16.16. ctypes — A foreign function library for Python — Python 3.6.5 documentation
— A foreign function library for Python
is a foreign function library for Python.
It provides C compatible
data types, and allows calling functions in DLLs or shared libraries.
used to wrap these libraries in pure Python.
16.16.1. ctypes tutorial
Note: The code samples in this tutorial use
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
On platforms
where sizeof(long) == sizeof(int) it is an alias to .
So, you should not be confused if
is printed if you would expect
— they are actually the same type.
16.16.1.1. Loading dynamic link libraries
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 cdecl calling
convention, while windll libraries call functions using the stdcall
calling convention. oledll also uses the stdcall calling convention, and
assumes the functions return a Windows HRESULT error code. The error
code is used to automatically raise an
exception when the
function call fails.
Changed in version 3.3: Windows errors used to raise , which is now an alias
Here are some examples for Windows. Note that msvcrt is the MS standard C
library containing most standard C functions, and uses the cdecl calling
convention:
&&& from ctypes import *
&&& print(windll.kernel32)
&WinDLL 'kernel32', handle ... at ...&
&&& print(cdll.msvcrt)
&CDLL 'msvcrt', handle ... at ...&
&&& libc = cdll.msvcrt
Windows appends the usual .dll file suffix automatically.
Accessing the standard C library through cdll.msvcrt will use an
outdated version of the library that may be incompatible with the one
being used by Python. Where possible, use native Python functionality,
or else import and use the msvcrt module.
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
LoadLibrary() method of the dll loaders should be used, or you should load
the library by creating an instance of CDLL by calling the constructor:
&&& cdll.LoadLibrary(&libc.so.6&)
&CDLL 'libc.so.6', handle ... at ...&
&&& libc = CDLL(&libc.so.6&)
&CDLL 'libc.so.6', handle ... at ...&
16.16.1.2. Accessing functions from loaded dlls
Functions are accessed as attributes of dll objects:
&&& from ctypes import *
&&& libc.printf
&_FuncPtr object at 0x...&
&&& print(windll.kernel32.GetModuleHandleA)
&_FuncPtr object at 0x...&
&&& print(windll.kernel32.MyOwnFunction)
Traceback (most recent call last):
File &&stdin&&, line 1, in &module&
File &ctypes.py&, line 239, in __getattr__
func = _StdcallFuncPtr(name, self)
AttributeError: function 'MyOwnFunction' not found
Note that win32 system dlls like kernel32 and user32 often export ANSI
as well as UNICODE versions of a function. The UNICODE version is exported with
an W appended to the name, while the ANSI version is exported with an A
appended to the name. The win32 GetModuleHandle 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 GetModuleHandle depending on whether
UNICODE is defined or not:
/* ANSI version */
HMODULE GetModuleHandleA(LPCSTR lpModuleName);
/* UNICODE version */
HMODULE GetModuleHandleW(LPCWSTR lpModuleName);
windll does not try to select one of them by magic, you must access the
version you need by specifying GetModuleHandleA or GetModuleHandleW
explicitly, and then call it with bytes or string objects respectively.
Sometimes, dlls export functions with names which aren’t valid Python
identifiers, like &??2@YAPAXI@Z&. In this case you have to use
to retrieve the function:
&&& getattr(cdll.msvcrt, &??2@YAPAXI@Z&)
&_FuncPtr object at 0x...&
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:
&&& cdll.kernel32[1]
&_FuncPtr object at 0x...&
&&& cdll.kernel32[0]
Traceback (most recent call last):
File &&stdin&&, line 1, in &module&
File &ctypes.py&, line 310, in __getitem__
func = _StdcallFuncPtr(name, self)
AttributeError: function ordinal 0 not found
16.16.1.3. Calling functions
You can call these functions like any other Python callable. This example uses
the time() function, which returns system time in seconds since the Unix
epoch, and the GetModuleHandleA() function, which returns a win32 module
This example calls both functions with a NULL pointer (None should be used
as the NULL pointer):
&&& print(libc.time(None))
&&& print(hex(windll.kernel32.GetModuleHandleA(None)))
0x1d000000
may raise a
after calling the function, if
it detects that an invalid number of arguments were passed.
This behavior
should not be relied upon.
It is deprecated in 3.6.2, and will be removed
is raised when you call an stdcall function with the
cdecl calling convention, or vice versa:
&&& cdll.kernel32.GetModuleHandleA(None)
Traceback (most recent call last):
File &&stdin&&, line 1, in &module&
ValueError: Procedure probably called with not enough arguments (4 bytes missing)
&&& windll.msvcrt.printf(b&spam&)
Traceback (most recent call last):
File &&stdin&&, line 1, in &module&
ValueError: Procedure probably called with too many arguments (4 bytes in excess)
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,
uses win32 structured exception handling to prevent
crashes from general protection faults when functions are called with invalid
argument values:
&&& windll.kernel32.GetModuleHandleA(32)
Traceback (most recent call last):
File &&stdin&&, line 1, in &module&
OSError: exception: access violation reading 0x
There are, however, enough ways to crash Python with , so you
should be careful anyway.
module can be helpful in
debugging crashes (e.g. from segmentation faults produced by erroneous C library
None, integers, bytes objects and (unicode) strings are the only native
Python objects that can directly be used as parameters in these function calls.
None is passed as a C NULL pointer, bytes objects and strings are passed
as pointer to the memory block that contains their data (char * or
wchar_t *).
Python integers are passed as the platforms default C
int 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
data types.
16.16.1.4. Fundamental data types
defines a number of primitive C compatible data types:
1-character bytes object
1-character string
unsigned char
unsigned short
unsigned int
unsigned long
__int64 or long long
unsigned __int64 or
unsigned long long
ssize_t or
Py_ssize_t
long double
char * (NUL terminated)
bytes object or None
wchar_t * (NUL terminated)
string or None
int or None
The constructor accepts any object with a truth value.
