如何使用sendmsg（）通过2个进程之间的套接字发送文件描述符？

Question

@cnicutar在此问题上回答了我之后，我尝试将文件描述符从父进程发送到其子进程。基于此示例，我编写了以下代码：

int socket_fd ,accepted_socket_fd, on = 1;
int server_sd, worker_sd, pair_sd[2];
struct sockaddr_in client_address;
struct sockaddr_in server_address;

/* =======================================================================
 * Setup the network socket.
 * =======================================================================
 */

if((socket_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
{
    perror("socket()");
    exit(EXIT_FAILURE);
}

if((setsockopt(socket_fd, SOL_SOCKET,  SO_REUSEADDR, (char *) &on, sizeof(on))) < 0)
{
    perror("setsockopt()");
    exit(EXIT_FAILURE);
}

server_address.sin_family = AF_INET;                 /* Internet address type */
server_address.sin_addr.s_addr = htonl(INADDR_ANY);  /* Set for any local IP */
server_address.sin_port = htons(port);               /* Set to the specified port */

if(bind(socket_fd, (struct sockaddr *) &server_address, sizeof(server_address)) < 0)
{
    perror("bind()");
    exit(EXIT_FAILURE);
}

if(listen(socket_fd, buffers) < 0)
{
    perror("listen()");
    exit(EXIT_FAILURE);
}

if(socketpair(AF_UNIX, SOCK_DGRAM, 0, pair_sd) < 0)
{
    socketpair("bind()");
    exit(EXIT_FAILURE);
}

server_sd = pair_sd[0];
worker_sd = pair_sd[1];



/* =======================================================================
 * Worker processes
 * =======================================================================
 */

struct iovec   iov[1];
struct msghdr  child_msg;
char   msg_buffer[80];
int pass_sd, rc;


/* Here the parent process create a pool of worker processes (its children) */
for(i = 0; i < processes; i++)
{
    if(fork() == 0)
    {
        // ...

        /* Loop forever, serving the incoming request */
        for(;;)
        {

            memset(&child_msg,   0, sizeof(child_msg));
            memset(iov,    0, sizeof(iov));

            iov[0].iov_base = msg_buffer;
            iov[0].iov_len  = sizeof(msg_buffer);
            child_msg.msg_iov     = iov;
            child_msg.msg_iovlen  = 1;
            child_msg.msg_name    = (char *) &pass_sd;
            child_msg.msg_namelen = sizeof(pass_sd);

            printf("Waiting on recvmsg\n");
            rc = recvmsg(worker_sd, &child_msg, 0);
            if (rc < 0)
            {
               perror("recvmsg() failed");
               close(worker_sd);
               exit(-1);
            }
            else if (child_msg.msg_namelen <= 0)
            {
               printf("Descriptor was not received\n");
               close(worker_sd);
               exit(-1);
            }
            else
            {
               printf("Received descriptor = %d\n", pass_sd);
            }

            //.. Here the child process can handle the passed file descriptor
        }
    }

}



/* =======================================================================
 * The parent process
 * =======================================================================
 */

struct msghdr parent_msg;
size_t length;

/* Here the parent will accept the incoming requests and passed it to its children*/
for(;;)
{
    length = sizeof(client_address);
    if((accepted_socket_fd = accept(socket_fd, NULL, NULL)) < 0)
    {
        perror("accept()");
        exit(EXIT_FAILURE);
    }

    memset(&parent_msg, 0, sizeof(parent_msg));
    parent_msg.msg_name  = (char *) &accepted_socket_fd;
    parent_msg.msg_namelen = sizeof(accepted_socket_fd);

    if((sendmsg(server_sd, &parent_msg, 0)) < 0)
    {
        perror("sendmsg()");
        exit(EXIT_FAILURE);
    }

}

但是不幸的是，我得到了这个错误：

sendmsg(): Invalid argument

我应该怎么做才能解决这个问题？我msghdr是否正确使用该结构？因为在我上面提到的示例中，它们使用msg_accrights和msg_accrightslen，当我使用它们时遇到了一些错误，因此我不得不使用msg_name和msg_namelen。

Answer 1

这是很难做到的。我建议仅使用为您提供帮助的图书馆。最简单的一种是libancillary。它提供了两种功能，一种是通过UNIX域套接字发送文件描述符，另一种是接收文件描述符。它们使用起来非常荒谬。