How to pass an array of strings from python to C using ctypes and modify that array values in C

Question

Following is the C function para_trans_test.

void para_trans_test(char x [] [100])
{
    strncpy(x[0],"zzz",100);
}

Following is the python code which doesnt work.

lib.para_trans_test.argtypes= [ctypes.POINTER(ctypes.c_char_p)]
numParams=2
L=numpy.array(['xxx','yyy'])
print(type(L))
py_s1 = (ctypes.c_char_p * numParams) ()
py_s1[:]=L
print("py_s1=",py_s1)
lib.para_trans_test(py_s1)
print(py_s1)

Initially array L is ('xxx','yyy').

After calling the C function para_trans_test I want array L to be ('zzz','yyy')

which doesn't work how?

Antti Haapala
– Antti Haapala

2017-08-24 05:00:10 +00:00
Commented Aug 24, 2017 at 5:00 — Antti Haapala
– Antti Haapala, Commented Aug 24, 2017 at 5:00

Mark Tolonen · Accepted Answer · 2017-08-25 01:35:36Z

3

The argument type is wrong. POINTER(c_char_p) is equivalent to char**. What is needed is a pointer to a c_char array:

Test DLL:

#include <string.h>
__declspec(dllexport) void para_trans_test(char x [] [100])
{
    strncpy(x[0],"zzz",100);
}

Python:

from ctypes import *
lib = CDLL('test')
lib.para_trans_test.argtypes = [POINTER(c_char * 100)]
py_s1 = (c_char * 100 * 2)()
py_s1[0].value = b'xxx'
py_s1[1].value = b'yyy'
print(py_s1[0].value,py_s1[1].value)
lib.para_trans_test(py_s1)
print(py_s1[0].value,py_s1[1].value)

Output:

b'xxx' b'yyy'
b'zzz' b'yyy'

answered Aug 25, 2017 at 1:35

Mark Tolonen

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