All these types can be created by calling them with an optional initializer of
the correct type and value:
&&& c_int()
&&& c_wchar_p(&Hello, World&)
c_wchar_p(392)
&&& c_ushort(-3)
c_ushort(65533)
Since these types are mutable, their value can also be changed afterwards:
&&& i = c_int(42)
&&& print(i)
c_long(42)
&&& print(i.value)
&&& i.value = -99
&&& print(i.value)
Assigning a new value to instances of the pointer types ,
changes the memory location they
point to, not the contents of the memory block (of course not, because Python
bytes objects are immutable):
&&& s = &Hello, World&
&&& c_s = c_wchar_p(s)
&&& print(c_s)
c_wchar_p(344)
&&& print(c_s.value)
Hello World
&&& c_s.value = &Hi, there&
&&& print(c_s)
# the memory location has changed
c_wchar_p(904)
&&& print(c_s.value)
&&& print(s)
# first object is unchanged
Hello, World
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
function which creates these in various ways.
current memory block contents can be accessed (or changed) with the raw
if you want to access it as NUL terminated string, use the value
&&& from ctypes import *
&&& p = create_string_buffer(3)
# create a 3 byte buffer, initialized to NUL bytes
&&& print(sizeof(p), repr(p.raw))
3 b'\x00\x00\x00'
&&& p = create_string_buffer(b&Hello&)
# create a buffer containing a NUL terminated string
&&& print(sizeof(p), repr(p.raw))
6 b'Hello\x00'
&&& print(repr(p.value))
b'Hello'
&&& p = create_string_buffer(b&Hello&, 10) # create a 10 byte buffer
&&& print(sizeof(p), repr(p.raw))
10 b'Hello\x00\x00\x00\x00\x00'
&&& p.value = b&Hi&
&&& print(sizeof(p), repr(p.raw))
10 b'Hi\x00lo\x00\x00\x00\x00\x00'
function replaces the c_buffer() function
(which is still available as an alias), as well as the c_string() function
from earlier ctypes releases.
To create a mutable memory block containing
unicode characters of the C type wchar_t use the
16.16.1.5. Calling functions, continued
Note that printf prints to the real standard output channel, not to
, so these examples will only work at the console prompt, not
from within IDLE or PythonWin:
&&& printf = libc.printf
&&& printf(b&Hello, %s\n&, b&World!&)
Hello, World!
&&& printf(b&Hello, %S\n&, &World!&)
Hello, World!
&&& printf(b&%d bottles of beer\n&, 42)
42 bottles of beer
&&& printf(b&%f bottles of beer\n&, 42.5)
Traceback (most recent call last):
File &&stdin&&, line 1, in &module&
ArgumentError: argument 2: exceptions.TypeError: Don't know how to convert parameter 2
As has been mentioned before, all Python types except integers, strings, and
bytes objects have to be wrapped in their corresponding
that they can be converted to the required C data type:
&&& printf(b&An int %d, a double %f\n&, 1234, c_double(3.14))
An int 1234, a double 3.140000
16.16.1.6. Calling functions with your own custom data types
You can also customize
argument conversion to allow instances of
your own classes be used as function arguments.
looks for an
_as_parameter_ attribute and uses this as the function argument.
course, it must be one of integer, string, or bytes:
&&& class Bottles:
def __init__(self, number):
self._as_parameter_ = number
&&& bottles = Bottles(42)
&&& printf(b&%d bottles of beer\n&, bottles)
42 bottles of beer
If you don’t want to store the instance’s data in the _as_parameter_
instance variable, you could define a
which makes the
attribute available on request.
16.16.1.7. Specifying the required argument types (function prototypes)
It is possible to specify the required argument types of functions exported from
DLLs by setting the argtypes attribute.
argtypes must be a sequence of C data types (the printf 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):
&&& printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
&&& printf(b&String '%s', Int %d, Double %f\n&, b&Hi&, 10, 2.2)
String 'Hi', Int 10, Double 2.200000
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:
&&& printf(b&%d %d %d&, 1, 2, 3)
Traceback (most recent call last):
File &&stdin&&, line 1, in &module&
ArgumentError: argument 2: exceptions.TypeError: wrong type
&&& printf(b&%s %d %f\n&, b&X&, 2, 3)
X 2 3.000000
If you have defined your own classes which you pass to function calls, you have
to implement a from_param() class method for them to be able to use them
in the argtypes sequence. The from_param() 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 _as_parameter_ attribute, or whatever you want to
pass as the C function argument in this case. Again, the result should be an
integer, string, bytes, a
instance, or an object with an
_as_parameter_ attribute.
16.16.1.8. Return types
By default functions are assumed to return the C int type.
return types can be specified by setting the restype attribute of the
function object.
Here is a more advanced example, it uses the strchr function, which expects
a string pointer and a char, and returns a pointer to a string:
&&& strchr = libc.strchr
&&& strchr(b&abcdef&, ord(&d&))
&&& strchr.restype = c_char_p
# c_char_p is a pointer to a string
&&& strchr(b&abcdef&, ord(&d&))
b'def'
&&& print(strchr(b&abcdef&, ord(&x&)))
If you want to avoid the ord(&x&) calls above, you can set the
argtypes attribute, and the second argument will be converted from a
single character Python bytes object into a C char:
&&& strchr.restype = c_char_p
&&& strchr.argtypes = [c_char_p, c_char]
&&& strchr(b&abcdef&, b&d&)
'def'
&&& strchr(b&abcdef&, b&def&)
Traceback (most recent call last):
File &&stdin&&, line 1, in &module&
ArgumentError: argument 2: exceptions.TypeError: one character string expected
&&& print(strchr(b&abcdef&, b&x&))
&&& strchr(b&abcdef&, b&d&)
'def'
You can also use a callable Python object (a function or a class for example) as
the restype attribute, if the foreign function returns an integer.
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:
&&& GetModuleHandle = windll.kernel32.GetModuleHandleA
&&& def ValidHandle(value):
if value == 0:
raise WinError()
return value
&&& GetModuleHandle.restype = ValidHandle
&&& GetModuleHandle(None)
&&& GetModuleHandle(&something silly&)
Traceback (most recent call last):
File &&stdin&&, line 1, in &module&
File &&stdin&&, line 3, in ValidHandle
OSError: [Errno 126] The specified module could not be found.
WinError is a function which will call Windows FormatMessage() api to
get the string representation of an error code, and returns an exception.
WinError takes an optional error code parameter, if no one is used, it calls
to retrieve it.
Please note that a much more powerful error checking mechanism is available
through the errcheck see the reference manual for details.
16.16.1.9. Passing pointers (or: passing parameters by reference)
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.
exports the
function which is used to pass parameters
by reference.
