate">sizeof(int) == sizeof(long)</code> it is an alias for <a class="reference internal" href="#ctypes.c_ulong" title="ctypes.c_ulong"><code class="xref py py-class docutils literal notranslate">c_ulong</code></a>.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_uint8">
class <code class="descclassname">ctypes.</code><code class="descname">c_uint8</code><a class="headerlink" href="#ctypes.c_uint8" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C 8-bit <code class="xref c c-type docutils literal notranslate">unsigned int</code> datatype. Usually an alias for
<a class="reference internal" href="#ctypes.c_ubyte" title="ctypes.c_ubyte"><code class="xref py py-class docutils literal notranslate">c_ubyte</code></a>.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_uint16">
class <code class="descclassname">ctypes.</code><code class="descname">c_uint16</code><a class="headerlink" href="#ctypes.c_uint16" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C 16-bit <code class="xref c c-type docutils literal notranslate">unsigned int</code> datatype. Usually an alias for
<a class="reference internal" href="#ctypes.c_ushort" title="ctypes.c_ushort"><code class="xref py py-class docutils literal notranslate">c_ushort</code></a>.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_uint32">
class <code class="descclassname">ctypes.</code><code class="descname">c_uint32</code><a class="headerlink" href="#ctypes.c_uint32" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C 32-bit <code class="xref c c-type docutils literal notranslate">unsigned int</code> datatype. Usually an alias for
<a class="reference internal" href="#ctypes.c_uint" title="ctypes.c_uint"><code class="xref py py-class docutils literal notranslate">c_uint</code></a>.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_uint64">
class <code class="descclassname">ctypes.</code><code class="descname">c_uint64</code><a class="headerlink" href="#ctypes.c_uint64" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C 64-bit <code class="xref c c-type docutils literal notranslate">unsigned int</code> datatype. Usually an alias for
<a class="reference internal" href="#ctypes.c_ulonglong" title="ctypes.c_ulonglong"><code class="xref py py-class docutils literal notranslate">c_ulonglong</code></a>.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_ulong">
class <code class="descclassname">ctypes.</code><code class="descname">c_ulong</code><a class="headerlink" href="#ctypes.c_ulong" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">unsigned long</code> datatype. The constructor accepts an
optional integer initializer; no overflow checking is done.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_ulonglong">
class <code class="descclassname">ctypes.</code><code class="descname">c_ulonglong</code><a class="headerlink" href="#ctypes.c_ulonglong" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">unsigned long long</code> datatype. The constructor
accepts an optional integer initializer; no overflow checking is done.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_ushort">
class <code class="descclassname">ctypes.</code><code class="descname">c_ushort</code><a class="headerlink" href="#ctypes.c_ushort" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">unsigned short</code> datatype. The constructor accepts
an optional integer initializer; no overflow checking is done.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_void_p">
class <code class="descclassname">ctypes.</code><code class="descname">c_void_p</code><a class="headerlink" href="#ctypes.c_void_p" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">void *</code> type. The value is represented as integer.
The constructor accepts an optional integer initializer.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_wchar">
class <code class="descclassname">ctypes.</code><code class="descname">c_wchar</code><a class="headerlink" href="#ctypes.c_wchar" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">wchar_t</code> datatype, and interprets the value as a
single character unicode string. The constructor accepts an optional string
initializer, the length of the string must be exactly one character.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_wchar_p">
class <code class="descclassname">ctypes.</code><code class="descname">c_wchar_p</code><a class="headerlink" href="#ctypes.c_wchar_p" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <code class="xref c c-type docutils literal notranslate">wchar_t *</code> datatype, which must be a pointer to a
zero-terminated wide character string. The constructor accepts an integer
address, or a string.
</dd></dl>

<dl class="class">
<dt id="ctypes.c_bool">
class <code class="descclassname">ctypes.</code><code class="descname">c_bool</code><a class="headerlink" href="#ctypes.c_bool" title="Permalink to this definition">¶</a></dt>
<dd>Represent the C <code class="xref c c-type docutils literal notranslate">bool</code> datatype (more accurately, <code class="xref c c-type docutils literal notranslate">_Bool</code> from
C99). Its value can be <code class="docutils literal notranslate">True</code> or <code class="docutils literal notranslate">False</code>, and the constructor accepts any object
that has a truth value.
</dd></dl>

