ctypes tutorial
"overview":index.html :: tutorial :: "codegenerator":codegen.html ::
"reference":reference.html :: "faq":faq.html
( Work in progress: "COM":com.html :: "COM sample":sum_sample.html )
This tutorial describes version 0.6.3 of 'ctypes'. There have been
quite some changes to version 0.4.x, the most important are
listed "here":changes.html.
Loading dynamic link libraries
'ctypes' exports the 'cdll', and on Windows also 'windll'
and 'oledll' objects to load 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 'WindowsError' Python exceptions when the
function call fails.
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
>>> print cdll.msvcrt
In principle the same way should work on Linux, but most of the
time it seems required to specify the search path in this way. So
this example shows also how to load libraries by specifying their
filename::
>>> from ctypes import *
>>> libc = cdll.LoadLibrary("/lib/libc.so.6")
>>>
This tutorial uses windows in its examples, however, functions
from the standard C library like 'strchr' and 'printf' should also
work on Linux and other systems.
Accessing functions from loaded dlls
Functions are accessed as attributes of dll objects::
>>> from ctypes import *
>>> print cdll.msvcrt.printf
>>> print windll.kernel32.GetModuleHandleA
>>> print windll.kernel32.MyOwnFunction
Traceback (most recent call last):
File "", line 1, in ?
File "ctypes.py", line 239, in __getattr__
func = _StdcallFuncPtr(name, self)
Attribute: function 'MyOwnFunction' not found
ctypes version 0.6.2 and above raise AttributeErrors when a symbol
is not found in a dll, before ValueError was raised.
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' explicitely, and then call it with normal
strings or unicode strings respectively.
Sometimes, dlls export functions with names which aren't valid
Python identifiers, like '"??2@YAPAXI@Z"'. In this case you have
to use 'getattr' to retrieve the function (XXX Better example?)::
>>> getattr(cdll.msvcrt, "??2@YAPAXI@Z")
>>>
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 handle.
This example calls both functions with a NULL pointer ('None'
should be used as the NULL pointer)::
>>> from ctypes import *
>>> print cdll.msvcrt.time(None)
1048777320
>>> print hex(windll.kernel32.GetModuleHandleA(None))
0x1d000000
'ctypes' tries at its best to protect you from calling functions
with the wrong number of arguments. Unfortunately this only works
on Windows. It does this by examining the stack after the function
returns::
>>> windll.kernel32.GetModuleHandleA()
Traceback (most recent call last):
File "", line 1, in ?
ValueError: Procedure probably called with not enough arguments
>>> windll.kernel32.GetModuleHandleA(0, 0)
Traceback (most recent call last):
File "", line 1, in ?
ValueError: Procedure probably called with too many arguments
>>>
On Windows, 'ctypes' 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 "", line 1, in ?
WindowsError: exception: access violation
>>>
There are, however, enough ways to crash Python with 'ctypes',
so you should be careful anyway.
Python integers, strings and unicode strings are the only objects
that can directly be used as parameters in these function calls.
Before we move on calling functions with other parameter types, we
have to learn more about 'ctypes' data types.
Simple data types
'ctypes' defines a number of primitive C compatible data types :
|---------------------------------------------|
|ctypes' type |C type |Python type|
|=============================================|
|'c_char' |'char' |character |
|---------------------------------------------|
|'c_byte' |'char' |integer |
|---------------------------------------------|
|'c_ubyte' |'unsigned char' |integer |
|---------------------------------------------|
|'c_short' |'short' |integer |
|---------------------------------------------|
|'c_ushort' |'unsigned short' |integer |
|---------------------------------------------|
|'c_int' |'int' |integer |
|---------------------------------------------|
|'c_uint' |'unsigned int' |integer |
|---------------------------------------------|
|'c_long' |'long' |integer |
|---------------------------------------------|
|'c_ulong' |'unsigned long' |long |
|---------------------------------------------|
|'c_longlong' |'__int64' or |long |
| |'long long' | |
|---------------------------------------------|
|'c_ulonglong'|'unsigned __int64' or|long |
| |'unsigned long long'| |
|---------------------------------------------|
|'c_float' |'float' |float |
|---------------------------------------------|
|'c_double' |'double' |float |
|---------------------------------------------|
|'c_char_p' |'char *' |string or |
| |(NUL terminated) |'None' |
|---------------------------------------------|
|'c_wchar_p' |'wchar_t *' |unicode or |
| |(NUL terminated) |'None' |
|---------------------------------------------|
|'c_void_p' |'void *' |integer or |
| | |'None' |
|---------------------------------------------|
All these types can be created by calling them with an optional
initializer of the correct type and value::
>>> c_int()
c_int(0)
>>> c_char_p("Hello World")
c_char_p('Hello, World')
>>> c_uint(-3)
Traceback (most recent call last):
File "", line 1, in ?
