Converting String() to c_char array in Python

Question

I'm using ctypes in order send the request to C++ application. For this I created the same structure in Python and C header file like this

C

typedef struct {
   int msg_id;
   unsigned char name[20];
} c_struct;

Python

from ctypes import *

class py_struct(BigEndianStructure):
    def __init__(self):
        BigEndianStructure.__init__(self)

    _fields_ = [('msg_id', c_int), ('name', c_char*20)]


.....

self.name = StringVar()
Entry(self, textvariable=self.name)

.....

def send(self):
   cmd = py_struct()
   cmd.name = self.name.get()

It doesn't work and raises a TypeError exception.

another code also doesn't work for me like this

name = (c_char*20)(self.name.get())
cmd.name = name

or

name = (c_char*20)(self.name.get().encode('utf-8'))
cmd.name = name

I've tried many ways to convert the Python string object with ctypes.

ctypes.c_char_p
ctypes.create_string_buffer

Can someone help me to convert a string object to c_char*20 array? Thanks in advance.

Eli · Accepted Answer · 2020-07-24 06:03:40Z

1

I've found the solution

name = bytes(self.name.get().encode('utf-8'))
cmd.name = name

answered Jul 24, 2020 at 6:03

Eli

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1 Comment

Mark Tolonen Over a year ago

.encode() returns a bytes object and defaults to utf-8 so cmd.name = self.name.get().encode() should be sufficient.

Mark Tolonen · Accepted Answer · 2020-07-24 21:12:34Z

0

Here's a reproducible example. .encode() should be all that's needed:

from tkinter import *
from ctypes import *

class Demo(Structure):
    _fields_ = [('name',c_char*20)]

root = Tk()
s = StringVar()
s.set('Hello, world!')

d = Demo()
d.name = s.get().encode()
print(d.name)

Output:

b'Hello, world!'

answered Jul 24, 2020 at 21:12

Mark Tolonen

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