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 <div class="section" id="module-ctypes">
<h1>15.15. <tt class="xref docutils literal">ctypes</tt> &#8212; A foreign function library for Python<a class="headerlink" href="#module-ctypes" title="Permalink to this headline">¶</a></h1>

New in version 2.5.
<tt class="xref docutils literal">ctypes</tt> 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>15.15.1. ctypes tutorial<a class="headerlink" href="#ctypes-tutorial" title="Permalink to this headline">¶</a></h2>
Note: The code samples in this tutorial use <a title="Test pieces of code within docstrings." class="reference external" href="doctest.html#module-doctest"><tt class="xref docutils literal">doctest</tt></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 title="ctypes.c_int" class="reference internal" href="#ctypes.c_int"><tt class="xref docutils literal">c_int</tt></a> type. This type is
an alias for the <a title="ctypes.c_long" class="reference internal" href="#ctypes.c_long"><tt class="xref docutils literal">c_long</tt></a> type on 32-bit systems. So, you should not be
confused if <a title="ctypes.c_long" class="reference internal" href="#ctypes.c_long"><tt class="xref docutils literal">c_long</tt></a> is printed if you would expect <a title="ctypes.c_int" class="reference internal" href="#ctypes.c_int"><tt class="xref docutils literal">c_int</tt></a> &#8212;
they are actually the same type.
<div class="section" id="loading-dynamic-link-libraries">
<h3>15.15.1.1. Loading dynamic link libraries<a class="headerlink" href="#loading-dynamic-link-libraries" title="Permalink to this headline">¶</a></h3>
<tt class="xref docutils literal">ctypes</tt> 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 <tt class="docutils literal">cdecl</tt> calling
convention, while windll libraries call functions using the <tt class="docutils literal">stdcall</tt>
calling convention. oledll also uses the <tt class="docutils literal">stdcall</tt> calling convention, and
assumes the functions return a Windows <tt class="xref docutils literal">HRESULT</tt> error code. The error
code is used to automatically raise a <tt class="xref docutils literal">WindowsError</tt> exception when the
function call fails.
Here are some examples for Windows. Note that <tt class="docutils literal">msvcrt</tt> is the MS standard C
library containing most standard C functions, and uses the cdecl calling
convention:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import *
&gt;&gt;&gt; print windll.kernel32 # doctest: +WINDOWS
&lt;WinDLL &#39;kernel32&#39;, handle ... at ...&gt;
&gt;&gt;&gt; print cdll.msvcrt # doctest: +WINDOWS
&lt;CDLL &#39;msvcrt&#39;, handle ... at ...&gt;
&gt;&gt;&gt; libc = cdll.msvcrt # doctest: +WINDOWS
&gt;&gt;&gt;
</pre></div>
</div>
Windows appends the usual <tt class="docutils literal">.dll</tt> file suffix automatically.
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
<tt class="xref docutils literal">LoadLibrary()</tt> 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-python"><div class="highlight"><pre>&gt;&gt;&gt; cdll.LoadLibrary(&quot;libc.so.6&quot;) # doctest: +LINUX
&lt;CDLL &#39;libc.so.6&#39;, handle ... at ...&gt;
&gt;&gt;&gt; libc = CDLL(&quot;libc.so.6&quot;) # doctest: +LINUX
&gt;&gt;&gt; libc # doctest: +LINUX
&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>15.15.1.2. 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-python"><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 # doctest: +WINDOWS
&lt;_FuncPtr object at 0x...&gt;
&gt;&gt;&gt; print windll.kernel32.MyOwnFunction # doctest: +WINDOWS
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
 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 <tt class="docutils literal">kernel32</tt> and <tt class="docutils literal">user32</tt> often export ANSI
as well as UNICODE versions of a function. The UNICODE version is exported with
an <tt class="docutils literal">W</tt> appended to the name, while the ANSI version is exported with an <tt class="docutils literal">A</tt>
appended to the name. The win32 <tt class="docutils literal">GetModuleHandle</tt> 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 <tt class="docutils literal">GetModuleHandle</tt> depending on whether
UNICODE is defined or not:
<div class="highlight-python"><pre>/* ANSI version */
HMODULE GetModuleHandleA(LPCSTR lpModuleName);
/* UNICODE version */
HMODULE GetModuleHandleW(LPCWSTR lpModuleName);</pre>
</div>
windll does not try to select one of them by magic, you must access the
version you need by specifying <tt class="docutils literal">GetModuleHandleA</tt> or <tt class="docutils literal">GetModuleHandleW</tt>
explicitly, and then call it with strings or unicode strings
respectively.
Sometimes, dlls export functions with names which aren&#8217;t valid Python
identifiers, like <tt class="docutils literal">&quot;??2&#64;YAPAXI&#64;Z&quot;</tt>. In this case you have to use
<a title="getattr" class="reference external" href="functions.html#getattr"><tt class="xref docutils literal">getattr()</tt></a> to retrieve the function:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; getattr(cdll.msvcrt, &quot;??2@YAPAXI@Z&quot;) # doctest: +WINDOWS
&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-python"><div class="highlight"><pre>&gt;&gt;&gt; cdll.kernel32[1] # doctest: +WINDOWS
&lt;_FuncPtr object at 0x...&gt;
&gt;&gt;&gt; cdll.kernel32[0] # doctest: +WINDOWS
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
 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>15.15.1.3. 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 <tt class="docutils literal">time()</tt> function, which returns system time in seconds since the Unix
epoch, and the <tt class="docutils literal">GetModuleHandleA()</tt> function, which returns a win32 module
handle.
This example calls both functions with a NULL pointer (<tt class="xref docutils literal">None</tt> should be used
as the NULL pointer):
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; print libc.time(None) # doctest: +SKIP
1150640792
&gt;&gt;&gt; print hex(windll.kernel32.GetModuleHandleA(None)) # doctest: +WINDOWS
0x1d000000
&gt;&gt;&gt;
</pre></div>
</div>
<tt class="xref docutils literal">ctypes</tt> tries to protect you from calling functions with the wrong number
of arguments or the wrong calling convention. Unfortunately this only works on
Windows. It does this by examining the stack after the function returns, so
although an error is raised the function has been called:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; windll.kernel32.GetModuleHandleA() # doctest: +WINDOWS
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
ValueError: Procedure probably called with not enough arguments (4 bytes missing)
&gt;&gt;&gt; windll.kernel32.GetModuleHandleA(0, 0) # doctest: +WINDOWS
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
ValueError: Procedure probably called with too many arguments (4 bytes in excess)
&gt;&gt;&gt;
</pre></div>
</div>
The same exception is raised when you call an <tt class="docutils literal">stdcall</tt> function with the
<tt class="docutils literal">cdecl</tt> calling convention, or vice versa:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; cdll.kernel32.GetModuleHandleA(None) # doctest: +WINDOWS
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
ValueError: Procedure probably called with not enough arguments (4 bytes missing)
&gt;&gt;&gt;