The same effect can be achieved with the
does a lot more work since it constructs a real pointer
object, so it is faster to use
if you don’t need the pointer
object in Python itself:
&&& i = c_int()
&&& f = c_float()
&&& s = create_string_buffer(b'\000' * 32)
&&& print(i.value, f.value, repr(s.value))
0 0.0 b''
&&& libc.sscanf(b&1 3.14 Hello&, b&%d %f %s&,
byref(i), byref(f), s)
&&& print(i.value, f.value, repr(s.value))
1 3. b'Hello'
16.16.1.10. Structures and unions
Structures and unions must derive from the
base classes which are defined in the
module. Each subclass must
define a _fields_ attribute.
_fields_ must be a list of
2-tuples, containing a field name and a field type.
The field type must be a
type like , or any other
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:
&&& from ctypes import *
&&& class POINT(Structure):
_fields_ = [(&x&, c_int),
(&y&, c_int)]
&&& point = POINT(10, 20)
&&& print(point.x, point.y)
&&& point = POINT(y=5)
&&& print(point.x, point.y)
&&& POINT(1, 2, 3)
Traceback (most recent call last):
File &&stdin&&, line 1, in &module&
ValueError: too many initializers
You can, however, build much more complicated structures.
A structure can
itself contain other structures by using a structure as a field type.
Here is a RECT structure which contains two POINTs named upperleft and
lowerright:
&&& class RECT(Structure):
_fields_ = [(&upperleft&, POINT),
(&lowerright&, POINT)]
&&& rc = RECT(point)
&&& print(rc.upperleft.x, rc.upperleft.y)
&&& print(rc.lowerright.x, rc.lowerright.y)
Nested structures can also be initialized in the constructor in several ways:
&&& r = RECT(POINT(1, 2), POINT(3, 4))
&&& r = RECT((1, 2), (3, 4))
Field s can be retrieved from the class, they are useful
for debugging because they can provide useful information:
&&& print(POINT.x)
&Field type=c_long, ofs=0, size=4&
&&& print(POINT.y)
&Field type=c_long, ofs=4, size=4&
does not support passing unions or structures with bit-fields
to functions by value.
While this may work on 32-bit x86, it’s not
guaranteed by the library to work in the general case.
Unions and
structures with bit-fields should always be passed to functions by pointer.
16.16.1.11. Structure/union alignment and byte order
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
_pack_ 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 #pragma pack(n) also does in MSVC.
uses the native byte order for Structures and Unions.
structures with non-native byte order, you can use one of the
BigEndianUnion, and LittleEndianUnion base classes.
classes cannot contain pointer fields.
16.16.1.12. Bit fields in structures and unions
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 _fields_ tuples:
&&& class Int(Structure):
_fields_ = [(&first_16&, c_int, 16),
(&second_16&, c_int, 16)]
&&& print(Int.first_16)
&Field type=c_long, ofs=0:0, bits=16&
&&& print(Int.second_16)
&Field type=c_long, ofs=0:16, bits=16&
16.16.1.13. Arrays
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:
TenPointsArrayType = POINT * 10
Here is an example of a somewhat artificial data type, a structure containing 4
POINTs among other stuff:
&&& from ctypes import *
&&& class POINT(Structure):
_fields_ = (&x&, c_int), (&y&, c_int)
&&& class MyStruct(Structure):
_fields_ = [(&a&, c_int),
(&b&, c_float),
(&point_array&, POINT * 4)]
&&& print(len(MyStruct().point_array))
Instances are created in the usual way, by calling the class:
arr = TenPointsArrayType()
for pt in arr:
print(pt.x, pt.y)
The above code print a series of 0 0 lines, because the array contents is
initialized to zeros.
Initializers of the correct type can also be specified:
&&& from ctypes import *
&&& TenIntegers = c_int * 10
&&& ii = TenIntegers(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
&&& print(ii)
&c_long_Array_10 object at 0x...&
&&& for i in ii: print(i, end=& &)
1 2 3 4 5 6 7 8 9 10
16.16.1.14. Pointers
Pointer instances are created by calling the
function on a
&&& from ctypes import *
&&& i = c_int(42)
&&& pi = pointer(i)
Pointer instances have a
attribute which
returns the object to which the pointer points, the i object above:
&&& pi.contents
c_long(42)
does not have OOR (original object return), it constructs a
new, equivalent object each time you retrieve an attribute:
&&& pi.contents is i
&&& pi.contents is pi.contents
Assigning another
instance to the pointer’s contents attribute
would cause the pointer to point to the memory location where this is stored:
&&& i = c_int(99)
&&& pi.contents = i
&&& pi.contents
c_long(99)
Pointer instances can also be indexed with integers:
Assigning to an integer index changes the pointed to value:
&&& print(i)
c_long(99)
&&& pi[0] = 22
&&& print(i)
c_long(22)
It is also possible to use indexes different from 0, but you must know what
you’re doing, just as in C: You can access or change arbitrary memory locations.
Generally you only use this feature if you receive a pointer from a C function,
and you know that the pointer actually points to an array instead of a single
Behind the scenes, the
function does more than simply create
pointer instances, it has to create pointer types first. This is done with the
function, which accepts any
type, and returns a
&&& PI = POINTER(c_int)
&class 'ctypes.LP_c_long'&
&&& PI(42)
Traceback (most recent call last):
File &&stdin&&, line 1, in &module&
TypeError: expected c_long instead of int
&&& PI(c_int(42))
&ctypes.LP_c_long object at 0x...&
Calling the pointer type without an argument creates a NULL pointer.
NULL pointers have a False boolean value:
&&& null_ptr = POINTER(c_int)()
&&& print(bool(null_ptr))
checks for NULL when dereferencing pointers (but dereferencing
invalid non-NULL pointers would crash Python):
&&& null_ptr[0]
Traceback (most recent call last):
ValueError: NULL pointer access
&&& null_ptr[0] = 1234
Traceback (most recent call last):
ValueError: NULL pointer access
16.16.1.15. Type conversions
Usually, ctypes does strict type checking.
