Basic echo between Python client and C server via socket using Python ctypes

.python-c-echo

Basic echo between Python3 client and C server, and vice versa, via socket using Python ctypes

client.c

/* client.c */

#include <sys/socket.h>
#include <arpa/inet.h> //inet_addr
#include <unistd.h>    //write
#include <time.h>

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#pragma pack(1)

typedef struct payload_t {
    uint32_t id;
    uint32_t counter;
    float temp;
} payload;

#pragma pack()


void sendMsg(int sock, void* msg, uint32_t msgsize)
{
    if (write(sock, msg, msgsize) < 0)
    {
        printf("Can't send message.\n");
        close(sock);
        exit(1);
    }
    printf("Message sent (%d bytes).\n", msgsize);
    return;
}

int main()
{
    const int PORT = 2300;
    const char* SERVERNAME = "localhost";
    int BUFFSIZE = sizeof(payload);
    char buff[BUFFSIZE];
    int sock;
    int nread;
    float mintemp = -10.0;
    float maxtemp = 30.0;
    time_t t;

    srand((unsigned) time(&t));
    
    struct sockaddr_in server_address;
    memset(&server_address, 0, sizeof(server_address));
    server_address.sin_family = AF_INET;
    inet_pton(AF_INET, SERVERNAME, &server_address.sin_addr);
    server_address.sin_port = htons(PORT);

    if ((sock = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
        printf("ERROR: Socket creation failed\n");
        return 1;
    }

    if (connect(sock, (struct sockaddr*)&server_address, sizeof(server_address)) < 0) {
        printf("ERROR: Unable to connect to server\n");
        return 1;
    }

    printf("Connected to %s\n", SERVERNAME);

    payload data;
    for(int i = 0; i < 5; i++) {
        data.id = 1;
        data.counter = i;
        data.temp = mintemp + rand() / (RAND_MAX / (maxtemp - mintemp + 1.0) + 1.0);

        printf("\nSending id=%d, counter=%d, temp=%f\n", data.id, data.counter, data.temp);
        sendMsg(sock, &data, sizeof(payload));

        bzero(buff, BUFFSIZE);
        nread = read(sock, buff, BUFFSIZE);
        printf("Received %d bytes\n", nread);
        payload *p = (payload*) buff;
        printf("Received id=%d, counter=%d, temp=%f\n",
                p->id, p->counter, p->temp);
    }
    
    // close the socket
    close(sock);
    return 0;

}

client.py

#!/usr/bin/env python3

""" client.py - Echo client for sending/receiving C-like structs via socket

References:
- Ctypes: https://docs.python.org/3/library/ctypes.html
- Sockets: https://docs.python.org/3/library/socket.html
"""

import socket
import sys
import random
from ctypes import *


""" This class defines a C-like struct """
class Payload(Structure):
    _fields_ = [("id", c_uint32),
                ("counter", c_uint32),
                ("temp", c_float)]


def main():
    server_addr = ('localhost', 2300)
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

    try:
        s.connect(server_addr)
        print("Connected to {:s}".format(repr(server_addr)))

        for i in range(5):
            print("")
            payload_out = Payload(1, i, random.uniform(-10, 30))
            print("Sending id={:d}, counter={:d}, temp={:f}".format(payload_out.id,
                                                              payload_out.counter,
                                                              payload_out.temp))
            nsent = s.send(payload_out)
            # Alternative: s.sendall(...): coontinues to send data until either
            # all data has been sent or an error occurs. No return value.
            print("Sent {:d} bytes".format(nsent))

            buff = s.recv(sizeof(Payload))
            payload_in = Payload.from_buffer_copy(buff)
            print("Received id={:d}, counter={:d}, temp={:f}".format(payload_in.id,
                                                               payload_in.counter,
                                                               payload_in.temp))
    except AttributeError as ae:
        print("Error creating the socket: {}".format(ae))
    except socket.error as se:
        print("Exception on socket: {}".format(se))
    finally:
        print("Closing socket")
        s.close()


if __name__ == "__main__":
    main()

server.c

/* server.c */

#include <sys/socket.h>
#include <arpa/inet.h> //inet_addr
#include <unistd.h>    //write

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#pragma pack(1)

typedef struct payload_t {
    uint32_t id;
    uint32_t counter;
    float temp;
} payload;