&gt;&gt;&gt; windll.msvcrt.printf(&quot;spam&quot;) # doctest: +WINDOWS
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
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, <tt class="xref docutils literal">ctypes</tt> uses win32 structured exception handling to prevent
crashes from general protection faults when functions are called with invalid
argument values:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; windll.kernel32.GetModuleHandleA(32) # doctest: +WINDOWS
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
WindowsError: exception: access violation reading 0x00000020
&gt;&gt;&gt;
</pre></div>
</div>
There are, however, enough ways to crash Python with <tt class="xref docutils literal">ctypes</tt>, so you
should be careful anyway.
<tt class="xref docutils literal">None</tt>, integers, longs, byte strings and unicode strings are the only native
Python objects that can directly be used as parameters in these function calls.
<tt class="xref docutils literal">None</tt> is passed as a C <tt class="docutils literal">NULL</tt> pointer, byte strings and unicode strings are
passed as pointer to the memory block that contains their data (<tt class="xref docutils literal">char *</tt>
or <tt class="xref docutils literal">wchar_t *</tt>). Python integers and Python longs are passed as the
platforms default C <tt class="xref docutils literal">int</tt> 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 <tt class="xref docutils literal">ctypes</tt> data types.
</div>
<div class="section" id="fundamental-data-types">
<h3>15.15.1.4. Fundamental data types<a class="headerlink" href="#fundamental-data-types" title="Permalink to this headline">¶</a></h3>
<tt class="xref docutils literal">ctypes</tt> defines a number of primitive C compatible data types :
<table border="1" class="docutils">
<colgroup>
<col width="24%" />
<col width="44%" />
<col width="31%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">ctypes type</th>
<th class="head">C type</th>
<th class="head">Python type</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><a title="ctypes.c_char" class="reference internal" href="#ctypes.c_char"><tt class="xref docutils literal">c_char</tt></a></td>
<td><tt class="xref docutils literal">char</tt></td>
<td>1-character string</td>
</tr>
<tr><td><a title="ctypes.c_wchar" class="reference internal" href="#ctypes.c_wchar"><tt class="xref docutils literal">c_wchar</tt></a></td>
<td><tt class="xref docutils literal">wchar_t</tt></td>
<td>1-character unicode string</td>
</tr>
<tr><td><a title="ctypes.c_byte" class="reference internal" href="#ctypes.c_byte"><tt class="xref docutils literal">c_byte</tt></a></td>
<td><tt class="xref docutils literal">char</tt></td>
<td>int/long</td>
</tr>
<tr><td><a title="ctypes.c_ubyte" class="reference internal" href="#ctypes.c_ubyte"><tt class="xref docutils literal">c_ubyte</tt></a></td>
<td><tt class="xref docutils literal">unsigned char</tt></td>
<td>int/long</td>
</tr>
<tr><td><a title="ctypes.c_short" class="reference internal" href="#ctypes.c_short"><tt class="xref docutils literal">c_short</tt></a></td>
<td><tt class="xref docutils literal">short</tt></td>
<td>int/long</td>
</tr>
<tr><td><a title="ctypes.c_ushort" class="reference internal" href="#ctypes.c_ushort"><tt class="xref docutils literal">c_ushort</tt></a></td>
<td><tt class="xref docutils literal">unsigned short</tt></td>
<td>int/long</td>
</tr>
<tr><td><a title="ctypes.c_int" class="reference internal" href="#ctypes.c_int"><tt class="xref docutils literal">c_int</tt></a></td>
<td><tt class="xref docutils literal">int</tt></td>
<td>int/long</td>
</tr>
<tr><td><a title="ctypes.c_uint" class="reference internal" href="#ctypes.c_uint"><tt class="xref docutils literal">c_uint</tt></a></td>
<td><tt class="xref docutils literal">unsigned int</tt></td>
<td>int/long</td>
</tr>
<tr><td><a title="ctypes.c_long" class="reference internal" href="#ctypes.c_long"><tt class="xref docutils literal">c_long</tt></a></td>
<td><tt class="xref docutils literal">long</tt></td>
<td>int/long</td>
</tr>
<tr><td><a title="ctypes.c_ulong" class="reference internal" href="#ctypes.c_ulong"><tt class="xref docutils literal">c_ulong</tt></a></td>
<td><tt class="xref docutils literal">unsigned long</tt></td>
<td>int/long</td>
</tr>
<tr><td><a title="ctypes.c_longlong" class="reference internal" href="#ctypes.c_longlong"><tt class="xref docutils literal">c_longlong</tt></a></td>
<td><tt class="xref docutils literal">__int64</tt> or <tt class="xref docutils literal">long long</tt></td>
<td>int/long</td>
</tr>
<tr><td><a title="ctypes.c_ulonglong" class="reference internal" href="#ctypes.c_ulonglong"><tt class="xref docutils literal">c_ulonglong</tt></a></td>
<td><tt class="xref docutils literal">unsigned __int64</tt> or
<tt class="xref docutils literal">unsigned long long</tt></td>
<td>int/long</td>
</tr>
<tr><td><a title="ctypes.c_float" class="reference internal" href="#ctypes.c_float"><tt class="xref docutils literal">c_float</tt></a></td>
<td><tt class="xref docutils literal">float</tt></td>
<td>float</td>
</tr>
<tr><td><a title="ctypes.c_double" class="reference internal" href="#ctypes.c_double"><tt class="xref docutils literal">c_double</tt></a></td>
<td><tt class="xref docutils literal">double</tt></td>
<td>float</td>
</tr>
<tr><td><a title="ctypes.c_longdouble" class="reference internal" href="#ctypes.c_longdouble"><tt class="xref docutils literal">c_longdouble</tt></a></td>
<td><tt class="xref docutils literal">long double</tt></td>
<td>float</td>
</tr>
<tr><td><a title="ctypes.c_char_p" class="reference internal" href="#ctypes.c_char_p"><tt class="xref docutils literal">c_char_p</tt></a></td>
<td><tt class="xref docutils literal">char *</tt> (NUL terminated)</td>
<td>string or <tt class="xref docutils literal">None</tt></td>
</tr>
<tr><td><a title="ctypes.c_wchar_p" class="reference internal" href="#ctypes.c_wchar_p"><tt class="xref docutils literal">c_wchar_p</tt></a></td>
<td><tt class="xref docutils literal">wchar_t *</tt> (NUL terminated)</td>
<td>unicode or <tt class="xref docutils literal">None</tt></td>
</tr>
<tr><td><a title="ctypes.c_void_p" class="reference internal" href="#ctypes.c_void_p"><tt class="xref docutils literal">c_void_p</tt></a></td>
<td><tt class="xref docutils literal">void *</tt></td>
<td>int/long or <tt class="xref docutils literal">None</tt></td>
</tr>
</tbody>
</table>
All these types can be created by calling them with an optional initializer of
the correct type and value:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; c_int()
c_long(0)
&gt;&gt;&gt; c_char_p(&quot;Hello, World&quot;)
c_char_p(&#39;Hello, World&#39;)
&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-python"><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 title="ctypes.c_char_p" class="reference internal" href="#ctypes.c_char_p"><tt class="xref docutils literal">c_char_p</tt></a>,
<a title="ctypes.c_wchar_p" class="reference internal" href="#ctypes.c_wchar_p"><tt class="xref docutils literal">c_wchar_p</tt></a>, and <a title="ctypes.c_void_p" class="reference internal" href="#ctypes.c_void_p"><tt class="xref docutils literal">c_void_p</tt></a> changes the memory location they
point to, not the contents of the memory block (of course not, because Python
strings are immutable):
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; s = &quot;Hello, World&quot;
&gt;&gt;&gt; c_s = c_char_p(s)
&gt;&gt;&gt; print c_s
c_char_p(&#39;Hello, World&#39;)
&gt;&gt;&gt; c_s.value = &quot;Hi, there&quot;
&gt;&gt;&gt; print c_s
c_char_p(&#39;Hi, there&#39;)
&gt;&gt;&gt; print s # first string 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 title="ctypes.create_string_buffer" class="reference internal" href="#ctypes.create_string_buffer"><tt class="xref docutils literal">create_string_buffer()</tt></a> function which creates these in various ways. The
current memory block contents can be accessed (or changed) with the <tt class="docutils literal">raw</tt>
property; if you want to access it as NUL terminated string, use the <tt class="docutils literal">value</tt>
property:
<div class="highlight-python"><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 &#39;\x00\x00\x00&#39;
&gt;&gt;&gt; p = create_string_buffer(&quot;Hello&quot;) # create a buffer containing a NUL terminated string
&gt;&gt;&gt; print sizeof(p), repr(p.raw)
6 &#39;Hello\x00&#39;
&gt;&gt;&gt; print repr(p.value)
&#39;Hello&#39;
&gt;&gt;&gt; p = create_string_buffer(&quot;Hello&quot;, 10) # create a 10 byte buffer
&gt;&gt;&gt; print sizeof(p), repr(p.raw)
10 &#39;Hello\x00\x00\x00\x00\x00&#39;
&gt;&gt;&gt; p.value = &quot;Hi&quot;
&gt;&gt;&gt; print sizeof(p), repr(p.raw)
10 &#39;Hi\x00lo\x00\x00\x00\x00\x00&#39;
&gt;&gt;&gt;
</pre></div>
</div>
The <a title="ctypes.create_string_buffer" class="reference internal" href="#ctypes.create_string_buffer"><tt class="xref docutils literal">create_string_buffer()</tt></a> function replaces the <tt class="xref docutils literal">c_buffer()</tt> function
(which is still available as an alias), as well as the <tt class="xref docutils literal">c_string()</tt> function
from earlier ctypes releases. To create a mutable memory block containing
unicode characters of the C type <tt class="xref docutils literal">wchar_t</tt> use the
<a title="ctypes.create_unicode_buffer" class="reference internal" href="#ctypes.create_unicode_buffer"><tt class="xref docutils literal">create_unicode_buffer()</tt></a> function.
</div>
<div class="section" id="calling-functions-continued">
<h3>15.15.1.5. 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 title="sys.stdout" class="reference external" href="sys.html#sys.stdout"><tt class="xref docutils literal">sys.stdout</tt></a>, so these examples will only work at the console prompt, not
from within IDLE or PythonWin:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; printf = libc.printf
&gt;&gt;&gt; printf(&quot;Hello, %s\n&quot;, &quot;World!&quot;)
Hello, World!
14
&gt;&gt;&gt; printf(&quot;Hello, %S\n&quot;, u&quot;World!&quot;)
Hello, World!
14
&gt;&gt;&gt; printf(&quot;%d bottles of beer\n&quot;, 42)
42 bottles of beer
19
&gt;&gt;&gt; printf(&quot;%f bottles of beer\n&quot;, 42.5)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
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
unicode strings have to be wrapped in their corresponding <tt class="xref docutils literal">ctypes</tt> type, so
that they can be converted to the required C data type:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; printf(&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>15.15.1.6. 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 <tt class="xref docutils literal">ctypes</tt> argument conversion to allow instances of
your own classes be used as function arguments. <tt class="xref docutils literal">ctypes</tt> looks for an
<tt class="xref docutils literal">_as_parameter_</tt> attribute and uses this as the function argument. Of
course, it must be one of integer, string, or unicode:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; class Bottles(object):
... def __init__(self, number):
... self._as_parameter_ = number
...
&gt;&gt;&gt; bottles = Bottles(42)
&gt;&gt;&gt; printf(&quot;%d bottles of beer\n&quot;, bottles)
42 bottles of beer
19
&gt;&gt;&gt;
</pre></div>
</div>
If you don&#8217;t want to store the instance&#8217;s data in the <tt class="xref docutils literal">_as_parameter_</tt>
instance variable, you could define a <a title="property" class="reference external" href="functions.html#property"><tt class="xref docutils literal">property()</tt></a> which makes the data
available.
</div>
<div class="section" id="specifying-the-required-argument-types-function-prototypes">
<h3>15.15.1.7. 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 <tt class="xref docutils literal">argtypes</tt> attribute.
<tt class="xref docutils literal">argtypes</tt> must be a sequence of C data types (the <tt class="docutils literal">printf</tt> 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-python"><div class="highlight"><pre>&gt;&gt;&gt; printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
&gt;&gt;&gt; printf(&quot;String &#39;%s&#39;, Int %d, Double %f\n&quot;, &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-python"><div class="highlight"><pre>&gt;&gt;&gt; printf(&quot;%d %d %d&quot;, 1, 2, 3)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
ArgumentError: argument 2: exceptions.TypeError: wrong type
&gt;&gt;&gt; printf(&quot;%s %d %f\n&quot;, &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 <tt class="xref docutils literal">from_param()</tt> class method for them to be able to use them
in the <tt class="xref docutils literal">argtypes</tt> sequence. The <tt class="xref docutils literal">from_param()</tt> 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 <tt class="xref docutils literal">_as_parameter_</tt> attribute, or whatever you want to
pass as the C function argument in this case. Again, the result should be an
integer, string, unicode, a <tt class="xref docutils literal">ctypes</tt> instance, or an object with an
<tt class="xref docutils literal">_as_parameter_</tt> attribute.
</div>
<div class="section" id="return-types">
<h3>15.15.1.8. Return types<a class="headerlink" href="#return-types" title="Permalink to this headline">¶</a></h3>
By default functions are assumed to return the C <tt class="xref docutils literal">int</tt> type. Other
return types can be specified by setting the <tt class="xref docutils literal">restype</tt> attribute of the
function object.
Here is a more advanced example, it uses the <tt class="docutils literal">strchr</tt> function, which expects
a string pointer and a char, and returns a pointer to a string:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; strchr = libc.strchr
&gt;&gt;&gt; strchr(&quot;abcdef&quot;, ord(&quot;d&quot;)) # doctest: +SKIP
8059983
&gt;&gt;&gt; strchr.restype = c_char_p # c_char_p is a pointer to a string
&gt;&gt;&gt; strchr(&quot;abcdef&quot;, ord(&quot;d&quot;))
&#39;def&#39;
&gt;&gt;&gt; print strchr(&quot;abcdef&quot;, ord(&quot;x&quot;))
None
&gt;&gt;&gt;
</pre></div>
</div>
If you want to avoid the <tt class="docutils literal">ord(&quot;x&quot;)</tt> calls above, you can set the
<tt class="xref docutils literal">argtypes</tt> attribute, and the second argument will be converted from a
single character Python string into a C char:
<div class="highlight-python"><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(&quot;abcdef&quot;, &quot;d&quot;)
&#39;def&#39;
&gt;&gt;&gt; strchr(&quot;abcdef&quot;, &quot;def&quot;)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
ArgumentError: argument 2: exceptions.TypeError: one character string expected
&gt;&gt;&gt; print strchr(&quot;abcdef&quot;, &quot;x&quot;)
None
&gt;&gt;&gt; strchr(&quot;abcdef&quot;, &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 <tt class="xref docutils literal">restype</tt> 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-python"><div class="highlight"><pre>&gt;&gt;&gt; GetModuleHandle = windll.kernel32.GetModuleHandleA # doctest: +WINDOWS
&gt;&gt;&gt; def ValidHandle(value):
... if value == 0:
... raise WinError()
... return value
...
&gt;&gt;&gt;
&gt;&gt;&gt; GetModuleHandle.restype = ValidHandle # doctest: +WINDOWS
&gt;&gt;&gt; GetModuleHandle(None) # doctest: +WINDOWS
486539264
&gt;&gt;&gt; GetModuleHandle(&quot;something silly&quot;) # doctest: +WINDOWS
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
 File &quot;&lt;stdin&gt;&quot;, line 3, in ValidHandle
WindowsError: [Errno 126] The specified module could not be found.
&gt;&gt;&gt;
</pre></div>
</div>
<tt class="docutils literal">WinError</tt> is a function which will call Windows <tt class="docutils literal">FormatMessage()</tt> api to
get the string representation of an error code, and returns an exception.
<tt class="docutils literal">WinError</tt> takes an optional error code parameter, if no one is used, it calls
<a title="ctypes.GetLastError" class="reference internal" href="#ctypes.GetLastError"><tt class="xref docutils literal">GetLastError()</tt></a> to retrieve it.
Please note that a much more powerful error checking mechanism is available
through the <tt class="xref docutils literal">errcheck</tt> attribute; see the reference manual for details.
</div>
<div class="section" id="passing-pointers-or-passing-parameters-by-reference">
<h3>15.15.1.9. 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.
<tt class="xref docutils literal">ctypes</tt> exports the <a title="ctypes.byref" class="reference internal" href="#ctypes.byref"><tt class="xref docutils literal">byref()</tt></a> function which is used to pass
parameters by reference. The same effect can be achieved with the
<a title="ctypes.pointer" class="reference internal" href="#ctypes.pointer"><tt class="xref docutils literal">pointer()</tt></a> function, although <a title="ctypes.pointer" class="reference internal" href="#ctypes.pointer"><tt class="xref docutils literal">pointer()</tt></a> does a lot more work since it
constructs a real pointer object, so it is faster to use <a title="ctypes.byref" class="reference internal" href="#ctypes.byref"><tt class="xref docutils literal">byref()</tt></a> if you
don&#8217;t need the pointer object in Python itself:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; i = c_int()
&gt;&gt;&gt; f = c_float()
&gt;&gt;&gt; s = create_string_buffer(&#39;\000&#39; * 32)
&gt;&gt;&gt; print i.value, f.value, repr(s.value)
0 0.0 &#39;&#39;
&gt;&gt;&gt; libc.sscanf(&quot;1 3.14 Hello&quot;, &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 &#39;Hello&#39;
&gt;&gt;&gt;
</pre></div>
</div>
</div>
<div class="section" id="structures-and-unions">
<h3>15.15.1.10. 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 title="ctypes.Structure" class="reference internal" href="#ctypes.Structure"><tt class="xref docutils literal">Structure</tt></a> and <a title="ctypes.Union" class="reference internal" href="#ctypes.Union"><tt class="xref docutils literal">Union</tt></a>
base classes which are defined in the <tt class="xref docutils literal">ctypes</tt> module. Each subclass must
define a <tt class="xref docutils literal">_fields_</tt> attribute. <tt class="xref docutils literal">_fields_</tt> must be a list of
2-tuples, containing a field name and a field type.
The field type must be a <tt class="xref docutils literal">ctypes</tt> type like <a title="ctypes.c_int" class="reference internal" href="#ctypes.c_int"><tt class="xref docutils literal">c_int</tt></a>, or any other
derived <tt class="xref docutils literal">ctypes</tt> 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-python"><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 ?
ValueError: too many initializers
&gt;&gt;&gt;
</pre></div>
</div>
You can, however, build much more complicated structures. Structures 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-python"><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-python"><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 external" 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-python"><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>
<div class="section" id="structure-union-alignment-and-byte-order">
<h3>15.15.1.11. 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
<tt class="xref docutils literal">_pack_</tt> 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 <tt class="docutils literal">#pragma pack(n)</tt> also does in MSVC.
<tt class="xref docutils literal">ctypes</tt> uses the native byte order for Structures and Unions. To build
structures with non-native byte order, you can use one of the
<a title="ctypes.BigEndianStructure" class="reference internal" href="#ctypes.BigEndianStructure"><tt class="xref docutils literal">BigEndianStructure</tt></a>, <a title="ctypes.LittleEndianStructure" class="reference internal" href="#ctypes.LittleEndianStructure"><tt class="xref docutils literal">LittleEndianStructure</tt></a>,
<tt class="xref docutils literal">BigEndianUnion</tt>, and <tt class="xref docutils literal">LittleEndianUnion</tt> base classes. These
classes cannot contain pointer fields.
</div>
<div class="section" id="bit-fields-in-structures-and-unions">
<h3>15.15.1.12. 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 <tt class="xref docutils literal">_fields_</tt> tuples:
<div class="highlight-python"><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>15.15.1.13. 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-python"><div class="highlight"><pre>TenPointsArrayType = POINT * 10
</pre></div>
</div>
Here is an example of an somewhat artificial data type, a structure containing 4
POINTs among other stuff:
<div class="highlight-python"><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-python"><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 <tt class="docutils literal">0 0</tt> lines, because the array contents is
initialized to zeros.
Initializers of the correct type can also be specified:
<div class="highlight-python"><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,
...
1 2 3 4 5 6 7 8 9 10
&gt;&gt;&gt;
</pre></div>
</div>
</div>
<div class="section" id="pointers">
<h3>15.15.1.14. Pointers<a class="headerlink" href="#pointers" title="Permalink to this headline">¶</a></h3>
Pointer instances are created by calling the <a title="ctypes.pointer" class="reference internal" href="#ctypes.pointer"><tt class="xref docutils literal">pointer()</tt></a> function on a
<tt class="xref docutils literal">ctypes</tt> type:
<div class="highlight-python"><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 <tt class="xref docutils literal">contents</tt> attribute which returns the object to
which the pointer points, the <tt class="docutils literal">i</tt> object above:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; pi.contents
c_long(42)
&gt;&gt;&gt;
</pre></div>
</div>
Note that <tt class="xref docutils literal">ctypes</tt> does not have OOR (original object return), it constructs a
new, equivalent object each time you retrieve an attribute:
<div class="highlight-python"><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 title="ctypes.c_int" class="reference internal" href="#ctypes.c_int"><tt class="xref docutils literal">c_int</tt></a> instance to the pointer&#8217;s contents attribute
would cause the pointer to point to the memory location where this is stored:
<div class="highlight-python"><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-python"><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-python"><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&#8217;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 title="ctypes.pointer" class="reference internal" href="#ctypes.pointer"><tt class="xref docutils literal">pointer()</tt></a> function does more than simply create
pointer instances, it has to create pointer types first. This is done with
the <a title="ctypes.POINTER" class="reference internal" href="#ctypes.POINTER"><tt class="xref docutils literal">POINTER()</tt></a> function, which accepts any <tt class="xref docutils literal">ctypes</tt> type, and returns
a new type:
<div class="highlight-python"><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 ?
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 <tt class="docutils literal">NULL</tt> pointer.
<tt class="docutils literal">NULL</tt> pointers have a <tt class="xref docutils literal">False</tt> boolean value:
<div class="highlight-python"><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>
<tt class="xref docutils literal">ctypes</tt> checks for <tt class="docutils literal">NULL</tt> when dereferencing pointers (but dereferencing
invalid non-<tt class="docutils literal">NULL</tt> pointers would crash Python):
<div class="highlight-python"><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>15.15.1.15. 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
<tt class="docutils literal">POINTER(c_int)</tt> in the <tt class="xref docutils literal">argtypes</tt> 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 <tt class="docutils literal">POINTER(c_int)</tt>, ctypes accepts an array of c_int:
<div class="highlight-python"><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>
To set a POINTER type field to <tt class="docutils literal">NULL</tt>, you can assign <tt class="xref docutils literal">None</tt>:
<div class="highlight-python"><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. <tt class="xref docutils literal">ctypes</tt> provides a <a title="ctypes.cast" class="reference internal" href="#ctypes.cast"><tt class="xref docutils literal">cast()</tt></a> function which can be
used in the same way. The <tt class="docutils literal">Bar</tt> structure defined above accepts
<tt class="docutils literal">POINTER(c_int)</tt> pointers or <a title="ctypes.c_int" class="reference internal" href="#ctypes.c_int"><tt class="xref docutils literal">c_int</tt></a> arrays for its <tt class="docutils literal">values</tt> field,
but not instances of other types:
<div class="highlight-python"><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 ?
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 title="ctypes.cast" class="reference internal" href="#ctypes.cast"><tt class="xref docutils literal">cast()</tt></a> function is handy.
The <a title="ctypes.cast" class="reference internal" href="#ctypes.cast"><tt class="xref docutils literal">cast()</tt></a> function can be used to cast a ctypes instance into a pointer
to a different ctypes data type. <a title="ctypes.cast" class="reference internal" href="#ctypes.cast"><tt class="xref docutils literal">cast()</tt></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-python"><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 title="ctypes.cast" class="reference internal" href="#ctypes.cast"><tt class="xref docutils literal">cast()</tt></a> can be used to assign to the <tt class="docutils literal">values</tt> field of <tt class="docutils literal">Bar</tt> the
structure:
<div class="highlight-python"><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>15.15.1.16. 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-python"><pre>struct cell; /* forward declaration */