This means, if you have
POINTER(c_int) in the argtypes 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 POINTER(c_int), ctypes accepts an array of c_int:
&&& class Bar(Structure):
_fields_ = [(&count&, c_int), (&values&, POINTER(c_int))]
&&& bar = Bar()
&&& bar.values = (c_int * 3)(1, 2, 3)
&&& bar.count = 3
&&& for i in range(bar.count):
print(bar.values[i])
In addition, if a function argument is explicitly declared to be a pointer type
(such as POINTER(c_int)) in argtypes, an object of the pointed
type (c_int in this case) can be passed to the function.
ctypes will apply
the required
conversion in this case automatically.
To set a POINTER type field to NULL, you can assign None:
&&& bar.values = None
Sometimes you have instances of incompatible types.
In C, you can cast one type
into another type.
provides a
function which can be
used in the same way.
The Bar structure defined above accepts
POINTER(c_int) pointers or
arrays for its values field,
but not instances of other types:
&&& bar.values = (c_byte * 4)()
Traceback (most recent call last):
File &&stdin&&, line 1, in &module&
TypeError: incompatible types, c_byte_Array_4 instance instead of LP_c_long instance
For these cases, the
function is handy.
function can be used to cast a ctypes instance into a pointer
to a different ctypes data type.
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:
&&& a = (c_byte * 4)()
&&& cast(a, POINTER(c_int))
&ctypes.LP_c_long object at ...&
can be used to assign to the values field of Bar the
structure:
&&& bar = Bar()
&&& bar.values = cast((c_byte * 4)(), POINTER(c_int))
&&& print(bar.values[0])
16.16.1.16. Incomplete Types
Incomplete Types are structures, unions or arrays whose members are not yet
specified. In C, they are specified by forward declarations, which are defined
struct cell; /* forward declaration */
struct cell {
char *name;
struct cell *next;
The straightforward translation into ctypes code would be this, but it does not
&&& class cell(Structure):
_fields_ = [(&name&, c_char_p),
(&next&, POINTER(cell))]
Traceback (most recent call last):
File &&stdin&&, line 1, in &module&
File &&stdin&&, line 2, in cell
NameError: name 'cell' is not defined
because the new class cell is not available in the class statement itself.
In , we can define the cell class and set the _fields_
attribute later, after the class statement:
&&& from ctypes import *
&&& class cell(Structure):
&&& cell._fields_ = [(&name&, c_char_p),
(&next&, POINTER(cell))]
Lets try it. We create two instances of cell, and let them point to each
other, and finally follow the pointer chain a few times:
&&& c1 = cell()
&&& c1.name = &foo&
&&& c2 = cell()
&&& c2.name = &bar&
&&& c1.next = pointer(c2)
&&& c2.next = pointer(c1)
&&& p = c1
&&& for i in range(8):
print(p.name, end=& &)
p = p.next[0]
foo bar foo bar foo bar foo bar
16.16.1.17. Callback functions
allows creating C callable function pointers from Python callables.
These are sometimes called callback functions.
First, you must create a class for the callback function. The class knows the
calling convention, the return type, and the number and types of arguments this
function will receive.
factory function creates types for callback functions
using the cdecl calling convention. On Windows, the
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’s
qsort() function, that is used to sort items with the help of a callback
qsort() will be used to sort an array of integers:
&&& IntArray5 = c_int * 5
&&& ia = IntArray5(5, 1, 7, 33, 99)
&&& qsort = libc.qsort
&&& qsort.restype = None
qsort() must be called with a pointer to the data to sort, the number of
items in the data array, the size of one item, and a pointer to the comparison
function, the callback. The callback will then be called with two pointers to
items, and it must return a negative integer if the first item is smaller than
the second, a zero if they are equal, and a positive integer otherwise.
So our callback function receives pointers to integers, and must return an
integer. First we create the type for the callback function:
&&& CMPFUNC = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
To get started, here is a simple callback that shows the values it gets
&&& def py_cmp_func(a, b):
print(&py_cmp_func&, a[0], b[0])
&&& cmp_func = CMPFUNC(py_cmp_func)
The result:
&&& qsort(ia, len(ia), sizeof(c_int), cmp_func)
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 5 7
py_cmp_func 1 7
Now we can actually compare the two items and return a useful result:
&&& def py_cmp_func(a, b):
print(&py_cmp_func&, a[0], b[0])
return a[0] - b[0]
&&& qsort(ia, len(ia), sizeof(c_int), CMPFUNC(py_cmp_func))
py_cmp_func 5 1
py_cmp_func 33 99
py_cmp_func 7 33
py_cmp_func 1 7
py_cmp_func 5 7
As we can easily check, our array is sorted now:
&&& for i in ia: print(i, end=& &)
1 5 7 33 99
Make sure you keep references to
objects as long as they
are used from C code.
doesn’t, and if you don’t, they may be
garbage collected, crashing your program when a callback is made.
Also, note that if the callback function is called in a thread created
outside of Python’s control (e.g. by the foreign code that calls the
callback), ctypes creates a new dummy Python thread on every invocation. This
behavior is correct for most purposes, but it means that values stored with
will not survive across different callbacks, even when
those calls are made from the same C thread.
16.16.1.18. Accessing values exported from dlls
Some shared libraries not only export functions, they also export variables. An
example in the Python library itself is the Py_OptimizeFlag, an integer
set to 0, 1, or 2, depending on the
flag given on
can access values like this with the in_dll() class methods of
pythonapi is a predefined symbol giving access to the Python C
&&& opt_flag = c_int.in_dll(pythonapi, &Py_OptimizeFlag&)
&&& print(opt_flag)
If the interpreter would have been started with , the sample would
have printed c_long(1), or c_long(2) if
would have been
specified.
An extended example which also demonstrates the use of pointers accesses the
pointer exported by Python.
Quoting the docs for that value:
This pointer is initialized to point to an array of struct _frozen
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 :
&&& from ctypes import *
&&& class struct_frozen(Structure):
_fields_ = [(&name&, c_char_p),
(&code&, POINTER(c_ubyte)),
(&size&, c_int)]
We have defined the struct _frozen data type, so we can get the pointer
to the table:
&&& FrozenTable = POINTER(struct_frozen)
&&& table = FrozenTable.in_dll(pythonapi, &PyImport_FrozenModules&)
Since table is a pointer to the array of struct_frozen records, we
can iterate over it, but we just have to make sure that our loop terminates,
because pointers have no size. Sooner or later it would probably crash with an
access violation or whatever, so it’s better to break out of the loop when we
hit the NULL entry:
&&& for item in table:
if item.name is None:
print(item.name.decode(&ascii&), item.size)
_frozen_importlib 31764
_frozen_importlib_external 41499
__hello__ 161
__phello__ -161
__phello__.spam 161
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 import __hello__ for example.