ValueError: Value out of range
Since these types are mutable, their value can also be changed afterwards::
>>> i = c_int(42)
>>> print i
c_int(42)
>>> print i.value
42
>>> i.value = -99
>>> print i.value
-99
Assigning a new value to instances of the pointer types
'c_char_p', 'c_wchar_p', and 'c_void_p' changes the *memory
location* they point to, *not the contents* of the memory block
(of course not, because Python strings are immutable)::
>>> s = "Hello, World"
>>> c_s = c_char_p(s)
>>> print c_s
c_char_p('Hello, World')
>>> c_s.value = "Hi, there"
>>> print c_s
c_char_p('Hi, there')
>>> print s # first string 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 'create_string_buffer' function which creates these in
various ways. The current memory block contents can be accessed
(or changed) with the 'raw' property, if you want to access it as
NUL terminated string, use the 'string' property::
>>> from ctypes import *
>>> p = create_string_buffer(3) # create a 3 byte buffer, initialized to NUL bytes
>>> print sizeof(p), repr(p.raw)
3 '\x00\x00\x00'
>>> p = create_string_buffer("Hello") # create a buffer containing a NUL terminated string
>>> print sizeof(p), repr(p.raw)
6 'Hello\x00'
>>> print repr(p.value)
'Hello'
>>> p = create_string_buffer("Hello", 10) # create a 10 byte buffer
>>> print sizeof(p), repr(p.raw)
10 'Hello\x00\x00\x00\x00\x00'
>>> p.value = "Hi"
>>> print sizeof(p), repr(p.raw)
10 'Hi\x00lo\x00\x00\x00\x00\x00'
>>>
The 'create_string_buffer' function replaces the 'c_buffer'
function (which is still available as an alias to the new
function), 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 'create_unicode_buffer'
function.
Calling functions, continued
Note that printf prints to the real standard output channel, *not*
to 'sys.stdout', so these examples will only work at the console
prompt, not from within *IDLE* or *PythonWin*::
>>> from ctypes import *; printf = cdll.msvcrt.printf
>>> printf("Hello, %s\n", "World!")
Hello, World!
14
>>> printf("Hello, %S", u"World!") # Note the upper case S!
Hello, World!
14
>>> printf("%d bottles of beer\n", 42)
42 bottles of beer
19
>>> printf("%f bottles of beer\n", 42.5)
Traceback (most recent call last):
File "", line 1, in ?
TypeError: Don't know how to convert parameter 2
>>>
As has been mentioned before, all Python types except intergers,
strings, and unicode strings have to be wrapped in their
corresponding 'ctypes' type, so that they can be converted to the
required C data type::
>>> from ctypes import *
>>> printf = cdll.msvcrt.printf
>>> printf("An int %d, a double %f\n", 1234, c_double(3.14))
Integer 1234, double 3.1400001049
34
>>>
Calling functions with your own custom data types
You can also customize 'ctypes' argument conversion to allow
instances of your own classes be used as function arguments.
'ctypes' looks for an '_as_parameter_' attribute and uses this as
the function argument. Of course, it must be one of integer,
string, or unicode::
>>> class Bottles(object):
... def __init__(self, number):
... self._as_parameter_ = number
...
>>> bottles = Bottles(42)
>>> from ctypes import *
>>> printf = cdll.msvcrt.printf
>>> printf("%d bottles of beer\n", bottles)
42 bottles of beer
19
>>>
If you don't want to store the instance's data in the
'_as_parameter_' instance variable, you could define a 'property'
which makes the data avaiblable.
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)::
>>> from ctypes import *
>>> printf = cdll.msvcrt.printf
>>> printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
>>> printf("String '%s', Int %d, Double %f\n", "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("%d %d %d", 1, 2, 3)
Traceback (most recent call last):
File "", line 1, in ?
TypeError: string expected instead of int instance
>>> printf("%s %d %f", "X", 2, 3)
X 2 3.00000012
>>>
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, it's '_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, unicode, a 'ctypes' instance, or
something having the '_as_parameter_' attribute.
Return types
By default functions are assumed to return integers. Other return
types can be specified by setting the 'restype' attribute of the
function object.