struct {
 char *name;
 struct cell *next;
} cell;</pre>
</div>
The straightforward translation into ctypes code would be this, but it does not
work:
<div class="highlight-python"><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 ?
 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 <tt class="docutils literal">class cell</tt> is not available in the class statement itself.
In <tt class="xref docutils literal">ctypes</tt>, we can define the <tt class="docutils literal">cell</tt> class and set the <tt class="xref docutils literal">_fields_</tt>
attribute later, after the class statement:
<div class="highlight-python"><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 <tt class="docutils literal">cell</tt>, and let them point to each
other, and finally follow the pointer chain a few times:
<div class="highlight-python"><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,
... 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>15.15.1.17. Callback functions<a class="headerlink" href="#callback-functions" title="Permalink to this headline">¶</a></h3>
<tt class="xref docutils literal">ctypes</tt> allows to create 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 CFUNCTYPE factory function creates types for callback functions using the
normal cdecl calling convention, and, on Windows, the WINFUNCTYPE factory
function creates types for callback functions using the stdcall 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&#8217;s <tt class="xref docutils literal">qsort()</tt>
function, this is used to sort items with the help of a callback function.
<tt class="xref docutils literal">qsort()</tt> will be used to sort an array of integers:
<div class="highlight-python"><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>
<tt class="xref docutils literal">qsort()</tt> 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 else.
So our callback function receives pointers to integers, and must return an
integer. First we create the <tt class="docutils literal">type</tt> for the callback function:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; CMPFUNC = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
&gt;&gt;&gt;
</pre></div>
</div>
For the first implementation of the callback function, we simply print the
arguments we get, and return 0 (incremental development ;-):
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def py_cmp_func(a, b):
... print &quot;py_cmp_func&quot;, a, b
... return 0
...
&gt;&gt;&gt;
</pre></div>
</div>
Create the C callable callback:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; cmp_func = CMPFUNC(py_cmp_func)
&gt;&gt;&gt;
</pre></div>
</div>
And we&#8217;re ready to go:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; qsort(ia, len(ia), sizeof(c_int), cmp_func) # doctest: +WINDOWS
py_cmp_func &lt;ctypes.LP_c_long object at 0x00...&gt; &lt;ctypes.LP_c_long object at 0x00...&gt;
py_cmp_func &lt;ctypes.LP_c_long object at 0x00...&gt; &lt;ctypes.LP_c_long object at 0x00...&gt;
py_cmp_func &lt;ctypes.LP_c_long object at 0x00...&gt; &lt;ctypes.LP_c_long object at 0x00...&gt;
py_cmp_func &lt;ctypes.LP_c_long object at 0x00...&gt; &lt;ctypes.LP_c_long object at 0x00...&gt;
py_cmp_func &lt;ctypes.LP_c_long object at 0x00...&gt; &lt;ctypes.LP_c_long object at 0x00...&gt;
py_cmp_func &lt;ctypes.LP_c_long object at 0x00...&gt; &lt;ctypes.LP_c_long object at 0x00...&gt;
py_cmp_func &lt;ctypes.LP_c_long object at 0x00...&gt; &lt;ctypes.LP_c_long object at 0x00...&gt;
py_cmp_func &lt;ctypes.LP_c_long object at 0x00...&gt; &lt;ctypes.LP_c_long object at 0x00...&gt;
py_cmp_func &lt;ctypes.LP_c_long object at 0x00...&gt; &lt;ctypes.LP_c_long object at 0x00...&gt;
py_cmp_func &lt;ctypes.LP_c_long object at 0x00...&gt; &lt;ctypes.LP_c_long object at 0x00...&gt;
&gt;&gt;&gt;
</pre></div>
</div>
We know how to access the contents of a pointer, so lets redefine our callback:
<div class="highlight-python"><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>
Here is what we get on Windows:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; qsort(ia, len(ia), sizeof(c_int), cmp_func) # doctest: +WINDOWS
py_cmp_func 7 1
py_cmp_func 33 1
py_cmp_func 99 1
py_cmp_func 5 1
py_cmp_func 7 5
py_cmp_func 33 5
py_cmp_func 99 5
py_cmp_func 7 99
py_cmp_func 33 99
py_cmp_func 7 33
&gt;&gt;&gt;
</pre></div>
</div>
It is funny to see that on linux the sort function seems to work much more
efficiently, it is doing less comparisons:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; qsort(ia, len(ia), sizeof(c_int), cmp_func) # doctest: +LINUX
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>
Ah, we&#8217;re nearly done! The last step is to actually compare the two items and
return a useful result:
<div class="highlight-python"><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;
</pre></div>
</div>
Final run on Windows:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func)) # doctest: +WINDOWS
py_cmp_func 33 7
py_cmp_func 99 33
py_cmp_func 5 99
py_cmp_func 1 99
py_cmp_func 33 7
py_cmp_func 1 33
py_cmp_func 5 33
py_cmp_func 5 7
py_cmp_func 1 7
py_cmp_func 5 1
&gt;&gt;&gt;
</pre></div>
</div>
and on Linux:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func)) # doctest: +LINUX
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>
It is quite interesting to see that the Windows <tt class="xref docutils literal">qsort()</tt> function needs
more comparisons than the linux version!
As we can easily check, our array is sorted now:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; for i in ia: print i,
...
1 5 7 33 99
&gt;&gt;&gt;
</pre></div>
</div>
Important note for callback functions:
Make sure you keep references to CFUNCTYPE objects as long as they are used from
C code. <tt class="xref docutils literal">ctypes</tt> doesn&#8217;t, and if you don&#8217;t, they may be garbage collected,
crashing your program when a callback is made.
</div>
<div class="section" id="accessing-values-exported-from-dlls">
<h3>15.15.1.18. 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 <tt class="docutils literal">Py_OptimizeFlag</tt>, an integer set
to 0, 1, or 2, depending on the <a class="reference external" href="../using/cmdline.html#cmdoption-O">-O</a> or <a class="reference external" href="../using/cmdline.html#cmdoption-OO">-OO</a> flag given on
startup.
<tt class="xref docutils literal">ctypes</tt> can access values like this with the <tt class="xref docutils literal">in_dll()</tt> class methods of
the type. pythonapi is a predefined symbol giving access to the Python C
api:
<div class="highlight-python"><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 external" href="../using/cmdline.html#cmdoption-O">-O</a>, the sample would
have printed <tt class="docutils literal">c_long(1)</tt>, or <tt class="docutils literal">c_long(2)</tt> if <a class="reference external" href="../using/cmdline.html#cmdoption-OO">-OO</a> would have been
specified.
An extended example which also demonstrates the use of pointers accesses the
<tt class="docutils literal">PyImport_FrozenModules</tt> pointer exported by Python.
Quoting the Python docs: This pointer is initialized to point to an array of
&#8220;struct _frozen&#8221; 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.
So manipulating this pointer could even prove useful. To restrict the example
size, we show only how this table can be read with <tt class="xref docutils literal">ctypes</tt>:
<div class="highlight-python"><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 <tt class="docutils literal">struct _frozen</tt> data type, so we can get the pointer to
the table:
<div class="highlight-python"><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 <tt class="docutils literal">table</tt> is a <tt class="docutils literal">pointer</tt> to the array of <tt class="docutils literal">struct_frozen</tt> 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&#8217;s better to break out of the loop when we
hit the NULL entry:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; for item in table:
... print item.name, item.size
... if item.name is None:
... break
...
__hello__ 104
__phello__ -104
__phello__.spam 104
None 0
&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 <tt class="docutils literal">import __hello__</tt> for example.
</div>
<div class="section" id="surprises">
<h3>15.15.1.19. Surprises<a class="headerlink" href="#surprises" title="Permalink to this headline">¶</a></h3>
There are some edges in <tt class="xref docutils literal">ctypes</tt> where you may be expect something else than
what actually happens.
Consider the following example:
<div class="highlight-python"><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 <tt class="docutils literal">3 4 1 2</tt>. What
happened? Here are the steps of the <tt class="docutils literal">rc.a, rc.b = rc.b, rc.a</tt> line above:
<div class="highlight-python"><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 <tt class="docutils literal">temp0</tt> and <tt class="docutils literal">temp1</tt> are objects still using the internal buffer of
the <tt class="docutils literal">rc</tt> object above. So executing <tt class="docutils literal">rc.a = temp0</tt> copies the buffer
contents of <tt class="docutils literal">temp0</tt> into <tt class="docutils literal">rc</tt> &#8216;s buffer. This, in turn, changes the
contents of <tt class="docutils literal">temp1</tt>. So, the last assignment <tt class="docutils literal">rc.b = temp1</tt>, doesn&#8217;t have
the expected effect.
Keep in mind that retrieving sub-objects from Structure, Unions, and Arrays
doesn&#8217;t copy the sub-object, instead it retrieves a wrapper object accessing
the root-object&#8217;s underlying buffer.
Another example that may behave different from what one would expect is this:
<div class="highlight-python"><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 <tt class="xref docutils literal">False</tt>? ctypes instances are objects containing a memory
block plus some <a class="reference external" 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 <tt class="docutils literal">contents</tt> 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>15.15.1.20. Variable-sized data types<a class="headerlink" href="#variable-sized-data-types" title="Permalink to this headline">¶</a></h3>
<tt class="xref docutils literal">ctypes</tt> provides some support for variable-sized arrays and structures.
The <a title="ctypes.resize" class="reference internal" href="#ctypes.resize"><tt class="xref docutils literal">resize()</tt></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 title="exceptions.ValueError" class="reference external" href="exceptions.html#exceptions.ValueError"><tt class="xref docutils literal">ValueError</tt></a> is raised if this is tried:
<div class="highlight-python"><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-python"><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 <tt class="xref docutils literal">ctypes</tt> 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>15.15.2. ctypes reference<a class="headerlink" href="#ctypes-reference" title="Permalink to this headline">¶</a></h2>
<div class="section" id="finding-shared-libraries">
<h3>15.15.2.1. 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 <tt class="xref docutils literal">find_library()</tt> function is to locate a library in a way
similar to what the compiler 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 <tt class="xref docutils literal">ctypes.util</tt> module provides a function which can help to determine the
library to load.
<dl class="data">
<dt>
<tt class="descclassname">ctypes.util.</tt><tt class="descname">find_library</tt><big>(</big>name<big>)</big></dt>
<dd>Try to find a library and return a pathname. name is the library name without
any prefix like lib, suffix like <tt class="docutils literal">.so</tt>, <tt class="docutils literal">.dylib</tt> or version number (this
is the form used for the posix linker option -l). If no library can
be found, returns <tt class="xref docutils literal">None</tt>.</dd></dl>