16.16.1.19. Surprises
There are some edges in
where you might expect something other
than what actually happens.
Consider the following example:
&&& from ctypes import *
&&& class POINT(Structure):
_fields_ = (&x&, c_int), (&y&, c_int)
&&& class RECT(Structure):
_fields_ = (&a&, POINT), (&b&, POINT)
&&& p1 = POINT(1, 2)
&&& p2 = POINT(3, 4)
&&& rc = RECT(p1, p2)
&&& print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
&&& # now swap the two points
&&& rc.a, rc.b = rc.b, rc.a
&&& print(rc.a.x, rc.a.y, rc.b.x, rc.b.y)
Hm. We certainly expected the last statement to print 3 4 1 2. What
happened? Here are the steps of the rc.a, rc.b = rc.b, rc.a line above:
&&& temp0, temp1 = rc.b, rc.a
&&& rc.a = temp0
&&& rc.b = temp1
Note that temp0 and temp1 are objects still using the internal buffer of
the rc object above. So executing rc.a = temp0 copies the buffer
contents of temp0 into rc ‘s buffer.
This, in turn, changes the
contents of temp1. So, the last assignment rc.b = temp1, doesn’t have
the expected effect.
Keep in mind that retrieving sub-objects from Structure, Unions, and Arrays
doesn’t copy the sub-object, instead it retrieves a wrapper object accessing
the root-object’s underlying buffer.
Another example that may behave different from what one would expect is this:
&&& s = c_char_p()
&&& s.value = &abc def ghi&
&&& s.value
'abc def ghi'
&&& s.value is s.value
Why is it printing False?
ctypes instances are objects containing a memory
block plus some s accessing the contents of the memory.
Storing a Python object in the memory block does not store the object itself,
instead the contents of the object is stored.
Accessing the contents again
constructs a new Python object each time!
16.16.1.20. Variable-sized data types
provides some support for variable-sized arrays and structures.
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
is raised if this is tried:
&&& short_array = (c_short * 4)()
&&& print(sizeof(short_array))
&&& resize(short_array, 4)
Traceback (most recent call last):
ValueError: minimum size is 8
&&& resize(short_array, 32)
&&& sizeof(short_array)
&&& sizeof(type(short_array))
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:
&&& short_array[:]
[0, 0, 0, 0]
&&& short_array[7]
Traceback (most recent call last):
IndexError: invalid index
Another way to use variable-sized data types with
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.
16.16.2. ctypes reference
16.16.2.1. Finding shared libraries
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 find_library() function is to locate a library in a way
similar to what the compiler or runtime loader does (on platforms with several
versions of a shared library the most recent should be loaded), while the ctypes
library loaders act like when a program is run, and call the runtime loader
The ctypes.util module provides a function which can help to determine
the library to load.
ctypes.util.find_library(name)
Try to find a library and return a pathname.
name is the library name without
any prefix like lib, suffix like .so, .dylib or version number (this
is the form used for the posix linker option -l).
If no library can
be found, returns None.
The exact functionality is system dependent.
On Linux, find_library() tries to run external programs
(/sbin/ldconfig, gcc, objdump and ld) to find the library file.
It returns the filename of the library file.
Changed in version 3.6: On Linux, the value of the environment variable LD_LIBRARY_PATH is used
when searching for libraries, if a library cannot be found by any other means.
Here are some examples:
&&& from ctypes.util import find_library
&&& find_library(&m&)
'libm.so.6'
&&& find_library(&c&)
'libc.so.6'
&&& find_library(&bz2&)
'libbz2.so.1.0'
On OS X, find_library() tries several predefined naming schemes and paths
to locate the library, and returns a full pathname if successful:
&&& from ctypes.util import find_library
&&& find_library(&c&)
'/usr/lib/libc.dylib'
&&& find_library(&m&)
'/usr/lib/libm.dylib'
&&& find_library(&bz2&)
'/usr/lib/libbz2.dylib'
&&& find_library(&AGL&)
'/System/Library/Frameworks/AGL.framework/AGL'
On Windows, find_library() searches along the system search path, and
returns the full pathname, but since there is no predefined naming scheme a call
like find_library(&c&) will fail and return None.
If wrapping a shared library with , it may be better to determine
the shared library name at development time, and hardcode that into the wrapper
module instead of using find_library() to locate the library at runtime.
16.16.2.2. Loading shared libraries
There are several ways to load shared libraries into the Python process.
way is to instantiate one of the following classes:
class ctypes.CDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)
Instances of this class represent loaded shared libraries. Functions in these
libraries use the standard C calling convention, and are assumed to return
class ctypes.OleDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)
Windows only: Instances of this class represent loaded shared libraries,
functions in these libraries use the stdcall calling convention, and are
assumed to return the windows specific
values contain information specifying whether the function call failed or
succeeded, together with additional error code.
If the return value signals a
failure, an
is automatically raised.
Changed in version 3.3:
used to be raised.
class ctypes.WinDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False)
Windows only: Instances of this class represent loaded shared libraries,
functions in these libraries use the stdcall calling convention, and are
assumed to return int by default.
On Windows CE only the standard calling convention is used, for convenience the
use the standard calling convention on this
The Python
is released before calling any
function exported by these libraries, and reacquired afterwards.
class ctypes.PyDLL(name, mode=DEFAULT_MODE, handle=None)
Instances of this class behave like
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.
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 handle named
parameter, otherwise the underlying platforms dlopen or LoadLibrary
function is used to load the library into the process, and to get a handle to
The mode parameter can be used to specify how the library is loaded.
details, consult the dlopen(3) manpage.
On Windows, mode is
On posix systems, RTLD_NOW is always added, and is not
configurable.
The use_errno parameter, when set to true, enables a ctypes mechanism that
allows accessing the system
error number in a safe way.
maintains a thread-local copy of the systems
if you call foreign functions created with use_errno=True then the
value before the function call is swapped with the ctypes private
copy, the same happens immediately after the function call.
