Questions tagged [ctypes]
`ctypes` is a Python package that wraps C .dll/.so libraries in pure Python.
ctypes
4,157
questions
318
votes
12
answers
91k
views
Wrapping a C library in Python: C, Cython or ctypes?
I want to call a C library from a Python application. I don't want to wrap the whole API, only the functions and datatypes that are relevant to my case. As I see it, I have three choices:
Create an ...
- 50.5k
138
votes
3
answers
142k
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ctypes - Beginner
I have the task of "wrapping" a c library into a python class. The docs are incredibly vague on this matter. It seems they expect only advanced python users would implement ctypes.
Some step ...
- 4,677
74
votes
5
answers
68k
views
How to use C++ classes with ctypes?
I'm just getting started with ctypes and would like to use a C++ class that I have exported in a dll file from within python using ctypes.
So lets say my C++ code looks something like this:
class ...
- 3,281
69
votes
7
answers
58k
views
ctypes error: libdc1394 error: Failed to initialize libdc1394
I'm trying to compile my program to a shared library that I can use from within Python code using ctypes.
The library compiles fine using this command:
g++ -shared -Wl,-soname,mylib -O3 -o mylib.so -...
- 33.3k
67
votes
2
answers
39k
views
Passing Numpy arrays to a C function for input and output
Oh my word I'm a fool.
I was simply omitting the second and third arguments when calling the function.
Like a fool.
Because that's what I am.
Original silly question follows:
This seems like it must ...
- 1,920
64
votes
10
answers
27k
views
Python: SWIG vs ctypes
In python, under what circumstances is SWIG a better choice than ctypes for calling entry points in shared libraries? Let's assume you don't already have the SWIG interface file(s). What are the ...
- 12.7k
64
votes
4
answers
77k
views
How do I convert a Python list into a C array by using ctypes?
If I have the follow 2 sets of code, how do I glue them together?
void
c_function(void *ptr) {
int i;
for (i = 0; i < 10; i++) {
printf("%p", ptr[i]);
}
return;
}
def ...
- 2,864
62
votes
3
answers
69k
views
Can I access ImageMagick API with Python?
I need to use ImageMagick as PIL does not have the amount of image functionality available that I am looking for. However, I am wanting to use Python.
The python bindings (PythonMagick) have not been ...
- 763
56
votes
8
answers
20k
views
How do I prevent a C shared library to print on stdout in python?
I work with a python lib that imports a C shared library that prints on stdout. I want a clean output in order to use it with pipes or to redirect in files. The prints are done outside of python, in ...
- 2,502
52
votes
8
answers
62k
views
list exported functions from dll with ctypes
Is there any way to know which functions are exported from the dll through python foreign function library ctypes?
And if possible to know details about the exported functions through ctypes.
If yes,...
- 581
51
votes
15
answers
157k
views
WindowsError: [Error 126] The specified module could not be found
I am loading a dll in python using following code:
if os.path.exists(dll_path):
my_dll = ctypes.cdll.LoadLibrary(dll_path)
But I am continuously getting the following error
WindowsError: [...
- 2,797
49
votes
5
answers
49k
views
Changing LD_LIBRARY_PATH at runtime for ctypes
How do you update this environment variable at runtime so that ctypes can load a library wherever? I've tried the following and neither seem to work.
from ctypes import *
os.environ['LD_LIBRARY_PATH']...
- 5,215
49
votes
4
answers
37k
views
How to pack and unpack using ctypes (Structure <-> str)
This might be a silly question but I couldn't find a good answer in the docs or anywhere.
If I use struct to define a binary structure, the struct has 2 symmetrical methods for serialization and ...
- 493
49
votes
3
answers
45k
views
Error loading DLL in python, not a valid win32 application [duplicate]
I am trying to load a DLL in python to call functions.
import ctypes
from ctypes import *
dsusb = ctypes.WinDLL('c:\python27\dsusb.dll')
I get the following error in my stack.
C:\Python27>...
- 2,826
45
votes
3
answers
82k
views
Pointers and arrays in Python ctypes
I have a DLL containing a C function with a prototype like this:
int c_read_block(uint32 addr, uint32 *buf, uint32 num);
I want to call it from Python using ctypes. The function expects a pointer to ...
- 4,728
44
votes
6
answers
39k
views
Getting data from ctypes array into numpy
I am using a Python (via ctypes) wrapped C library to run a series of computation. At different stages of the running, I want to get data into Python, and specifically numpy arrays.