The exact functionality is system dependent.
On Linux, <tt class="xref docutils literal">find_library()</tt> tries to run external programs
(<tt class="docutils literal">/sbin/ldconfig</tt>, <tt class="docutils literal">gcc</tt>, and <tt class="docutils literal">objdump</tt>) to find the library file. It
returns the filename of the library file. Here are some examples:
<div class="highlight-python"><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, <tt class="xref docutils literal">find_library()</tt> tries several predefined naming schemes and paths
to locate the library, and returns a full pathname if successful:
<div class="highlight-python"><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, <tt class="xref docutils literal">find_library()</tt> searches along the system search path, and
returns the full pathname, but since there is no predefined naming scheme a call
like <tt class="docutils literal">find_library(&quot;c&quot;)</tt> will fail and return <tt class="xref docutils literal">None</tt>.
If wrapping a shared library with <tt class="xref docutils literal">ctypes</tt>, it may be better to determine
the shared library name at development type, and hardcode that into the wrapper
module instead of using <tt class="xref docutils literal">find_library()</tt> to locate the library at runtime.
</div>
<div class="section" id="loading-shared-libraries">
<h3>15.15.2.2. Loading shared libraries<a class="headerlink" href="#loading-shared-libraries" title="Permalink to this headline">¶</a></h3>
There are several ways to loaded shared libraries into the Python process. One
way is to instantiate one of the following classes:
<dl class="class">
<dt id="ctypes.CDLL">
class <tt class="descclassname">ctypes.</tt><tt class="descname">CDLL</tt><big>(</big>name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False<big>)</big><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
<tt class="xref docutils literal">int</tt>.</dd></dl>