<dl class="class">
<dt id="ctypes.HRESULT">
class <code class="descclassname">ctypes.</code><code class="descname">HRESULT</code><a class="headerlink" href="#ctypes.HRESULT" title="Permalink to this definition">¶</a></dt>
<dd>Windows only: Represents a <code class="xref c c-type docutils literal notranslate">HRESULT</code> value, which contains success or
error information for a function or method call.
</dd></dl>

<dl class="class">
<dt id="ctypes.py_object">
class <code class="descclassname">ctypes.</code><code class="descname">py_object</code><a class="headerlink" href="#ctypes.py_object" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal notranslate">PyObject *</code></a> datatype. Calling this without an
argument creates a <code class="docutils literal notranslate">NULL</code> <a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal notranslate">PyObject *</code></a> pointer.
</dd></dl>

The <code class="xref py py-mod docutils literal notranslate">ctypes.wintypes</code> module provides quite some other Windows specific
data types, for example <code class="xref c c-type docutils literal notranslate">HWND</code>, <code class="xref c c-type docutils literal notranslate">WPARAM</code>, or <code class="xref c c-type docutils literal notranslate">DWORD</code>. Some
useful structures like <code class="xref c c-type docutils literal notranslate">MSG</code> or <code class="xref c c-type docutils literal notranslate">RECT</code> are also defined.
</div>
<div class="section" id="structured-data-types">
<h3>16.16.2.8. Structured data types<a class="headerlink" href="#structured-data-types" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="ctypes.Union">
class <code class="descclassname">ctypes.</code><code class="descname">Union</code>(*args, **kw)<a class="headerlink" href="#ctypes.Union" title="Permalink to this definition">¶</a></dt>
<dd>Abstract base class for unions in native byte order.
</dd></dl>

<dl class="class">
<dt id="ctypes.BigEndianStructure">
class <code class="descclassname">ctypes.</code><code class="descname">BigEndianStructure</code>(*args, **kw)<a class="headerlink" href="#ctypes.BigEndianStructure" title="Permalink to this definition">¶</a></dt>
<dd>Abstract base class for structures in big endian byte order.
</dd></dl>

<dl class="class">
<dt id="ctypes.LittleEndianStructure">
class <code class="descclassname">ctypes.</code><code class="descname">LittleEndianStructure</code>(*args, **kw)<a class="headerlink" href="#ctypes.LittleEndianStructure" title="Permalink to this definition">¶</a></dt>
<dd>Abstract base class for structures in little endian byte order.
</dd></dl>

Structures with non-native byte order cannot contain pointer type fields, or any
other data types containing pointer type fields.
<dl class="class">
<dt id="ctypes.Structure">
class <code class="descclassname">ctypes.</code><code class="descname">Structure</code>(*args, **kw)<a class="headerlink" href="#ctypes.Structure" title="Permalink to this definition">¶</a></dt>
<dd>Abstract base class for structures in native byte order.
Concrete structure and union types must be created by subclassing one of these
types, and at least define a <a class="reference internal" href="#ctypes.Structure._fields_" title="ctypes.Structure._fields_"><code class="xref py py-attr docutils literal notranslate">_fields_</code></a> class variable. <a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> will
create <a class="reference internal" href="../glossary.html#term-descriptor">descriptor</a>s which allow reading and writing the fields by direct
attribute accesses. These are the
<dl class="attribute">
<dt id="ctypes.Structure._fields_">
<code class="descname">_fields_</code><a class="headerlink" href="#ctypes.Structure._fields_" title="Permalink to this definition">¶</a></dt>
<dd>A sequence defining the structure fields. The items must be 2-tuples or
3-tuples. The first item is the name of the field, the second item
specifies the type of the field; it can be any ctypes data type.
For integer type fields like <a class="reference internal" href="#ctypes.c_int" title="ctypes.c_int"><code class="xref py py-class docutils literal notranslate">c_int</code></a>, a third optional item can be
given. It must be a small positive integer defining the bit width of the
field.
Field names must be unique within one structure or union. This is not
checked, only one field can be accessed when names are repeated.
It is possible to define the <a class="reference internal" href="#ctypes.Structure._fields_" title="ctypes.Structure._fields_"><code class="xref py py-attr docutils literal notranslate">_fields_</code></a> class variable after the
class statement that defines the Structure subclass, this allows creating
data types that directly or indirectly reference themselves:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class List(Structure):
 pass
List._fields_ = [(&quot;pnext&quot;, POINTER(List)),
 ...
 ]
</pre></div>
</div>
The <a class="reference internal" href="#ctypes.Structure._fields_" title="ctypes.Structure._fields_"><code class="xref py py-attr docutils literal notranslate">_fields_</code></a> class variable must, however, be defined before the
type is first used (an instance is created, <a class="reference internal" href="#ctypes.sizeof" title="ctypes.sizeof"><code class="xref py py-func docutils literal notranslate">sizeof()</code></a> is called on it,
and so on). Later assignments to the <a class="reference internal" href="#ctypes.Structure._fields_" title="ctypes.Structure._fields_"><code class="xref py py-attr docutils literal notranslate">_fields_</code></a> class variable will
raise an AttributeError.
It is possible to defined sub-subclasses of structure types, they inherit
the fields of the base class plus the <a class="reference internal" href="#ctypes.Structure._fields_" title="ctypes.Structure._fields_"><code class="xref py py-attr docutils literal notranslate">_fields_</code></a> defined in the
sub-subclass, if any.
</dd></dl>