The function
returns the value of the ctypes private
copy, and the function
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
SetLastError() Windows API
are used to request and change the ctypes private
copy of the windows error code.
ctypes.RTLD_GLOBAL
Flag to use as mode parameter.
On platforms where this flag is not available,
it is defined as the integer zero.
ctypes.RTLD_LOCAL
Flag to use as mode parameter.
On platforms where this is not available, it
is the same as RTLD_GLOBAL.
ctypes.DEFAULT_MODE
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.
Instances of these classes have no public methods.
Functions exported by the
shared library can be accessed as attributes or by index.
Please note that
accessing the function through an attribute caches the result and therefore
accessing it repeatedly returns the same object each time.
On the other hand,
accessing it through an index returns a new object each time:
&&& libc.time == libc.time
&&& libc['time'] == libc['time']
The following public attributes are available, their name starts with an
underscore to not clash with exported function names:
PyDLL._handle
The system handle used to access the library.
PyDLL._name
The name of the library passed in the constructor.
Shared libraries can also be loaded by using one of the prefabricated objects,
which are instances of the
class, either by calling the
LoadLibrary() method, or by retrieving the library as attribute of the
loader instance.
class ctypes.LibraryLoader(dlltype)
Class which loads shared libraries.
dlltype should be one of the
has special behavior: It allows loading a shared library by
accessing it as attribute of a library loader instance.
The result is cached,
so repeated attribute accesses return the same library each time.
LoadLibrary(name)
Load a shared library into the process and return it.
This method always
returns a new instance of the library.
These prefabricated library loaders are available:
ctypes.cdll
instances.
ctypes.windll
Windows only: Creates
instances.
ctypes.oledll
Windows only: Creates
instances.
ctypes.pydll
instances.
For accessing the C Python api directly, a ready-to-use Python shared library
object is available:
ctypes.pythonapi
An instance of
that exposes Python C API functions as
attributes.
Note that all these functions are assumed to return C
int, which is of course not always the truth, so you have to assign
the correct restype attribute to use these functions.
16.16.2.3. Foreign functions
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:
class ctypes._FuncPtr
Base class for C callable foreign functions.
Instances of foreign functions are also C c they
represent C function pointers.
This behavior can be customized by assigning to special attributes of the
foreign function object.
Assign a ctypes type to specify the result type of the foreign function.
Use None for void, 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 int, and
the callable will be called with this integer, allowing further
processing or error checking.
Using this is deprecated, for more flexible
post processing or error checking use a ctypes data type as
and assign a callable to the
attribute.
Assign a tuple of ctypes types to specify the argument types that the
function accepts.
Functions using the stdcall calling convention can
only be called with the same number of arguments as the length of this
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
from_param() class method of the items in the
tuple, this method allows adapting the actual argument to an object that
the foreign function accepts.
For example, a
tuple will convert a string passed as argument into
a bytes object using ctypes conversion rules.
New: It is now possible to put items in argtypes which are not ctypes
types, but each item must have a from_param() method which returns a
value usable as argument (integer, string, ctypes instance).
This allows
defining adapters that can adapt custom objects as function parameters.
Assign a Python function or another callable to this attribute. The
callable will be called with three or more arguments:
callable(result, func, arguments)
result is what the foreign function returns, as specified by the
restype attribute.
func is the foreign function object itself, this allows reusing the
same callable object to check or post process the results of several
functions.
arguments is a tuple containing the parameters originally passed to
the function call, this allows specializing the behavior on the
arguments used.
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.
exception ctypes.ArgumentError
This exception is raised when a foreign function call cannot convert one of the
passed arguments.
16.16.2.4. Function prototypes
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.
ctypes.CFUNCTYPE(restype, *argtypes, use_errno=False, use_last_error=False)
The returned function prototype creates functions that use the standard C
calling convention.
The function will release the GIL during the call.
use_errno is set to true, the ctypes private copy of the system
variable is exchanged with the real
value before
use_last_error does the same for the Windows error
ctypes.WINFUNCTYPE(restype, *argtypes, use_errno=False, use_last_error=False)
Windows only: The returned function prototype creates functions that use the
stdcall calling convention, except on Windows CE where
is the same as .
The function will
release the GIL during the call.
use_errno and use_last_error have the
same meaning as above.
ctypes.PYFUNCTYPE(restype, *argtypes)
The returned function prototype creates functions that use the Python calling
convention.
The function will not release the GIL during the call.
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:
prototype(address)
Returns a foreign function at the specified address which must be an integer.
prototype(callable)
Create a C callable function (a callback function) from a Python callable.
prototype(func_spec[, paramflags])
Returns a foreign function exported by a shared library. func_spec must
be a 2-tuple (name_or_ordinal, library). 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.
prototype(vtbl_index, name[, paramflags[, iid]])
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 argtypes tuple.
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 argtypes.
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:
Specifies an input parameter to the function.
Output parameter.
The foreign function fills in a value.
Input parameter which defaults to the integer zero.
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.
This example demonstrates how to wrap the Windows MessageBoxW function so
that it supports default parameters and named arguments. The C declaration from
the windows header file is this:
WINUSERAPI int WINAPI
MessageBoxW(
HWND hWnd,
LPCWSTR lpText,
LPCWSTR lpCaption,
UINT uType);
Here is the wrapping with :
&&& from ctypes import c_int, WINFUNCTYPE, windll
&&& from ctypes.wintypes import HWND, LPCWSTR, UINT
&&& prototype = WINFUNCTYPE(c_int, HWND, LPCWSTR, LPCWSTR, UINT)
&&& paramflags = (1, &hwnd&, 0), (1, &text&, &Hi&), (1, &caption&, &Hello from ctypes&), (1, &flags&, 0)
&&& MessageBox = prototype((&MessageBoxW&, windll.user32), paramflags)
The MessageBox foreign function can now be called in these ways:
&&& MessageBox()
&&& MessageBox(text=&Spam, spam, spam&)
&&& MessageBox(flags=2, text=&foo bar&)
A second example demonstrates output parameters.
The win32 GetWindowRect
function retrieves the dimensions of a specified window by copying them into
RECT structure that the caller has to supply.
