On 5/11/23, Jason Qian via Python-list <
python-list@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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