<dl class="attribute">
<dt id="ctypes.Structure._pack_">
<code class="descname">_pack_</code><a class="headerlink" href="#ctypes.Structure._pack_" title="Permalink to this definition">¶</a></dt>
<dd>An optional small integer that allows overriding the alignment of
structure fields in the instance. <a class="reference internal" href="#ctypes.Structure._pack_" title="ctypes.Structure._pack_"><code class="xref py py-attr docutils literal notranslate">_pack_</code></a> must already be defined
when <a class="reference internal" href="#ctypes.Structure._fields_" title="ctypes.Structure._fields_"><code class="xref py py-attr docutils literal notranslate">_fields_</code></a> is assigned, otherwise it will have no effect.
</dd></dl>

<dl class="attribute">
<dt id="ctypes.Structure._anonymous_">
<code class="descname">_anonymous_</code><a class="headerlink" href="#ctypes.Structure._anonymous_" title="Permalink to this definition">¶</a></dt>
<dd>An optional sequence that lists the names of unnamed (anonymous) fields.
<a class="reference internal" href="#ctypes.Structure._anonymous_" title="ctypes.Structure._anonymous_"><code class="xref py py-attr docutils literal notranslate">_anonymous_</code></a> must be already defined when <a class="reference internal" href="#ctypes.Structure._fields_" title="ctypes.Structure._fields_"><code class="xref py py-attr docutils literal notranslate">_fields_</code></a> is
assigned, otherwise it will have no effect.
The fields listed in this variable must be structure or union type fields.
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> will create descriptors in the structure type that allows
accessing the nested fields directly, without the need to create the
structure or union field.
Here is an example type (Windows):
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class _U(Union):
 _fields_ = [(&quot;lptdesc&quot;, POINTER(TYPEDESC)),
 (&quot;lpadesc&quot;, POINTER(ARRAYDESC)),
 (&quot;hreftype&quot;, HREFTYPE)]

class TYPEDESC(Structure):
 _anonymous_ = (&quot;u&quot;,)
 _fields_ = [(&quot;u&quot;, _U),
 (&quot;vt&quot;, VARTYPE)]
</pre></div>
</div>
The <code class="docutils literal notranslate">TYPEDESC</code> structure describes a COM data type, the <code class="docutils literal notranslate">vt</code> field
specifies which one of the union fields is valid. Since the <code class="docutils literal notranslate">u</code> field
is defined as anonymous field, it is now possible to access the members
directly off the TYPEDESC instance. <code class="docutils literal notranslate">td.lptdesc</code> and <code class="docutils literal notranslate">td.u.lptdesc</code>
are equivalent, but the former is faster since it does not need to create
a temporary union instance:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>td = TYPEDESC()
td.vt = VT_PTR
td.lptdesc = POINTER(some_type)
td.u.lptdesc = POINTER(some_type)
</pre></div>
</div>
</dd></dl>