Here is the C declaration:
WINUSERAPI BOOL WINAPI
GetWindowRect(
HWND hWnd,
LPRECT lpRect);
Here is the wrapping with :
&&& from ctypes import POINTER, WINFUNCTYPE, windll, WinError
&&& from ctypes.wintypes import BOOL, HWND, RECT
&&& prototype = WINFUNCTYPE(BOOL, HWND, POINTER(RECT))
&&& paramflags = (1, &hwnd&), (2, &lprect&)
&&& GetWindowRect = prototype((&GetWindowRect&, windll.user32), paramflags)
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 errcheck protocol to do
further output processing and error checking.
The win32 GetWindowRect api
function returns a BOOL to signal success or failure, so this function could
do the error checking, and raises an exception when the api call failed:
&&& def errcheck(result, func, args):
if not result:
raise WinError()
return args
&&& GetWindowRect.errcheck = errcheck
If the errcheck function returns the argument tuple it receives
unchanged,
continues the normal processing it does on the output
parameters.
If you want to return a tuple of window coordinates instead of a
RECT instance, you can retrieve the fields in the function and return them
instead, the normal processing will no longer take place:
&&& def errcheck(result, func, args):
if not result:
raise WinError()
rc = args[1]
return rc.left, rc.top, rc.bottom, rc.right
&&& GetWindowRect.errcheck = errcheck
16.16.2.5. Utility functions
ctypes.addressof(obj)
Returns the address of the memory buffer as integer.
obj must be an
instance of a ctypes type.
ctypes.alignment(obj_or_type)
Returns the alignment requirements of a ctypes type. obj_or_type must be a
ctypes type or instance.
ctypes.byref(obj[, offset])
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.
byref(obj, offset) corresponds to this C code:
(((char *)&obj) + offset)
The returned object can only be used as a foreign function call parameter.
It behaves similar to pointer(obj), but the construction is a lot faster.
ctypes.cast(obj, type)
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
ctypes.create_string_buffer(init_or_size, size=None)
This function creates a mutable character buffer. The returned object is a
ctypes array of .
init_or_size must be an integer which specifies the size of the array, or a
bytes object which will be used to initialize the array items.
If a bytes object is specified as first argument, the buffer is made one item
larger than its length so that the last element in the array is a NUL
termination character. An integer can be passed as second argument which allows
specifying the size of the array if the length of the bytes should not be used.
ctypes.create_unicode_buffer(init_or_size, size=None)
This function creates a mutable unicode character buffer. The returned object is
a ctypes array of .
init_or_size must be an integer which specifies the size of the array, or a
string which will be used to initialize the array items.
If a string is specified as first argument, the buffer is made one item
larger than the length of the string so that the last element in the array is a
NUL termination character. An integer can be passed as second argument which
allows specifying the size of the array if the length of the string should not
ctypes.DllCanUnloadNow()
Windows only: This function is a hook which allows implementing in-process
COM servers with ctypes.
It is called from the DllCanUnloadNow function that
the _ctypes extension dll exports.
ctypes.DllGetClassObject()
Windows only: This function is a hook which allows implementing in-process
COM servers with ctypes.
It is called from the DllGetClassObject function
that the _ctypes extension dll exports.
ctypes.util.find_library(name)
Try to find a library and return a pathname.
name is the library name
without any prefix like lib, suffix like .so, .dylib or version
number (this is the form used for the posix linker option -l).
no library can be found, returns None.
The exact functionality is system dependent.
ctypes.util.find_msvcrt()
Windows only: return the filename of the VC runtime library used by Python,
and by the extension modules.
If the name of the library cannot be
determined, None is returned.
If you need to free memory, for example, allocated by an extension module
with a call to the free(void *), it is important that you use the
function in the same library that allocated the memory.
ctypes.FormatError([code])
Windows only: Returns a textual description of the error code code.
error code is specified, the last error code is used by calling the Windows
api function GetLastError.
ctypes.GetLastError()
Windows only: Returns the last error code set by Windows in the calling thread.
This function calls the Windows GetLastError() function directly,
it does not return the ctypes-private copy of the error code.
ctypes.get_errno()
Returns the current value of the ctypes-private copy of the system
variable in the calling thread.
ctypes.get_last_error()
Windows only: returns the current value of the ctypes-private copy of the system
LastError variable in the calling thread.
ctypes.memmove(dst, src, count)
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.
ctypes.memset(dst, c, count)
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.
ctypes.POINTER(type)
This factory function creates and returns a new ctypes pointer type. Pointer
types are cached and reused internally, so calling this function repeatedly is
cheap. type must be a ctypes type.
ctypes.pointer(obj)
This function creates a new pointer instance, pointing to obj. The returned
object is of the type POINTER(type(obj)).
Note: If you just want to pass a pointer to an object to a foreign function
call, you should use byref(obj) which is much faster.
ctypes.resize(obj, size)
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 sizeof(type(obj)),
but it is possible to enlarge the buffer.
ctypes.set_errno(value)
Set the current value of the ctypes-private copy of the system
variable in the calling thread to value and return the previous value.
ctypes.set_last_error(value)
Windows only: set the current value of the ctypes-private copy of the system
LastError variable in the calling thread to value and return the
previous value.
ctypes.sizeof(obj_or_type)
Returns the size in bytes of a ctypes type or instance memory buffer.
Does the same as the C sizeof operator.
ctypes.string_at(address, size=-1)
This function returns the C string starting at memory address address as a bytes
object. If size is specified, it is used as size, otherwise the string is assumed
to be zero-terminated.
ctypes.WinError(code=None, descr=None)
Windows only: this function is probably the worst-named thing in ctypes. It
creates an instance of OSError.
If code is not specified,
GetLastError is called to determine the error code. If descr is not
specified,
is called to get a textual description of the
Changed in version 3.3: An instance of
used to be created.
ctypes.wstring_at(address, size=-1)
This function returns the wide character string starting at memory address
address as a string.
If size is specified, it is used as the number of
characters of the string, otherwise the string is assumed to be
zero-terminated.
16.16.2.6. Data types
class ctypes._CData
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 the address of the memory block is returned by the
helper function. Another instance variable is exposed as
; 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 ):
from_buffer(source[, offset])
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
so the default is zero.
If the source buffer is not
large enough a
is raised.
from_buffer_copy(source[, offset])
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 so the default
If the source buffer is not large enough a
from_address(address)
This method returns a ctypes type instance using the memory specified by
address which must be an integer.
from_param(obj)
This method adapts obj to a ctypes type.
