如何从接受调用获取套接字端口号（C UNIX）

Question

i've done a simple client/server program where the server wait for an external connection and return the connection-socket if the port number of the client is in the range of [1025-2048] otherwise return -1. The problem is that when i get the port number by the client adress (which should be stored in the sockaddr structure) it says me that the client port number is zero, but in the client program i've set the client portnumber to 1999.

SERVER

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/signal.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <errno.h>


int function(int fd_socket) {

    int fd_socket_acc;
    int len;
    int port;
    struct sockaddr_in client_addr;

    puts("WAITING FOR CLIENT...");

    fd_socket_acc = accept(fd_socket, (struct sockaddr*)&client_addr, &len);

    puts("CONNECTION DONE.");

    port = ntohs (client_addr.sin_port);
    printf("client port number: %d \n", port);


    if (port >= 1024 && port <= 2048) {

        close (fd_socket_acc);
        return fd_socket_acc;
    }   
    else {
        close(fd_socket_acc);
        return -1;
    }
}

int main(int argc, char *argv[]) {


    int fd_socket;
    struct sockaddr_in local_addr;

    fd_socket = socket(AF_INET, SOCK_STREAM, 0);

    local_addr.sin_family = AF_INET;
    local_addr.sin_port = htons(1887);
    local_addr.sin_addr.s_addr = INADDR_ANY;

    bind(fd_socket, (struct sockaddr*)&local_addr, sizeof(local_addr));

    listen(fd_socket, 3);

    function(fd_socket);

    //close(fd_socket);

}

CLIENT

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/signal.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <errno.h>

int main(int argc, char *argv[]) {

    int fd_socket;
    struct sockaddr_in local_addr;
    struct sockaddr_in server_addr;
    struct hostent *hp;

    fd_socket = socket(AF_INET, SOCK_STREAM, 0);

    local_addr.sin_family = AF_INET;
    local_addr.sin_port = htons(1999);
    local_addr.sin_addr.s_addr = INADDR_ANY;

    bind(fd_socket, (struct sockaddr*)&local_addr, sizeof(local_addr));

    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(1887);
    //hostname is "ubuntu"
    hp = gethostbyname("ubuntu");
    bcopy(hp->h_addr, &server_addr.sin_addr, 4);

    printf("%d \n", ntohs(local_addr.sin_port));

    connect(fd_socket, (struct sockaddr*)&server_addr, sizeof(server_addr));

    wait(2);

    close(fd_socket);

}

If i get the port number in client with a printf("%d", ntohs(local_addr.sin_port)) it stamps correctly 1999 , but if i get the port number of client in server with printf("%d", ntohs(client_addr.sin_port)) it stamps 0 . Why?

thanks in advance!

Answer 1

In order to obtain the client port number in client_addr through accept you have to tell accept how big that buffer is by setting

socklen_t len = sizeof(client_addr);

You can alternatively retrieve it by calling afterwards

len = sizeof(client_addr);
getpeername(fd_socket_acc, (struct sockaddr*)&client_addr, &len);

Answer 2

Maybe because you do not set the variable len to anything, and I suspect that your compiler sets it to 0.
What happens is that you try to accept with an undefined len size.

Adding len=sizeof( struct sockaddr_in ); before making a call to accept would help to fill the passed client_addr correctly.