///Passing File Descriptors Between Processes Using Sendmsg() and Recvmsg() Using this technique you can pass file descriptors between processes using ``sendmsg()`` and ``recvmsg()`` functions using UNIX Domain Protocol. Any descriptor can be passed using this method not just a file descriptor. This is quite useful when you want to balance load in a multi-core system. Other way of passing file descriptors is by forking the process, but in this case you can pass file descriptors between different processes at anytime. ## Creating the UNIX Domain Protocol server ``SOCKET_PATH`` was set to ``/tmp/unix_socket`` ```c int create_server() { struct sockaddr_un addr; int fd; if ((fd = socket(AF_LOCAL, SOCK_STREAM, 0)) < 0) { log_error("Failed to create server socket"); return fd; } memset(&addr, 0, sizeof(addr)); addr.sun_family = AF_LOCAL; unlink(SOCKET_PATH); strcpy(addr.sun_path, SOCKET_PATH); if (bind(fd, (struct sockaddr *) &(addr), sizeof(addr)) < 0) { log_error("Failed to bind server socket"); return -1; } if (listen(fd, MAX_PENDING) < 0) { log_error("Failed to listen on server socket"); return -1; } setnonblocking(fd); /* Add handler to handle events on fd */ return fd; } I used epoll for handling events, so adding a handler was something like this. ```c hash_set(ioloop->handlers, fd, handler); e.data.fd = fd; e.events = EPOLLIN; if(epoll_ctl(ioloop->epfd, EPOLL_CTL_ADD, fd, &e) < 0) { log_error("Failed to insert handler to epoll"); return -1; } Connections to the server should be accepted with accept() similar to TCP sockets. ## Connecting to the server ```c int connect_server() { struct sockaddr_un addr; int fd; if ((fd = socket(AF_LOCAL, SOCK_STREAM, 0)) < 0) { log_error("Failed to create client socket"); return fd; } memset(&addr, 0, sizeof(addr)); addr.sun_family = AF_LOCAL; strcpy(addr.sun_path, SOCKET_PATH); if (connect(fd, (struct sockaddr *) &(addr), sizeof(addr)) < 0) { log_error("Failed to connect to server"); return -1; } setnonblocking(fd); /* Add handler to handle events */ return fd; } I was adding ``EPOLLOUT`` listener to the socket whenever there were file descriptors to be passed to the other process. ## Receiving a file descriptor ```c static int recv_file_descriptor( int socket) /* Socket from which the file descriptor is read */ { int sent_fd; struct msghdr message; struct iovec iov[1]; struct cmsghdr *control_message = NULL; char ctrl_buf[CMSG_SPACE(sizeof(int))]; char data[1]; int res; memset(&message, 0, sizeof(struct msghdr)); memset(ctrl_buf, 0, CMSG_SPACE(sizeof(int))); /* For the dummy data */ iov[0].iov_base = data; iov[0].iov_len = sizeof(data); message.msg_name = NULL; message.msg_namelen = 0; message.msg_control = ctrl_buf; message.msg_controllen = CMSG_SPACE(sizeof(int)); message.msg_iov = iov; message.msg_iovlen = 1; if((res = recvmsg(socket, &message, 0)) <= 0) return res; /* Iterate through header to find if there is a file descriptor */ for(control_message = CMSG_FIRSTHDR(&message); control_message != NULL; control_message = CMSG_NXTHDR(&message, control_message)) { if( (control_message->cmsg_level == SOL_SOCKET) && (control_message->cmsg_type == SCM_RIGHTS) ) { return *((int *) CMSG_DATA(control_message)); } } return -1; } ## Sending a file descriptor ```c static int send_file_descriptor( int socket, /* Socket through which the file descriptor is passed */ int fd_to_send) /* File descriptor to be passed, could be another socket */ { struct msghdr message; struct iovec iov[1]; struct cmsghdr *control_message = NULL; char ctrl_buf[CMSG_SPACE(sizeof(int))]; char data[1]; memset(&message, 0, sizeof(struct msghdr)); memset(ctrl_buf, 0, CMSG_SPACE(sizeof(int))); /* We are passing at least one byte of data so that recvmsg() will not return 0 */ data[0] = ' '; iov[0].iov_base = data; iov[0].iov_len = sizeof(data); message.msg_name = NULL; message.msg_namelen = 0; message.msg_iov = iov; message.msg_iovlen = 1; message.msg_controllen = CMSG_SPACE(sizeof(int)); message.msg_control = ctrl_buf; control_message = CMSG_FIRSTHDR(&message); control_message->cmsg_level = SOL_SOCKET; control_message->cmsg_type = SCM_RIGHTS; control_message->cmsg_len = CMSG_LEN(sizeof(int)); *((int *) CMSG_DATA(control_message)) = fd_to_send; return sendmsg(socket, &message, 0); }