It is called with the actual
object used in a foreign function call when the type is present in the
foreign function’s argtypes 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.
types accept other objects as well.
in_dll(library, name)
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.
Common instance variables of ctypes data types:
Sometimes ctypes data instances do not own the memory block they contain,
instead they share part of the memory block of a base object.
read-only member is the root ctypes object that owns the
memory block.
_b_needsfree_
This read-only variable is true when the ctypes data instance has
allocated the memory block itself, false otherwise.
This member is either None or a dictionary containing Python objects
that need to be kept alive so that the memory block contents is kept
This object is only e never modify the
contents of this dictionary.
16.16.2.7. Fundamental data types
class ctypes._SimpleCData
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.
is a subclass of
, so it inherits their methods and attributes. ctypes data
types that are not and do not contain pointers can now be pickled.
Instances have a single attribute:
This attribute contains the actual value of the instance. For integer and
pointer types, it is an integer, for character types, it is a single
character bytes object or string, for character pointer types it is a
Python bytes object or string.
When the value attribute is retrieved from a ctypes instance, usually
a new object is returned each time.
does not implement
original object return, always a new object is constructed.
The same is
true for all other ctypes object instances.
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
restype of , you will always receive a Python bytes
object, not a
Subclasses of fundamental data types do not inherit this behavior. So, if a
foreign functions restype is a subclass of , 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 value attribute.
These are the fundamental ctypes data types:
class ctypes.c_byte
Represents the C signed char datatype, and interprets the value as
small integer.
The constructor accepts an optional no
overflow checking is done.
class ctypes.c_char
Represents the C char 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.
class ctypes.c_char_p
Represents the C char * datatype when it points to a zero-terminated
For a general character pointer that may also point to binary data,
POINTER(c_char) must be used.
The constructor accepts an integer
address, or a bytes object.
class ctypes.c_double
Represents the C double datatype.
The constructor accepts an
optional float initializer.
class ctypes.c_longdouble
Represents the C long double datatype.
The constructor accepts an
optional float initializer.
On platforms where sizeof(long double) ==
sizeof(double) it is an alias to .
class ctypes.c_float
Represents the C float datatype.
The constructor accepts an
optional float initializer.
class ctypes.c_int
Represents the C signed int datatype.
The constructor accepts an
optional no overflow checking is done.
On platforms
where sizeof(int) == sizeof(long) it is an alias to .
class ctypes.c_int8
Represents the C 8-bit signed int datatype.
Usually an alias for
class ctypes.c_int16
Represents the C 16-bit signed int datatype.
Usually an alias for
class ctypes.c_int32
Represents the C 32-bit signed int datatype.
Usually an alias for
class ctypes.c_int64
Represents the C 64-bit signed int datatype.
Usually an alias for
class ctypes.c_long
Represents the C signed long datatype.
The constructor accepts an
optional no overflow checking is done.
class ctypes.c_longlong
Represents the C signed long long datatype.
The constructor accepts
an optional no overflow checking is done.
class ctypes.c_short
Represents the C signed short datatype.
The constructor accepts an
optional no overflow checking is done.
class ctypes.c_size_t
Represents the C size_t datatype.
class ctypes.c_ssize_t
Represents the C ssize_t datatype.
New in version 3.2.
class ctypes.c_ubyte
Represents the C unsigned char datatype, it interprets the value as
small integer.
The constructor accepts an optional no
overflow checking is done.
class ctypes.c_uint
Represents the C unsigned int datatype.
The constructor accepts an
optional no overflow checking is done.
On platforms
where sizeof(int) == sizeof(long) it is an alias for .
class ctypes.c_uint8
Represents the C 8-bit unsigned int datatype.
Usually an alias for
class ctypes.c_uint16
Represents the C 16-bit unsigned int datatype.
Usually an alias for
class ctypes.c_uint32
Represents the C 32-bit unsigned int datatype.
Usually an alias for
class ctypes.c_uint64
Represents the C 64-bit unsigned int datatype.
Usually an alias for
class ctypes.c_ulong
Represents the C unsigned long datatype.
The constructor accepts an
optional no overflow checking is done.
class ctypes.c_ulonglong
Represents the C unsigned long long datatype.
The constructor
accepts an optional no overflow checking is done.
class ctypes.c_ushort
Represents the C unsigned short datatype.
The constructor accepts
an optional no overflow checking is done.
class ctypes.c_void_p
Represents the C void * type.
The value is represented as integer.
The constructor accepts an optional integer initializer.
class ctypes.c_wchar
Represents the C wchar_t 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.
class ctypes.c_wchar_p
Represents the C wchar_t * datatype, which must be a pointer to a
zero-terminated wide character string.
The constructor accepts an integer
address, or a string.
class ctypes.c_bool
Represent the C bool datatype (more accurately, _Bool from
Its value can be True or False, and the constructor accepts any object
that has a truth value.
class ctypes.HRESULT
Windows only: Represents a HRESULT value, which contains success or
error information for a function or method call.
class ctypes.py_object
Represents the C
Calling this without an
argument creates a NULL
The ctypes.wintypes module provides quite some other Windows specific
data types, for example HWND, WPARAM, or DWORD.
useful structures like MSG or RECT are also defined.
16.16.2.8. Structured data types
class ctypes.Union(*args, **kw)
Abstract base class for unions in native byte order.
class ctypes.BigEndianStructure(*args, **kw)
Abstract base class for structures in big endian byte order.
class ctypes.LittleEndianStructure(*args, **kw)
Abstract base class for structures in little endian byte order.
Structures with non-native byte order cannot contain pointer type fields, or any
other data types containing pointer type fields.
class ctypes.Structure(*args, **kw)
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
class variable.
create s which allow reading and writing the fields by direct
attribute accesses.
These are the
A sequence defining the structure fields.
The items must be 2-tuples or
The first item is the name of the field, the second item
specifies t it can be any ctypes data type.
For integer type fields like , a third optional item can be
It must be a small positive integer defining the bit width of the
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
class variable after the
class statement that defines the Structure subclass, this allows creating
data types that directly or indirectly reference themselves:
class List(Structure):
List._fields_ = [(&pnext&, POINTER(List)),
class variable must, however, be defined before the
type is first used (an instance is created,
is called on it,
and so on).
Later assignments to the
class variable will
raise an AttributeError.
It is p