C++ client not receiving any data from Python server

Question

I'm working on a project with a C++ client and a Python server.

int connecting(SOCKET s){
    WSADATA wsa;
    struct sockaddr_in server;
    if (WSAStartup(MAKEWORD(2,2),&wsa) != 0)
    {
        std::cout<<"WSA error";
        return 1;
    }
    if((s=socket(AF_INET, SOCK_STREAM, 0))==INVALID_SOCKET){
        std::cout<<"invalid socket";
        return 1;
    }
    server.sin_addr.s_addr = inet_addr("192.168.1.10");
    server.sin_family = AF_INET;
    server.sin_port = htons(2424);

    if(connect(s , (struct sockaddr *)&server , sizeof(server))!=0){
        std::cout<<"error connecting";
        return 1;
    }
    else{
        return 0;
    }
}

void recv_data(SOCKET socket){
    char buf[buflen];
    recv(socket, buf, buflen, 0);
    std::cout<<buf;
}

int main(int argc, char* argv[]){
   SOCKET s;
   connecting(s);
   recv_data(s);
}

Here's the very basic Python server.

import socket
import struct

s=socket.socket()
s.bind(("192.168.1.10", 2424))
s.listen(1)
c, a=s.accept()
print(f"{a}")
data="Hi from server"
c.send(data.encode())

After establishing the connection in the server, the a variable is printed. Nothing is received nor printed in the client-side. I tried putting in a loop the recv() function in the client but it does not work.

Answer 1

You misunderstood argument passing by value...

void f(int x) {
    x = 5;
}

int main() {
    int j = 7;
    f(j);
    printf("%d\n", j);
}

What is j? NOT 5! The number 7 was passed into f, where it was stored in the variable x, and then the number 5 was stored in the variable x, which is not the variable j, so j was not changed.

You have this in your program:

int main(int argc, char* argv[]){
   SOCKET s;
   connecting(s);
   recv_data(s);
}

After the call connecting(s) what is s? NOT A SOCKET! Only the variable s inside the function connecting held a socket handle, and the variable s in main is not that variable, so it never got to hold a socket handle. You pass some uninitialized value to recv_data.

The function recv probably reads this random uninitialized value and returns an error code meaning "hey, that isn't a socket handle" but since your code never checked whether it returned an error code, you wouldn't know about this.