How to use sendmsg() to send a file-descriptor via sockets between 2 processes?
Solution 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.
Solution 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.
Comments
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Eng.Fouad almost 2 years
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 usemsg_accrights
andmsg_accrightslen
and I got some error when I use them so I had to usemsg_name
andmsg_namelen
instead. -
Nakedible over 11 yearsThe user is not using AF_INET for socket passing, but an AF_UNIX socket created by socketpair.
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Jeff Learman over 8 yearsAfter sendmsg, what's the state of the fd in the server? Is it open and needs to be closed, or does sendmsg/cmsg close it? In example code I never see server closing the fd, which would be necessary if sendmsg doesn't close it.
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fuzzyTew almost 4 yearsI believe it stays open and the OS closes it when the process terminates if you don't first. Haven't fully checked that though.