<dl class="class">
<dt id="ctypes.OleDLL">
class <tt class="descclassname">ctypes.</tt><tt class="descname">OleDLL</tt><big>(</big>name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False<big>)</big><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 <tt class="docutils literal">stdcall</tt> calling convention, and are
assumed to return the windows specific <a title="ctypes.HRESULT" class="reference internal" href="#ctypes.HRESULT"><tt class="xref docutils literal">HRESULT</tt></a> code. <a title="ctypes.HRESULT" class="reference internal" href="#ctypes.HRESULT"><tt class="xref docutils literal">HRESULT</tt></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 <tt class="xref docutils literal">WindowsError</tt> is automatically raised.</dd></dl>

<dl class="class">
<dt id="ctypes.WinDLL">
class <tt class="descclassname">ctypes.</tt><tt class="descname">WinDLL</tt><big>(</big>name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False<big>)</big><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 <tt class="docutils literal">stdcall</tt> calling convention, and are
assumed to return <tt class="xref docutils literal">int</tt> by default.
On Windows CE only the standard calling convention is used, for convenience the
<a title="ctypes.WinDLL" class="reference internal" href="#ctypes.WinDLL"><tt class="xref docutils literal">WinDLL</tt></a> and <a title="ctypes.OleDLL" class="reference internal" href="#ctypes.OleDLL"><tt class="xref docutils literal">OleDLL</tt></a> use the standard calling convention on this
platform.
</dd></dl>

The Python <a class="reference external" 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 <tt class="descclassname">ctypes.</tt><tt class="descname">PyDLL</tt><big>(</big>name, mode=DEFAULT_MODE, handle=None<big>)</big><a class="headerlink" href="#ctypes.PyDLL" title="Permalink to this definition">¶</a></dt>
<dd>Instances of this class behave like <a title="ctypes.CDLL" class="reference internal" href="#ctypes.CDLL"><tt class="xref docutils literal">CDLL</tt></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 <tt class="docutils literal">handle</tt> named
parameter, otherwise the underlying platforms <tt class="docutils literal">dlopen</tt> or <tt class="docutils literal">LoadLibrary</tt>
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.
The use_errno parameter, when set to True, enables a ctypes mechanism that
allows to access the system <tt class="xref docutils literal">errno</tt> error number in a safe way.
<tt class="xref docutils literal">ctypes</tt> maintains a thread-local copy of the systems <tt class="xref docutils literal">errno</tt>
variable; if you call foreign functions created with <tt class="docutils literal">use_errno=True</tt> then the
<tt class="xref docutils literal">errno</tt> value before the function call is swapped with the ctypes private
copy, the same happens immediately after the function call.
The function <a title="ctypes.get_errno" class="reference internal" href="#ctypes.get_errno"><tt class="xref docutils literal">ctypes.get_errno()</tt></a> returns the value of the ctypes private
copy, and the function <a title="ctypes.set_errno" class="reference internal" href="#ctypes.set_errno"><tt class="xref docutils literal">ctypes.set_errno()</tt></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 title="ctypes.GetLastError" class="reference internal" href="#ctypes.GetLastError"><tt class="xref docutils literal">GetLastError()</tt></a> and
<tt class="xref docutils literal">SetLastError()</tt> Windows API functions; <a title="ctypes.get_last_error" class="reference internal" href="#ctypes.get_last_error"><tt class="xref docutils literal">ctypes.get_last_error()</tt></a> and
<a title="ctypes.set_last_error" class="reference internal" href="#ctypes.set_last_error"><tt class="xref docutils literal">ctypes.set_last_error()</tt></a> are used to request and change the ctypes private
copy of the windows error code.

New in version 2.6: The use_last_error and use_errno optional parameters were added.
<dl class="data">
<dt>
<tt class="descclassname">ctypes.</tt><tt class="descname">RTLD_GLOBAL</tt></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>
<tt class="descclassname">ctypes.</tt><tt class="descname">RTLD_LOCAL</tt></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>
<tt class="descclassname">ctypes.</tt><tt class="descname">DEFAULT_MODE</tt></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, however <a title="object.__getattr__" class="reference external" href="../reference/datamodel.html#object.__getattr__"><tt class="xref docutils literal">__getattr__()</tt></a>
and <a title="object.__getitem__" class="reference external" href="../reference/datamodel.html#object.__getitem__"><tt class="xref docutils literal">__getitem__()</tt></a> have special behavior: functions exported by the shared
library can be accessed as attributes of by index. Please note that both
<a title="object.__getattr__" class="reference external" href="../reference/datamodel.html#object.__getattr__"><tt class="xref docutils literal">__getattr__()</tt></a> and <a title="object.__getitem__" class="reference external" href="../reference/datamodel.html#object.__getitem__"><tt class="xref docutils literal">__getitem__()</tt></a> cache their result, so calling them
repeatedly returns the same object each time.
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">
<tt class="descclassname">PyDLL.</tt><tt class="descname">_handle</tt><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">
<tt class="descclassname">PyDLL.</tt><tt class="descname">_name</tt><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 title="ctypes.LibraryLoader" class="reference internal" href="#ctypes.LibraryLoader"><tt class="xref docutils literal">LibraryLoader</tt></a> class, either by calling the
<tt class="xref docutils literal">LoadLibrary()</tt> method, or by retrieving the library as attribute of the
loader instance.
<dl class="class">
<dt id="ctypes.LibraryLoader">
class <tt class="descclassname">ctypes.</tt><tt class="descname">LibraryLoader</tt><big>(</big>dlltype<big>)</big><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 title="ctypes.CDLL" class="reference internal" href="#ctypes.CDLL"><tt class="xref docutils literal">CDLL</tt></a>, <a title="ctypes.PyDLL" class="reference internal" href="#ctypes.PyDLL"><tt class="xref docutils literal">PyDLL</tt></a>, <a title="ctypes.WinDLL" class="reference internal" href="#ctypes.WinDLL"><tt class="xref docutils literal">WinDLL</tt></a>, or <a title="ctypes.OleDLL" class="reference internal" href="#ctypes.OleDLL"><tt class="xref docutils literal">OleDLL</tt></a> types.
<a title="object.__getattr__" class="reference external" href="../reference/datamodel.html#object.__getattr__"><tt class="xref docutils literal">__getattr__()</tt></a> has special behavior: It allows to load 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">
<tt class="descname">LoadLibrary</tt><big>(</big>name<big>)</big><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>
<tt class="descclassname">ctypes.</tt><tt class="descname">cdll</tt></dt>
<dd>Creates <a title="ctypes.CDLL" class="reference internal" href="#ctypes.CDLL"><tt class="xref docutils literal">CDLL</tt></a> instances.</dd></dl>

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

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

<dl class="data">
<dt>
<tt class="descclassname">ctypes.</tt><tt class="descname">pydll</tt></dt>
<dd>Creates <a title="ctypes.PyDLL" class="reference internal" href="#ctypes.PyDLL"><tt class="xref docutils literal">PyDLL</tt></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>
<tt class="descclassname">ctypes.</tt><tt class="descname">pythonapi</tt></dt>
<dd>An instance of <a title="ctypes.PyDLL" class="reference internal" href="#ctypes.PyDLL"><tt class="xref docutils literal">PyDLL</tt></a> that exposes Python C API functions as
attributes. Note that all these functions are assumed to return C
<tt class="xref docutils literal">int</tt>, which is of course not always the truth, so you have to assign
the correct <tt class="xref docutils literal">restype</tt> attribute to use these functions.</dd></dl>

</div>
<div class="section" id="foreign-functions">
<h3>15.15.2.3. 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 <tt class="descclassname">ctypes.</tt><tt class="descname">_FuncPtr</tt><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">
<tt class="descname">restype</tt><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 <tt class="xref docutils literal">None</tt> for <tt class="xref docutils literal">void</tt>, 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 <tt class="xref docutils literal">int</tt>, and
the callable will be called with this integer, allowing to do further
processing or error checking. Using this is deprecated, for more flexible
post processing or error checking use a ctypes data type as
<a title="ctypes._FuncPtr.restype" class="reference internal" href="#ctypes._FuncPtr.restype"><tt class="xref docutils literal">restype</tt></a> and assign a callable to the <a title="ctypes._FuncPtr.errcheck" class="reference internal" href="#ctypes._FuncPtr.errcheck"><tt class="xref docutils literal">errcheck</tt></a> attribute.
</dd></dl>