#pragma pack()


int createSocket(int port)
{
    int sock, err;
    struct sockaddr_in server;

    if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    {
        printf("ERROR: Socket creation failed\n");
        exit(1);
    }
    printf("Socket created\n");

    bzero((char *) &server, sizeof(server));
    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(port);
    if (bind(sock, (struct sockaddr *)&server , sizeof(server)) < 0)
    {
        printf("ERROR: Bind failed\n");
        exit(1);
    }
    printf("Bind done\n");

    listen(sock , 3);

    return sock;
}

void closeSocket(int sock)
{
    close(sock);
    return;
}

void sendMsg(int sock, void* msg, uint32_t msgsize)
{
    if (write(sock, msg, msgsize) < 0)
    {
        printf("Can't send message.\n");
        closeSocket(sock);
        exit(1);
    }
    printf("Message sent (%d bytes).\n", msgsize);
    return;
}

int main()
{
    int PORT = 2300;
    int BUFFSIZE = 512;
    char buff[BUFFSIZE];
    int ssock, csock;
    int nread;
    struct sockaddr_in client;
    int clilen = sizeof(client);

    ssock = createSocket(PORT);
    printf("Server listening on port %d\n", PORT);

    while (1)
    {
        csock = accept(ssock, (struct sockaddr *)&client, &clilen);
        if (csock < 0)
        {
            printf("Error: accept() failed\n");
            continue;
        }

        printf("Accepted connection from %s\n", inet_ntoa(client.sin_addr));
        bzero(buff, BUFFSIZE);
        while ((nread=read(csock, buff, BUFFSIZE)) > 0)
        {
            printf("\nReceived %d bytes\n", nread);
            payload *p = (payload*) buff;
            printf("Received contents: id=%d, counter=%d, temp=%f\n",
                    p->id, p->counter, p->temp);

            printf("Sending it back.. ");
            sendMsg(csock, p, sizeof(payload));
        }
        printf("Closing connection to client\n");
        printf("----------------------------\n");
        closeSocket(csock);
    }

    closeSocket(ssock);
    printf("bye");
    return 0;
}

server.py

#!/usr/bin/env python3

""" server.py - Echo server for sending/receiving C-like structs via socket

References:
- Ctypes: https://docs.python.org/3/library/ctypes.html
- Sockets: https://docs.python.org/3/library/socket.html
"""

import socket
import sys
import random
from ctypes import *


""" This class defines a C-like struct """
class Payload(Structure):
    _fields_ = [("id", c_uint32),
                ("counter", c_uint32),
                ("temp", c_float)]


def main():
    PORT = 2300
    server_addr = ('localhost', PORT)
    ssock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    print("Socket created")

    try:
        # bind the server socket and listen
        ssock.bind(server_addr)
        print("Bind done")
        ssock.listen(3)
        print("Server listening on port {:d}".format(PORT))

        while True:
            csock, client_address = ssock.accept()
            print("Accepted connection from {:s}".format(client_address[0]))

            buff = csock.recv(512)
            while buff:
                print("\nReceived {:d} bytes".format(len(buff)))
                payload_in = Payload.from_buffer_copy(buff)
                print("Received contents id={:d}, counter={:d}, temp={:f}".format(payload_in.id,
                                                            payload_in.counter,
                                                            payload_in.temp))
                print("Sending it back.. ", end='')
                nsent = csock.send(payload_in)
                print("Sent {:d} bytes".format(nsent))
                buff = csock.recv(512)

            print("Closing connection to client")
            print("----------------------------")
            csock.close()

    except AttributeError as ae:
        print("Error creating the socket: {}".format(ae))
    except socket.error as se:
        print("Exception on socket: {}".format(se))
    except KeyboardInterrupt:
        ssock.close()
    finally:
        print("Closing socket")
        ssock.close()


if __name__ == "__main__":
    main()

Yes sure, you can include the uint32 array inside a Payload structure, along with other data. Not sure how to handle dynamic size array in that case, though

Hello, I'm trying to sent a str from client to server using the ctype structure but I couldn't, could you help me?

@Mario-RC Hello, I am trying the same but to no result. Have you been able to do it?

@Mario-RC Hello, I am trying the same but to no result. Have you been able to do it?

@xicocana Hi! I finally decided not to use ctype structure, instead I encoded the message and sent it at a byte level.

You can check my code here: https://github.com/Mario-RC/UDP-sockets

@Mario-RC thanks for the fast reply! Yesterday I ended up doing the same. I'll still star your rep it looks useful for the future! thanks again

@xicocana It is a pleasure, thank you very much!

Hey, thanks for the gist. Can you guide me on the vice-versa process? C client to Python server?

Hi @pra-dan, the payload handling is quite similar, just need to switch roles. I added the files to show an example of C client and Python server.

Any ideas on how to modify the above to communicate with FastAPI Websockets either directly from the C client or from the Python server?

Something like this:
C client code ----> Python server.py <----> FastAPI
OR
C client code ----> FastAPI

@marcom04 @Mario-RC

Awesome code! Thanks mate!