The wrapping I ...
- 3,068
43
votes
6
answers
69k
views
Access memory address in python
My question is: How can I read the content of a memory address in python?
example:
ptr = id(7)
I want to read the content of memory pointed by ptr.
Thanks.
- 4,024
41
votes
3
answers
34k
views
40
votes
3
answers
56k
views
Converting python string object to c char* using ctypes
I am trying to send 2 strings from Python (3.2) to C using ctypes. This is a small part of my project on my Raspberry Pi. To test if the C function received the strings correctly, I place one of them ...
- 1,107
39
votes
2
answers
7k
views
Why is printf() giving a strange output in python?
I tried to use the C-function printf() in the python command line on Linux. To make that work, I imported ctypes. My problem is: If I create an object of CDLL to use the printf()-function in a loop, I ...
- 1,511
36
votes
6
answers
59k
views
Calling C functions in Python
I have a bunch of functions that I've written in C and I'd like some code I've written in Python to be able to access those functions.
I've read several questions on here that deal with a similar ...
- 1,904
31
votes
5
answers
50k
views
Python ctypes: loading DLL from from a relative path
I have a Python module, wrapper.py, that wraps a C DLL. The DLL lies in the same folder as the module. Therefore, I use the following code to load it:
myDll = ctypes.CDLL("MyCDLL.dll")
This works if ...
- 36.1k
31
votes
6
answers
5k
views
Safer way to expose a C-allocated memory buffer using numpy/ctypes?
I'm writing Python bindings for a C library that uses shared memory buffers to store its internal state. The allocation and freeing of these buffers is done outside of Python by the library itself, ...
- 73.1k
31
votes
3
answers
4k
views
Building a ctypes-"based" C library with distutils
Following this recommendation, I have written a native C extension library to optimise part of a Python module via ctypes. I chose ctypes over writing a CPython-native library because it was quicker ...
- 6,610
30
votes
2
answers
2k
views
Clean way of structuring ctypes class
I've defined a ctypes class and an associated convenience function like so:
class BNG_FFITuple(Structure):
_fields_ = [("a", c_uint32),
("b", c_uint32)]
class BNG_FFIArray(...
- 26k
29
votes
2
answers
28k
views
How to return array from C++ function to Python using ctypes
I am using ctypes to implement a C++ function in Python. The C++ function should return a pointer to an array. Unfortunately I haven't figured out, how to access the array in Python. I tried numpy....
- 663
29
votes
6
answers
17k
views
How do I pass large numpy arrays between python subprocesses without saving to disk?
Is there a good way to pass a large chunk of data between two python subprocesses without using the disk? Here's a cartoon example of what I'm hoping to accomplish:
import sys, subprocess, numpy
...
- 2,882
29
votes
1
answer
31k
views
Python and ctypes: how to correctly pass "pointer-to-pointer" into DLL?
I have a DLL that allocates memory and returns it. Function in DLL is like this:
void Foo( unsigned char** ppMem, int* pSize )
{
* pSize = 4;
* ppMem = malloc( * pSize );
for( int i = 0; i < ...
- 41.3k
28
votes
6
answers
26k
views
ctypes loading a c shared library that has dependencies
On Linux, I have a c shared library that depends on other libs. LD_LIBRARY_PATH is properly set to allow the linker to load all the libraries. When I do:
libgidcwf = ctypes.cdll.LoadLibrary(...
- 562
28
votes
3
answers
46k
views
Python & Ctypes: Passing a struct to a function as a pointer to get back data
I've looked through other answers but can't seem to get this to work. I'm trying to call a function within a DLL for communicating with SMBus devices. This function takes a pointer to a struct, which ...
- 307
27
votes
1
answer
14k
views
ctypes: construct pointer from arbitrary integer
For low-level purposes, I need to construct a ctypes pointer from an arbitrary address, given as an integer. For instance:
INTP = ctypes.POINTER(ctypes.c_int)
p = INTP(0x12345678) # i *know* this is ...
- 1,275
27
votes
2
answers
9k
views
ctypes vs C extension
I have a few functions written in C for a game project. These functions get called quite a lot (about 2000-4000 times per second). The functions are written in C for raw speed.
Now, the easiest way ...
- 114k
26
votes
6
answers
31k
views
How can I unload a DLL using ctypes in Python?
I'm using ctypes to load a DLL in Python. This works great.
Now we'd like to be able to reload that DLL at runtime.
