Help on ctypes.POINTER for Python array
Jason Qian
jqian at tibco.com
Thu May 11 13:58:33 EDT 2023
Awesome, thanks!
On Thu, May 11, 2023 at 1:47 PM Eryk Sun <eryksun at gmail.com> wrote:
> On 5/11/23, Jason Qian via Python-list <python-list at python.org> wrote:
> >
> > in the Python, I have a array of string
> > var_array=["Opt1=DG","Opt1=DG2"]
> > I need to call c library and pass var_array as parameter
> > In the argtypes, how do I set up ctypes.POINTER(???) for var_array?
> >
> > func.argtypes=[ctypes.c_void_p,ctypes.c_int, ctypes.POINTER(????)]
> >
> > In the c code:
> > int func (void* obj, int index, char** opt)
>
> The argument type is ctypes.POINTER(ctypes.c_char_p), but that's not
> sufficient. It doesn't implement converting a list of str objects into
> an array of c_char_p pointers that reference byte strings. You could
> write a wrapper function that implements the conversion before calling
> func(), or you could set the argument type to a custom subclass of
> ctypes.POINTER(ctypes.c_char_p) that implements the conversion via the
> from_param() class method.
>
> https://docs.python.org/3/library/ctypes.html#ctypes._CData.from_param
>
> Here's an example of the latter.
>
> C library:
>
> #include <stdio.h>
>
> int
> func(void *obj, int index, char **opt)
> {
> int length;
> for (length=0; opt[length]; length++);
> if (index < 0 || index >= length) {
> return -1;
> }
> return printf("%s\n", opt[index]);
> }
>
>
> Python:
>
> import os
> import ctypes
>
> lib = ctypes.CDLL('./lib.so')
> BaseOptions = ctypes.POINTER(ctypes.c_char_p)
>
> class Options(BaseOptions):
> @classmethod
> def from_param(cls, param):
> if isinstance(param, list):
> new_param = (ctypes.c_char_p * (len(param) + 1))()
> for i, p in enumerate(param):
> new_param[i] = os.fsencode(p)
> param = new_param
> return BaseOptions.from_param(param)
>
> lib.func.argtypes = (ctypes.c_void_p, ctypes.c_int, Options)
>
>
> demo:
>
> >>> opts = ['Opt1=DG', 'Opt1=DG2']
> >>> lib.func(None, 0, opts)
> Opt1=DG
> 8
> >>> lib.func(None, 1, opts)
> Opt1=DG2
> 9
>
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