It is possible to defined sub-subclasses of structures, they inherit the
fields of the base class. If the subclass definition has a separate
<a class="reference internal" href="#ctypes.Structure._fields_" title="ctypes.Structure._fields_"><code class="xref py py-attr docutils literal notranslate">_fields_</code></a> variable, the fields specified in this are appended to the
fields of the base class.
Structure and union constructors accept both positional and keyword
arguments. Positional arguments are used to initialize member fields in the
same order as they are appear in <a class="reference internal" href="#ctypes.Structure._fields_" title="ctypes.Structure._fields_"><code class="xref py py-attr docutils literal notranslate">_fields_</code></a>. Keyword arguments in the
constructor are interpreted as attribute assignments, so they will initialize
<a class="reference internal" href="#ctypes.Structure._fields_" title="ctypes.Structure._fields_"><code class="xref py py-attr docutils literal notranslate">_fields_</code></a> with the same name, or create new attributes for names not
present in <a class="reference internal" href="#ctypes.Structure._fields_" title="ctypes.Structure._fields_"><code class="xref py py-attr docutils literal notranslate">_fields_</code></a>.
</dd></dl>

</div>
<div class="section" id="arrays-and-pointers">
<h3>16.16.2.9. Arrays and pointers<a class="headerlink" href="#arrays-and-pointers" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="ctypes.Array">
class <code class="descclassname">ctypes.</code><code class="descname">Array</code>(*args)<a class="headerlink" href="#ctypes.Array" title="Permalink to this definition">¶</a></dt>
<dd>Abstract base class for arrays.
The recommended way to create concrete array types is by multiplying any
<a class="reference internal" href="#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> data type with a positive integer. Alternatively, you can subclass
this type and define <a class="reference internal" href="#ctypes.Array._length_" title="ctypes.Array._length_"><code class="xref py py-attr docutils literal notranslate">_length_</code></a> and <a class="reference internal" href="#ctypes.Array._type_" title="ctypes.Array._type_"><code class="xref py py-attr docutils literal notranslate">_type_</code></a> class variables.
Array elements can be read and written using standard
subscript and slice accesses; for slice reads, the resulting object is
not itself an <a class="reference internal" href="#ctypes.Array" title="ctypes.Array"><code class="xref py py-class docutils literal notranslate">Array</code></a>.
<dl class="attribute">
<dt id="ctypes.Array._length_">
<code class="descname">_length_</code><a class="headerlink" href="#ctypes.Array._length_" title="Permalink to this definition">¶</a></dt>
<dd>A positive integer specifying the number of elements in the array.
Out-of-range subscripts result in an <a class="reference internal" href="exceptions.html#IndexError" title="IndexError"><code class="xref py py-exc docutils literal notranslate">IndexError</code></a>. Will be
returned by <a class="reference internal" href="functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate">len()</code></a>.
</dd></dl>

<dl class="attribute">
<dt id="ctypes.Array._type_">
<code class="descname">_type_</code><a class="headerlink" href="#ctypes.Array._type_" title="Permalink to this definition">¶</a></dt>
<dd>Specifies the type of each element in the array.
</dd></dl>

Array subclass constructors accept positional arguments, used to
initialize the elements in order.
</dd></dl>

<dl class="class">
<dt id="ctypes._Pointer">
class <code class="descclassname">ctypes.</code><code class="descname">_Pointer</code><a class="headerlink" href="#ctypes._Pointer" title="Permalink to this definition">¶</a></dt>
<dd>Private, abstract base class for pointers.
Concrete pointer types are created by calling <a class="reference internal" href="#ctypes.POINTER" title="ctypes.POINTER"><code class="xref py py-func docutils literal notranslate">POINTER()</code></a> with the
type that will be pointed to; this is done automatically by
<a class="reference internal" href="#ctypes.pointer" title="ctypes.pointer"><code class="xref py py-func docutils literal notranslate">pointer()</code></a>.
If a pointer points to an array, its elements can be read and
written using standard subscript and slice accesses. Pointer objects
have no size, so <a class="reference internal" href="functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate">len()</code></a> will raise <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>. Negative
subscripts will read from the memory before the pointer (as in C), and
out-of-range subscripts will probably crash with an access violation (if
you’re lucky).
<dl class="attribute">
<dt id="ctypes._Pointer._type_">
<code class="descname">_type_</code><a class="headerlink" href="#ctypes._Pointer._type_" title="Permalink to this definition">¶</a></dt>
<dd>Specifies the type pointed to.
</dd></dl>