<dl class="attribute">
<dt id="ctypes._FuncPtr.argtypes">
<tt class="descname">argtypes</tt><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 <tt class="docutils literal">stdcall</tt> 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
<tt class="xref docutils literal">from_param()</tt> class method of the items in the <a title="ctypes._FuncPtr.argtypes" class="reference internal" href="#ctypes._FuncPtr.argtypes"><tt class="xref docutils literal">argtypes</tt></a>
tuple, this method allows to adapt the actual argument to an object that
the foreign function accepts. For example, a <a title="ctypes.c_char_p" class="reference internal" href="#ctypes.c_char_p"><tt class="xref docutils literal">c_char_p</tt></a> item in
the <a title="ctypes._FuncPtr.argtypes" class="reference internal" href="#ctypes._FuncPtr.argtypes"><tt class="xref docutils literal">argtypes</tt></a> tuple will convert a unicode string passed as
argument into an byte string 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 <tt class="xref docutils literal">from_param()</tt> method which returns a
value usable as argument (integer, string, ctypes instance). This allows
to define adapters that can adapt custom objects as function parameters.
</dd></dl>

<dl class="attribute">
<dt id="ctypes._FuncPtr.errcheck">
<tt class="descname">errcheck</tt><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>
<tt class="descname">callable</tt><big>(</big>result, func, arguments<big>)</big></dt>
<dd>result is what the foreign function returns, as specified by the
<a title="ctypes._FuncPtr.restype" class="reference internal" href="#ctypes._FuncPtr.restype"><tt class="xref docutils literal">restype</tt></a> attribute.
func is the foreign function object itself, this allows to reuse 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 to specialize 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 <tt class="descclassname">ctypes.</tt><tt class="descname">ArgumentError</tt><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>15.15.2.4. 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.
<dl class="function">
<dt id="ctypes.CFUNCTYPE">
<tt class="descclassname">ctypes.</tt><tt class="descname">CFUNCTYPE</tt><big>(</big>restype, *argtypes, use_errno=False, use_last_error=False<big>)</big><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
<tt class="xref docutils literal">errno</tt> variable is exchanged with the real <tt class="xref docutils literal">errno</tt> value before
and after the call; use_last_error does the same for the Windows error
code.

Changed in version 2.6: The optional use_errno and use_last_error parameters were added.
</dd></dl>

<dl class="function">
<dt id="ctypes.WINFUNCTYPE">
<tt class="descclassname">ctypes.</tt><tt class="descname">WINFUNCTYPE</tt><big>(</big>restype, *argtypes, use_errno=False, use_last_error=False<big>)</big><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
<tt class="docutils literal">stdcall</tt> calling convention, except on Windows CE where
<a title="ctypes.WINFUNCTYPE" class="reference internal" href="#ctypes.WINFUNCTYPE"><tt class="xref docutils literal">WINFUNCTYPE()</tt></a> is the same as <a title="ctypes.CFUNCTYPE" class="reference internal" href="#ctypes.CFUNCTYPE"><tt class="xref docutils literal">CFUNCTYPE()</tt></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">
<tt class="descclassname">ctypes.</tt><tt class="descname">PYFUNCTYPE</tt><big>(</big>restype, *argtypes<big>)</big><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>
<dl class="function">
<dt>
<tt class="descname">prototype</tt><big>(</big>address<big>)</big></dt>
<dd>Returns a foreign function at the specified address which must be an integer.</dd></dl>

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

<dl class="function">
<dt>
<tt class="descname">prototype</tt><big>(</big>func_spec[, paramflags]<big>)</big></dt>
<dd>Returns a foreign function exported by a shared library. func_spec must be a
2-tuple <tt class="docutils literal">(name_or_ordinal, library)</tt>. 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>
<tt class="descname">prototype</tt><big>(</big>vtbl_index, name[, paramflags[, iid]]<big>)</big></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 <tt class="xref docutils literal">argtypes</tt> 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 <tt class="xref docutils literal">argtypes</tt>.
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>
<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>
</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.
</blockquote>
This example demonstrates how to wrap the Windows <tt class="docutils literal">MessageBoxA</tt> function so
that it supports default parameters and named arguments. The C declaration from
the windows header file is this:
<div class="highlight-python"><pre>WINUSERAPI int WINAPI
MessageBoxA(
 HWND hWnd ,
 LPCSTR lpText,
 LPCSTR lpCaption,
 UINT uType);</pre>
</div>
Here is the wrapping with <tt class="xref docutils literal">ctypes</tt>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; from ctypes import c_int, WINFUNCTYPE, windll
&gt;&gt;&gt; from ctypes.wintypes import HWND, LPCSTR, UINT
&gt;&gt;&gt; prototype = WINFUNCTYPE(c_int, HWND, LPCSTR, LPCSTR, UINT)
&gt;&gt;&gt; paramflags = (1, &quot;hwnd&quot;, 0), (1, &quot;text&quot;, &quot;Hi&quot;), (1, &quot;caption&quot;, None), (1, &quot;flags&quot;, 0)
&gt;&gt;&gt; MessageBox = prototype((&quot;MessageBoxA&quot;, windll.user32), paramflags)
&gt;&gt;&gt;
</pre></div>
</div>
The MessageBox foreign function can now be called in these ways:
<div class="highlight-python"><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;)
&gt;&gt;&gt;
</pre></div>
</div>
A second example demonstrates output parameters. The win32 <tt class="docutils literal">GetWindowRect</tt>
function retrieves the dimensions of a specified window by copying them into
<tt class="docutils literal">RECT</tt> structure that the caller has to supply. Here is the C declaration:
<div class="highlight-python"><pre>WINUSERAPI BOOL WINAPI
GetWindowRect(
 HWND hWnd,
 LPRECT lpRect);</pre>
</div>
Here is the wrapping with <tt class="xref docutils literal">ctypes</tt>:
<div class="highlight-python"><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 <tt class="xref docutils literal">errcheck</tt> protocol to do
further output processing and error checking. The win32 <tt class="docutils literal">GetWindowRect</tt> api
function returns a <tt class="docutils literal">BOOL</tt> 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-python"><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 <tt class="xref docutils literal">errcheck</tt> function returns the argument tuple it receives
unchanged, <tt class="xref docutils literal">ctypes</tt> continues the normal processing it does on the output
parameters. If you want to return a tuple of window coordinates instead of a
<tt class="docutils literal">RECT</tt> instance, you can retrieve the fields in the function and return them
instead, the normal processing will no longer take place:
<div class="highlight-python"><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>15.15.2.5. Utility functions<a class="headerlink" href="#utility-functions" title="Permalink to this headline">¶</a></h3>
<dl class="function">
<dt id="ctypes.addressof">
<tt class="descclassname">ctypes.</tt><tt class="descname">addressof</tt><big>(</big>obj<big>)</big><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">
<tt class="descclassname">ctypes.</tt><tt class="descname">alignment</tt><big>(</big>obj_or_type<big>)</big><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">
<tt class="descclassname">ctypes.</tt><tt class="descname">byref</tt><big>(</big>obj[, offset]<big>)</big><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.
<tt class="docutils literal">byref(obj, offset)</tt> corresponds to this C code:
<div class="highlight-python"><pre>(((char *)&amp;obj) + offset)</pre>
</div>
The returned object can only be used as a foreign function call
parameter. It behaves similar to <tt class="docutils literal">pointer(obj)</tt>, but the
construction is a lot faster.

New in version 2.6: The offset optional argument was added.
</dd></dl>

<dl class="function">
<dt id="ctypes.cast">
<tt class="descclassname">ctypes.</tt><tt class="descname">cast</tt><big>(</big>obj, type<big>)</big><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">
<tt class="descclassname">ctypes.</tt><tt class="descname">create_string_buffer</tt><big>(</big>init_or_size[, size]<big>)</big><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 title="ctypes.c_char" class="reference internal" href="#ctypes.c_char"><tt class="xref docutils literal">c_char</tt></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
to specify the size of the array if the length of the string should not be used.
If the first parameter is a unicode string, it is converted into an 8-bit string
according to ctypes conversion rules.
</dd></dl>

<dl class="function">
<dt id="ctypes.create_unicode_buffer">
<tt class="descclassname">ctypes.</tt><tt class="descname">create_unicode_buffer</tt><big>(</big>init_or_size[, size]<big>)</big><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 title="ctypes.c_wchar" class="reference internal" href="#ctypes.c_wchar"><tt class="xref docutils literal">c_wchar</tt></a>.
init_or_size must be an integer which specifies the size of the array, or a
unicode string which will be used to initialize the array items.
If a unicode 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 to specify the size of the array if the length of the string should not
be used.
If the first parameter is a 8-bit string, it is converted into an unicode string
according to ctypes conversion rules.
</dd></dl>

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

<dl class="function">
<dt id="ctypes.util.find_library">
<tt class="descclassname">ctypes.util.</tt><tt class="descname">find_library</tt><big>(</big>name<big>)</big><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 <tt class="docutils literal">lib</tt>, suffix like <tt class="docutils literal">.so</tt>, <tt class="docutils literal">.dylib</tt> or version
number (this is the form used for the posix linker option -l). If
no library can be found, returns <tt class="xref docutils literal">None</tt>.
The exact functionality is system dependent.

Changed in version 2.6: Windows only: <tt class="docutils literal">find_library(&quot;m&quot;)</tt> or <tt class="docutils literal">find_library(&quot;c&quot;)</tt> return the
result of a call to <tt class="docutils literal">find_msvcrt()</tt>.
</dd></dl>

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

New in version 2.6.
</dd></dl>

<dl class="function">
<dt id="ctypes.FormatError">
<tt class="descclassname">ctypes.</tt><tt class="descname">FormatError</tt><big>(</big>[code]<big>)</big><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">
<tt class="descclassname">ctypes.</tt><tt class="descname">GetLastError</tt><big>(</big><big>)</big><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">
<tt class="descclassname">ctypes.</tt><tt class="descname">get_errno</tt><big>(</big><big>)</big><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
<tt class="xref docutils literal">errno</tt> variable in the calling thread.

New in version 2.6.
</dd></dl>

<dl class="function">
<dt id="ctypes.get_last_error">
<tt class="descclassname">ctypes.</tt><tt class="descname">get_last_error</tt><big>(</big><big>)</big><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
<tt class="xref docutils literal">LastError</tt> variable in the calling thread.

