CODEWITHSUNDEEP
pythoncctypes
Orca
Orca

Reputation: 177

How to produce a character array in ctypes and pass its pointer as a function argument

I want to use some information in C++ that is contained in a serialised buffer in a list of characters in Python, using ctypes. I have tried the following, but I don't understand why my argument types are throwing an error.

I have a C wrapper to some C++ code, of the form

extern "C" {
    special_object *special_object_new() { return new special_object(); }

    int function(special_object *O, int buffer_size, char *buffer) {
        char *buf = (char*) malloc(buffer_size);
        for(int i = 0; i < buffer_size; i++) buf[i] = buffer[i];
        O->do_something_in_cpp_with_buf(buf);
        free(buf);
        buf = NULL;
        return 0;
    }
}

I would like to pass function a character buffer buf from Python ctypes. This character buffer originally appears in Python as a list of characters, so I first convert it to a C character array using

import ctypes
import cdll
buf_c = (ctypes.c_char*len(buf))(*buf)

and then cast it to a pointer using

buf_p = ctypes.cast(buf_c, ctypes.POINTER(ctypes.c_char))

which has a type

>>> <class 'ctypes.LP_c_char'>

I already know the size of this buffer, buf_size, as an integer.

I execute the following (loading library and declaring function arguments),

lib = cdll.LoadLibrary('PATH_TO_LIB/lib.so')
lib.function.restype = ctypes.c_int
lib.function.argtypes = [ctypes.c_int, ctypes.POINTER(ctypes.c_char)]

Then I make the class containing the function,

class Special(object):
    def __init__(self):
        self.obj = lib.special_object_new()

    def py_function(self, buffer_size, buffer):
        res = lib.function(self.obj, buffer_size, buffer)

Finally, I try to call it, as

s = Special()
s.py_function(ctypes.c_int(buf_size), buffer)

but I get the error

ArgumentError: argument 1: <type 'exceptions.TypeError'>: wrong type

I can't work out what I am doing wrong when I make the pointer? Any help would be appreciated!

Upvotes: 1

Views: 1782

Answers (1)

Mark Tolonen
Mark Tolonen

Reputation: 177971

You must define all the arguments for each function. Here's a working example:

test.cpp

#ifdef _WIN32
#   define API __declspec(dllexport)
#else
#   define API
#endif

#include <stdio.h>

struct special_object {
    special_object() {
        printf("created\n");
    }

    ~special_object() {
        printf("destroyed\n");
    }

    void print(char* buffer, int buffer_size) {
        for(int i = 0; i < buffer_size; i++)
            printf("%02X ",static_cast<unsigned char>(buffer[i]));
        printf("\n");
    }
};

extern "C" {
    API special_object *special_object_new() {
        return new special_object();
    }

    API void special_object_print(special_object *O, char *buffer, int buffer_size) {
        O->print(buffer, buffer_size);
    }

    API void special_object_delete(special_object *O) {
        delete O;
    }
}

test.py

from ctypes import *

lib = CDLL('test')
lib.special_object_new.argtypes = ()
lib.special_object_new.restype = c_void_p
lib.special_object_print.argtypes = c_void_p, c_char_p, c_int
lib.special_object_print.restype = None
lib.special_object_delete.argtypes = c_void_p,
lib.special_object_delete.restype = None

class Special:
    def __init__(self):
        self.obj = lib.special_object_new()
    def __del__(self):
        lib.special_object_delete(self.obj)
    def print(self,buffer):
        lib.special_object_print(self.obj,buffer,len(buffer))

s = Special()
s.print(bytes([0x01,0x02,0xaa,0x55]))
s.print(b'hello, world!')

Output:

created
01 02 AA 55
68 65 6C 6C 6F 2C 20 77 6F 72 6C 64 21
destroyed

Upvotes: 3

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