Very Computer

> I used to have a program which will fork several
> child processes, and then listen on the net waiting
> for client's requests. After the program gets a request
> it will then pass the file descriptor of the opened socket
> to one of the child processes. The program works fine
> on Sun OS 4.1.4, Solaris 2.5, and HPUX.

> After I got my own Linux box set up, I tried to port the
> program using the msghdr, cmsghdr method, however it
> seems that the child process has problem getting the
> file descriptor from the parent. Don't know what
> the problem is, have no idea whether Linux support
> this or not. Can anybody help me here?

> Linux does support this.  I got it working from the examples out of
> Richard Stevens' "UNIX Network Programming" and "Advanced Programming in
> the UNIX Environment," with the following code:

> #define CONTROLLEN      (sizeof(struct cmsghdr) + sizeof(int))
> #define CMSG_DATA(cmsg) ((u_char *)((cmsg) + 1))

> // sockfd is the socket between the processes.
> // fd is the new file descriptor to be passed.
> int send_fd(int sockfd, int fd)
> {
>     struct iovec        iov[1];
>     char buf[2];
>     struct msghdr       msg;
>     int                 *ptr;
>     static struct cmsghdr       *cmptr = NULL;

>     buf[0] = 'w';
>     buf[1] = '\0';
>     iov[0].iov_base = buf;      // No data to send
>     iov[0].iov_len  = 2;

>     if (cmptr == NULL &&
>         (cmptr = (struct cmsghdr *) malloc(CONTROLLEN)) == NULL)
>         return -1;

>     cmptr->cmsg_level = SOL_SOCKET;
>     cmptr->cmsg_type  = SCM_RIGHTS;
>     cmptr->cmsg_len   = CONTROLLEN;
>     *(int *)CMSG_DATA(cmptr) = fd;

>     msg.msg_iov        = iov;
>     msg.msg_iovlen     = 1;
>     msg.msg_name       = (caddr_t) 0;
>     msg.msg_namelen    = 0;
>     msg.msg_control    = (caddr_t) cmptr;
>     msg.msg_controllen = CONTROLLEN;

>     if (sendmsg(sockfd, &msg, 0) < 0)
>         return -1;

>     return 0;
> }

> // sockfd is the socket used to communicate with the other process
> // returns the new file descriptor
> int recv_fd(int sockfd)
> {
>     int fd;
>     int *ptr;
>     char buf[10];
>     struct iovec iov[1];
>     struct msghdr msg;
>     static struct cmsghdr *cmptr = NULL;

>     if (cmptr == NULL &&
>         (cmptr = (struct cmsghdr *) malloc(CONTROLLEN)) == NULL)
>         return -1;

>     iov[0].iov_base = buf;
>     iov[0].iov_len = 10;
>     msg.msg_iov = iov;
>     msg.msg_iovlen = 1;
>     msg.msg_name = (caddr_t) 0;
>     msg.msg_namelen = 0;
>     msg.msg_control = (caddr_t) cmptr;
>     msg.msg_controllen = CONTROLLEN;

>     if (recvmsg(sockfd, &msg, 0) < 0)
>         return -1;

>     fd = *(int *)CMSG_DATA(cmptr);

>     return(fd);
> }

> Eric