New in version 2.6.
</dd></dl>

<dl class="function">
<dt id="ctypes.memmove">
<tt class="descclassname">ctypes.</tt><tt class="descname">memmove</tt><big>(</big>dst, src, count<big>)</big><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">
<tt class="descclassname">ctypes.</tt><tt class="descname">memset</tt><big>(</big>dst, c, count<big>)</big><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">
<tt class="descclassname">ctypes.</tt><tt class="descname">POINTER</tt><big>(</big>type<big>)</big><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 an reused internally, so calling this function repeatedly is
cheap. type must be a ctypes type.</dd></dl>

<dl class="function">
<dt id="ctypes.pointer">
<tt class="descclassname">ctypes.</tt><tt class="descname">pointer</tt><big>(</big>obj<big>)</big><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 <tt class="docutils literal">POINTER(type(obj))</tt>.
Note: If you just want to pass a pointer to an object to a foreign function
call, you should use <tt class="docutils literal">byref(obj)</tt> which is much faster.
</dd></dl>

<dl class="function">
<dt id="ctypes.resize">
<tt class="descclassname">ctypes.</tt><tt class="descname">resize</tt><big>(</big>obj, size<big>)</big><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 <tt class="docutils literal">sizeof(type(obj))</tt>,
but it is possible to enlarge the buffer.</dd></dl>

<dl class="function">
<dt id="ctypes.set_conversion_mode">
<tt class="descclassname">ctypes.</tt><tt class="descname">set_conversion_mode</tt><big>(</big>encoding, errors<big>)</big><a class="headerlink" href="#ctypes.set_conversion_mode" title="Permalink to this definition">¶</a></dt>
<dd>This function sets the rules that ctypes objects use when converting between
8-bit strings and unicode strings. encoding must be a string specifying an
encoding, like <tt class="docutils literal">'utf-8'</tt> or <tt class="docutils literal">'mbcs'</tt>, errors must be a string
specifying the error handling on encoding/decoding errors. Examples of
possible values are <tt class="docutils literal">&quot;strict&quot;</tt>, <tt class="docutils literal">&quot;replace&quot;</tt>, or <tt class="docutils literal">&quot;ignore&quot;</tt>.
<a title="ctypes.set_conversion_mode" class="reference internal" href="#ctypes.set_conversion_mode"><tt class="xref docutils literal">set_conversion_mode()</tt></a> returns a 2-tuple containing the previous
conversion rules. On windows, the initial conversion rules are <tt class="docutils literal">('mbcs',
'ignore')</tt>, on other systems <tt class="docutils literal">('ascii', 'strict')</tt>.
</dd></dl>

<dl class="function">
<dt id="ctypes.set_errno">
<tt class="descclassname">ctypes.</tt><tt class="descname">set_errno</tt><big>(</big>value<big>)</big><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 <tt class="xref docutils literal">errno</tt>
variable in the calling thread to value and return the previous value.

New in version 2.6.
</dd></dl>

<dl class="function">
<dt id="ctypes.set_last_error">
<tt class="descclassname">ctypes.</tt><tt class="descname">set_last_error</tt><big>(</big>value<big>)</big><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
<tt class="xref docutils literal">LastError</tt> variable in the calling thread to value and return the
previous value.

New in version 2.6.
</dd></dl>

<dl class="function">
<dt id="ctypes.sizeof">
<tt class="descclassname">ctypes.</tt><tt class="descname">sizeof</tt><big>(</big>obj_or_type<big>)</big><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 <tt class="docutils literal">sizeof()</tt> function.</dd></dl>

<dl class="function">
<dt id="ctypes.string_at">
<tt class="descclassname">ctypes.</tt><tt class="descname">string_at</tt><big>(</big>address[, size]<big>)</big><a class="headerlink" href="#ctypes.string_at" title="Permalink to this definition">¶</a></dt>
<dd>This function returns the string starting at memory address address. 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">
<tt class="descclassname">ctypes.</tt><tt class="descname">WinError</tt><big>(</big>code=None, descr=None<big>)</big><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 WindowsError. If code is not specified,
<tt class="docutils literal">GetLastError</tt> is called to determine the error code. If <tt class="docutils literal">descr</tt> is not
specified, <a title="ctypes.FormatError" class="reference internal" href="#ctypes.FormatError"><tt class="xref docutils literal">FormatError()</tt></a> is called to get a textual description of the
error.</dd></dl>

<dl class="function">
<dt id="ctypes.wstring_at">
<tt class="descclassname">ctypes.</tt><tt class="descname">wstring_at</tt><big>(</big>address[, size]<big>)</big><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 unicode 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>15.15.2.6. Data types<a class="headerlink" href="#data-types" title="Permalink to this headline">¶</a></h3>
<dl class="class">
<dt id="ctypes._CData">
class <tt class="descclassname">ctypes.</tt><tt class="descname">_CData</tt><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 title="ctypes.addressof" class="reference internal" href="#ctypes.addressof"><tt class="xref docutils literal">addressof()</tt></a> helper function. Another instance variable is exposed as
<a title="ctypes._CData._objects" class="reference internal" href="#ctypes._CData._objects"><tt class="xref docutils literal">_objects</tt></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 external" href="../glossary.html#term-metaclass">metaclass</a>):
<dl class="method">
<dt id="ctypes._CData.from_buffer">
<tt class="descname">from_buffer</tt><big>(</big>source[, offset]<big>)</big><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 title="exceptions.ValueError" class="reference external" href="exceptions.html#exceptions.ValueError"><tt class="xref docutils literal">ValueError</tt></a> is raised.

New in version 2.6.
</dd></dl>

<dl class="method">
<dt id="ctypes._CData.from_buffer_copy">
<tt class="descname">from_buffer_copy</tt><big>(</big>source[, offset]<big>)</big><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 title="exceptions.ValueError" class="reference external" href="exceptions.html#exceptions.ValueError"><tt class="xref docutils literal">ValueError</tt></a> is
raised.

New in version 2.6.
</dd></dl>

<dl class="method">
<dt id="ctypes._CData.from_address">
<tt class="descname">from_address</tt><big>(</big>address<big>)</big><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">
<tt class="descname">from_param</tt><big>(</big>obj<big>)</big><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&#8217;s <tt class="xref docutils literal">argtypes</tt> 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">
<tt class="descname">in_dll</tt><big>(</big>library, name<big>)</big><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_">
<tt class="descname">_b_base_</tt><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 title="ctypes._CData._b_base_" class="reference internal" href="#ctypes._CData._b_base_"><tt class="xref docutils literal">_b_base_</tt></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_">
<tt class="descname">_b_needsfree_</tt><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">
<tt class="descname">_objects</tt><a class="headerlink" href="#ctypes._CData._objects" title="Permalink to this definition">¶</a></dt>
<dd>This member is either <tt class="xref docutils literal">None</tt> 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>15.15.2.7. 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 <tt class="descclassname">ctypes.</tt><tt class="descname">_SimpleCData</tt><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 title="ctypes._SimpleCData" class="reference internal" href="#ctypes._SimpleCData"><tt class="xref docutils literal">_SimpleCData</tt></a> is a subclass of
<a title="ctypes._CData" class="reference internal" href="#ctypes._CData"><tt class="xref docutils literal">_CData</tt></a>, so it inherits their methods and attributes.

Changed in version 2.6: 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">
<tt class="descname">value</tt><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 string, for character pointer types it is a Python string or
unicode string.
When the <tt class="docutils literal">value</tt> attribute is retrieved from a ctypes instance, usually
a new object is returned each time. <tt class="xref docutils literal">ctypes</tt> 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
<tt class="xref docutils literal">restype</tt> of <a title="ctypes.c_char_p" class="reference internal" href="#ctypes.c_char_p"><tt class="xref docutils literal">c_char_p</tt></a>, you will always receive a Python string,
not a <a title="ctypes.c_char_p" class="reference internal" href="#ctypes.c_char_p"><tt class="xref docutils literal">c_char_p</tt></a> instance.
Subclasses of fundamental data types do not inherit this behavior. So, if a
foreign functions <tt class="xref docutils literal">restype</tt> is a subclass of <a title="ctypes.c_void_p" class="reference internal" href="#ctypes.c_void_p"><tt class="xref docutils literal">c_void_p</tt></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 <tt class="docutils literal">value</tt> attribute.
These are the fundamental ctypes data types:
<dl class="class">
<dt id="ctypes.c_byte">
class <tt class="descclassname">ctypes.</tt><tt class="descname">c_byte</tt><a class="headerlink" href="#ctypes.c_byte" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <tt class="xref docutils literal">signed char</tt> 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 <tt class="descclassname">ctypes.</tt><tt class="descname">c_char</tt><a class="headerlink" href="#ctypes.c_char" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <tt class="xref docutils literal">char</tt> 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 <tt class="descclassname">ctypes.</tt><tt class="descname">c_char_p</tt><a class="headerlink" href="#ctypes.c_char_p" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <tt class="xref docutils literal">char *</tt> datatype when it points to a zero-terminated
string. For a general character pointer that may also point to binary data,
<tt class="docutils literal">POINTER(c_char)</tt> must be used. The constructor accepts an integer
address, or a string.</dd></dl>

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

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

New in version 2.6.
</dd></dl>

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

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

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

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

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

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

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

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

<dl class="class">
<dt id="ctypes.c_short">
class <tt class="descclassname">ctypes.</tt><tt class="descname">c_short</tt><a class="headerlink" href="#ctypes.c_short" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <tt class="xref docutils literal">signed short</tt> 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 <tt class="descclassname">ctypes.</tt><tt class="descname">c_size_t</tt><a class="headerlink" href="#ctypes.c_size_t" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <tt class="xref docutils literal">size_t</tt> datatype.</dd></dl>

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

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

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

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

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

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

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

<dl class="class">
<dt id="ctypes.c_ushort">
class <tt class="descclassname">ctypes.</tt><tt class="descname">c_ushort</tt><a class="headerlink" href="#ctypes.c_ushort" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <tt class="xref docutils literal">unsigned short</tt> 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 <tt class="descclassname">ctypes.</tt><tt class="descname">c_void_p</tt><a class="headerlink" href="#ctypes.c_void_p" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <tt class="xref docutils literal">void *</tt> 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 <tt class="descclassname">ctypes.</tt><tt class="descname">c_wchar</tt><a class="headerlink" href="#ctypes.c_wchar" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <tt class="xref docutils literal">wchar_t</tt> 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 <tt class="descclassname">ctypes.</tt><tt class="descname">c_wchar_p</tt><a class="headerlink" href="#ctypes.c_wchar_p" title="Permalink to this definition">¶</a></dt>
<dd>Represents the C <tt class="xref docutils literal">wchar_t *</tt> 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 <tt class="descclassname">ctypes.</tt><tt class="descname">c_bool</tt><a class="headerlink" href="#ctypes.c_bool" title="Permalink to this definition">¶</a></dt>
<dd>Represent the C <tt class="xref docutils literal">bool</tt> datatype (more accurately, <tt class="xref docutils literal">_Bool</tt> from
C99). Its value can be True or False, and the constructor accepts any object
that has a truth value.