Allowed values for 'restype' are simple data types like 'c_int',
'c_long', 'c_char' and so on as well as pointers to other data
types. Functions returning structures are not yet supported.
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::
>>> from ctypes import *
>>> strchr = cdll.msvcrt.strchr
>>> strchr("abcdef", ord("d"))
8059983
>>> strchr.restype = c_char_p # c_char_p is a pointer to a string
>>> strchr("abcdef", ord("d"))
'def'
>>> print strchr("abcdef", ord("x"))
None
>>>
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 string into a C char::
>>> from ctypes import *
>>> msvcrt = cdll.msvcrt
>>> msvcrt.strchr.restype = "s"
>>> msvcrt.strchr.argtypes = [c_char_p, c_char]
>>> msvcrt.strchr("abcdef", "d")
'def'
>>> msvcrt.strchr("abcdef", "def")
Traceback (most recent call last):
File "", line 1, in ?
TypeError: one character string expected
>>> print msvcrt.strchr("abcdef", "x")
None
>>> print msvcrt.strchr("abcdef", "d")
"def"
>>>
You can also use a callable Python object (a function or a class
for example) as the 'restype' attribute. It 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::
>>> from ctypes import *
>>> GetModuleHandle = windll.kernel32.GetModuleHandleA
>>> def ValidHandle(value):
... if value == 0:
... raise WinError()
... return value
...
>>>
>>> GetModuleHandle.restype = ValidHandle
>>> GetModuleHandle(None)
486539264
>>> GetModuleHandle("something silly")
Traceback (most recent call last):
File "", line 1, in ?
File "", line 3, in ValidHandle
WindowsError: [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 'GetLastError()' to
retrieve it.
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*.
'ctypes' exports the 'byref' function which is used to pass
parameters by reference. The same effect can be achieved with the
'pointer' function, although 'pointer' does a lot more work since
it constructs a real pointer object, so it is faster to use
'byref' if you don't need the pointer object in Python itself::
>>> from ctypes import *
>>> msvcrt = cdll.msvcrt
>>> i = c_int()
>>> f = c_float()
>>> s = create_string_buffer('\000' * 32)
>>> print i.value, f.value, repr(s.value)
0 0.0 ''
>>> msvcrt.sscanf("1 3.14 Hello", "%d %f %s",
... byref(i), byref(f), s)
3
>>> print i.value, f.value, repr(s.value)
1 3.1400001049 'Hello'
**Note**
It seems to be a difficult issue, the mailing list gets quite
some questions about how to call functions expecting pointers.
If you have suggestions for improvements for the preceeding
section, please post to
"ctypes-users":mailto:ctypes-users@lists.sourceforge.net.
Structures and Unions
Structures and unions must derive from the 'Structure' and 'Union'
base classes which are defined in the 'ctypes' 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 'ctypes' type like 'c_int', or any other
derived 'ctypes' 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
10 20
>>> point = POINT(y=5)
>>> print point.x, point.y
0 5
>>> POINT(1, 2, 3)
Traceback (most recent call last):
File "", line 1, in ?
ValueError: too many initializers
>>>
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' ::
>>> class RECT(Structure):
... _fields_ = [("upperleft", POINT),
... ("lowerright", POINT)]
...
>>> rc = RECT(point)
>>> print rc.upperleft.x, rc.upperleft.y
10 20
>>> print rc.lowerright.x, rc.lowerright.y
0 0
>>>
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))
Fields descriptors can be retrieved from the *class*, they have
readonly 'size' and 'offset' attributes describing the size in
bytes and the offset of this field from the beginning of the
internal memory buffer::
>>> print POINT.x.size, POINT.x.offset
0 4
>>> print POINT.y.size, POINT.y.offset
4 4
>>>
Structure and Union fields are normally aligned in the same way
the C compiler would do it by default. It is possible to override
this behaviour be specifying a '_pack_' class attribute in the
subclass, it must be set to a positive integer and specifies the
maximum alignment for the fields. I believe this is what '#pragma
pack(n)' also does in MSVC.
**New in version 0.6.2**: Structures and unions can also be passed
*by value* to function calls.
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::
TenPointsArray = POINT * 10
Here is an example of an somewhat artifical 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", float),
... ("point_array", POINT * 4)]
>>>
>>> print len(MyStruct().point_array)
4
Instances are created in the usual way, by calling the class::
arr = TenPointsArray()
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)
<__main__.c_int_Array_10 object at 0x009119F0>
>>> for i in ii: print i,
...