<dl class="attribute">
<dt id="ctypes._Pointer.contents">
<code class="descname">contents</code><a class="headerlink" href="#ctypes._Pointer.contents" title="Permalink to this definition">¶</a></dt>
<dd>Returns the object to which to pointer points. Assigning to this
attribute changes the pointer to point to the assigned object.
</dd></dl>

</dd></dl>

</div>
</div>
</div>

</div>
 </div>
 </div>
 <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">16.16. <code class="docutils literal notranslate">ctypes</code> — A foreign function library for Python</a><ul>
<li><a class="reference internal" href="#ctypes-tutorial">16.16.1. ctypes tutorial</a><ul>
<li><a class="reference internal" href="#loading-dynamic-link-libraries">16.16.1.1. Loading dynamic link libraries</a></li>
<li><a class="reference internal" href="#accessing-functions-from-loaded-dlls">16.16.1.2. Accessing functions from loaded dlls</a></li>
<li><a class="reference internal" href="#calling-functions">16.16.1.3. Calling functions</a></li>
<li><a class="reference internal" href="#fundamental-data-types">16.16.1.4. Fundamental data types</a></li>
<li><a class="reference internal" href="#calling-functions-continued">16.16.1.5. Calling functions, continued</a></li>
<li><a class="reference internal" href="#calling-functions-with-your-own-custom-data-types">16.16.1.6. Calling functions with your own custom data types</a></li>
<li><a class="reference internal" href="#specifying-the-required-argument-types-function-prototypes">16.16.1.7. Specifying the required argument types (function prototypes)</a></li>
<li><a class="reference internal" href="#return-types">16.16.1.8. Return types</a></li>
<li><a class="reference internal" href="#passing-pointers-or-passing-parameters-by-reference">16.16.1.9. Passing pointers (or: passing parameters by reference)</a></li>
<li><a class="reference internal" href="#structures-and-unions">16.16.1.10. Structures and unions</a></li>
<li><a class="reference internal" href="#structure-union-alignment-and-byte-order">16.16.1.11. Structure/union alignment and byte order</a></li>
<li><a class="reference internal" href="#bit-fields-in-structures-and-unions">16.16.1.12. Bit fields in structures and unions</a></li>
<li><a class="reference internal" href="#arrays">16.16.1.13. Arrays</a></li>
<li><a class="reference internal" href="#pointers">16.16.1.14. Pointers</a></li>
<li><a class="reference internal" href="#type-conversions">16.16.1.15. Type conversions</a></li>
<li><a class="reference internal" href="#incomplete-types">16.16.1.16. Incomplete Types</a></li>
<li><a class="reference internal" href="#callback-functions">16.16.1.17. Callback functions</a></li>
<li><a class="reference internal" href="#accessing-values-exported-from-dlls">16.16.1.18. Accessing values exported from dlls</a></li>
<li><a class="reference internal" href="#surprises">16.16.1.19. Surprises</a></li>
<li><a class="reference internal" href="#variable-sized-data-types">16.16.1.20. Variable-sized data types</a></li>
</ul>
</li>
<li><a class="reference internal" href="#ctypes-reference">16.16.2. ctypes reference</a><ul>
<li><a class="reference internal" href="#finding-shared-libraries">16.16.2.1. Finding shared libraries</a></li>
<li><a class="reference internal" href="#loading-shared-libraries">16.16.2.2. Loading shared libraries</a></li>
<li><a class="reference internal" href="#foreign-functions">16.16.2.3. Foreign functions</a></li>
<li><a class="reference internal" href="#function-prototypes">16.16.2.4. Function prototypes</a></li>
<li><a class="reference internal" href="#utility-functions">16.16.2.5. Utility functions</a></li>
<li><a class="reference internal" href="#data-types">16.16.2.6. Data types</a></li>
<li><a class="reference internal" href="#ctypes-fundamental-data-types-2">16.16.2.7. Fundamental data types</a></li>
<li><a class="reference internal" href="#structured-data-types">16.16.2.8. Structured data types</a></li>
<li><a class="reference internal" href="#arrays-and-pointers">16.16.2.9. Arrays and pointers</a></li>
</ul>
</li>
</ul>
</li>
</ul>

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