通过 python 套接字接收 ctypes `c_wchar_p` 的问题（python 3.7.7）

Question

(Windows 10, x64, PyCharm)

short story: I have a problem with payload_in.event and payload_out.event in the python server script. Both are from type c_wchar_p which is from ctypes .

long story: I am working on a client and a server in python currently. They communicate over TCP/IP sockets on 127.0.01:6666. I am using the module Ctypes to send and receive a c-like struct. Everything works perfectly, I can send/recv arrays, integers, floats and work with them on both sides but I found out that there is a bug when printing payload_in.event and payload_out.event in the python server script, both of type c_wchar_p .

In the client script, I can access and print payload_in.event and payload_out.event perfectly ***but my server script can not even though the code is the same. . .

In the server script, printing payload_in.event and payload_out.event causes both programs to pause for 10 seconds or so and then the client console tells me that the connection to the server was closed.

Everything works fine if I comment out the printing of payload_in.event and payload_out.event in the python server script. I don't know how to fix this behaviour and would be glad if someone could help me out.

beagleServer.py

import socket
from ctypes import *


class Struct(Structure):
    _fields_ = [("id", c_uint32),  # 4B
                ("prio", c_uint32),  # 4B
                ("event", c_wchar_p),  # 8B
                ("data", c_float * 10)]  # 40B


def main():
    BUFFSIZE = sizeof(Struct)
    host = '127.0.0.1'
    port = 6666

    beagleServer = socket.socket()
    beagleServer.bind((host, port))
    beagleServer.listen(1)

    print("Server is listening on " + host + ":" + str(port) + "!")
    client, clientAddress = beagleServer.accept()
    print("Connection from: " + str(clientAddress[0]) + ":" + str(clientAddress[1]))

    # receiving payload
    buffer = client.recv(BUFFSIZE)
    payload_in = Struct.from_buffer_copy(buffer)
    print("\nPayload of ", sizeof(payload_in), " Bytes has been received successfully: ", payload_in,
          "\nPayload.id: ", payload_in.id,
          "\nPayload.prio: ", payload_in.prio,
          "\nPayload.event: ", payload_in.event,
          "\nPayload.data: ", list(payload_in.data),
          "\nReceived buffer: ", buffer)

    # sending back payload
    payload_out = payload_in
    transferredBytes = client.send(payload_out)
    print("\nPayload of ", transferredBytes, " Bytes has been sent successfully: ", payload_out,
          "\nPayload.id: ", payload_out.id,
          "\nPayload.prio: ", payload_out.prio,
          "\nPayload.event: ", payload_out.event,
          "\nPayload.data: ", list(payload_out.data))

    print("Closing server socket!")
    client.close()

if __name__ == '__main__':
    main()

beagleClient.py

import socket
from ctypes import *


class Struct(Structure):
    _fields_ = [("id", c_uint32),  # 4B
                ("prio", c_uint32),  # 4B
                ("event", c_wchar_p),  # 8B
                ("data", c_float * 10)]  # 40B


beagleServer = ('127.0.0.1', 6666)
beagleClient = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
beagleClient.connect(beagleServer)
print("Connection with server has been established!")

# Creating the Struct
payload_out = Struct(390897,
                     0,
                     "activateMode1",
                     (3.12, 4.21, 1.23))
print("\nPayload of ", sizeof(payload_out), " Bytes has been created successfully: ", payload_out,
      "\nPayload.id: ", payload_out.id,
      "\nPayload.prio: ", payload_out.prio,
      "\nPayload.event: ", payload_out.event,
      "\nPayload.data: ", list(payload_out.data))

# Sending
bytesTransferred = beagleClient.send(payload_out)
print("\nPayload of ", bytesTransferred, " Bytes has been sent!")

# Receiving
BUFFSIZE = sizeof(Struct)
buffer = beagleClient.recv(BUFFSIZE)
payload_in = Struct.from_buffer_copy(buffer)
print("\nPayload of ", sizeof(payload_in), " Bytes has been received successfully: ", payload_in,
      "\nPayload.id: ", payload_in.id,
      "\nPayload.prio: ", payload_in.prio,
      "\nPayload.event: ", payload_in.event,
      "\nPayload.data: ", list(payload_in.data))

print("Closing the connection to ", beagleServer, "!")
beagleClient.close()

Answer 1

Struct.event is a pointer. A pointer transmitted between two processes is just an address that has no meaning in the other process. Use an array instead:

("event", c_wchar * 80)

It can still be initialized the same way, but you can't exceed the array size for the string transmitted. Also c_wchar size varies between architectures, so you may want to use c_char * 80 instead, and then encode/decode the string, eg payload.event = 'some string'.encode() (or just use a byte string: b'some string' ).