Dec 21 -- Let the Children Speak up!

Article:  https://ibr.cs.tu-bs.de/advent/21-sendfd/
Workload: ~155 source-code lines

21-sendfd/Makefile

+PROG=server
+
+all: ${PROG} client
+
+%: %.c
+	gcc $< -o  $@ -Wall -g
+
+run: ${PROG}
+	./${PROG}
+
+strace: ${PROG}
+	strace ./${PROG}
+
+clean:
+	rm -f ./${PROG} client

21-sendfd/client.c

+#include <sys/socket.h>
+#include <sys/signalfd.h>
+#include <sys/un.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <stdbool.h>
+#include <signal.h>
+
+
+#define die(msg) do { perror(msg); exit(EXIT_FAILURE); } while(0)
+
+// Receive a message with an attached file descriptor from a
+// connected UNIX domain socket. The message is stored in the buffer,
+// and the file descriptor in *fd. The function returns the length of
+// the message.
+//
+// sock_fd:      Connected UNIX domain socket
+// buf, bufsize: Buffer for the received message
+// fd:           Where to store the file descriptor
+int recvfd(int sock_fd, char *buf, size_t bufsize, int *fd) {
+    return 0;
+}
+
+int main(int argc, char *argv[]) {
+    // FIXME: Create a socket(2) with AF_UNIX, SOCK_SEQPACKET
+    // FIXME: Connect to the domain socket "./socket"
+    // FIXME: Receive the file descriptor with recvfd()
+    // FIXME: Add a read/write loop that transfers data from your stdin to the received file descriptor (like cat)
+}

21-sendfd/server.c

+#define _GNU_SOURCE
+#include <stdio.h>
+#include <unistd.h>
+#include <errno.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <assert.h>
+#include <limits.h>
+#include <sys/un.h>
+#include <sys/socket.h>
+
+#define die(msg) do { perror(msg); exit(EXIT_FAILURE); } while(0)
+
+// Send a file descriptor over an connected UNIX domain socket. The
+// file descriptor is send as auxiliary data attached to a regular
+// buffer. With Linux, we have to send at least one byte to transfer a
+// file descriptor.
+//
+// sockfd:      connected UNIX domain socket
+// buf, buflen: Message to send, arbitrary data
+// fd:          file descriptor to transfer
+void sendfd(int sockfd, void *buf, size_t buflen, int fd) {
+    // FIXME: Prepare a struct msghdr with an msg_control buffer
+    // FIXME: Attach the file descriptor as cmsg (see cmsg(3), unix(7))
+    // FIXME: Use sendmsg(2) to send the file descriptor and the message to the other process.
+}
+
+int main() {
+    // FIXME: Create an socket with AF_UNIX and SOCK_SEQPACKET
+    // FIXME: Bind it to a filename and listen
+    // FIXME: Accept clients, send your STDOUT, and directly close the connection again
+}
+