pointers - set c_char_p in Python -

in c header file have:

long test_api callandsave(        ___out_ char param1[31],      ___out_ char param2[5],      ___out_ char param3[21],      ___out_ char* pointerparam     ); 

i trying set , pass parameters in python , pass them callandsave:

import ctypes ctypes import * lib = cdll.loadlibrary('./testlib.so')  param1 = (ctypes.c_char*31)() param2 = (ctypes.c_char*5)() param3 = (ctypes.c_char*21)() pointerparam = ctypes.c_char_p()  lib.efttrx(param1,param2,param3,pointerparam) 

but getting "output parameter null" error pointerparam. how should define "___out_ char* pointerparam" in python correctly?

___out_ char* pointerparam 

the ___out_ in specification means pointer have data written it. must allocate buffer write to. ctypes.c_char_p() doesn't make buffer - it's null pointer.

look ctypes.create_string_buffer. must figure out how big of buffer callandsave expects.