New in version 2.6.
</dd></dl>

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

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

The <tt class="xref docutils literal">ctypes.wintypes</tt> module provides quite some other Windows specific
data types, for example <tt class="xref docutils literal">HWND</tt>, <tt class="xref docutils literal">WPARAM</tt>, or <tt class="xref docutils literal">DWORD</tt>. Some
useful structures like <tt class="xref docutils literal">MSG</tt> or <tt class="xref docutils literal">RECT</tt> are also defined.
</div>
<div class="section" id="structured-data-types">
<h3>15.15.2.8. 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 <tt class="descclassname">ctypes.</tt><tt class="descname">Union</tt><big>(</big>*args, **kw<big>)</big><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 <tt class="descclassname">ctypes.</tt><tt class="descname">BigEndianStructure</tt><big>(</big>*args, **kw<big>)</big><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 <tt class="descclassname">ctypes.</tt><tt class="descname">LittleEndianStructure</tt><big>(</big>*args, **kw<big>)</big><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 <tt class="descclassname">ctypes.</tt><tt class="descname">Structure</tt><big>(</big>*args, **kw<big>)</big><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 title="ctypes.Structure._fields_" class="reference internal" href="#ctypes.Structure._fields_"><tt class="xref docutils literal">_fields_</tt></a> class variable. <tt class="xref docutils literal">ctypes</tt> will
create <a class="reference external" 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_">
<tt class="descname">_fields_</tt><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 title="ctypes.c_int" class="reference internal" href="#ctypes.c_int"><tt class="xref docutils literal">c_int</tt></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 title="ctypes.Structure._fields_" class="reference internal" href="#ctypes.Structure._fields_"><tt class="xref docutils literal">_fields_</tt></a> class variable after the
class statement that defines the Structure subclass, this allows to create
data types that directly or indirectly reference themselves:
<div class="highlight-python"><div class="highlight"><pre>class List(Structure):
 pass
List._fields_ = [(&quot;pnext&quot;, POINTER(List)),
 ...
 ]
</pre></div>
</div>
The <a title="ctypes.Structure._fields_" class="reference internal" href="#ctypes.Structure._fields_"><tt class="xref docutils literal">_fields_</tt></a> class variable must, however, be defined before the
type is first used (an instance is created, <tt class="docutils literal">sizeof()</tt> is called on it,
and so on). Later assignments to the <a title="ctypes.Structure._fields_" class="reference internal" href="#ctypes.Structure._fields_"><tt class="xref docutils literal">_fields_</tt></a> class variable will
raise an AttributeError.
Structure and union subclass constructors accept both positional and named
arguments. Positional arguments are used to initialize the fields in the
same order as they appear in the <a title="ctypes.Structure._fields_" class="reference internal" href="#ctypes.Structure._fields_"><tt class="xref docutils literal">_fields_</tt></a> definition, named
arguments are used to initialize the fields with the corresponding name.
It is possible to defined sub-subclasses of structure types, they inherit
the fields of the base class plus the <a title="ctypes.Structure._fields_" class="reference internal" href="#ctypes.Structure._fields_"><tt class="xref docutils literal">_fields_</tt></a> defined in the
sub-subclass, if any.
</dd></dl>

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

<dl class="attribute">
<dt id="ctypes.Structure._anonymous_">
<tt class="descname">_anonymous_</tt><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 title="ctypes.Structure._anonymous_" class="reference internal" href="#ctypes.Structure._anonymous_"><tt class="xref docutils literal">_anonymous_</tt></a> must be already defined when <a title="ctypes.Structure._fields_" class="reference internal" href="#ctypes.Structure._fields_"><tt class="xref docutils literal">_fields_</tt></a> is
assigned, otherwise it will have no effect.
The fields listed in this variable must be structure or union type fields.
<tt class="xref docutils literal">ctypes</tt> will create descriptors in the structure type that allows to
access the nested fields directly, without the need to create the
structure or union field.
Here is an example type (Windows):
<div class="highlight-python"><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 <tt class="docutils literal">TYPEDESC</tt> structure describes a COM data type, the <tt class="docutils literal">vt</tt> field
specifies which one of the union fields is valid. Since the <tt class="docutils literal">u</tt> field
is defined as anonymous field, it is now possible to access the members
directly off the TYPEDESC instance. <tt class="docutils literal">td.lptdesc</tt> and <tt class="docutils literal">td.u.lptdesc</tt>
are equivalent, but the former is faster since it does not need to create
a temporary union instance:
<div class="highlight-python"><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 title="ctypes.Structure._fields_" class="reference internal" href="#ctypes.Structure._fields_"><tt class="xref docutils literal">_fields_</tt></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 title="ctypes.Structure._fields_" class="reference internal" href="#ctypes.Structure._fields_"><tt class="xref docutils literal">_fields_</tt></a>. Keyword arguments in the
constructor are interpreted as attribute assignments, so they will initialize
<a title="ctypes.Structure._fields_" class="reference internal" href="#ctypes.Structure._fields_"><tt class="xref docutils literal">_fields_</tt></a> with the same name, or create new attributes for names not
present in <a title="ctypes.Structure._fields_" class="reference internal" href="#ctypes.Structure._fields_"><tt class="xref docutils literal">_fields_</tt></a>.
</dd></dl>

</div>
<div class="section" id="arrays-and-pointers">
<h3>15.15.2.9. Arrays and pointers<a class="headerlink" href="#arrays-and-pointers" title="Permalink to this headline">¶</a></h3>
Not yet written - please see the sections <a class="reference internal" href="#ctypes-pointers">Pointers</a> and section
<a class="reference internal" href="#ctypes-arrays">Arrays</a> in the tutorial.
</div>
</div>
</div>

</div>
 </div>
 </div>
 <div class="sphinxsidebar">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference external" href="#">15.15. <tt class="docutils literal">ctypes</tt> &#8212; A foreign function library for Python</a><ul>
<li><a class="reference external" href="#ctypes-tutorial">15.15.1. ctypes tutorial</a><ul>
<li><a class="reference external" href="#loading-dynamic-link-libraries">15.15.1.1. Loading dynamic link libraries</a></li>
<li><a class="reference external" href="#accessing-functions-from-loaded-dlls">15.15.1.2. Accessing functions from loaded dlls</a></li>
<li><a class="reference external" href="#calling-functions">15.15.1.3. Calling functions</a></li>
<li><a class="reference external" href="#fundamental-data-types">15.15.1.4. Fundamental data types</a></li>
<li><a class="reference external" href="#calling-functions-continued">15.15.1.5. Calling functions, continued</a></li>
<li><a class="reference external" href="#calling-functions-with-your-own-custom-data-types">15.15.1.6. Calling functions with your own custom data types</a></li>
<li><a class="reference external" href="#specifying-the-required-argument-types-function-prototypes">15.15.1.7. Specifying the required argument types (function prototypes)</a></li>
<li><a class="reference external" href="#return-types">15.15.1.8. Return types</a></li>
<li><a class="reference external" href="#passing-pointers-or-passing-parameters-by-reference">15.15.1.9. Passing pointers (or: passing parameters by reference)</a></li>
<li><a class="reference external" href="#structures-and-unions">15.15.1.10. Structures and unions</a></li>
<li><a class="reference external" href="#structure-union-alignment-and-byte-order">15.15.1.11. Structure/union alignment and byte order</a></li>
<li><a class="reference external" href="#bit-fields-in-structures-and-unions">15.15.1.12. Bit fields in structures and unions</a></li>
<li><a class="reference external" href="#arrays">15.15.1.13. Arrays</a></li>
<li><a class="reference external" href="#pointers">15.15.1.14. Pointers</a></li>
<li><a class="reference external" href="#type-conversions">15.15.1.15. Type conversions</a></li>
<li><a class="reference external" href="#incomplete-types">15.15.1.16. Incomplete Types</a></li>
<li><a class="reference external" href="#callback-functions">15.15.1.17. Callback functions</a></li>
<li><a class="reference external" href="#accessing-values-exported-from-dlls">15.15.1.18. Accessing values exported from dlls</a></li>
<li><a class="reference external" href="#surprises">15.15.1.19. Surprises</a></li>
<li><a class="reference external" href="#variable-sized-data-types">15.15.1.20. Variable-sized data types</a></li>
</ul>
</li>
<li><a class="reference external" href="#ctypes-reference">15.15.2. ctypes reference</a><ul>
<li><a class="reference external" href="#finding-shared-libraries">15.15.2.1. Finding shared libraries</a></li>
<li><a class="reference external" href="#loading-shared-libraries">15.15.2.2. Loading shared libraries</a></li>
<li><a class="reference external" href="#foreign-functions">15.15.2.3. Foreign functions</a></li>
<li><a class="reference external" href="#function-prototypes">15.15.2.4. Function prototypes</a></li>
<li><a class="reference external" href="#utility-functions">15.15.2.5. Utility functions</a></li>
<li><a class="reference external" href="#data-types">15.15.2.6. Data types</a></li>
<li><a class="reference external" href="#ctypes-fundamental-data-types-2">15.15.2.7. Fundamental data types</a></li>
<li><a class="reference external" href="#structured-data-types">15.15.2.8. Structured data types</a></li>
<li><a class="reference external" href="#arrays-and-pointers">15.15.2.9. Arrays and pointers</a></li>
</ul>
</li>
</ul>
</li>
</ul>

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 title="previous chapter">15.14. <tt class="docutils literal">errno</tt> &#8212; Standard errno system symbols</a>
 <h4>Next topic</h4>
 <a href="someos.html"
 title="next chapter">16. Optional Operating System Services</a>
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