1 2 3 4 5 6 7 8 9 10
>>>
Pointers
Pointer instances are created by calling the 'pointer' function
on a 'ctypes' type::
>>> from ctypes import *
>>> i = c_int(42)
>>> pi = pointer(i)
>>>
Pointer instances have a 'contents' attribute which returns the
ctypes' type pointed to, the 'c_int(42)' in the above case::
>>> pi.contents
c_int(42)
>>>
Assigning another 'c_int' instance to the pointer's contents
attribute would cause the pointer to point to the memory location
where this is stored::
>>> pi.contents = c_int(99)
>>> pi.contents
c_int(99)
>>>
Pointer instances can also be indexed with integers::
>>> pi[0]
99
>>>
Assigning to an integer index changes the pointed to value::
>>> i2 = pi[0]
>>> i2
c_int(99)
>>> pi[0] = 22
>>> i2
c_int(22)
>>>
It is also possible to use indexes different from 0, but you must
know what you're doing when you use this: You access or change
arbitrary memory locations when you do this. 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.
Pointer classes/types
Behind the scenes, the 'pointer' function does more than simply
create pointer instances, it has to create pointer *types* first.
This is done with the 'POINTER' function, which accepts any
'ctypes' type, and returns a new type::
>>> from ctypes import *
>>> PI = POINTER(c_int)
>>> PI
>>> PI(42)
Traceback (most recent call last):
File "", line 1, in ?
TypeError: expected c_int instead of int
>>> PI(c_int(42))
>>>
Incomplete Types
**Note:** This code actually works now with ctypes 0.6.3
*Incomplete Types* are structures, unions or arrays whose members
are not yet specified. In the 'ctypes' context, you can create
types representing pointers to these incomplete types by passing
their name (as a string) to the POINTER function, and complete the
result subclass later.
Consider this example (C-code)::
struct cell;
struct {
char *name;
struct cell *next;
} cell;
The straightforward translation into ctypes code would be this,
but it does not work::
>>> class cell(Structure):
... _fields_ = [("name", c_char_p),
... ("next", POINTER(cell))]
...
Traceback (most recent call last):
File "", line 1, in ?
File "", line 2, in cell
NameError: name 'cell' is not defined
>>>
because the new 'class cell' is not available in the class
statement itself.
We can do it by creating an *incomplete pointer type* by
calling POINTER with the class _name_, and later setting the
complete type after it is defined::
>>> from ctypes import *
>>> lpcell = POINTER("cell")
>>> class cell(Structure):
... _fields_ = [("name", c_char_p),
... ("next", lpcell)]
...
>>> SetPointerType(lpcell, 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(c2)
>>> p = c1
>>> for i in range(8):
... print p.name,
... p = p.next[0]
...
foo bar foo bar foo bar foo bar
>>>
Callback functions
(This example is too long, I should have used a shorter
array)
'ctypes' 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 result type the function
has to return, and the number and types of the arguments this
function will receive.
'ctypes' provides the CFUNCTYPE factory function to create types
for callback functions using the normal cdecl calling convention,
and, on Windows, the WINFUNCTYPE factory function to create 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, this is used to sort items with the help of a
callback function. 'qsort' will be used to sort an array of integers::
>>> from ctypes import *
>>> IntArray10 = c_int * 10
>>> ia = IntArray10(5, 4, 3, 1, 7, 9, 33, 2, 99, 0)
>>> qsort = cdll.msvcrt.qsort
>>>
'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 the
sort function, which is the callback. The callback function 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 0
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 'type' for the callback
function::
>>> CMPFUNC = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int))
>>>
For the first implementation of the callback function, we simply
print the arguments we get, and return 0 (incremental development)::
>>> def py_cmp_func(a, b):
... print "py_cmp_func", a, b
... return 0
...
>>>
Create the C callable function::
>>> cmp_func = CMPFUNC(py_cmp_func)
>>>
And we're ready to go::
>>> qsort(ia, len(ia), sizeof(c_int), cmp_func)
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
py_cmp_func
-1
>>>
We know how to access the contents of a pointer, so lets redefine our callback::
>>> def py_cmp_func(a, b):
... print "py_cmp_func", a[0], b[0]
... return 0
...