The straightforward approach would seem to be:
1. Unload DLL
2. Load DLL
...
- 4,794
24
votes
6
answers
14k
views
Passing a list of strings to from python/ctypes to C function expecting char **
I have a C function which expects a list \0 terminated strings as input:
void external_C( int length , const char ** string_list) {
// Inspect the content of string_list - but not modify it.
}
...
- 241
24
votes
4
answers
24k
views
How do I interact with MATLAB from Python?
A friend asked me about creating a small web interface that accepts some inputs, sends them to MATLAB for number crunching and outputs the results. I'm a Python/Django developer by trade, so I can ...
- 2,583
24
votes
2
answers
2k
views
Loading and accessing multiple ctype instances
I have some existing C code that I am working with in Python. I am able to load the library using the following commands:
library_path = '/full/path/to/my/library.dylib'
lib1 = cdll.LoadLibrary(...
- 493
22
votes
7
answers
5k
views
Prototyping with Python code before compiling
I have been mulling over writing a peak-fitting library for a while. I know Python fairly well and plan on implementing everything in Python to begin with but envisage that I may have to re-implement ...
- 19k
22
votes
1
answer
6k
views
Swig python - c++ how to use type int8_t
I have a C function that takes as paramenter an 8 bit integer
int8_t foo( int8_t x );
I would like to call this function from my python code using a swig interface but int8_t type do not exists in ...
- 4,011
21
votes
6
answers
57k
views
Can't import dll module in Python
I've been stressin for a few days trying to compile a modified version of libuvc on windows and now that I've finally done it, I can't seem to load it on Python. This lib that I've already compiled ...
- 247
21
votes
5
answers
2k
views
Building Self-Referencing Tuples
After seeing a conversation in a forum from many years ago that was never resolved, it caused me to wonder how one would correctly create a tuple that referenced itself. Technically, this is a very ...
- 21.7k
21
votes
1
answer
6k
views
What is the difference between ctypes.pointer, ctypes.POINTER, and ctypes.byref?
In ctypes, what is the difference between pointer and byref? They both seem like a way to pass a pointer to a function, for example as an output parameter.
- 41.2k
19
votes
2
answers
36k
views
How to use NumPy array with ctypes?
I am still writing on a python interface for my c code with ctypes. Today I substituted my file reading function with a python version, which was programmed by somebody else using NumPy. The 'old' c ...
- 34.3k
19
votes
3
answers
36k
views
Python | accessing dll using ctypes
I'm trying to access some functions in a dll (nss3.dll) that ships with Firefox web browser. To handle this task I have used ctypes in Python. The problem is that it fails at the initial point which ...
- 15.1k
19
votes
2
answers
18k
views
Python ctypes: how to free memory? Getting invalid pointer error
I want to get some string from a C/C++ library with ctypes into python. My code looks like this:
Code in lib:
const char* get(struct something *x)
{
[...]
// buf is a stringstream
return ...
- 1,954
19
votes
2
answers
12k
views
Callbacks with ctypes (How to call a python function from C)
Is it possible to call a Python function from a C dll function?
We consider this C function:
void foo( void (*functionPtr)(int,int) , int a, int b);
On Python, I would like to call foo and set the ...
- 27.4k
19
votes
1
answer
16k
views
Different behaviour of ctypes c_char_p?
I am confused with this behaviour of different versions of python and dont understand why ?
Python 2.7.5 (default, Aug 25 2013, 00:04:04)
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on ...
- 1,281
19
votes
1
answer
23k
views
How to use malloc and free with python ctypes?
I have a function in my C library, say runsim() which takes pointer to struct repdata as one of the arguments, where struct repdata is given by
struct repdata {
int *var1;
int *var2;
int *...
- 32.7k
19
votes
1
answer
744
views
SWIG Python bindings to native code not working with OpenCV 2.1
I have an OpenCV project mixing Python and C. After changing to OpenCV 2.1, my calls to C code are not working any more, probably because OpenCV is no more using SWIG bindings.
From Python, I was ...
- 1,758
18
votes
1
answer
22k
views
How to convert pointer to c array to python array
I have a C++ callback function that calls into Python using ctypes. This function's parameters are a pointer to an array of double and the number of elements.
There are a lot of elements, ...
- 359
18
votes
5
answers
82k
views
Python import dll
How would I import a winDLL into python and be able to use all of its functions? It only needs doubles and strings.
- 1,808