问题
After @cnicutar answers me on this question, I tried to send a file-descriptor from the parent process to its child. Based on this example, I wrote this code:
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);
}
}
But unfortunately, I got this error:
sendmsg(): Invalid argument
What should I do to fix this problem? and am I using the msghdr structure correctly? because in the example I mentioned above, they use msg_accrights and msg_accrightslen and I got some error when I use them so I had to use msg_name and msg_namelen instead.
回答1:
This is extremely hard to get right. I'd recommend just using a library that does it for you. One of the simplest is libancillary. It gives you two functions, one to send a file descriptor over a UNIX-domain socket and one to receive one. They are absurdly simple to use.
回答2:
The problem is you are passing the file descriptor in a msg_name field. This is an address field, and it is not intended to pass arbitrary data.
In fact, the file descriptors should be passed in a special way so the kernel could duplicate the file descriptor for the receiving process (and maybe the descriptor will have another value after the duplicating). That's why there is a special ancillary message type (SCM_RIGHTS) to pass file descriptors.
The following would work (I omitted some of the error handling). In client:
memset(&child_msg, 0, sizeof(child_msg));
char cmsgbuf[CMSG_SPACE(sizeof(int))];
child_msg.msg_control = cmsgbuf; // make place for the ancillary message to be received
child_msg.msg_controllen = sizeof(cmsgbuf);
printf("Waiting on recvmsg\n");
rc = recvmsg(worker_sd, &child_msg, 0);
struct cmsghdr *cmsg = CMSG_FIRSTHDR(&child_msg);
if (cmsg == NULL || cmsg -> cmsg_type != SCM_RIGHTS) {
printf("The first control structure contains no file descriptor.\n");
exit(0);
}
memcpy(&pass_sd, CMSG_DATA(cmsg), sizeof(pass_sd));
printf("Received descriptor = %d\n", pass_sd);
In server:
memset(&parent_msg, 0, sizeof(parent_msg));
struct cmsghdr *cmsg;
char cmsgbuf[CMSG_SPACE(sizeof(accepted_socket_fd))];
parent_msg.msg_control = cmsgbuf;
parent_msg.msg_controllen = sizeof(cmsgbuf); // necessary for CMSG_FIRSTHDR to return the correct value
cmsg = CMSG_FIRSTHDR(&parent_msg);
cmsg->cmsg_level = SOL_SOCKET;
cmsg->cmsg_type = SCM_RIGHTS;
cmsg->cmsg_len = CMSG_LEN(sizeof(accepted_socket_fd));
memcpy(CMSG_DATA(cmsg), &accepted_socket_fd, sizeof(accepted_socket_fd));
parent_msg.msg_controllen = cmsg->cmsg_len; // total size of all control blocks
if((sendmsg(server_sd, &parent_msg, 0)) < 0)
{
perror("sendmsg()");
exit(EXIT_FAILURE);
}
See also man 3 cmsg, there are some examples.
回答3:
You cannot send file descriptors over AF_INET. Use a UNIX domain socket.
来源：https://stackoverflow.com/questions/8481138/how-to-use-sendmsg-to-send-a-file-descriptor-via-sockets-between-2-processes