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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.
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 <WinDLL 'kernel32', handle 77e80000 at 7ecfe8> >>> print cdll.msvcrt <CDLL 'msvcrt', handle 78000000 at 80b010>
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") <CDLL '/lib/libc.so.6', handle 40018c28 at 4019978c> >>>
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.
Functions are accessed as attributes of dll objects:
>>> from ctypes import * >>> print cdll.msvcrt.printf <ctypes._CdeclFuncPtr object ar 0x00905F68> >>> print windll.kernel32.GetModuleHandleA <ctypes._StdcallFuncPtr object ar 0x008E6D28> >>> print windll.kernel32.MyOwnFunction Traceback (most recent call last): File "<stdin>", 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") <ctypes._CdeclFuncPtr object at 0x00905EE0> >>>
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 "<stdin>", line 1, in ? ValueError: Procedure probably called with not enough arguments >>> windll.kernel32.GetModuleHandleA(0, 0) Traceback (most recent call last): File "<stdin>", 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 "<stdin>", 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.
ctypes
defines a number of primitive C compatible data types :
ctypes' type |
C type |
Python type |
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character |
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integer |
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integer |
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integer |
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integer |
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integer |
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integer |
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integer |
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long |
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long |
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long |
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string or
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unicode or
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integer or
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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 "<stdin>", 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 c_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 = c_buffer(3) # create a 3 byte buffer, initialized to NUL bytes >>> print sizeof(p), repr(p.raw) 3 '\x00\x00\x00' >>> p = c_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 = c_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' >>>
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 "<stdin>", 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 >>>
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.
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 "<stdin>", 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.
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 "<stdin>", 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 "<stdin>", line 1, in ? File "<stdin>", 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.
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 = c_string('\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'
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.
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 "<stdin>", 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 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 >>>
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.
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 <class 'ctypes.LP_c_int> >>> PI(42) Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: expected c_int instead of int >>> PI(c_int(42)) <ctypes.LP_c_int object at 0x008ECCE8> >>>
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 "<stdin>", line 1, in ? 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.
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 >>>
(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 <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> py_cmp_func <ctypes.LP_c_int object at 0x009127E8> <ctypes.LP_c_int object at 0x00927DF8> -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!
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.
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.
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.
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.