>>> cmp_func = CMPFUNC(py_cmp_func)
>>> qsort(ia, len(ia), sizeof(c_int), cmp_func)
py_cmp_func 5 9
py_cmp_func 5 0
py_cmp_func 9 0
py_cmp_func 4 9
py_cmp_func 3 9
py_cmp_func 1 9
py_cmp_func 7 9
py_cmp_func 33 9
py_cmp_func 2 9
py_cmp_func 99 9
py_cmp_func 0 9
py_cmp_func 99 9
py_cmp_func 99 9
py_cmp_func 2 9
py_cmp_func 33 9
py_cmp_func 7 9
py_cmp_func 1 9
py_cmp_func 3 9
py_cmp_func 4 9
-1
>>>
Ah, we're nearly done! Last refinements::
>>> 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 9
py_cmp_func 5 0
py_cmp_func 9 5
py_cmp_func 4 5
py_cmp_func 3 5
py_cmp_func 1 5
py_cmp_func 7 5
py_cmp_func 99 5
py_cmp_func 2 5
py_cmp_func 33 5
py_cmp_func 33 5
py_cmp_func 2 5
py_cmp_func 7 33
py_cmp_func 99 33
py_cmp_func 9 99
py_cmp_func 7 33
py_cmp_func 9 33
py_cmp_func 7 9
py_cmp_func 4 0
py_cmp_func 3 4
py_cmp_func 1 4
py_cmp_func 2 4
py_cmp_func 2 0
py_cmp_func 3 2
py_cmp_func 1 3
py_cmp_func 2 0
py_cmp_func 1 2
py_cmp_func 1 0
-1
>>>
So, is our array sorted now::
>>> for i in ia: print i,
...
0 1 2 3 4 5 7 9 33 99
>>>
Yep, it worked!
A warning for callback functions
**Important Note**:
Make sure you keep references to CFUNCTYPE objects as long as they
are used from C code. ctypes doesn't, and if you don't, they may
be garbage collected, crashing your program when a callback is
made.
Accessing values exported from dlls
Sometimes, a dll not only exports functions, it also exports
values. Examples in the Python dll itself are the
'Py_OptimizeFlag', an integer set to 0, 1, or 2, depending on the
'-O' or '-OO' flag given on startup.
Starting with version 0.6.1, 'ctypes' can access values like this
with the 'in_dll' class methods of the types. The following
examples only work on Windows::
>>> from ctypes import *
>>> pydll = cdll.python22
>>> opt_flag = c_int.in_dll(pydll, "Py_OptimizeFlag")
>>> print opt_flag
c_int(0)
>>>
If the interpreter would have been started with '-O', the sample
would have printed 'c_int(1)', or 'c_int(2)' if '-OO' would have
been specified.
A somewhat extended example which also demontrates the use of
pointers accesses the 'PyImport_FrozenModules' pointer exported by
Python.
Quoting the Python docs: *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
'ctypes'::
>>> from ctypes import *
>>> pydll = cdll.python22
>>>
>>> class struct_frozen(Structure):
... _fields_ = [("name", c_char_p),
... ("code", POINTER(c_ubyte)),
... ("size", c_int)]
...
>>>
We have 'loaded' the Python dll and defined the 'struct _frozen'
data type, so we can get the pointer to the table::
>>> FrozenTable = POINTER(struct_frozen)
>>> table = FrozenTable.in_dll(pdll, "PyImport_FrozenModules")
>>>
Since 'table' is a 'pointer' to the 'struct_frozen' records, we
can iterate over it, 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:
... print item.name, item.size
... if item.name is None:
... break
...
__hello__ 100
__phello__ -100
__phello__.spam 100
None 0
>>>
The fact that standard Python has a frozen module and a frozen
package (indicated by the negative size member) is not wellknown,
AFAIK it is used for testing. Try it out with 'import __hello__'
for example.
XXX Describe how to access the 'code' member fields, which contain
the byte code for the modules.
Surprises
There are some corners in 'ctypes' where you may be expect
something else than what actually happens.
Consider the following example::
>>> from ctypes import *
>>> class POINT(Structure):
... _fields_ = ("x", "i"), ("y", "i")
...
>>> 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
1 2 3 4
>>> # 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
3 4 3 4
Hm. We certainly expected the last statement to print '3 4 1 2'.
What happended? 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 subobjects from Structure, Unions,
and Arrays doesn't *copy* the subobject, it does more retrieve a
wrapper object accessing the root-object's underlying buffer.
Bugs, ToDo and non-implemented things
Bitfields are not implemented.
Enumeration types are not implemented. You can do it easily
yourself, using 'c_int' as the base class.
'long double' is not implemented.