c - Sending file descriptor over UNIX domain socket, and select()

Question

I'm using a UNIX domain socket to transfer a file descriptor to another process. This works fine, but when I first try to see if socket is writeable using select(), the sendmsg() call fails with a Bad File Descriptor error.

The sendmsg() function does work fine in combination with select() if I don't add the file descriptor info to the msghdr struct, so the conflict seems to be between select() and transferring file descriptors.

I couldn't find any info on this in the man pages for select(), recvmsg(), or any other. Since this needs to become a server which hands out file descriptors to multiple processes, I'd still like to be able to use select().

Is there anything I can do to make this work, or does anyone know of alternative solutions?

Platform is Ubuntu 10.4.

This is the code that initializes the structures:

struct cmsghdr_fd : public cmsghdr
{
  int fd;
};

int sendfd(int sock, int fd)
{
  struct msghdr hdr;
  struct iovec data;
  struct cmsghdr_fd msgdata;

  char dummy = '*';
  data.iov_base = &dummy;
  data.iov_len = sizeof(dummy);

  hdr.msg_name = NULL;
  hdr.msg_namelen = 0;
  hdr.msg_iov = &data;
  hdr.msg_iovlen = 1;
  hdr.msg_flags = 0;

  hdr.msg_control = &msgdata;
  hdr.msg_controllen = sizeof(msgdata);

  struct cmsghdr* cmsg = CMSG_FIRSTHDR(&hdr);
  cmsg->cmsg_len   = hdr.msg_controllen;
  cmsg->cmsg_level = SOL_SOCKET;
  cmsg->cmsg_type  = SCM_RIGHTS;

  *(int*)CMSG_DATA(cmsg) = fd;

  int n = sendmsg(sock, &hdr, 0);

  if(n == -1)
    printf("sendmsg() failed: %s (socket fd = %d)
", strerror(errno), sock);

  return n;
}

Again, this works, as long as I don't call select() first to check whether the socket is ready for writing.

Answer 1

I tried the sendfd code at this page, which was kindly provided by nos, and even though it's only slightly different, it works even when I use it in combination with select(). This is what the code looks like now:

    int sendfd(int sock, int fd)
    {
      struct msghdr hdr;
      struct iovec data;

      char cmsgbuf[CMSG_SPACE(sizeof(int))];

      char dummy = '*';
      data.iov_base = &dummy;
      data.iov_len = sizeof(dummy);

      memset(&hdr, 0, sizeof(hdr));
      hdr.msg_name = NULL;
      hdr.msg_namelen = 0;
      hdr.msg_iov = &data;
      hdr.msg_iovlen = 1;
      hdr.msg_flags = 0;

      hdr.msg_control = cmsgbuf;
      hdr.msg_controllen = CMSG_LEN(sizeof(int));

      struct cmsghdr* cmsg = CMSG_FIRSTHDR(&hdr);
      cmsg->cmsg_len   = CMSG_LEN(sizeof(int));
      cmsg->cmsg_level = SOL_SOCKET;
      cmsg->cmsg_type  = SCM_RIGHTS;

      *(int*)CMSG_DATA(cmsg) = fd;

      int n = sendmsg(sock, &hdr, 0);

      if(n == -1)
        printf("sendmsg() failed: %s (socket fd = %d)
", strerror(errno), sock);

      return n;
        }