通过 UNIX 域套接字发送文件描述符和 select()
我正在使用 UNIX 域套接字将文件描述符传输到另一个进程。这工作正常,但是当我第一次尝试使用 select() 查看套接字是否可写时,sendmsg() 调用失败并出现错误文件描述符错误。
如果我不将文件描述符信息添加到 msghdr 结构中,则 sendmsg() 函数与 select() 结合使用效果很好,因此冲突似乎发生在 select() 和传输文件描述符之间。
我在 select()、recvmsg() 或任何其他的手册页中找不到任何相关信息。由于这需要成为一个向多个进程分发文件描述符的服务器,我仍然希望能够使用 select()。
我可以做些什么来实现这项工作,或者有人知道替代解决方案吗?
平台是Ubuntu 10.4。
这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;
}
同样,只要我不首先调用 select() 来检查套接字是否准备好写入,这就可以工作。
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)\n", 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.
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我在此页面尝试了sendfd代码,其中由 nos 善意提供,尽管它只是略有不同,但即使我将它与 select( 结合使用,它也能工作。 )。这就是代码现在的样子:
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: