通过UNIX域socket发送文件描述符，并选择（） [英] Sending file descriptor over UNIX domain socket, and select()

问题描述

我使用UNIX域套接字将文件描述符传输到另一个进程。这可以正常工作，但是当我第一次尝试使用select（）来查看套接字是否可写时，sendmsg（）调用失败，并显示错误的文件描述符错误。

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.

如果我不将文件描述符信息添加到msghdr结构体中，sendmsg（）函数可以与select（）结合使用，因此冲突似乎是在select（）和转移文件描述符之间。

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.

在这个手册页中找不到任何有关select（），recvmsg（）或任何其他信息的信息。因为这需要成为将文件描述符传递给多个进程的服务器，我仍然希望能够使用select（）。

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().

有什么我可以做这个工作，还是有人知道其他解决方案吗？

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

平台是Ubuntu 10.4。

Platform is Ubuntu 10.4.

是初始化结构的代码：

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)\n", strerror(errno), sock);

  return n;
}

再次，这个工作，只要我不调用select（）首先检查套接字是否准备好写入。

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

推荐答案

我尝试了此页面，由 nos ，即使它只是略有不同，它甚至当我将它与select（）组合使用时也是有效的。这就是现在的代码：

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)\n", strerror(errno), sock);

      return n;
        }

这篇关于通过UNIX域socket发送文件描述符，并选择（）的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！