socket close inside fork c++

Question

I am trying socket programming and with the server code:

    while(1) {
    sin_size = sizeof (their_addr);
    new_fd = accept(sockfd,(struct sockaddr *)&their_addr, &sin_size);
    if(new_fd == -1) {
        perror("accept");
        continue;
    }

    inet_ntop(their_addr.ss_family, get_in_addr((struct sockaddr*)&their_addr),s,sizeof(s));
    cout<<"got connection from" << s << endl;

    if((pid = fork()) == 0){//child
        close(sockfd);
        if(send(new_fd,"hello world!",12,0) == -1) {
            perror("send");
            close(new_fd);
            exit(0);
        }
        char buf[50];
        int numbytes;
        if((numbytes = recv(new_fd,&buf,50,0)) == -1) {
            perror("receive");
            close(new_fd);
            exit(0);
        }
        buf[numbytes] = '\0';
        cout<<"numbytes" << numbytes <<endl;
        cout<<"server received " << buf <<endl;
    }
    close(new_fd);
}

this code gives me a bad file descriptor, however when i comment the close(sockfd), the code runs fine. Since I am forking to a new child process and closing the listening port , why am I getting a bad file descriptor? Is there something I am missing here.

I'm assuming that `sockfd` is the passive listening socket, and `new_fd` is a newly accepted socket? — Some programmer dude, Feb 23 '13 at 04:58
_Where_ are you getting the "bad file descriptor" error? Which call? — Some programmer dude, Feb 23 '13 at 04:59
i guess it is at the recv part since I get hello world on the client — Ravi Nankani, Feb 23 '13 at 05:03
You _guess_ it's the receive part? Do you get the `perror` output from there? Add error checking to the `close` calls as well. — Some programmer dude, Feb 23 '13 at 05:25
i tried debugging the child process from gdb and it there was no error there. What is my guess is that when i close the sockfd from the client, it gets closed for the parent process as well leading to bad file descriptor. — Ravi Nankani, Feb 23 '13 at 05:35

score 1 · Answer 1 · answered Feb 23 '13 at 05:09

1

You close new_fd twice. After you receive you close it once, then the child process continues and close it again. You should put that second close only in the parent process, or not close at all inside the if (fork()) body.

answered Feb 23 '13 at 05:09

Some programmer dude

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i changed that to only closing the new_fd in parent, but the problem persists. – Ravi Nankani Feb 23 '13 at 05:15

score 1 · Answer 2 · answered Feb 17 '15 at 15:08

I know it is almost 2 years since the question was posed, but I had the same issue, and after googling around and not finding anything I decided to try working on it for a while. I found the issue to be that you don't close the child process unless there's a send error. So instead of having this:

if (send(new_fd,"hello world!",12,0) == -1) {
    perror("send");
    close(new_fd);
    exit(0);
}

Move the closing bracket right below the perror like this:

if (send(new_fd,"hello world!",12,0) == -1) {
    perror("send");
}    
close(new_fd);
exit(0);

By doing this, you force the child process to close right after sending the message. I'm not entirely sure, but I think that the Bad file descriptor comes from the call to accept, by the forked child process, which hasn't been closed but has a Bad file descriptor, since you've closed the listening socket for this process.

socket close inside fork c++

2 Answers2