ctypes is a python built-in library that invokes exported functions from native compiled libraries.
Note: Since this library handles compiled code, it is relatively OS dependent.
# ctypes arrays
As any good C programmer knows, a single value won't get you that far. What will really get us going are arrays!
>>> c_int * 16 <class '__main__.c_long_Array_16'>
This is not an actual array, but it's pretty darn close! We created a class that denotes an array of 16
Now all we need to do is to initialize it:
>>> arr = (c_int * 16)(*range(16)) >>> arr <__main__.c_long_Array_16 object at 0xbaddcafe>
arr is an actual array that contains the numbers from 0 to 15.
They can be accessed just like any list:
>>> arr 5 >>> arr = 20 >>> arr 20
And just like any other
ctypes object, it also has a size and a location:
>>> sizeof(arr) 64 # sizeof(c_int) * 16 >>> hex(addressof(arr)) '0xc000l0ff'
# Wrapping functions for ctypes
In some cases, a C function accepts a function pointer. As avid
ctypes users, we would like to use those functions, and even pass python function as arguments.
Let's define a function:
>>> def max(x, y): return x if x >= y else y
Now, that function takes two arguments and returns a result of the same type. For the sake of the example, let's assume that type is an int.
Like we did on the array example, we can define an object that denotes that prototype:
>>> CFUNCTYPE(c_int, c_int, c_int) <CFunctionType object at 0xdeadbeef>
That prototype denotes a function that returns an
c_int (the first argument), and accepts two
c_int arguments (the other arguments).
Now let's wrap the function:
>>> CFUNCTYPE(c_int, c_int, c_int)(max) <CFunctionType object at 0xdeadbeef>
Function prototypes have on more usage: They can wrap
ctypes function (like
libc.ntohl) and verify that the correct arguments are used when invoking the function.
>>> libc.ntohl() # garbage in - garbage out >>> CFUNCTYPE(c_int, c_int)(libc.ntohl)() Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: this function takes at least 1 argument (0 given)
# Basic usage
Let's say we want to use
First, we must load
>>> from ctypes import * >>> libc = cdll.LoadLibrary('libc.so.6') >>> libc <CDLL 'libc.so.6', handle baadf00d at 0xdeadbeef>
Then, we get the function object:
>>> ntohl = libc.ntohl >>> ntohl <_FuncPtr object at 0xbaadf00d>
And now, we can simply invoke the function:
>>> ntohl(0x6C) 1811939328 >>> hex(_) '0x6c000000'
Which does exactly what we expect it to do.
# Common pitfalls
# Failing to load a file
The first possible error is failing to load the library. In that case an OSError is usually raised.
This is either because the file doesn't exists (or can't be found by the OS):
>>> cdll.LoadLibrary("foobar.so") Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/lib/python3.5/ctypes/__init__.py", line 425, in LoadLibrary return self._dlltype(name) File "/usr/lib/python3.5/ctypes/__init__.py", line 347, in __init__ self._handle = _dlopen(self._name, mode) OSError: foobar.so: cannot open shared object file: No such file or directory
As you can see, the error is clear and pretty indicative.
The second reason is that the file is found, but is not of the correct format.
>>> cdll.LoadLibrary("libc.so") Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/lib/python3.5/ctypes/__init__.py", line 425, in LoadLibrary return self._dlltype(name) File "/usr/lib/python3.5/ctypes/__init__.py", line 347, in __init__ self._handle = _dlopen(self._name, mode) OSError: /usr/lib/i386-linux-gnu/libc.so: invalid ELF header
In this case, the file is a script file and not a
.so file. This might also happen when trying to open a
.dll file on a Linux machine or a 64bit file on a 32bit python interpreter. As you can see, in this case the error is a bit more vague, and requires some digging around.
# Failing to access a function
Assuming we successfully loaded the
.so file, we then need to access our function like we've done on the first example.
When a non-existing function is used, an
AttributeError is raised:
>>> libc.foo Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/lib/python3.5/ctypes/__init__.py", line 360, in __getattr__ func = self.__getitem__(name) File "/usr/lib/python3.5/ctypes/__init__.py", line 365, in __getitem__ func = self._FuncPtr((name_or_ordinal, self)) AttributeError: /lib/i386-linux-gnu/libc.so.6: undefined symbol: foo
# Basic ctypes object
The most basic object is an int:
>>> obj = ctypes.c_int(12) >>> obj c_long(12)
obj refers to a chunk of memory containing the value 12.
That value can be accessed directly, and even modified:
>>> obj.value 12 >>> obj.value = 13 >>> obj c_long(13)
obj refers to a chunk of memory, we can also find out it's size and location:
>>> sizeof(obj) 4 >>> hex(addressof(obj)) '0xdeadbeef'
# Complex usage
Let's combine all of the examples above into one complex scenario: using
For more details about the function, read the man page. I urge you to read it before going on.
First, we'll define the proper prototypes:
>>> compar_proto = CFUNCTYPE(c_int, POINTER(c_int), POINTER(c_int)) >>> lfind_proto = CFUNCTYPE(c_void_p, c_void_p, c_void_p, POINTER(c_uint), c_uint, compar_proto)
Then, let's create the variables:
>>> key = c_int(12) >>> arr = (c_int * 16)(*range(16)) >>> nmemb = c_uint(16)
And now we define the comparison function:
>>> def compar(x, y): return x.contents.value - y.contents.value
POINTER(c_int), so we need to dereference them and take their values in order to actually compare the value stored in the memory.
Now we can combine everything together:
>>> lfind = lfind_proto(libc.lfind) >>> ptr = lfind(byref(key), byref(arr), byref(nmemb), sizeof(c_int), compar_proto(compar))
ptr is the returned void pointer. If
key wasn't found in
arr, the value would be
None, but in this case we got a valid value.
Now we can convert it and access the value:
>>> cast(ptr, POINTER(c_int)).contents c_long(12)
Also, we can see that
ptr points to the correct value inside
>>> addressof(arr) + 12 * sizeof(c_int) == ptr True